Loudspeaker Driver Parameters Archive

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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.

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What Power Driver Do You Need?

Posted By Andy Kos

What’s up with the Watts?business path choice

Choosing the right driver for your speakers can be something of a minefield, particularly if its your first time. There are a number of things to be aware of, one of which is the power rating (specified in Watts)

Technological advances in materials are allowing Loudspeaker drivers to be developed with larger power handling, at the same time we have seen some loudspeakers drivers have their power ratings changed, with increases of 25% or more, but with apparently no change to the driver. Add to this the confusion of RMS Power, Continuous Power, Program Power, and Peak Power and it’s not suprising some of you are getting confused. This article aims to explain some of terms used, and dispel a few myths.

Q: My amp is rated at 400W per channel, will a 600W driver break my amp by drawing too much power?

A: NO. The power rating of your amplifier is a measure of how many watts it can deliver to the speakers before it reaches the limitations of its internal power supply and starts to clip/distort.  The power rating of your speakers is the maximum power they can accept from an amplifier before they are in danger of overheating and burning out. Providing you have the corrent impedance load, your speakers are unable to draw more power than the amplifier is willing to give. https://speakerwizard.co.uk/impedance-faqs/

Q: If I replace my 400W speakers with 450W speakers will they go louder?

A: Not necessarily, the limiting factor is generally your amplifier, if you amp is rated at 400 Watts, you wont get any more than 400W output without severely distorting the sound, and potentially damaging your amplifier. If your amp can output 450W, then an increase in power handling may make your speakers go a little louder, but its possible they may be no louder, or in some cases quieter. The key factor here is efficiency, some speakers are more efficient than others. If you have two speakers operating at the same power level, and one is more efficient at converting electricity into sound, it doesnt take a rocket scientist to figure out which of the two will be louder. Most manufacturers give an indication of efficiency using the sensitivity figure measured in db@1W/1M (link to sensitivity)

Q: Which power rating should I look at?

If you’re reading this, chances are you are a novice, so for simplicity we suggest you use the continuous RMS power rating. Luckily, this is the one most manufacturers specify. You may also see Music Program Power Ratings, typically these are double the  RMS power rating. If you see a peak power rating, its often meaningless, and serves little purpose. Peak power ratings are often four time the RMS rating, so if you see this anywhere, divide it by 4 to give you an idea of the real power rating.

Q: What do the power ratings mean?

RMS Power.

Sometimes referred to as Average Power, or Continuous Power. The term RMS here is incorrectly used, it is not RMS power in the true sense of the term, as Power does vary from positive to negative, it is the Voltage. The RMS Voltage is used in the power calculation, hence giving rise to the term ‘RMS Power’.

Over the years, various standards have been used in this ‘RMS Power’ calculation, including;

The IEC268-5 (1978) standard (IEC = International Electrotechnical Commission)

The EIA RS-426-A (1980) standard (EID = Electronic Industries Association)

The AES2-1984 standard (AES = Audio Engineering Society)

The AES2-2012 standard – which is becoming the most commonly adopted standard. Previously many manufacturers used the EIA standard, re-ratings some speakers using AES saw increases in power ratings of 25%, sometimes more. One example we are aware of was a high power 18″ driver, which was previously rated at 600W, and had the power rating changed to 800W overnight – with no changes whatsoever to the speaker design.

How is this possible? The AES power rating has a higher crest factor than the EIA rating, and is also for a shorter duration 2 hours for AES whereas the EIA rating was over an 8 hour test period. Some would argue this is not a totally realistic test. However for the layman, it is a useful benchmark for matching up driver power to speaker power. We’ll explain why:

Music Power:

Also know as Program Power, this is an indicator of the power rating of speaker use with ‘typical program materal’, which most of us call music. The RMS Power test is not done with music, it is generally done with bandwidth limited pink noise, which is a continuous signal. When was the last time you played music that sounded like static? Not often I bet. Music power takes into consideration that a bass beat is not continuous, it is a series of pulses. In between the pulses, there is no power being applied to the speaker, so when you average out the power over time, you can handle much more power.

What does this mean? Well you can exceed RMS Power for short periods of time, but not with a continuous signal. So if you play average music, you can usually run above RMS Power level quite happily in most instances, and with appropriate limiters in place, somewhere between RMS power and Music Power is generally a safe level. This is why I’m saying the Music Power is a useful benchmark – it helps determine amplifier power choice ‘roughly’ and is a good level to aim at to keep your drivers operating within safe parameters, if like most people, you try to push things just a little harder every now and then, you should still be fine, just as long as you don’t start treating music power as a long term continuous power rating, as you will almost certainly cause your speakers to fail if you do this.

Peak Power:

This is the maximum short term power that can be applied to the driver, and is typically calculated to be four time the RMS Power. I recommend you dont use Peak Power for anything except bragging to someone who doesnt know anything about sound.

Should I buy the most powerful speakers I can afford?

Probably not – It’s best to get speakers appropriate to your requirement. This is partly due to how speakers work. High power speakers are designed to have a high excursion (thats lots of cone movement) – in order to be able to handle the extra power, they typically have stiffer components, particularly with regard to the suspension. These high power speakers require a certain amount of power to overcome the mechanical resistances of the suspension. Comparing two extreme examples, of say a 100W speaker and a 1000W speaker running bass. The 100W speaker will have a low output for say, the first 5-10W put into it, once you are putting in 50W or so, the speaker will be at half its rated power, and will be (depending on sensitivity) fairly loud, ramp it up to 90-100W and the driver is giving all it can, potentially operating at its most efficient point. Lets suppose you only have a 100W amplifier, the 100W speaker will give you more output than the 1000W speaker. If you put your 1000W woofer on the end of your 100W amplifierm the first 40-50W of power will be used inefficiently, just to overcome the stiffness and resistance of the suspension, ramp it up to 100W, and you are still only tickling the 1000W woofer, you will find that you may need 200W running through the 1000W woofer to be as loud as the 100W woofer. As you keep ramping up the power, the 1000W woofer will ultimately create a lot more sound output than the 100W speaker ever will, but if you only have a small amplifier, you are better off with an appropriately matched driver.

So what if I exceed the power ratings?

You run the risk of overheating the voice coil of the speaker, and causing it to fail – but be careful – just keeping an driver within it’s recommended power rating is no guarantee of longevity. You also need to be away of excursion (often specified using Xmax and Xlim) as you can damage an driver through over-excursion without exceeding the power rating.

What about Power Compression?

Power compression is the little bugbear that can upset your best laid plans, and give you reason to throw the manufacturer’s specifications out the window. Many manufacturers choose to ignore power compression, some actively avoid specifying it or even mentioning it.

The sensitivity (efficiency) manufacturers specify is measured at a power level of 1W at a distance of 1m from the speaker. At 1W, very little heat is lost within the voice coil, so the driver is more effective at converting electricity into sound.

Its very common for loudspeaker voice coils to be wound from copper, which has a positive temperature co-efficient of +0.393% per degree C. It’s quite feasible for the voice coil of a high power speaker to reach 200 degrees Celsius, which could mean a change in the resistance of the wire to increase of 50% or more. Your 8 ohm driver will no longer be an 8 ohm driver, and the nominal impedance could rise to as high as 13 or 14 ohms. At full power, many drivers are no longer operating at their stated impedance, and they generating a lot of heat.

A good quality driver, with low power compression at its full rated power, could be 3-4 dB louder than a driver that suffers badly from power compression. Many modern designs are taking account of this, and making efforts to ensure driver cooling is maximised to counter the effects of 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.

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

Rms – Mechanical Resistance of Suspension

Posted By Andy Kos

Rms, or mechanical resistance, describes how much damping the speaker’s suspension provides to control cone movement. Think of it like shock absorbers in a car—too much resistance, and the suspension is stiff and unyielding; too little, and it becomes too loose, leading to uncontrolled movement.

Rms is directly linked to Cms and Fs, as can be seen in the formula below:

What Does Rms Actually Do?

Rms affects how quickly the cone stops moving after being displaced. A higher Rms means more damping, which helps prevent unwanted resonances but can reduce efficiency. A lower Rms means less damping, allowing for more movement but potentially leading to excessive ringing or overshoot.

How Rms Affects Speaker Performance

  • Higher Rms (More Damping) →
    • The cone stops moving quickly after a signal ends
    • Prevents excessive ringing and improves transient response
    • Often found in PA and midrange drivers, where control is crucial
    • Can reduce efficiency because more energy is absorbed
  • Lower Rms (Less Damping) →
    • The cone moves more freely, leading to longer decay times
    • More efficient at converting electrical energy into sound
    • Often found in subwoofers, where extended low-frequency response is desirable
    • May require careful tuning to avoid unwanted resonances

How Rms Relates to Qms

Rms directly affects Qms, the mechanical quality factor of a driver.

  • A low Rms results in a high Qms, meaning the driver has lower mechanical losses and rings for longer.
  • A high Rms leads to a low Qms, meaning mechanical energy is dissipated more quickly, reducing ringing.

For example:

  • A PA midrange driver may have Rms = 5 kg/s and Qms = 2-3 for precise, controlled response.
  • A subwoofer may have Rms = 1.5 kg/s and Qms = 7-10 to allow for more free movement and extended bass.

Rms and Speaker Efficiency

Higher Rms means more energy is absorbed as heat in the suspension, making the driver less efficient. That’s why high-efficiency speakers (like horn-loaded designs) often have low Rms, reducing mechanical losses.

Real-World Example of Rms in Different Drivers

Driver TypeTypical Rms (kg/s)Effect on Performance
PA Midrange4 – 6Tight control, minimal ringing
Hi-Fi Woofer2 – 4Balanced damping for clarity and bass extension
Subwoofer1 – 2More excursion, deeper bass, less damping

Final Thoughts

Rms might not be the most commonly discussed Thiele-Small parameter, as typically most people focus on Cms or Vas. These are ways of quantifiying more or less the same thing, just in slightly different ways.

Filed in Loudspeaker Driver Parameters, TS Parameters

Cms – Compliance: The Suspension’s Flexibility

Posted By Andy Kos

Cms, or compliance, is a measure of how flexible the suspension of a speaker driver is. If you’ve ever tapped on a speaker cone and noticed how easily (or not) it moves, you’ve just gotten a feel for Cms in action. It’s essentially the inverse of stiffness—a higher Cms means a more flexible suspension, while a lower Cms indicates a stiffer suspension.

How Does Cms Affect Speaker Performance?

Cms plays a critical role in determining a speaker’s resonant frequency (Fs). The relationship is simple:

  • High Cms (soft suspension) → Lower Fs (good for deep bass)
  • Low Cms (stiff suspension) → Higher Fs (better for midrange or high-output designs)

If you’re designing a subwoofer, you typically want higher Cms so the cone moves freely and reaches low frequencies more easily. On the other hand, PA midrange drivers often have low Cms, giving them a stiffer suspension for better control at higher frequencies.

Cms, Excursion, and Control

While a softer suspension helps a driver reach lower frequencies, it comes with trade-offs. If the suspension is too compliant, the cone may overshoot and take longer to return to rest, leading to poor transient response (i.e., sloppy bass). On the flip side, a stiff suspension keeps movement tight and controlled, but it also limits how deep the driver can go in the bass range.

This is why Cms is not just about flexibility, but about balancing flexibility with control—just like a car’s suspension. Too soft, and you’re bouncing all over the road. Too stiff, and every bump feels like a punch.

The Link Between Cms and Vas

If you’ve seen Vas (Equivalent Compliance Volume) on spec sheets, it’s directly related to Cms. Higher Cms leads to a larger Vas because a soft suspension behaves as if it’s moving a large volume of air. Likewise, a stiff suspension gives a smaller Vas.

In practical terms:

  • A high Cms (large Vas) driver generally needs a big box to work properly.
  • A low Cms (small Vas) driver can function well in a compact enclosure.

This is why subwoofers with soft suspensions are often found in huge cabinets—because they need that extra air volume to perform correctly.

You can calculate Cms from Vas and Sd using the formula below: ρ (air density) = 1.184 kg/m³ and c (speed of sound) = 343 m/s

Cms and Speaker Longevity

A well-designed suspension must not only be flexible enough to allow movement but also durable enough to maintain its characteristics over time. Over years of use, the spider and surround—the two key suspension components—can loosen up slightly, increasing Cms and slightly lowering Fs.

This is why some people notice their subwoofers playing deeper and looser after a break-in period—because the compliance has increased slightly as the suspension softens.

Final Thoughts

Cms is one of those Thiele-Small parameters that ties into everything—excursion, Fs, Vas, and even long-term speaker performance. It’s not just about how soft or stiff the suspension is, but how well it’s balanced for the intended application. Whether you’re tuning a subwoofer for deep bass or a midrange driver for accuracy, Cms is a key factor that shapes the final result. In modern speaker designs, with high BL, the motor force can over-ride the effects of Cms in determining the box size, as usual there are trade-offs and exception with all parts of speaker design.

Filed in Loudspeaker Driver Parameters, TS Parameters

Driver TS Parameters: BL (Motor Strength)

Posted By Andy Kos

BL is the product of the magnetic field strength (B) in the voice coil gap and the length of wire (L) within that field, measured in tesla-meters (T·m). It represents the force generated to move the cone.

For basic speaker design, motor strength (BL) isn’t a primary concern since manufacturers optimize it for the driver’s intended purpose. However, understanding BL helps in designing high-performance enclosures and matching drivers to specific applications.

Simply put, BL quantifies the strength of the motor system. It is calculated as the product of the magnetic field (B) and the active length of the voice coil (L) within that field. A stronger BL means greater force to move the cone, resulting in better control and efficiency. For a more detailed explanation, with diagrams, please see the article on Xmax: https://speakerwizard.co.uk/driver-ts-parameters-xmax/ which includes diagrams of the magnet structure, and magnetic fields.

With ferrite magnets, increasing motor strength typically requires a larger magnet, which adds weight and cost. However, magnet size alone isn’t an indicator of quality—some large-magnet drivers are poorly designed and inefficient, and the huge magnet is just for show – although a large magnet can have a side benefit of providing a large thermal mass and surface area for cooling.

Neodymium magnets offer high motor strength with minimal weight, making them ideal for portable and high-performance systems. However, they are significantly more expensive than ferrite alternatives.

BL depends not just on magnet strength but also on the voice coil’s wire length within the field. This means coil height, diameter, wire gauge, and even multi-layer winding techniques influence BL. Many modern drivers use inside-outside wound voice coils to maximize wire length while maintaining compact designs.

BL can be thought of as the “muscle” of the speaker motor. A high BL means the motor exerts greater force on the cone, improving control and efficiency. Drivers with high BL values tend to deliver tight, accurate sound, while low BL drivers may sound softer or “looser.”

To help put things into context, for a typical 18″ woofer a high BL figure would be considered 30 or higher. A driver with a BL in this range will exhibit very price cone control. A low BL figure would be 20 or less, a driver with a low BL will be significantly less able to control the cone accurately.

Depending on your application, you may still be wondering why you should care about BL? If you are planning on building a horn loaded bass bin, or scoop bin, a high BL is pretty much essential, you wont get away with just chucking any old driver into the cabinet and get the right results. If you consider that in most horn loaded applications you are having to compress air, you want a driver that exhibits a high force to achieve this compression, a strong motor makes this possible.

In other applications, if you were to make a listening comparison between a high BL driver with a low BL driver, you would find the high BL driver will sound much tighter and more accurate. Generally for live music applications this type of sound is preferred as it will more accurately reproduce the instrument sounds. Low BL driver can sound ‘woolly’ or ‘muddy’ because the cone does not respond to transients as quickly, and has the potential to introduce some distortion. Some people prefer this sound as it can give a warmer bass sound.

High BL drivers are generally needed for high power bass applications, where a large heavy cone is used. High BL drivers are usually more efficient, as the higher motor strength equates to more pushing power. For mid-range drivers, it is normal to use a much lighter cone, and a high BL is not necessarily needed, but can contribute to a more accurate response.

High BL drivers make it possible to use smaller sealed boxes in some designs, because the motor strength is so high, the restoring force of the air in the box, and the suspension of the driver, are small in comparison, making them relatively insignificant.

High BL increases electromagnetic damping, which can be problematic in bass reflex enclosures. Excessive damping may reduce the effectiveness of the port, requiring design adjustments for optimal performance. With bass reflex designs a slightly lower BL is often more appropriate to get a more balanced result.

We’ll discuss BL and voice coil geometry more in another article.

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