Why Thickness and Air Gap Matter
Sound absorption is frequency-dependent. A material absorbs most effectively at frequencies whose wavelength is comparable to its thickness. Because low frequencies have long wavelengths, controlling them requires either thicker material or an air gap behind the panel.
How Thickness Affects Absorption
Thin panels (roughly 12–25 mm) absorb mid and high frequencies well but little low-frequency energy. Thicker panels (50–100 mm) extend absorption down into the low-mid and low frequencies. The exact values depend on the material, so always refer to the manufacturer's tested absorption data.
How the Air Gap Affects Absorption
Mounting a panel a short distance off the wall creates an air cavity that behaves like additional thickness. A modest air gap (e.g. 50–100 mm) meaningfully improves low-frequency absorption without adding material. This is why suspended baffles and panels mounted on stand-offs often outperform directly-glued panels at low frequencies.
Practical Guidance
- Speech / high-frequency echo: thin panels (12–25 mm) are often sufficient.
- Music / low-frequency problems: use thicker panels (50 mm+) and/or an air gap.
- Ceilings: suspended baffles and clouds provide an air gap naturally.
Relevant Standards
- ISO 354 / ASTM C423 — measurement of absorption (the test reports these values per thickness and mounting).
Limitations
Thickness and air gap are two of several factors; density, flow resistivity, and the complete mounting also matter. General guidance does not replace the manufacturer's tested absorption coefficients for the specific product and configuration.
Common Mistakes
- Assuming a thicker panel always absorbs more at all frequencies (thickness mainly helps low frequencies).
- Ignoring the air gap and gluing panels directly when low-frequency control is needed.
- Using a single NRC value to compare products of different thickness.
Related Yamamah Solutions
For help specifying panel thickness and mounting for your project, contact us for a free acoustic consultation.
Frequently Asked Questions
Does an air gap really help? Yes — a cavity behind the panel behaves like extra thickness and improves low-frequency absorption.
What thickness do I need for a meeting room? For speech echo, 25–50 mm panels are typically sufficient; confirm with tested data for the specific product.
Verified Sources
- ISO 354 — Measurement of sound absorption in a reverberation room.
- ASTM C423 — Sound absorption and absorption coefficients.