Subwoofer Vas Meaning and Explanation

Norvan Martin
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When designing an enclosure for a subwoofer, it is important to ensure that the enclosure volume is appropriate for the subwoofer’s optimal performance. One of the most important factors to consider in determining the size of the enclosure is the subwoofer’s Vas. 

Subwoofer VAS or Volume of Air Suspension refers to the volume of air that a subwoofer driver requires to achieve its maximum excursion or displacement. In other words, it is the inner capacity of the box required for that subwoofer to give maximum output at a stated frequency, usually bout 80 hertz.

Another way of thinking about Vas is the volume of air that the subwoofer’s suspension and driver assembly can displace when moving back and forth. Since it is a measure of the volume of air, it is often specified in liters (L) or cubic feet.

If a subwoofer has a high VAS, it will require a large enclosure volume to achieve its optimal performance. On the other hand, a subwoofer with a low VAS will require a smaller enclosure. 

In other words, each sub needs a certain box size to get optimum results.

What Does VAS Stand For?

VAS, or Volume of Air Suspension, is used as a technical term describing the mechanical properties of subwoofer drivers.

VAS measures how much air each subwoofer driver sees as it travels back and forth – thus being an invaluable parameter when designing subwoofers and determining their performance characteristics.

subwoofer cone vas

Vas can be more accurately described in this way:

Vas is the equivalent compliance (inverse of stiffness) volume seen by a given diameter speaker cone, meaning the volume of box air that gives the same stiffness as the driver’s suspension.

It is a measure of the overall stiffness, or resistance to motion of the cone, the surround, and the spider. It is specified in terms of the volume of air having the same compliance as the driver.

A small number corresponds to a small volume of air, which is stiffer than a larger volume of air. Thus, compliance and stiffness are inversely proportional.

Of course, Vas is usually measured in cubic feet and generally is equivalent to the internal volume of the enclosure.

Vas vs Box Volume

Vas and Box Volume are key concepts in loudspeaker engineering. Vas refers to the volume of air needed by loudspeaker drivers at a specific frequency, while box volume refers to internal volumes within speaker enclosures that impact tuning frequency and sound quality

Both concepts share many similarities but contain unique distinctions within their design concepts and functionality.

Vas is an essential factor when designing loudspeaker drivers as it can help determine low-frequency response and indicates how much air an enclosure or porting box can move; larger values indicate greater enclosure volume required for optimal low-frequency performance.

For many persons, determining the best box size for a subwoofer considering box, the process starts with selecting a box size. Next, you can model the driver and box size to see what the response looks like in modeling software.

A good example of such software is the Woofer Box and Circuit Designer 6.0. Other options include WinISD and  Torres Box Tuning Calculator.

Vas vs Box Volume

Box volume refers to the internal space within a speaker enclosure or box and plays an integral part in determining both the tuning frequency of a system, as well as overall sound quality.

Tuning frequency refers to when an enclosure resonates at resonance frequency which directly correlates with box volume – smaller box volumes tend to result in higher tuning frequencies while larger volumes result in lower tuning frequencies.

Moreover, it impacts other aspects such as frequency response, transient response capabilities, and power handling capacities of any speaker system.

Impact on Speaker Performance

Another critical difference between Vas and box volume is their impact on speaker performance. Both Vas and box volume are important factors that affect the low-frequency response of a speaker system. However, they have different effects on the overall performance of the system.

Vas values have an enormous effect on a speaker driver’s low-frequency response. A larger Vas value means more air can move at low frequencies, creating lower resonant frequencies and deeper bass responses; however, larger Vas values also require larger enclosure volumes for optimal low-frequency performance – thus restricting design options while making optimal performance even harder to attain.

Box volume can have a direct and decisive effect on the performance of speaker systems. Box volume directly determines tuning frequency – that frequency at which an enclosure resonates – which in turn impacts low-frequency response and overall sound quality of systems.

A lower tuning frequency leads to punchier bass response while larger box volumes produce longer bass responses due to decreased tuning frequencies, though larger volumes often make designing systems less efficient or challenging overall.

Importance in Loudspeaker Design

Vas and box volume differ most in their importance when designing loudspeaker systems, yet both need to be taken into consideration as integral parameters in this process. While both parameters play a part in designing speaker systems, their relative levels of importance vary and have distinct impacts it.

Vas is an integral component in designing speaker drivers. As its name implies, this measurement determines the low-frequency response of a driver and is therefore essential in reaching peak performance. Vas specifications typically include this parameter which helps determine an ideal enclosure volume for its driver.

However, box volume is an integral element in designing a speaker enclosure. It plays an integral role in setting the tuning frequency of the system and ensures resonance at an ideal frequency; furthermore, it affects overall sound quality, so it must be carefully considered during the design processes.

Furthermore, other factors like port tuning, driver placement, and damping materials must also be carefully taken into account during this process.

Subwoofer Vas Sealed vs Ported Subwoofer

Vas is considered separately if you are dealing with a sealed vs ported subwoofer. In the case of a sealed box, you can deviate from the optimal box size by up to 15%.

On the other hand, if you are working with a ported subwoofer, you need to follow the exact instructions for getting the precise size of the enclosure. 

Why is this the case?

Well, keep in mind that Vas is the point at which the sub will play like it’s in a free-air enclosure. For example, if a subwoofer has a Vas of 3.0cft, then a 3cft enclosure (sealed) would be the equivalent of a free air design.

This is because a large box or a ported box does not provide the same air suspension on the woofer cone that a small box or ported box would.

Keep in mind that the air inside the cabinet has its compliance. As such when you try to compress the air inside a box, you will encounter some amount of resistance. Now, if the box is small, the air is harder to compress and therefore less compliant. On the other hand, if the box is larger, the air is easier to compress and, therefore more compliant.

From the above, we can conclude that Vas describes the volume of the air inside the cabinet, where the compliance of the speaker matches the compliance of the air inside the box.

The following therefore also applies:

  • If you are designing an acoustic suspension box, the volume of the box will be less than Vas.
  • If it’s larger than Vas, it would be an infinite baffle

Vas Calculations and Measurement

Vas can be calculated as follows:

Vas = ρc2Cas

where Cas is the acoustic compliance of the driver’s suspension.

Vas can be tricky to measure because air pressure changes relative to humidity and temperature. This means you would need a properly controlled environment to measure Vas correctly.

Cms is measured in meters per Newton. Cms is the force exerted by the mechanical suspension of the speaker. It is simply a measurement of its stiffness.

Considering stiffness (Cms), in conjunction with the Q parameters gives rise to the kind of subjective decisions made by car manufacturers when tuning cars between comfort to carry the president and precision to go racing.

Think of the peaks and valleys of audio signals like a road surface then consider that the ideal speaker suspension is like a car suspension that can traverse the rockiest terrain with race-car precision and sensitivity at the speed of a fighter plane. It’s quite a challenge because focusing on any one discipline tends to have a detrimental effect on the others.

How Do You Calculate the Right VAS for a Subwoofer?

Finding the Perfect VAS for Your Subwoofer You can use online calculators to figure out the perfect VAS (Volume Acoustic Suspension) for your subwoofer since they do all the complicated math for you. But if you want to work it out by hand, just follow these steps:

1. Get The Details

Start by getting hold of some key mechanical details about your subwoofer, like:

  • Sd (Cone Surface Area): How much space does the subwoofer cone cover?
  • Cms (Speaker Suspension’s Flexibility): How bendy the subwoofer’s suspension system is.
  • ρ (Air Density): How heavy the air is around the subwoofer.
  • π (Pi): That number we all know from math class, roughly 3.14159.

2. Understand the Equation:

Make sure you know this formula for calculating VAS:

VAS = (Cms / ρ) × Sd^2 / (4 × π^2)

Here’s what each part of that formula means:

  • VAS: This is the “Equivalent Air Volume” which is like the amount of air that would push back just like the subwoofer’s suspension does when it’s squashed.
  • CMS: This is the “Suspension Flexibility,” and it tells you how easy or hard it is to move the subwoofer’s suspension. It’s usually in meters per newton (m/N).
  • ρ (Rho): This stands for “Air Density,” and it’s about how much air weighs in a certain space, calculated in kilograms per cubic meter (kg/m³).
  • Sd: “Cone Surface Area,” specifically how big the area of the cone on your subwoofer is, in square meters (m²).
  • π (Pi): Our familiar math constant, somewhere around 3.14159, is important for working out stuff with circles.

2. Apply the Equation:

Take all those numbers you found and plug them into the equation. Make sure they’re right; otherwise, your subwoofer box might not turn out like you expect.

The VAS value you get will be in cubic meters (m³). You may want to change it to liters (L), which could be more useful when building your subwoofer box.

Vas, Qts, and Fs Calculations for Sealed Boxes

When it comes to calculations, it is more complicated for a sealed box since you need to consider Vas, Fs, and Qts for a sealed box. Box volume is determined by 2 specs, Vas AND Qts. 

On the other hand, how deep a woofer will reach is determined by two specs, Fs and Qts.

Typically, the calculations would follow a procedure similar to below:

  1. Select the box volume, Vb (use the same units for Vb that you use for Vas
  2. Alpha = Vas/Vb
  3. Now Qtc = Qts * sqrt[(alpha^2) + 1] and Fc = Fs * sqrt[(alpha^2) + 1]

Now you are typically looking for a target Qtc of between 0.65 and .90. However you will find that more often than not, the “optimally flat” value for Qtc is 0.71.

Here are some examples:

Vb = 2.65 ft^3 (75 liter)
Qtc = 0.90
Fc = 40 Hz
F3 = 29 Hz
F10 = 18 Hz

Vb = 3.40 ft^3 (96 liter)
Qtc = 0.85
Fc = 37.5 Hz
F3 = 28.3 Hz
F10 = 17.2 Hz

Vb = 4.41 ft^3 (125 liter)
Qtc = 0.80
Fc = 35.3 Hz
F3 = 27.8 Hz
F10 = 16.4 Hz

Conclusion

Overall, Vas and box volume are two critical concepts in loudspeaker engineering that have significant ramifications on speaker performance and design. While they may appear similar at first glance, each offers distinct design concepts, impacts upon speaker performance, and levels of significance in design processes – it is therefore vitally important that when designing speaker systems that achieve optimal sound quality a consideration be made of both Vas and box volume as part of this design process.

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Norvan Martin is the founder of BoomSpeaker.com. He is a professional Electronics Engineer and is passionate about home theater systems and AV electronics. BoomSpeaker was created as an online hub to share his knowledge and experiences as it relates to home theaters and home audio electronics. My email: [email protected]  Connect on Pinterest and Linkedin