What Is Speaker Compliance? (Cms and Vas explained)

Speaker compliance is an interesting factor in speaker design. Although a relatively simple concept to understand compared to Thiele / Small parameters, it is an important parameter for effective speaker operation.

Speaker compliance, known as Cms, is an indicator of how stiff the suspension system of a speaker is. Compliance is the inverse of stiffness. Therefore, if the speaker suspension is stiff, then the speaker driver will have a lower compliance value (Cms).

The speaker suspension system refers to the speaker surround and speaker spider, which helps the speaker cone move and keeps the motion linear, with the voice coil centred in the speaker’s magnetic gap.

In this article, I will cover what Cms is, what Vas is and how they relate to the real world of speaker operation, covering:

  • What is compliance CMS of a speaker?
  • What is the importance of speaker compliance?
  • What is Vas of a speaker?

What Is Compliance CMS of A Speaker?

Every speaker will have a compliance associated with it. Cms is a measure of the stiffness of the speaker’s suspension system, which determines how much the cone will move when it receives an electrical signal.

In fact, there are two compliance values associated with every speaker:

  1. The compliance of the speaker driver itself (Cms)
  2. The compliance of the box within which the speaker driver is mounted (Vas)

We measure compliance in meters/Newtons (m/N).

To understand compliance, imagine you have a large spring hanging on the wall with nothing attached to it. You place a mark on the wall where the bottom of the spring sits.

Next, you attached a 1kg weight to the end of the spring. Place another mark on the wall, next to the bottom of the spring, in its new position.

Let’s imagine you measure exactly 1 meter between each of these marks.

With this information, you can now specify the compliance of the spring, as it is the distance it moved in response to 1kg of weight.

So in this case, the Cms of our spring is given by:

Compliance = Displacement / Force

Now, given this example, we can apply the same theory to the suspension system of a loudspeaker, which is in effect, just a spring made up of a surround and a suspension.

How much the speaker cone and suspension system move (displacement) in response to a force (input signal) will give us the compliance value of the speaker.

If the speaker cone and suspension system have a lot of movement, then it has a high compliance value.

If the speaker cone and suspension system hardly move, then the loudspeaker has low compliance.

What Is The Importance Of Speaker Compliance?

A speaker’s compliance value tells us how effectively the speaker cone can move in order to produce sound.

For example, a low compliance value indicates that the speaker cone is better at reproducing low-frequency sounds, as the speaker driver does not rely on the air spring effect of the enclosure as much. This gives you the opportunity to put your subwoofer into a larger box and bring out more bass!

Most importantly, speaker compliance has a direct impact on Fs, which is the resonant frequency of a speaker in free air. To learn more about Speaker Fs, I have covered this topic in greater detail in this article.

Regarding the relationship between compliance (Cms) and the speaker resonance frequency Fs, when Cms increases, Fs decreases.

When Cms decreases, Fs increases.

This inverse relationship between Cms and Fs is important to know when designing a loudspeaker, as how stiff the spider is will determine the Fs, the speaker’s resonance. [source]

What Is Vas Of A Speaker?

Vas, known as the equivalent compliance, describes the total internal volume of the speaker enclosure and its effect on the performance of a speaker. It is measured in litres (l) or an equivalent unit of volume.

The air inside a speaker enclosure has a “spring effect” on the speaker driver itself as it acts as a force against the speaker cone and suspension system.

For example, if you try to press down on the cone of a speaker driver that is mounted in a sealed box enclosure, you will meet resistance, as the internal air of the sealed enclosure pushes back.

As a general rule, the smaller the box, the stiffer the suspension system; therefore, the speaker is said to be less compliant.

Likewise, the larger the box, the weaker the suspension system; therefore, the speaker is said to be more compliant.

Cms is proportional to Vas. A higher Cms means a higher Vas and vice versa.

Vas is the volume of air inside the speaker enclosure where the speaker driver’s compliance and the compliance of the air inside the box are equal.

In other words, the enclosure air volume gives the same stiffness as the speaker driver suspension.

Final Thoughts

In conclusion, speaker compliance is an important factor to consider when designing a loudspeaker. It directly affects the Fs (resonant frequency) of the speaker, with higher compliance, resulting in lower Fs and vice versa.

Additionally, Vas (equivalent compliance) describes the total internal volume of the speaker enclosure and its effect on performance.

A smaller box will have a stiffer suspension system, while larger boxes will have weaker suspension systems that are more compliant.

Understanding these concepts can help you design speakers that provide optimal sound quality for your needs.

Happy listening!

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