Understanding Key Loudspeaker Parameters(5): Equivalent Compliance Volume (Vas)--The Air Spring Effect
Published by IWISTAO
In loudspeaker design, few Thiele–Small parameters influence enclosure size and low-frequency performance as strongly as Vas. Short for Equivalent Compliance Volume, Vas connects the mechanical flexibility of the speaker’s suspension with a volume of air that would exhibit the same acoustic compliance.
Whether you’re designing a sealed box, tuning a bass-reflex system, or selecting drivers for a DIY project, understanding Vas is essential for predicting enclosure behavior.
1. What Is Vas?
Vas represents the volume of air that has the same acoustic compliance (springiness) as the loudspeaker’s suspension system. It reflects how easily the cone, surround, and spider can be displaced.
- High Vas = soft suspension (high compliance)
- Low Vas = stiff suspension (low compliance)
Vas is expressed in liters (L) or cubic meters (m³).
2. Why Vas Matters
a. Enclosure Volume Requirements
- Large Vas drivers require large enclosures for proper bass reproduction.
- Small Vas drivers work well in compact boxes.
This is why a 15-inch woofer may have a Vas above 150 L, while a 3-inch full-range driver may have a Vas below 3 L.
b. Bass Performance
A high-Vas driver offers:
- Deeper bass extension
- Smoother LF roll-off
- Slower transient response
A low-Vas driver offers:
- Tighter bass
- Smaller enclosure compatibility
- Limited deep LF extension
c. Box Tuning (Sealed & Ported)
Vas directly affects:
- Sealed box system resonance (Fc)
- Bass-reflex tuning frequency (fb)
- Alignment tables (Butterworth, Chebyshev, QB3)
Incorrect Vas → incorrect enclosure design → poor bass response.
3. How Vas Relates to Cms and Sd
Vas links directly to mechanical compliance (Cms) and cone area (Sd) using:
Vas = ρ × c² × Sd² × Cms- Larger Sd → larger Vas
- Softer suspension (higher Cms) → larger Vas
- Stiff suspension → smaller Vas
4. Interpreting Vas Values
| Vas Value | Driver Type | Behavior | Enclosure Size |
|---|---|---|---|
| 1–5 L | Small full-range / midrange | Tight, limited LF | Very small box |
| 5–20 L | 4–6″ mid-woofers | Balanced LF | Small box |
| 20–60 L | 6–8″ woofers | Good LF extension | Medium box |
| 60–150 L | 10–12″ woofers | Deep bass | Large box |
| 150 L+ | 15–18″ subwoofers | Very deep LF | Very large box |
Vas is not a “quality” metric. It simply indicates how much enclosure volume the driver needs.
5. How to Measure Vas
Method 1 — Added Mass
- Measure resonance frequency (fo).
- Add known mass to the cone.
- Measure the new resonance frequency.
- Calculate Cms → Vas using T/S equations.
Method 2 — Known Test Box
- Mount the driver in a sealed box of known volume.
- Measure the system resonance (Fc).
- Calculate Vas from the shift in frequency.
Software tools like DATS, CLIO, and REW can compute Vas automatically.
6. Practical Examples
| Driver Model | Sd (cm²) | Cms | Vas | Description |
|---|---|---|---|---|
| 3″ Full-range | 35 | Low | 2.8 L | Suitable for ultra-compact enclosures |
| 6.5″ Woofer | 140 | Medium | 28 L | Common bookshelf speaker choice |
| 12″ Woofer | 530 | High | 120 L | Requires a large cabinet |
| 15″ Subwoofer | 880 | Very high | 220 L | Exceptional deep-bass capability |
7. Choosing the Right Vas for Your Project
- Sealed boxes: medium to high Vas → deeper LF
- Bass-reflex systems: match Vas reasonably with enclosure size
- Open-baffle designs: high Vas drivers perform best
Conclusion
Vas is one of the foundational Thiele–Small parameters. It determines how compliant the suspension is, how large the enclosure must be, and how the driver behaves at low frequencies. Understanding Vas empowers designers and audio enthusiasts to build speakers with accurate, powerful, and well-controlled bass performance.
