Air gap: how the distance from the wall increases the effectiveness of acoustic devices

Why is the distance of the panel from the wall so important? The mechanism of action

We often focus on the absorbing material – its density and thickness. However, the arrangement of panel + air gap + wall creates an entirely new acoustic system. The air gap acts like a air spring, which changes the acoustic impedance of the entire structure.

What does this mean in practice?

• Better matching to the environment: The system with an air gap is better "tuned" to the air in the room. This allows sound waves to more willingly penetrate the material rather than bounce off its surface.

• More effective energy absorption: When a sound wave passes through the panel and then hits the air gap, its particles slow down and are forced to vibrate. This causes the conversion of acoustic energy into heat, which is the essence of absorption.

This phenomenon is crucial where standard panels glued directly to the wall fail – in the range of mid and low frequencies.

How does the depth of the gap affect absorption? [Data and Practice]

The appropriate distance from the wall allows for precise "targeting" of specific frequency bands. Scientific research, including the work of Moesli Muhammad's team, as well as our practice in acoustic adaptation of rooms, confirm this relationship.

• Gap 10–20 mm: Even a small distance significantly increases the sound absorption coefficient in the range of 500–1500 Hz. This is an ideal solution for improving speech intelligibility and reducing office noise or echo in the living room.

• Gap 30–50 mm: Increasing the distance shifts the maximum effectiveness towards lower frequencies, covering the range of 250–500 Hz. This is the optimal choice for universal broadband panels in home recording studios, listening rooms, or rehearsal spaces.

• Gap 100–200 mm: Such distances are reserved for dedicated low-frequency structures. When combined with thick material (e.g., high-density mineral wool), they create effective bass traps, essential for controlling booming bass.

Note: Too small of a gap (below 10 mm) is unfavorable. It can lead to undesirable resonances that, instead of helping, worsen the performance of the arrangement in certain bands.

When can the air gap be harmful? Two mistakes to avoid

Although moving the panel away from the wall is a powerful tool, its improper use can cause more harm than good.

1. Error No. 1: Too small distance (<10 mm)
   As we mentioned, the minimum gap is crucial. Gluing the panel "almost" to the wall, but not quite, can create a narrow gap that will act unpredictably, limiting effectiveness. Either we mount the panel directly on the wall or maintain a conscious, calculated distance.

2. Error No. 2: Too large, uncontrolled distance (>200 mm)
   For standard broadband panels, excessive gap (e.g., 300 mm) weakens their performance in the mid and high ranges, while also failing to provide precise bass control. Such large distances only make sense in specialized, custom-designed resonant structures (e.g., Helmholtz resonators), where the gap is a key element tuning the whole system to a specific frequency.

Practical guide: What distance should you choose for your room?

To determine the optimal gap, first identify your goal.

• Goal: Reduction of echo, improvement of speech intelligibility (office, salon, conference room)

  • Recommended gap: 20–50 mm.

  • Why? This distance most effectively enhances the performance of panels in the range of the human voice and typical background noises.

• Goal: Control of mid and low tones (home studio, listening room, home theater)

  • Recommended gap: 50–100 mm for broadband panels.

  • Why? It shifts panel effectiveness down, helping to manage "muddiness" in lower registers of instruments and vocals.

• Goal: Precise battle against booming bass (corners of rooms)

  • Recommended gap: 100–200 mm (or even more in dedicated constructions).

  • Why? Only a large depth, combined with the appropriate material, allows for the creation of an effective bass trap. It is worth considering mounting the panels diagonally in the corners, which naturally creates a large air gap behind them.

Summary: Your simplest and free acoustic upgrade

The air gap behind the acoustic panel is not a detail but a fundamental element affecting its performance. Instead of investing in doubling the thickness of the material, it is often enough to wisely utilize the free space.

Remember these 3 rules:

1. Always leave an air gap (min. 20 mm), unless the specifics of installation require otherwise.

2. Match the depth of the gap to the problem you want to solve (greater gap = action on lower frequencies).

3. Avoid extremes – both too small and too large, unconsidered gaps can worsen the effect.

Proper use of the air gap is proof that in acoustics, the best solutions are often the simplest.

Figure 1 – Tested panels made from PET polyester felt, thickness 45 mm.
Installed on the floor of the reverberation chamber.

Figure 2 – Results of acoustic absorption tests on panels made from PET polyester felt, thickness 45 mm.
Installed on the floor of the reverberation chamber.

Figure 3 – Tested panels made from PET polyester felt, thickness 45 mm.
Installed 30 cm from the floor of the reverberation chamber.

Figure 4 – Results of acoustic absorption tests on panels made from PET polyester felt, thickness 45 mm,
installed 30 cm from the floor of the reverberation chamber.

Figure 5 – Tested panels made from PET polyester felt, thickness 45 mm,
installed 50 cm from the floor of the reverberation chamber.

Figure 6 – Results of acoustic absorption tests on panels made from PET polyester felt, thickness 45 mm,
installed 50 cm from the floor of the reverberation chamber.