Archive for April, 2014

2nd order Butterworth Passive Crossover Calculator

Posted April 23, 2014 By Andy Kos

To save you time, we’ve put together a calculator for one of the most common passive crossovers, a 2nd order Butterworth. You can configure the Impedance of the woofer and tweeter, and the crossover frequency.

C1, C2, L1 and L2 correspond to the relavant capacitors and inductors in the diagram below:

2nd order Crossover medium

Where Vin is connected to the positive terminal of your amplifier, and 0V is the negative terminal. The dots on the circuit signify where components/wires are connected. L2 connects to the input, and NOT to 0V, hence the ‘loop’ in the wire in the diagram.

 

 

 

Thiele Small Parameters

Posted April 22, 2014 By Andy Kos

The Thiele Small Parameters (often referred to as T/S Parameters) are provided in specification sheets by most manufacturers – but what are they for? If you read through the section on parameters you’ll get a more detailed explanation on the significance of each parameter, but put simply they are a set of parameters which define the electromechanical properties of the loudspeaker driver, which are a measure of how it behaves electrically, and mechanically.

Once you understand some of the basics of the Thiele Small parameters, you will know what to look for when it comes to choosing a loudspeaker driver. If you’re not interested in the nitty gritty, then it’s probably sufficient to be aware that the parameters are normally used when it comes to doing simulations of loudspeaker behaviour for purposes of optimising cabinet design.

For those of you who are keen to understand a little more about these parameter, you should find almost everything you need to know in this section.

The following are small signal mechanical parameters, and are measured at small signal levels

Since the above characteristics are not always easy to measure, it is often easier to measure other parameters, and derive any missing parameters from those that are available. Other parameters, known as small signal parameters, are as follows:

Large signal parameters, listed below, are used when predicting driver behaviour at high power levels:

 

 

 

Re is the DC resistance of the loudspeaker’s voice coil, you should not confuse this with impedance, although related, the two are different.

Measuring the resistance of a speaker with a multi-meter, the reading you get across the terminals should be close to the manufacturer’s specified Re, it’s not unusual to get a minor variation from manufacturer’s published specifications, chances are the Voice Coil resistance will actually be correct and your multi-meter is what’s wrong.

So why is Re around 5-6 ohms, but the impedance specified as 8 ohms. Let’s look at what Re actually signifies, it’s the DC resistance of the wire in the voice coil. If you were to unwind your voice coil, and run it out in a straight line, connect it to a battery or other power source you could predict the current through the wire using ohm’s law: V = IR, and this resistance would be an accurate measure of how much the wire resists the flow of DC electrical current through it.

All good so far, but when you wind copper into a voice coil, you create an inductor. An inductor will still behave like a straight wire when it comes to DC, (Direct Current) however, inductors behave differently when subjected to AC (Alternating current) signals. The output from your amplifier is an AC signal, as it alternates in polarity and varies in amplitude. Inductors resist change in current, when you apply an AC signal to an inductor it will create a back EMF to try to resist  the change of current, creating additional ‘reactance’, when you add the reactance to the resistance, you get the driver’s impedance. If you want a simple analogy for an inductor, you could compare it to the suspension of a car, putting weight in a car will make the shock absorbers compress a little, they resist the change, but then remain static doing nothing this would be like applying a DC signal to an inductor, it resists the initial flow of DC current, but once current is flowing the inductor does nothing. If you then drive over lots of bumps, the suspension works by creating a force that pushes back to resist all the little bumps, preventing the force of the bumps reaching the car body, in much the same way that an inductor blocks high frequency signals by creating a back EMF.

One thing to note, is that the reactive component from the voice coil can vary significantly, up to 200 ohms in extreme cases, there is a peak around the drivers resonant frequency, and then there is a increase at the upper end of the driver’s operating range. Typical impedance plot of a bass speaker:

impedance

 

The rising impedance from 1 kHz upwards is caused by the loudspeaker’s inductance. You can see from the above impedance curve that an amplifier will see a higher impedance of nearly 25 ohms at 5 kHz, which would significantly reduce the power that can be delivered to the speaker at those frequencies. To get better high frequency response, the inductance needs to be kept as low as possible by adjusting the voice coil geometry. In some cases this is not possible, so to improve high frequency response a common feature is to include a copper shorting ring in the pole piece of the speaker, this creates a short circuit for induced back EMF, reducing the impedance at higher frequencies and extending the mid-range response. You wont find copper shorting rings in drivers designed for sub-bass or bass applications, as it’s not needed, and they are most often found in higher quality drivers designed for better, smoother high frequency response. Used correctly, the shorting ring can extend higher frequency response by a few kHz.

In the above graph there is also a large peak just above 40Hz, this corresponds to the driver’s resonant frequency. The resonant frequency is the point that the driver will naturally move the most, where the compliance of the suspension and spider for the given moving mass are most susceptible to oscillate. The peak in impedance is caused by the back-emf of the moving coil. The more the cone moves, the greater the back-emf. Since the cone moves most at the resonant frequency, this is where the back-emf will be greatest. An impedance of 40 ohms or more will significantly reduce the power delivered to the speaker by the amplifier at those frequencies, but at the same time the speaker will naturally want to move more easily at the resonant frequency, requiring a little less power to achieve a given excursion.  You’ll notice also that the mid-band frequencies show a relatively flat response in impedance, this is where the excursion of the speaker is low, and the impedance will most closely match the ‘nominal impedance’ of 8 ohms.

The 8 ohm rating given to loudspeakers is the average impedance across the drivers main operating range. It’s useful for approximate power calculations and simpler designs, however for more advanced designs it is often desirable to measure the exact impedance at particular frequencies so that variations can be compensated for at the design stage.

Re is used in conjunction with Le (inductance) for purposes of simulating driver performance.

 

 

 

Driver TS Parameters: Mmd & Mms

Posted April 13, 2014 By Andy Kos

If you’re comparing drivers in detail, it will help to understand some of the more intricate TS Parameters, as over time it will help you differentiate between different drivers and identify which applications they are more suited for.

For example, for a high power 18″ horn loaded bass bin, you will probably be looking for a driver with a high BL, and also a good strong cone. A strong cone will generally also be a heavier cone, so you’ll be looking for a driver with a relatively heavy cone.

For a 12″ mid driver, that wont be doing any bass, but mainly focused on vocal reproduction you will want a driver with a precise response, and a light cone.

Mmd is the mass of the moving parts of the driver;  the diaphragm, dust dome and voice coil. The diaphragm is the paper cone in a standard speaker. The voice coil includes the former and the copper wire.  Most definitions online for Mmd seem to just be copies of each other, citing that the surround and spider are included in the moving mass. A bit of further research has suggested that only part of the mass of the spider and surround should be included, as the outside edge of the surround, and the outside edge of the spider are both glued to the chassis, and therefore DO NOT MOVE.

Mms is commonly used in loudspeaker modelling software. It is Mmd plus the ‘air load’. The air load is the air just in front, and just behind the speaker cone that will tend to move back and forth with the cone. It’s just a few grams of air, but for mathematical modelling of speaker performance, needs to be added in. A larger cone will have a larger air load.

Mms is used to calculate other TS Parameters, such as Qes and Qms.

There is one final significant point with regard to the Mms, and that is the relationship to Fs  (Free Air resonance). A heavier cone generally has a lower resonant frequency, a lighter cone will generally have a higher resonant frequency. The other factor in the equation is  Cms which is a measure of the suspension compliance. The formula which connects Fs to Mms is as follows:

Fs_Formula

Click here to read more about Thiele Small Parameters: Fs (Free Air Resonance)

We’ve mentioned this before, and just to hammer the point home here, you really, really should be aware now that all driver parameters are dependent on each other. The fact that cone weight, voice coil geometry, magnet strength, cone stiffness all interact and affect each other in both positive and negative ways means that any speaker design always has some compromises. Some might call this optimisation – a driver specifically designed for the best sub-bass response will sacrifice mid and upper bass response.

To get lower frequencies for better bass, you will use a heavier stiffer cone. Whilst in theory, for lower resonant frequency you can keep increasing the cone mass, the drawback often be lower efficiency, and to counter  this you need a stronger magnet, and also a longer voice coil for better motor strength. Its unsurprising that in many instances, a compromise is settled on, which balances efficiency, resonant frequency, and cost.

To improve the mid range response of a driver, you would look at making the cone lighter, but that would potentially increase the resonant frequency, fine for mid-range drivers, not so good for subwoofers. As with most things in life, you can’t have your cake and eat it, and the only way to cover the whole frequency range more effectively is to use different drivers optimised for specific tasks.

Driver TS Parameters: BL (Motor Strength)

Posted April 12, 2014 By Andy Kos

BL: The product of magnet field strength in the voice coil gap and the length of wire in the magnetic field, in tesla-metres (T·m)

Unless you’re getting into more advanced levels of speaker cabinet design, the motor strength isn’t normally something you need to worry about too much. To some extent, it’s safe to assume that the manufacturer will have set the motor strength appropriately for the design of the driver, but it will still be useful to have a basic understanding of this parameter, and it’s significance.

Put simply, the motor strength (BL) is a measure of how strong the magnetic field within the voice coil gap (B) multiplied by the length of voice coil that is active in the magnetic field (L). 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 standard ferrite magnets, a stronger magnetic field is generally achieved with a bigger magnet. The bigger the magnet, the heavier the magnet. The heavier the magnet is, the stronger the chassis needs to be, and you’ll also need to ensure strong glue to bond the magnet to the top plate, and the back plate.   Increasing the magnet size and chassis strength will make the driver more expensive, but don’t let a big magnet fool you, this  isn’t necessarily a sign of a high quality premium speaker though, I’ve seen a number of drivers with big magnets that are not much use for anything other than picking up nails off the floor.

In order to keep weight down, neodymium magnets have become more common, these have the ability to generate very strong magnetic fields from surprisingly small, light magnets. Using a neodymium magnet on a driver can have a massive impact on the overall driver weight, which is becoming quite desirable for small portable systems. You will have to pay for the privilege though, as neodymium magnets are often significantly more expensive that ferrite.

Don’t forget that BL is also dependent on the length of wire in the magnetic field, so the height of the magnetic gap will also affect the BL product. If you’ve read the article about Xmax you’ll hopefully start to realise that there are a number of loudspeaker parameters that are dependent on each other. The voice coil design will have an impact on the Xmax, and since BL takes into consideration the length of wire, it will also impact on BL.  The diameter of the voice coil and  the gauge of wire used (how thick the wire is) will have a very direct impact on the length of wire in the magnetic field. Beyond that, it’s also possible to have multiple layers of wire on the voice coil, and many modern drivers utilise inside-outside wound voice coils, where wire is wound and glued to the inside of the voice coil as well as the outside.

So what does the BL number actually mean? The figure is given in Tesla Meters, and you will find the following ‘definition’ in various places online:

BL: Think of this as how good a weightlifter the transducer is. A measured mass is applied to the cone forcing it back while the current required for the motor to force the mass back is measured. The formula is mass in grams divided by the current in amperes. A high BL figure indicates a very strong transducer that moves the cone with authority!

Personally I think that is a little too simplistic, but it’s sufficient for someone with casual interest to get a handle on some of the key TS Parameter.

To help put things into context, 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 causes high electromagnetic damping, and in some cases this is undesirable, in fact with reflex boxes it can cause problems with the reflex ports being under-damped and requiring modification, often with bass reflex designs a slightly lower BL is more appropriate to get a more balanced result.

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

 

Driver TS Parameters: Xmax

Posted April 12, 2014 By Andy Kos

Possibly one of the most misunderstood parameters, most people know Xmax concerns driver excursion, but dont really know  precisely what it means, and it is probably the name that confuses people, as it is slightly misleading.

We’re used to  letters X and Y denoting dimensions, and in this case, X does relate to a dimension, it’s to do with the distance a loudspeaker’s voice coil travels back and forth, so it’s all good so far – but the ‘max’ is what throws people. Its natural to assume max means maximum, and the conclusion most people reach is that X max means maximum excursion in dimension X, which is nearly right. What’s missing is the word linear. Xmax is generally regarded as maximum linear excursion – but what exactly does that mean?

Let’s look at a simplistic way of how Xmax is often calculated (this applies to overhung voice coils – which is most common in high power loudspeakers)

The Formula is: (HVC – HG) / 2

Where HVC is the Height of the Voice Coil, and  HG is the Height of the magnetic Gap.

To understand this we need to look at how the components of a loudspeaker fit together, this is a simplified cross-sectional diagram of a common loudspeaker.

 

Cross-sectional diagram of a typical loudspealer

Cross-sectional diagram of a typical loudspealer

 

Now, let’s look in more detail at the area around the voice coil, we’ve removed the right hand part of the voice coil to make the diagram clearer.

 

Voice coil in magnetic field, showing Xmax and Magnetic Gap Height

Voice coil in magnetic field, showing Xmax and Magnetic Gap Height

 

You can probably now see why Xmax is often referred to as Voice Coil Overhang. It’s the amount by which the voice coil overhangs the magnetic gap, but why is this significant?

Let’s take a closer look at the static magnetic fields in a loudspeaker. These are the fields generated by the magnet, rather than the fields generated by the voice coil.

Magnet structure and magnetic flux (simplified)

Magnet structure and magnetic flux (simplified)

This is a simplified diagram, intended to show the most significant path of magnetic flux. There will also be stray flux outside the speaker, and inside the air gap between the magnet and pole piece, and in a real speaker, the field lines are unlikely to quite as uniform as in the above diagram, but it should be sufficient to see the general principle of how the magnetic field is acting.  The permanent magnet has a north pole at one end, and a south pole at the other. Depending on the speaker manufacturer, it’s normal for the pole piece to become the ‘north pole’ and the top plate to become the south pole. The shape of the top plate, and pole piece helps focus the magnetic flux, and you will notice the lines of flux are closest together in the magnetic gap – where the voice coil would normally be.

Since ferrous materials are much more magnetically permeable than air, by a factor of about 400.  Magnetic flux will tend to take the route of least resistance, in much the same way as electricity does, this will mean the magnetix flux  will tend to want to travel through the metal parts of the speaker. Where it reaches the gap it will continue to go down the route of least resistance, which in this case will be the shortest distance through the air, ie straight across the gap. The flux is squeezed together across the gap, causing the flux to flow in parallel lines across the gap, creating a uniform, linear magnetic field.

Traditionally Xmax was calculated mathematically using the simplistic formula mentioned earlier, this is because many earlier speaker designs used relatively weak magnets, and it was assumed that the magnetic field would drop off very significantly just above or just below the gap, and would be of little or no use.

When you pass electrical current through the voice coil, it will create its own electromagnetic field, which will push against the magnetic flux in the voice coil gap, causing the voice coil to move. If you keep driver excursion within Xmax, there will always be the same height of voice coil within the gap. The diagrams below show the maximum excursion in each direction to keep within mathematically calculated Xmax. Since the magnetic field in the gap should be linear and uniform, and the height of voice coil within the gap remaining constant, mathematical models can be created to predict driver behaviour. Working outside Xmax will cause those mathematical models to become inaccurate, as well as potentially introducing distortion and other poor performance.

 

Maximum back excursion

Maximum back excursion

Maximum forward Excursion

Maximum forward Excursion

 

Moving the voice coil  any further up or down in either direction, as in the diagrams below, would cause the height of the voice coil that is within the magnetic gap to become shorter, shown by the red arrows.  You can clearly see this is less than the Gap Height. Less Voice Coil in the magnetic gap, means less pushing force moving the cone, which is where the non-linear behaviour starts, hence the term maximum linear excursion. The cone will still move, but it will no longer be optimal performance.

 

Xmax exceeded

Xmax exceeded

Xmax exceeded

Xmax exceeded

 

It is becoming increasingly common to use stronger magnets in modern designs, which can sometimes mean that useful magnetic flux (although slightly weaker) will also be present just outside the gap, and magnetic field strength may still be acceptable in this area. Depending on the magnet strength, and other factors,  Xmax  when consider to be a measure of maximum linear excursion can actually be 25%-40% larger than mathematically calculated Voice Coil Overhang.

So when you are comparing one brand of driver to another, you need to be aware that the Xmax figures may be calculated differently, and a driver with a specified Xmax of 7mm from one manufacturer (using Voice Coil Overhang) could in fact have a very similar performance to one from another manufacturer with an Xmax of 10mm, who has perhaps used a different mathematical model and/or tolerance to determine the limit of linear excursion.

The best solution may be to determine Xmax by measurement rather than simple maths, and there is a growing trend towards using Klippel Analysis to determine Xmax more accurately, the driver is progressively driven to high levels at low frequencies, and Xmax is determined by measuring excursion at a level where 10% THD is measure in the output. This is believed to better represent actual driver performance, however is quite time consuming, and can be difficult to measure, consequently many manufacturers do not bother.

 

What is the significance of Xmax?

Cone excursion is related to loudness, especially with deep bass frequencies in a bass reflex cabinets. To reproduce bass frequencies at high volume you need to move a lot of air, and to move that air your speaker cone needs to move a lot. A bass driver with a low Xmax will generally not be designed to reproduce bass frequencies at high power, as it simply can not move enough to do the job. There is an exception to this in horn loaded bass cabinets, where excursion can be restricted, and Xmax may be less critical, depending on the design.

Will exceeding Xmax damage the speaker?

Not necessarily, some manufacturers will also specify Xlim or Xdamage which is the maximum mechanical excursion before damage is expected, this can often be double Xmax. The two will often be related, a driver with a large Xmax designed for long excursion, will usually be designed such that Xlim is proportional to Xmax. Xlim is often regarded as maximum mechanical excursion, as this is the point where you will cause mechanical damage if you exceed this, most commonly with the end of the voice coil hitting the back of the speaker and damaging the voice coil former:

Exceeding Xmax

You can in most instances exceed Xmax without causing mechanical damage to the voice coil, however you should take note that exceeding Xmax can reduce the power handling due to detrimental effects on voice coil cooling.

Depending on the driver design, other things to consider when exceeding Xmax is the mechanical stresses on the speaker components, such as the spider, and where the spider joins the cone and coil former. There is potentially a large force acting on these components, stretching and pulling them beyond their designed limits. Whilst you can often exceed design parameters a little without causing damage, it would not be a sensible idea to exceed Xmax significantly as you will reduce the useful working life of your speaker.