Beyond the Specs: What Is an Air Gap, and Why Does It Help?

The empty space behind your panel isn't just empty.

Do acoustic panels need an air gap? No. They work without one, above all in the mids and highs. But for the lower notes, a gap really helps, and it is an upgrade that costs nothing and weighs nothing, because it is just air.

Here's why that invisible space matters, and where its usefulness runs out.

Felt Needs Moving Air

Felt absorbs sound by slowing moving air, turning that motion into a little heat. So it works best where the air is moving most.

At a wall, sound behaves like a rope tied to it. At the knot, nothing moves. A quarter of a wave out, the rope swings the most. For high notes, that quarter wave is a few centimetres, inside the panel itself. For low notes, it is far out in the room. At 100 to 300 Hz, a single wave is more than a metre long.

So a panel placed directly on a wall mostly catches the high and mid frequencies. The low notes reach it where the air is barely moving, and much of their energy reflects back into the room.

An Air Gap Adds Depth, Not Felt

Move the panel off the wall, and the same felt now sits where the air moves for lower notes. Nothing about the panel has changed. It has simply been moved to where there is more work to do.

Absorption is most effective where the air moves fastest.
That happens one-quarter of a wavelength from a hard surface, like a wall.

For example:

• At 250 Hz, the wavelength is about 1.37 m.
• One quarter of that is about 34 cm.
• Felt starts working well before a full quarter wave fits behind it, so even a few centimetres of gap help.

Quick Reference: Quarter Wavelengths

A rule of thumb, not a measurement: sound arriving head on, at a speed of 343 m/s.

For 160 Hz, the felt would ideally sit 54 cm from the wall. A panel hung there would need explaining to guests. Nobody has to go that far: the rule tells you which way to go, not where to stop. The deeper the felt sits, the lower it reaches.

The rule is also the head-on picture. In a real room, sound arrives from every angle, and air moving along a wall is not stopped by it, so felt close to a wall does a little better than the rule suggests. A little.

A panel on a gap is not a tuned device, like a Helmholtz resonator or the bass port on a speaker. It does not pick out one note. It moves the same felt to where the air moves more, so it helps a band of lower notes, not the whole low end. The gap itself absorbs nothing. All the absorbing is still done by the felt.

How much does the gap add to Surge Pro? We can't give you a measured figure. Surge Pro was tested on its spacer and has never been measured flat on a wall, so we have a measured after and no measured before. Here is the after.

In the laboratory, Surge Pro on a 35 mm gap absorbed 0.59 at 250 Hz, 0.95 at 400 Hz and 1.15 at 500 Hz (NTOU, ASTM C423-22, report A-10-2025). A coefficient of 0.59 means about 59% of the sound reaching the panel is absorbed.

The Panel + Gap Acts as a System

Think of the panel and the air gap as one absorber. The test report does: it lists the 35 mm of air as part of the product.

• The felt does all the absorbing, at every frequency.
• The gap decides where the felt sits, and so which notes it reaches.
• The gap adds no felt, so its help stops at a band of lower notes.

This is how we design at Massform. Every Surge Pro and Rhythm Pro panel comes with a 35 mm (about 1.4 in) air-gap spacer, the raise Surge Pro was tested on.

A Gap Is the Free Trick. More Felt Is the Real Fix.

For the real low end, you need more felt. The laboratory test measured exactly that step: Surge Pro panels, 50 mm deep, on a 35 mm gap, then each panel stacked on a 50 mm Basstrap Pro of the same size, 100 mm deep in all, on the same gap.

Laboratory absorption coefficients, by frequency:

• On the gap alone, absorption first reaches 1.0 at 500 Hz.
• With the Basstrap, it reaches 1.0 at 200 Hz, and at 160 Hz it rises from 0.21 to 0.83, about four times as much.
• At 125 Hz, even the deeper build reads 0.25. The deepest bass is a job for where the speakers and your seat go, more than for anything on a wall.

Values above 1.0 come from sound reaching the open edges of the raised test array as well as its face. Why is NRC over 1.0 possible? explains it. As single numbers, Surge Pro reads NRC 0.95 without a Basstrap and NRC 1.15 with one. One Basstrap layer is what was tested; we have not tested two.

Tested by National Taiwan Ocean University (NTOU) to ASTM C423-22, reports A-10-2025 and A-11-2025, April 2025: 19 panels, 10.999 m², raised 35 mm with open edges. With Basstrap, each panel sat on a 50 mm Basstrap.

Download the lab report A-10-2025, on a 35 mm gap (PDF)

Download the lab report A-11-2025, with a Basstrap (PDF)

What About Ceiling Clouds and Corners?

Both put more air behind the panel. Plan in 3D hangs a ceiling cloud 25 cm (10 in) below the ceiling on a Cloud Kit, and a panel on a Corner Kit spans the corner at 45 degrees, with a triangular cavity behind it.

Neither mounting has been measured, so we give no figure for either. Plan in 3D rates a cloud or a corner panel with the same wall-mounted test data, and says that probably understates their bass. Until one is measured, we claim no advantage for them over a panel on the wall.

Quick Answers

How big should the air gap be?
Deeper reaches lower, and every centimetre comes out of the room. A few centimetres already help, because felt starts working before a full quarter wave fits behind it. Surge Pro was tested on 35 mm, the spacer it comes with.

Does an air gap help with bass?
It helps the low mids, around 250 Hz, more than the deep bass. For the low end, add felt, such as a Basstrap behind the panel. For the deepest notes, move the speakers and your seat first. Bass traps, and what they can and can't do goes further.

How do you hang acoustic panels with an air gap?
On spacers, battens or a frame that holds the panel off the wall. Surge Pro and Rhythm Pro come with a 35 mm spacer, so the gap is part of hanging them.

Can I hear what panels would do in my room before buying?
Plan in 3D puts panels on the walls of a model of your room, with or without a Basstrap behind them, and plays a voice from where the sound starts to where you sit, before and after. It estimates from your room's size and surfaces; it does not measure your room. Every panel in it sits on its spacer, as tested, so it cannot play you the gap on its own.

TL;DR:

An air gap is the free trick.
More felt is the real fix.

Start with a gap. Add felt for the bass.

Corrections, October 2026: an earlier version of this post said absorption on the gap began ramping up at 315 Hz, which understated the laboratory report (0.59 at 250 Hz), and that a 3 cm panel on a 5 cm gap performs like an 8 cm panel, which had no source. The first is replaced above with the report's values; the second is gone, because we have no measurement of Surge Pro flat on a wall to compare with.