Archive for May, 2014

Cms is a measure of the suspension compliance. Compliance is the opposite of stiffness. A driver with a stiff cone suspension will have a low Cms, and a driver with a ‘loose’ cone suspension will have a higher Cms.

Vas is known as the compliance equivalent volume, and is specified in litres. Cms is proportional to Vas, a higher Cms will mean a higher Vas, but what does the Vas figure actually represent?

You could think of the stiffness of the suspension as providing a restoring force that brings the cone back to it’s central ‘neutral’ position. If you were to gently push the dust dome of a driver with your fingertips, one with a stiff suspension would push back harder than one with a loose suspension.

Imagine a situation where you have an imaginary driver  mounted in a sealed box, with an infinitely compliant suspension – ie a suspension that offers no resistance whatsoever to movement. If you were to push the driver cone back into the box, the cone moving back into the box would compress the air inside the box slightly, and when you release the driver cone, the air would push back to restore the cone to its original position. In a box with a small volume, the air would compress more, pushing back harder. As the box size increases, the same distance of movement of the cone will compress the air inside the box less, resulting in a smaller restoring force to push the cone back to it’s original position.

The Vas measurement in litres is the size of the ‘imaginary’ box described above, which has exactly the same restoring force as the suspension of the driver. Cms and Vas are effectively two different ways of describing exactly the same thing, the main reason for converting Cms to Vas is that Vas fits into a lot of formulae better, and allows easier modelling of driver performance.

Air temperature, air pressure and humidity can have a significant effect on Vas measurements, and it is quite common for variations of up to +/- 20% from published specifications. This is a combination of differences of measurement environment, and manufacturing tolerances.

Vas can be used for determining optimum box size for sealed speaker boxes (ie. NOT vented).

If your sealed box is too small, the when the driver cone moves backwards into the box, compressing the air inside the box, the restoring force will be higher than optimal, causing the driver to move back out a little too quickly and potentially gain too much speed, and overshoot, causing it to go further out of the box than it should do. On the return journey, the rarefaction of the air inside the box will pull the cone in too fast, potentially causing it to go in too far. This is known as underdamping, when the movement of the cone gets exaggerated and increased instead of being controlled. This is undesirable as it causes distortion, and potentially affects the cooling of the voice coil

If your sealed box is too big, the air inside the box will will slow the driver from returning to it’s central rest position rather than help it, this can make the box overdamped. Many people consider being slightly overdamped as the best option, as it gives a more accurate sound, but it will often reduce the output volume.

For Bass speakers, critical damping, or ‘perfect’ damping is often what is sought after, this gives the best compromise, where the air inside the box helps restore the driver to it’s natural position, but not too much, and not too little, but just right. A speaker box with a Q of 0.707 is generally regarded as perfectly damped, you could think of this as the Goldilocks Q, where it’s ‘just right’.

The ratio of Vas to box volume = (Qtc/Qts)2 – 1 : 1

Qtc is the desired Q of the speaker, let’s assume we are aiming for a Qtc of 0.7

Qts is the total Q of the driver, available from the manufacturer’s specification sheet. For purposes of some simple maths, let’s use an imaginary driver with a Qts of 0.35.

0.7/0.35 = 2 and 2 squared=4

So our formula gives:(4-1):1 or 3:1

If Vas for our imaginary driver was 180 litres, we would make our box 60 litres to achieve a Qtc of 0.7

Its not unusual to see boxes with a Qtc of up to 1.1, higher Qtc of around 1 is slightly underdamped and will cause a peak in bass response around the resonant frequency, giving the impression of a better bass response. In reality you will be sacrificing deep bass response for upper bass response, and with reduced sound quality and less control of the driver, increasing cone excursion and the possibility of over-excursion.

You may also find the formula written as

Qtc = Qts X (Square Root((Vas / Vb) + 1))

The results are the same, just the formula has been re-arranged. Vb is the box volume. Some people prefer to use Vc for closed box, and Vb for reflex box. As a general rule of thumb, drivers with a high Vas prefer a bigger box to get the best results, and you should take this into consideration when choosing the driver for your application.

The formulae here are for sealed boxes only, bass reflex boxes require different calculations which we will cover elsewhere.