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POWER – What’s a watt?

Posted By Andy Kos

Over the years in the audio industry, I have made numerous attempts to explain some of the concepts used with regard to speaker/amplifier power. Most are summarised below, with links to some of the articles covering each topic in more detail. If you’re serious about sound, and curious about power, these are well worth a read, and will help you make more sense of power.

What is Power (Watts)?

Power is a measure of how fast energy is being used or delivered. One watt is defined as one joule per second, which in audio terms is the rate at which energy is transferred from an amplifier to a loudspeaker. High power for a short burst and lower power delivered continuously can result in the same total energy, which is why power must be considered over time.

For example, a 4 W burst for 0.25 seconds followed by 0.75 seconds of silence delivers the same energy as a 2 W burst for 0.5 seconds followed by 0.5 seconds of silence, or 1 W delivered continuously for 1 second.

This simplified example is an analogy for understanding peak, program, and continuous power. Peak power represents short, high-energy bursts, program power represents longer bursts with more time at a high level, and continuous power represents energy delivered without breaks. Although real music does not follow fixed duty cycles, all three cases above average to the same energy rate: 4 W × 0.25 = 1, 2 W × 0.5 = 1, and 1 W × 1 = 1.

This is why loudspeaker power ratings typically follow the same pattern: program power is usually twice the continuous rating, and peak power is typically four times the continuous rating. The numbers relate to the same underlying energy, but describe how that power is delivered over time.

RMS Power

RMS is a mathematical method that works extremely well for steady sine waves, such as AC mains power, where voltage and current are continuous and predictable. Music is not like this, so “RMS power” is not ideal for describing real audio behaviour. Some amplifier manufacturers still use the term, but it often includes a hidden crest factor or burst condition, meaning the figure is not a true continuous power level but a calculated equivalent.
Read more…

AES / Continuous Power

AES power defines how much power a loudspeaker can handle on average over time using a standardised broadband noise signal. It represents the long-term thermal limit of the voice coil and is the most reliable figure for continuous operation. Unlike RMS-style ratings, AES power is designed specifically for real audio signals rather than steady test tones.

Program (Music) Power

Program power allows for higher short-term peaks while keeping the long-term average power the same as the AES rating. It reflects the dynamic nature of music, where loud transients are followed by quieter moments. Program power is headroom, not extra continuous power, and should never be treated as a sustained operating level.
Read More on AES / Program Power

How much Power do you need?

What’s up with the Watts?
Power ratings in audio can be confusing because music is dynamic, not constant. Its hard to know what you want, what’s best and how to use power figures sensibly when choosing speakers and amplifiers.
Read More…

Pe – Power Handling Capacity

Often seen in manufacturers technical data, Pe is the long term power handling capacity, usually measured using the AES standard (but not always) and some manufacturers have their own test criteria and will often name this ‘nominal power handling’ This is not necessarily comparable between all speaker brands – also a little explanation as to why MORE POWER does not necessarily mean MORE VOLUME
Read More..

Filed in Power, Science of Sound | Tagged: , , , , , , ,

RMS Power

Posted By Andy Kos

What does RMS actually mean?

RMS stands for Root Mean Square. It is a mathematical method, not a type of power. It is normally applied to voltage or current, but for many years it has been used in the audio industry to describe amplifier and loudspeaker power.

Why? Simply because it was the best available method at the time, and no better, widely agreed standard existed.

I am often asked “What’s the RMS power?” My usual answer is that RMS is not particularly suitable for audio. If you want to understand why, read on. Otherwise, just accept that AES power is the standard you should be using for loudspeakers.

Why do we have RMS at all?

RMS comes from electrical engineering, where it works extremely well for AC power systems. It allows an AC signal to be converted into an equivalent DC value that produces the same heating effect.

In the UK, mains electricity is described as 240 V AC. That figure is already an RMS value. In reality, the waveform swings to about 339 V peak, or 679 V peak-to-peak.

The RMS figure is very useful. If an electric heater draws 10 A from a 240 V supply, we call it a 2400 W heater. The voltage and current are constant, the waveform is a steady 50 Hz sine wave, and the power delivery is continuous. This is a perfect use case for RMS.

[Image: AC sine wave showing peak, peak-to-peak, and RMS level]

Why RMS doesn’t map cleanly to audio

Audio amplifiers and loudspeakers do not operate with constant sine waves. Music is dynamic, the amplitude changes constantly, and power delivery is anything but steady.

To work around this, various test standards were created using controlled noise signals instead of tones. For loudspeakers, common examples included EIA RS-426A and IEC 268-5.

With a known test signal, it is possible to calculate an equivalent RMS value using averaging and squaring maths. This is where the idea of “RMS power” for speakers came from. However, it was never especially accurate, and often resulted in unrealistically low power ratings.

The amplifier vs speaker mismatch

Over time, it became normal to match amplifier and speaker ratings directly. For example, using a 400 W RMS amplifier with a 400 W RMS speaker.

The problem is that the two numbers were not measuring the same thing.

Amplifiers were often tested at 1 kHz using a continuous sine wave into a resistive load. This frequently overstated real-world power, because at lower frequencies (around 40–100 Hz) the power supply could not always sustain the same output. In practice, usable power at 100 Hz could be 10% lower than at 1 kHz.

Meanwhile, loudspeakers could often tolerate short-term peaks above their RMS rating. This is why users traditionally chose a slightly larger amplifier, to provide headroom.

Conclusion

RMS was not useless, but it was never a complete or accurate way to relate amplifier power to loudspeaker power. There was always an element of estimation and experience involved.

This mismatch is exactly why modern standards moved on, and why RMS power is no longer the best reference point for real-world audio systems.

Filed in Power, Science of Sound | Tagged: , ,

Making sense of power

Posted By Andy Kos

AES Power, Program Power, and Amplifier Power Explained

Power ratings in audio are confusing because they try to describe something that is constantly changing. Music is not a steady signal, yet power ratings are often presented as if everything operates at a fixed level. To make sense of AES power, program power, and amplifier power ratings, it helps to think in terms of power over time, rather than a single number.

Summary

AES power describes how much power a loudspeaker can handle on average over time. Program (music) power allows for higher short-term peaks, not higher continuous power. Real music delivers power in bursts rather than as a constant load. Modern amplifiers, especially Class D designs, are very good at producing high peak power briefly, while long-term power is limited by heat, power supplies, and mains capacity. The aim of this article is to show how average power, burst power, and time are related, and why a headline figure such as a 2000 W peak can be entirely real while still not representing the long-term power demand of a system. By looking at how music behaves over time, and using simple visual examples, it becomes much easier to understand how loudspeaker ratings, amplifier ratings, and even 13 A mains plugs all make sense in practice.

AES Power vs Program (Music) Power

Modern loudspeaker drivers are usually specified with two related power figures: continuous power (often defined using the AES standard) and program or music power.

AES power represents the long-term average thermal capability of the loudspeaker. It indicates how much power the voice coil can safely dissipate as heat over time using a defined broadband test signal. In simple terms, AES power is a safe long-term operating limit.

Program (music) power is typically quoted as twice the AES power, which corresponds to a +3 dB increase. Importantly, this does not mean the loudspeaker can handle twice the average power. The long-term average power remains the same as the AES rating.

The difference between AES and program power is not extra heat capacity, but crest factor. Program power allows for higher short-term peaks while keeping the long-term average unchanged. The signal is allowed to get taller for brief moments, but not heavier overall.

To understand why this distinction matters, it helps to look at how power is delivered over time.

Power over time: short, high peaks

In the first diagram, the vertical axis represents instantaneous power, and the horizontal axis represents time. The shaded red areas show when power is being delivered.

The diagrams are illustrative rather than literal. Their purpose is to explain concepts, not to define exact test conditions or limits.

Figure 1: Short-duration, high-peak power delivered in brief bursts over time.

Each rectangle represents a short burst of power:

  • Peak power: 2000 W
  • Duration: 0.5 seconds

Power multiplied by time gives energy. A 2000 W burst lasting 0.5 seconds delivers the same energy as 1000 W delivered for a full second:

2000 W × 0.5 s = 1000 W × 1 s

Although the instantaneous power is high, it is only present briefly. When averaged over a longer time window, the effective power is much lower. This is the key idea behind program power: higher peaks are allowed, but they do not increase the long-term average.

This type of power delivery closely resembles real music. Bass hits, kick drums, and transients are short, intense bursts separated by quieter moments.

To make the diagrams easier to understand, I have intentionally used simple numbers, 2000W, 1000W, 0.5 seconds – it makes the maths much easier. You can see each red rectangle is divided up into 8 smaller rectangles, which is easy to visualise.

Same energy, delivered differently

In the Figure 2, the same total energy is delivered in a different way. Instead of short peaks, power is delivered continuously. This is similar to electrical systems such as heaters, power is delivered continuously to a static load, the power level does not go up or down, it remains constant. Music is not like this, amplifier and music power is dynamic, and constantly changing. The power an amplifier delivers to the speaker, and draws from the mains supply varies with time.

Figure 2: The same total energy delivered as lower, continuous power over the same time period.

  • Power level: 1000 W
  • Duration: 1 second

The shaded area is the same size as in the Figure 1, which means the total energy is identical. The average power is also the same. You can see this visually: the area still covers exactly eight grid squares, just arranged differently.

In this example, the continuous 1000 W case does not represent a significant challenge for the amplifier. An amplifier capable of delivering 2000 W peaks will typically have no difficulty sustaining 1000 W continuously, as the average power and thermal load remain well within its design limits.

The real limitation appears when high power is sustained for longer periods. While short bursts of 2000 W are easily achievable, maintaining that level for several seconds places extreme demands on the power supply and output stage. Voltage rails sag, current limiting engages, and thermal protection may begin to operate.

This is where many Class D amplifiers reach their limits. They are designed to deliver very high short-term power with ease, but they are not intended to sustain maximum output continuously, particularly at low frequencies.

What this means for loudspeakers

From the loudspeaker’s point of view, heating depends on average power over time, not peak power. The voice coil does not care whether energy arrives in short bursts or steadily; what matters is how much heat builds up overall.

AES power therefore describes a realistic long-term thermal limit. It represents the average power a loudspeaker can dissipate safely over time without overheating. In the simplified examples shown here, this is illustrated by the second diagram, where the average power level remains constant.

Program or music power acknowledges that real music is dynamic and contains peaks and lulls. This is illustrated in the Figure 3, where the total shaded area remains the same, but the instantaneous power rises to higher peaks. The average power is unchanged, the program material just has a higher crest factor with more pronounced peaks and lulls.

Figure 3: Illustrating crest factor for music/program power.

This simplified example shows how a loudspeaker can safely handle higher short-term peaks, provided the long-term average power remains within the AES rating. Program power just shows the speaker can handle higher short term peaks, as long as the long-term average power remains within the AES limit.

Problems arise when program power is treated as a continuous operating level. If the programme material has little dynamic range, or if heavy compression and limiting are applied, the crest factor is reduced and the average power rises toward the peak level. In this situation the loudspeaker is no longer operating within its intended thermal limits and the voice coil can overheat.

Program power is therefore best understood as a headroom allowance for dynamic signals, not as a sustained power rating. It best represents live music, particularly percussive sounds. Electronic, synthesised music is often compressed, and has long extended bass notes with low dynamics.

What this means for amplifiers

Modern amplifiers, particularly Class D designs, behave much like the first diagram rather than the second.

They are extremely good at delivering short bursts of high power thanks to high-voltage rails and efficient output stages. This is why many modern amplifiers are rated using standards such as EIAJ, which better reflect burst capability and musical crest factor.

What these amplifiers cannot do is sustain very high power indefinitely, especially at low frequencies. Long, continuous bass notes place heavy demands on the power supply, causing voltage sag, current limiting, or thermal protection to intervene.

This is why amplifier power ratings often look impressive on paper but drop significantly under continuous sine-wave testing, particularly into low impedances.

Matching amplifier power to speaker power

Program power is useful when choosing an amplifier because it indicates how much headroom is available for musical peaks. A common and sensible approach is to choose an amplifier capable of delivering somewhere between the AES power and the program power of the loudspeaker.

This provides enough headroom for dynamics without pushing the driver beyond its long-term thermal limits. However, once amplifier power approaches program ratings, proper use of limiters and compressors becomes essential to prevent excessive average power.

How can this make sense on a 13A plug?

The same power-over-time logic also explains why large amplifiers can operate from ordinary mains supplies. To take this one step further, it is useful to look at a more realistic musical signal rather than a simple rectangle, which represents power being fully on or fully off.

Figure 4 shows a simplified ADSR-style envelope, loosely resembling a typical percussive sound such as a drum hit. The instantaneous power still rises briefly to around 2000 W, but the time spent at high power is much shorter than in the rectangular examples.

Figure 4: A simplified percussive envelope showing brief high-power peak typical of a real musical signal.

As a result, the total shaded area is smaller, meaning less total energy is delivered overall. Despite the high peak, the average power remains relatively low. This is exactly the type of signal that modern Class D amplifiers handle extremely well: short, high-power bursts delivered cleanly without clipping and distortion. To clarify, this is not intended to suggest that a Class D amplifier cannot sustain peaks for longer than shown – indeed most can. The diagram is simply a representative example of real-world music, intended to show how musical signals translate into long-term average power, and why the ability to handle short bursts of high power is important for preserving dynamics.

This behaviour explains why large amplifiers can produce impressive peak power figures while still operating safely from standard mains connections. The peaks are brief, the average power is modest, and the electrical system only needs to support the long-term average rather than the instantaneous maximum.

For reference, the percussive example above has been approximated into a final diagram (Fig. 5) showing the same energy spread out as continuous power over time. Although the instantaneous peak reaches around 2000 W very briefly, the equivalent long-term average power is much lower, in the region of 600W.

Figure 5: The same energy from Figure 4 redistributed as continuous power, showing the equivalent long-term average.

This illustrates an important point: a short, high-power percussive event may look extreme when viewed instantaneously, but when averaged over time it represents a far more modest power demand. Even when additional sounds are present, the medium-term average power may only rise to around 800-900 W.

Applied across a four-channel amplifier, this suggests that even when all channels are working hard, the combined long-term average power is often closer to 3000W rather than the headline peak figures. While this approaches the practical limits of a 13 A mains supply, real music contains loud passages, quieter sections, and natural breaks. These variations reduce the long-term average further, keeping operation within safe limits.

This is why high-power amplifiers can operate from standard mains connections. Peak power figures describe short-term capability, not continuous demand. In the case of amplifiers such as the JAM Systems Q10, which is rated at up to 2500 W EIAJ per channel into 2 ohms, the apparent mismatch between output power and a 13 A plug disappears once power is considered over time rather than at its instantaneous maximum. Realistically this amp is at the limits of a 13A supply, which is why it comes with a heavy duty mains cable with 2.5mm cable and a heavy duty plug.

After being asked countless times how the JAM Systems Q10 can operate from a 13A plug, this article was written to explain exactly that. This article now serves as the standard explanation.

The key takeaway

Power ratings make far more sense when you consider how power is delivered over time. Peak power, program power, and amplifier ratings all describe different aspects of the same thing: short-term capability versus long-term limits.

Many people dismiss peak power figures because of how terms such as PMPO were misused in consumer hi-fi, often wildly overstating real capability. However, genuine high burst power serves a real purpose. It allows the dynamics of the original programme material to be preserved, delivering very large transients when required, but only for short periods of time.

AES power defines what is safe on average. Program power defines how much headroom is available for musical peaks. Understanding the difference makes amplifier choice, system setup, and real-world behaviour far easier to predict.

Filed in Power, Science of Sound | Tagged: , , , ,

Thiele Small Parameters – A Quick Overview

Posted By Andy Kos

We’re still updating and improving these pages, they are intended as general guidelines to help with the understanding of T&S Parameters and their relevance to speaker design. In some cases, a simplified explanation or example is used to illustrate a point, and may not be 100% accurate in all circumstances.,

BL – Motor Strength or Force Factor > read more

BL (force factor) represents the strength of the magnetic motor system in a loudspeaker. It is calculated as the product of magnetic flux density (B) and length of wire in the magnetic field (L), measured in Tesla meters (T·m). A higher BL indicates a stronger motor, which generally improves control over the cone.

Mms – Moving Mass > read more

Mms refers to the total mass of all moving components of the driver, including the cone, voice coil, dust cap, and the air load around the diaphragm. A higher Mms generally results in a lower resonant frequency (Fs), which can help extend bass response, but it also requires more energy to move. Conversely, a lower Mms allows for better transient response, making it ideal for midrange drivers.

Cms – Compliance of the Driver Suspension > read more

Cms represents the compliance of a speaker’s suspension system, essentially measuring its flexibility. It’s the inverse of stiffness; a higher Cms indicates a more pliable suspension, allowing the cone to move more freely. This flexibility affects the driver’s resonant frequency (Fs); a more compliant suspension results in a lower Fs, enabling better low-frequency reproduction.

Rms – Mechanical Resistance of the Suspension > read more

Rms denotes the mechanical resistance within the driver’s suspension, quantifying the inherent damping due to the materials and construction of components like the surround and spider. Higher Rms values indicate greater energy loss, which can dampen cone movement and affect the driver’s transient response. Balancing Rms is crucial for achieving desired sound characteristics, as excessive mechanical resistance can lead to reduced efficiency and dynamic range.

Sd – Effective Diaphragm Area > read more

Sd refers to the effective surface area of the driver’s diaphragm (cone) that actively moves air. It’s typically measured in square meters (m²). A larger Sd allows the driver to displace more air, contributing to higher sound pressure levels, especially at low frequencies. Accurately determining Sd involves measuring the cone’s diameter and accounting for the surround’s contribution to the moving area.

η₀ (Eta Zero) – Reference Efficiency > read more

η₀ represents the efficiency of the driver, given as a percentage, showing how well it converts electrical power into acoustic output. A higher η₀ means the driver is more efficient and produces more SPL for the same input power. Efficiency is closely related to BL, Sd, and Mms, with high-efficiency drivers typically having a strong motor (high BL) and lightweight moving parts (low Mms).

Fs – Free Air Resonance > read more

Fs is the frequency at which the driver naturally resonates when not mounted in an enclosure. It is determined by the moving mass (Mms) and compliance (Cms) of the driver. Lower Fs values indicate better suitability for subwoofers, while higher Fs values are typical for midrange and tweeters, where tight cone control is needed.

Z – Nominal Impedance > read more

Z is the nominal impedance of the driver, typically 4Ω, 8Ω, or 16Ω, and represents the average electrical resistance presented to an amplifier. While Re (DC resistance) is slightly lower, the impedance of a driver varies across different frequencies due to Le (voice coil inductance) and resonance effects.

Vas – Equivalent Compliance Volume > read more

Vas represents the volume of air that exhibits the same compliance as the driver’s suspension. Measured in liters, it provides insight into how the driver interacts with the air in an enclosure. A larger Vas suggests a more compliant suspension, often necessitating a larger enclosure for optimal performance. Understanding Vas aids in designing speaker cabinets that complement the driver’s mechanical properties.

Pe – Power Handling Capacity > read more

Pe indicates the thermal power handling capacity of the driver, measured in watts. It defines the maximum continuous power the driver can handle without incurring thermal damage to components like the voice coil. Exceeding Pe can lead to overheating and potential failure. It’s essential to match the amplifier’s output with the driver’s Pe to ensure reliability and longevity.

Xmax – Maximum Linear Excursion > read more

Xmax defines the maximum distance the driver’s cone can move linearly in one direction without significant distortion. Measured in millimeters, it reflects the limits of the voice coil’s travel within the magnetic gap. Exceeding Xmax can cause nonlinear behavior, leading to distortion and potential mechanical damage. Designing with an appropriate Xmax ensures the driver can handle desired sound pressure levels without compromising sound quality.

Xlim – Maximum Physical Excursion Before Damage > read more

Xlim, also known as Xmech or Xdamage, specifies the absolute maximum excursion the driver can endure before mechanical failure occurs. Surpassing Xlim can result in physical damage to components such as the voice coil, spider, or surround. It’s crucial to ensure that the driver operates within safe excursion limits, especially in high-power applications, to maintain durability and performance.

Le – Voice Coil Inductance > read more

Le measures the inductance of the voice coil, typically in millihenries (mH). This parameter affects the driver’s impedance at higher frequencies, influencing the crossover design and overall frequency response. A higher Le can lead to a roll-off in the high-frequency response, making it essential to consider in the design of midrange and high-frequency drivers.

Re – DC Resistance of the Voice Coil > read more

Re denotes the direct current (DC) resistance of the voice coil, measured in ohms (Ω). It’s a fundamental parameter that influences the driver’s electrical damping (Qes) and overall impedance. Accurate knowledge of Re is vital for matching the driver with the amplifier and designing appropriate crossover networks.

Qes – Electrical Quality Factor > read more

Qes represents the electrical damping of the driver at its resonant frequency (Fs). It reflects how the driver’s electrical characteristics control its resonance. A lower Qes indicates higher electrical damping, leading to tighter control over cone movement, which is desirable in achieving accurate sound reproduction.

Qms – Mechanical Quality Factor > read more

Qms quantifies the mechanical damping of the driver, considering losses in the suspension system. It indicates how the mechanical properties influence the driver’s resonance. A higher Qms suggests lower mechanical losses, resulting in a more resonant system. Balancing Qms with Qes is essential for achieving the desired total damping (Qts) and overall sound quality.

Qts – Total Quality Factor > read more

Qts is the combined quality factor that accounts for both electrical (Qes) and mechanical (Qms) damping. It provides a comprehensive understanding of the driver’s damping characteristics at resonance. The value of Qts influences enclosure design decisions:

  • Low Qts (< 0.3): Suitable for horn-loaded enclosures, offering tight, controlled bass.
  • Medium Qts (0.3 – 0.5): Ideal for bass reflex (ported) enclosures, balancing efficiency and bass

Vd – Peak Displacement Volume > read more

Vd is the product of the effective diaphragm area (Sd) and the maximum linear excursion (Xmax). It quantifies the maximum volume of air the driver can displace, directly influencing its ability to produce low-frequency sound. A higher Vd indicates greater potential for bass output, making it a useful parameter when selecting a suitable woofer.

EBP – Efficiency Bandwidth Product > read more

EBP (Efficiency Bandwidth Product) is a quick way to estimate whether a driver is better suited for a sealed or ported enclosure, calculated as Fs / Qes. A low EBP (< 50) suggests the driver works best in sealed enclosures, while a high EBP (> 100) indicates it is more suitable for ported or horn-loaded designs. While useful as a guideline, EBP isn’t an absolute rule—other factors like Vas, Xmax, and BL also influence enclosure choice.

Filed in Loudspeaker Driver Parameters, TS Parameters | Tagged: , , , , , , , , , , , ,

What Power Driver Do You Need?

Posted By Andy Kos

What’s up with the Watts?business path choice

Choosing the right loudspeaker driver can feel like a minefield, especially if you’re new to PA sound. One of the most confusing areas is power ratings, usually quoted in watts.

To make matters worse, advances in materials and testing standards have seen many drivers increase their quoted power ratings by 25% or more, sometimes with no physical changes to the driver at all. Add in terms like RMS, Continuous, Program, and Peak power, and it’s no surprise there’s confusion.
This article mains to explain what those power ratings actually mean, clears up some common myths, and helps you choose sensible amplifier and speaker combinations.

Q: My amp is rated at 400W per channel. Will a 600W driver damage it by drawing too much power?

A: No. An amplifier’s power rating describes the maximum power it can deliver to a speaker before it reaches the limits of its power supply and begins to clip or distort. A loudspeaker’s power rating describes the maximum power it can safely absorb before overheating or failing.

Speakers do not “pull” power from an amplifier. Provided the impedance load is correct, a speaker cannot draw more power than the amplifier is capable of supplying.

Learn more about impedance and why it matters

Q: If I replace my 400W speakers with 450W speakers will they go louder?
A: Not necessarily. The limiting factor is usually the amplifier. If your amp is rated at 400W, you will not get more than 400W of clean output without distortion and potential amplifier damage. If your amplifier can deliver 450W, higher power handling may allow slightly higher output, but it may make no difference at all, and in some cases the speaker may even be quieter.
The key factor is efficiency. Some speakers convert electrical power into sound more effectively than others. If two speakers are operating at the same power level, the more efficient one will be louder. This is normally indicated by the sensitivity rating, measured in dB @ 1W / 1m.
Efficiency and Sensitivity

Q: Which power rating should I look at?

If you’re new to this, the most useful figure is the continuous power rating, usually specified using the AES standard. This gives a sensible indication of how much power a driver can handle of ‘continuous sound’ under realistic conditions and is the figure most manufacturers now quote.

You may also see a program or music power rating, which is typically around twice the AES rating. Peak power ratings are often quoted as four times the continuous rating and are of little practical use. If you see a peak figure, dividing it by four will usually give a reasonable idea of the true continuous power capability.

Q: What do the power ratings mean?

Continuous / “RMS” Power. Historically, loudspeaker power ratings were often quoted using continuous sine-wave tests, sometimes at a single frequency such as 1 kHz. These tests were easy to define and repeat, but they were not representative of real music and placed unusually high thermal stress on the voice coil.

The term “RMS power” is technically incorrect, as power itself does not have an RMS value; RMS applies to voltage or current. The RMS voltage is used in the power calculation, which is where the term originated. While RMS is useful for steady, resistive loads such as heaters or cable thermal calculations, it is not an ideal way to describe how loudspeakers behave with dynamic audio signals.

As a result, RMS power has largely been replaced by standardised noise-based tests that better represent music and broadband programme material, most notably the AES standard.

Over the years, manufacturers have used several recognised standards (a few still reference older ones):

  • IEC 268-5 (1978) – International Electrotechnical Commission
  • EIA RS-426-A (1980) – Electronic Industries Association
  • AES2-1984 – Audio Engineering Society
  • AES2-2012 – now the most widely adopted standard

When many manufacturers moved from the EIA standard to AES testing, some drivers saw power ratings increase by 25% or more without any physical design changes. One example we are aware of is a high-power 18″ driver that was re-rated from 600W to 800W purely as a result of the change in test method.

How is this possible? The AES standard defines a broadband pink noise signal with a specified crest factor for power testing, which differs from older methods. Many manufacturers implement the AES test using controlled noise over a defined period to stress the voice coil thermally, and this often results in higher quoted power figures compared to older standards. While the exact test duration and setup can vary by manufacturer, the AES rating provides a more consistent benchmark for comparing loudspeaker power handling.

Music / Program Power.

Often quoted as Program or Music power, this figure is typically defined as twice the continuous (AES or equivalent) power rating, representing a +3 dB increase. This does not mean the loudspeaker can handle twice the average power. Instead, program power allows for higher short-term peaks while the long-term average power remains the same as the continuous rating. In practical terms, this corresponds to a signal with a higher crest factor than the standard continuous test signal.

Real music contains peaks and valleys rather than a constant energy level. Program power reflects this by permitting greater instantaneous power during musical transients, provided the average power over time does not exceed the continuous rating. For system design, program power is best viewed as a headroom figure rather than a usable continuous operating level. Treating program power as a sustained power rating will almost certainly result in loudspeaker damage, however program power is useful for calculating amplifier power to get sufficient headroom. Opinions vary but most people suggest getting an amplifier slightly larger than AES power is a good start. If you want decent headroom, maybe aim halfway between AES Power and Program power – but at this point you have to start exercising caution with compressors and limiters to ensure the long term power rating does not get too high.

Peak Power:

This is the maximum very short-term power a driver can survive, and is typically quoted as four times the continuous (AES) power rating, representing a +6 dB increase. It exists almost entirely as a theoretical limit and has little relevance to real-world system design.

Peak power should not be used for amplifier matching, system sizing, or safe operating levels. Its primary practical use is for marketing, or for impressing someone who doesn’t know anything about sound.

Should I buy the most powerful speakers I can afford?

Probably not. It’s usually better to choose speakers that are appropriate for your amplifier power and intended use.

High power drivers are typically designed to survive large amounts of electrical and mechanical stress. This often involves design trade-offs such as heavier moving parts, longer voice coils, and suspensions optimised for high excursion. While these features increase power handling, they do not automatically increase efficiency.

As a result, a very high power driver is not necessarily louder at low or moderate power levels than a lower power driver with higher sensitivity. If you compare two extreme examples, such as a 100W driver and a 1000W driver, both driven from a 100W amplifier, the lower power driver may actually produce more output simply because it converts the available power into sound more efficiently.

The higher power driver only begins to show its advantage when sufficient power is available to drive it closer to its intended operating range. With a larger amplifier, it will ultimately produce far more output than the smaller driver ever could. However, when amplifier power is limited, choosing a driver with power handling far in excess of what the amplifier can deliver offers little benefit, and could be detrimental. Typically a 1000W woofer can be heavy and inefficient compared to a 200W woofer. On a smaller amp of 200W, the 200W woofer could actually play louder than the 1000W woofer which is inefficient.

In short, more watts on the specification sheet do not guarantee more sound. Sensitivity and appropriate system matching matter far more than headline power ratings.

So what if I exceed the power ratings?

You run the risk of overheating the voice coil and causing thermal failure. However, staying within the recommended power rating is not a guarantee of reliability.

Loudspeakers can also be damaged mechanically through excessive cone movement. This is described by excursion limits such as Xmax and Xlim. It is entirely possible to destroy a driver through over-excursion without ever exceeding its rated power, particularly at low frequencies or in poorly controlled enclosures.

There is also an important interaction between excursion, bandwidth, and cooling. In some situations a driver can reach its thermal limits at power levels well below its rated AES power.

For example, running a loudspeaker over a very narrow pass band (such as 30–40 Hz) in a cabinet tuned close to that frequency can result in extremely low cone excursion. While this may reduce mechanical stress, it also reduces air movement around the voice coil. Since many drivers rely partly on cone motion to aid cooling, limited excursion can significantly reduce heat dissipation.

In these conditions, particularly in small enclosures with restricted airflow, the usable thermal power handling may be substantially lower than the published AES rating. In extreme cases it can be closer to 50% of the rated value, despite excursion remaining well within safe limits.

What about Power Compression?

Power compression is the hidden problem that can upset even the best-planned systems and make published specifications feel misleading. Many manufacturers choose not to quote power compression figures at all, and some avoid mentioning it entirely.

Loudspeaker sensitivity is specified at 1 W measured at 1 m. At this very low power level, the voice coil remains cool and the driver is highly efficient at converting electrical energy into sound.

In real use, voice coils heat up. Most are wound with copper, which has a positive temperature coefficient of approximately 0.39% per degree Celsius. It is entirely possible for the voice coil of a high-power driver to reach temperatures approaching 200°C, resulting in a resistance increase of 50% or more.

As the voice coil resistance rises, the effective impedance of the driver increases. An 8 Ω loudspeaker may behave more like a 13–14 Ω load at high power. The amplifier delivers less current, acoustic output drops, and a significant proportion of the input power is lost as heat rather than sound.

The practical result is reduced output at high drive levels. A well-designed driver with good thermal management and low power compression can be 3–4 dB louder at full power than an otherwise similar driver that suffers heavily from compression. For this reason, modern high-quality designs place increasing emphasis on cooling and heat dissipation to minimise power compression.

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Power Compression

Posted By Andy Kos

When selecting speakers, it’s common for people to just look at maximum power handling, and many manufacturers make a point of specifying seemingly unbelievable power handling capacity of 1000W or more. Its quite rare for manufacturers to specify power compression though, and it seems to be often overlooked by system designers.

It seems that loudspeakers to handle what appear to be insanely high levels of power compared to 10 or 15 years ago. Has there been some amazing technological breakthrough? Do we need to re-write the physics text books? No, it’s still just basic physics – so what are the changes?

Firstly, modern materials used in the construction of voice coils are able to withstand significantly higher temperatures before failing.  Why is this important? Well Cone loudspeakers are in fact very inefficient, with even the best transducers only converting around 5% of the electrical energy supplied into sound, the majority of the remainder is converted into heat. So a 1000W bass speaker running at full power may well be converting only 50W into acoustic power, and 950W of heat. Thats like having a 1kw bar heater in your bassbin! That’s a lot of heat, which can cause big problems.

Aside from improving construction materials, manufacturers are also refining designs to maximise heat transfer away from the voice oil, this also contributes to the increases in power handling capacity we are experiencing.

What’s all this got to do with power compression?

Enabling speakers to handle much higher temperatures might seem a good thing, as it increases maximum power handling, but it also has a detrimental effect. Most voice coils are made from copper or aluminium wire, both of which have a positive temperature co-efficient of around 0.4% per °C. What’s the significance of that? You will have heard of superconductors, which operate at extremely low temperatures in order to try to reduce and minimise resistance.  Loudspeaker voice coils  unfortunately work in the opposite way: as the temperature increases, the resistance also increases.

A modern state of the art voice coil is designed to withstand extremely high temperatures, often operating at up to 3000C or more when driven at full power. 0.4% may sound insignificant, but remember this is per °C – at only 2300C the voice coil DC resistance has almost doubled which causes the voice coil impedance to increase accordingly. Some simple maths and you can quickly see that the increase in temperature  can make your 8 ohm speaker start behaving more like a 16 ohm speaker.

So after setting your sound system carefully at the start of the night, an hour in, and it doesn’t sound as loud – you might wonder whats going on. Two things: firstly, your ears have a self defence mechanism: there are 2 tiny muscles in the middle ear that will contract when the ear is exposed to loud sounds. This contraction will reduce the loudness of the sounds reaching the inner ear, thereby protecting the inner ear against exposure to loud noises. This isn’t power compression, but it’s something to be aware of, as you may well be tempted to turn up the volume, I know from experience that a typical DJ will certainly try this, and end up running his mixer into overdrive in the attempt to get more volume.

The second factor is power compression, a typical loudspeaker can lose 3-6 dB of volume once power compression kicks in.

You could think of power compression a bit like the aerodynamics of driving a car. When you start moving, a certain level of power from your engine sets you hurtling forwards at high speed, but as you go faster, wind resistance increases, so you stop accelerating. You need to apply more power to increase speed, but wind resistance keeps increasing too, so you have to apply even more power.

If your amplifiers have headroom, your instincts will make you want to turn them up, to restore the original volume level. To some extent this will work, if you’re familiar with the maths, you’ll see whats going on. Your 8 ohm speaker at room temperature happily accepts 1000W from your amplifier, and gradually reaches an operating temperature of say 250°C. Your resistance has doubled, and your ‘new’ 16 ohm speaker will probably only be receiving around 500W from your amplifier. In a way, as the speaker reaches temperature, it ‘protects itself’ by reducing the power it is operating at, stopping it getting any hotter. If it were to cool a little, the power would increase again, causing it to heat up.

Lets suppose you turn the gain up on your amplifiers, determined to try to push 1000W through your speakers. As you apply more power, you will generate more heat,  perhaps reaching 350°C or more, with your speakers resistance continuing to increase to perhaps 20 or more ohms. Essentially you are fighting a losing battle, as you turn the gain up, the speaker fights back with a higher resistance. You will eventually reach a limit, either your amp will run out of headroom and you cant turn it any louder, or the other possibility, which happens all too often, is your speaker will overheat, and burn out causing catastrophic failure.

Now you know about power compression and the fact that speaker resistance increases with heat, you’ll probably realise that you actually have to push a speaker very hard in order to cause it fail – so if your speaker suddenly fails and you smell burning, the only person to blame is YOU, as you now know better than to try to fight power compression by applying more power.

Now consider what effect power compression will have. 3-6dB loss at full operating power is almost like switching off half your PA system. To achieve the same consistent volume you will need twice as many speakers!

What’s the solution? Either buy speakers with headroom, e.g. if you want to operate at around 500-600W, you might want to consider purchasing speakers rated at 800W or more. At 75% of rated power, the effects of power compression should be much less significant. Also, try to select speakers with improved cooling technology, that suffer less from power compression. Avoiding power compression could make your speakers twice as loud, meaning you could take half as many to the gig!

There are other side effects from the increased levels of heat in a speaker, T/S parameters can vary, bass can sound boomy and mid frequencies can sound muffled. For the best sound quality, its best to try to  minimise power compression effects,

 

 

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Impedance – FAQs

Posted By Andy Kos

How do I know what impedance load I have?

Most manufacturers will specify impedance, and will include it in the product specifications, often printing it on the speaker itself. If you don’t have this information, you can measure the DC resistance using a multi-meter (please note Resistance is NOT Impedance – find out why here: https://speakerwizard.co.uk/impedance-and-resistance-whats-the-difference/

You should only measure the resistance of speakers when they are not in use, and not connected to an amplifier. By putting your multi-meter probes on each terminal of the speaker you will get the DC resistance, which can be used as a guide to get the impedance. A DC resistance of 5-6 ohms is normal for a driver with 8 ohm impedance, around  12-13 ohms  is common for  a 16 ohm impedance driver, and  3 ohms DC resistance would be normal for a 4 ohm impedance. You may notice that moving the cone whilst checking the resistance will make the reading change, this is because the voice coil is moving in a magnetic field, which will induce a voltage in the  coil, which in turn will affect the multimeter’s measurement.

Many loudspeaker manufacturers will label the drivers to make identification easier, Eminence for example include a suffix on the drivers, for example the Beta12A is the standard model, and is 8 ohm impedance, the letter A designated 8 ohm impedance. The Beta 12B is 16 ohm impedance, and the Beta 12C is 4 ohm impedance. This same letter designation is used through the range of Eminence speakers.

I have more than one speaker in parallel – what’s the impedance?

First, let’s clarify what we mean by parallel, this is where the electrical paths through the drivers from + to – run in parallel to each other. If you trace a route from + to – you go through either one driver, or the other. The diagram below shows two speakers wired in parallel:

parallel

 To wire speakers in parallel, all you have to do is connect the + (positive or red terminal) on each speaker to the + (positive or red terminal) on your amplifier, and the corresponding – (minus or black terminal) on the speaker to the – (minus or black terminal) on your amplifier. If you plug several speakers into one amplifier, unless you have unusual cabling, this would be the standard way you would run several speakers off one amplifier.

Its normal to put speakers of the same impedance in parallel with each other, mismatching impedances isn’t a great idea unless you have a fairly advanced knowledge of speaker systems and are doing this for a specific purpose.

So what does this do to the impedance?

The impedance of each speaker stays the same, but the impedance load the amplifier sees will change. In the diagram above, if the two speakers were both 8 ohm impedance, the load the amplifier would see is 4 ohms. To think of this in simple terms, you could think of one loudspeaker as a busy road with a specific amount of traffic travelling along it, if you have two roads, the traffic can travel along either road, which presents less ‘resistance’ to the same amount of traffic. With a basic knowledge of maths, and using this analogy of two routes between start and finish, you can guess what the resistance of two parallel 8 ohm drivers would be, it’s half that of one 8 ohm driver, and is 4 ohms.

The formula for calculating parallel resistances is as follows:

parallel_formula_web

R1, R2, R3, are the individual resistances, the formula works for as many, or as few resistances there are in parallel, for two drivers in parallel, you use R1 and R2 only, for three drivers you use R1, R2 and R3.

RT is the total parallel resistance. For equal parallel resistances, the formula becomes very simple, as the table of parallel 8 ohm impedances shows:

No drivers Parallel Impedance Fraction
1 8 ohms 1/1
2 4 ohms 1/2
3 2.6 ohms 1/3
4 2 ohms 1/4
5 1.6 ohms 1/5
6 1.3 ohms 1/6

As you can see, 3 drivers gives a combined parallel impedance of one third of the original impedance of 8 ohms, and 4 drivers gives a combined parallel impedance of one quarter of the original impedance.

Very few amplifiers will run happily into impedances below 2 ohms, and there is a strong possibility you can damage the amplifier by plugging too many speakers into it. Some amplifiers will not work safely below 4 ohms, so it’s quite important to ensure you have the correct load on your amplifier.

How do I wire speakers in series?

The term series where things are arranged in sequence implies how you would arrange speakers in series, as per the diagram below you can see that the positive (+) terminal of the first speaker is connected  to the positive (+) of the amplifier as normal, but the negative  (-) terminal goes the the positive terminal of the second speaker. The last speaker in the series has it’s negative (-) terminal connected to the negative (-) terminal of the amplifier.

series_web

Series impedances work opposite to parallel, going back to the comparison with traffic, if your busy road has traffic lights in it, every extra set of traffic lights adds more resistance to traffic flow. In the same way, each loudspeaker in series adds to the impedance. To calculate the total impedance, simply add together the individual impedances, as shown in the table below. In most instances, its rare to have more than 2 drivers wired in series, as the increase in impedance will mean most amplifiers are able to deliver very little power to the drivers.

No drivers Series Impedance
1 8 ohms
2 16 ohms
3 24 ohms
4 32 ohms

 If we get less power, what’s the point of connecting drivers in series?

If you just have one pair of speakers, there isn’t much point, but it gets interesting when you have multiples of speakers. If for example you have four speakers, that are 8 ohms, and you want to run all four speakers off one amplifier, you could wire all four in parallel, to give a 2 ohm load, or all 4 in series to get a 32 ohm load. But what if your amplifier wont work below 4 ohms?

The solution is simple, a series-parallel combination:

series_parallel

 

Assuming all drivers are 8 ohms, some simple maths and you can see that each of the two series combinations has an impedance of 16 ohms. Two 16 ohm impedances in parallel have an overall impedance of 8 ohms. What this allows you to do is use four speakers where you would previously have only used one, giving you a significant increase in power handling.

Variations of series-parallel configurations are common in guitar speakers,  4 x10″ and 4 x 12″ cabinets are common, with different wiring to suit specific applications and impedance requirement. Many guitar cabinets utilise 16 ohm drivers in order to achieve the desired results.

Its sometimes advised that its best to avoid using series configurations with speakers, due to the fact that that you have two coils or inductors which can induce unwanted voltage and cause distortion. Series configurations are rarely used in hifi or studio systems.

 

 

 

 

 

 

 

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Impedance and Resistance – what’s the difference?

Posted By Andy Kos

Why does my 8 ohm speaker read 6 ohms when I measure it on a multimeter? It must be faulty right?

WRONG!

I’ve heard this so many times I’ve lost count, but there is a difference between impedance and resistance. When you measure resistance with a multimeter you are measuring DC resistance. The DC resistance is determined by the copper (or sometimes aluminium) wire in the voice coil of the speaker, and is actually as the name suggests; resistance to the passage of electric current through the copper. The key point here is that the electrical current travels in one direction only, and is fixed and does not change.

Impedance is equivalent to resistance, but for circuits where the voltage and current change, such as in a loudspeaker. An extra factor comes into play, which is the fact the the loudspeaker is based on a coil of wire. This coil of wire acts as an inductor. Without getting too involved in the science part of this, its sufficent to know the inductor creates an additional ‘reactance’ to alternating signals, which when added to the DC resistance of the voice coil, gives the overall Impedance.

To complicate matters further, the Impedance varies with frequency, so the 8 ohms specified for loudspeakers is not totally accurate, it is referred to as ‘nominal impedance’ – a kind of ‘average’ impedance figure that can be used for typical calculations involving loudspeakers. The graph below show a typical 18″ subwoofer, the impedance is shown on the scale on the left hand side.

impedance

For purposes of being able to run your own sound system, or building your own speakers, it’s sufficient to accept the manufacturer’s quoted impedance as being correct for your application. You don’t need to be concerned with the finer points of impedance unless you get into more serious aspects of speaker design, and if you’re at that level, I highly doubt you will have bothered read this far, as you will know all of this already!

 

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Qts – Total Quality Factor: The Balance Between Electrical and Mechanical Damping

Posted By Andy Kos

Qts is one of the most critical Thiele-Small parameters when designing a speaker system. It represents the total quality factor of a driver, combining both electrical damping (Qes) and mechanical damping (Qms) into a single value. Understanding Qts is useful for determining the best type of enclosure for a driver. Getting the right Qts for a bass reflex enclosure ensures efficient output, strong transient response, and extended bass performance.

What Exactly Is Qts?

Qts is a dimensionless number that describes how well a driver controls its own resonance. It is calculated using the following formula:

Where:

  • Qms = Mechanical quality factor (how well the suspension controls cone movement)
  • Qes = Electrical quality factor (how well the voice coil and magnet control movement)

A lower Qts means more damping, resulting in tighter, more controlled motion. A higher Qts means less damping, allowing the driver to resonate more freely. A higher Qts driver tends to demand a larger cabinet to operate most effectively, so choosing the right driver with the right Qts is very important for almost every speaker cabinet design.

The Best Qts Range for PA Speakers

For PA speakers, especially bass reflex (ported) enclosures, the ideal Qts range is:

✅ 0.30 – 0.45 → Best for ported PA subwoofers & woofers
✅ 0.35 – 0.38 → The sweet spot, balancing efficiency, transient response, and bass output
✅ Above 0.45 → Can still work in ported enclosures, but requires a larger cabinet

A Qts below 0.3 is generally found in horn-loaded enclosures, where tight cone control and efficiency are prioritized, and the driver will work happily with a small rear chamber. There are sometimes exceptions, these are intended as guidelines only, to help make an informed choice if you’re just starting blindly at a wall of numbers.

How Qts Affects Ported Enclosures

  • Qts 0.30 – 0.38 → Balanced sound with good transient response and deep bass.
  • Qts 0.38 – 0.45 → More extended bass possible, but less transient snap.
  • Qts above 0.45 → Requires a larger cabinet to compensate for weaker motor control.

For PA subwoofers and woofers, the ideal Qts keeps the cabinet size reasonable while ensuring powerful, clean bass.


PA Speaker Examples

Driver TypeTypical Qts RangeBest Enclosure Type
PA Subwoofer (Ported)0.30 – 0.38Bass Reflex (Ported)
General PA Woofer0.35 – 0.45Ported, some larger designs
Horn-Loaded Subwoofer0.15 – 0.30Horn-Loaded

🔹 Example 1: A Qts = 0.35 subwoofer is ideal for high-efficiency ported enclosures, delivering tight, punchy bass.
🔹 Example 2: A Qts = 0.42 woofer can still work in a ported cabinet, but may require a larger box to compensate.
🔹 Example 3: A Qts = 0.20 subwoofer would likely underperform in a ported box, but excels in a horn-loaded design.

Final Thoughts

For PA systems, getting the right Qts for a ported enclosure is crucial.

✅ The sweet spot for PA ported enclosures → 0.35 – 0.38 (from our experience)
✅ Avoid Qts above 0.45 unless using a very large cabinet
✅ Below 0.3 is best suited for horn-loaded designs



Filed in Loudspeaker Driver Parameters, TS Parameters

Sd – Effective Diaphragm Area

Posted By Andy Kos

What Is Sd?

Sd (Effective Diaphragm Area) is the active surface area of a speaker cone that moves air to produce sound. It’s usually measured in square meters (m²), but sometimes also specified in square centimeters (cm²). Sd is most often used for calculating other TS parameters, and its fairly common for all woofers with a certain diameter to have virtually the same Sd, this is because it can be calculated directly from the speakers diameter:

Where:

  • Sd = Effective diaphragm area (m²)
  • D = Effective cone diameter (meters)
  • π (pi) = 3.1416

Note: The effective diameter usually excludes the surround—only the part of the cone that actively moves air is considered – this can be hard to determine in some cases as some of the surround does move with the cone. For precise Sd, advanced methods are required to accurately determine the active surface area.

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