Understanding Sound Absorbers

Acoustic absorbers reduce sound reflections in a room by converting sound energy into heat. This helps minimize unwanted reverberation and improve clarity. How they work, and how effective they are, depends heavily on the frequencies you're trying to absorb.

How Absorbers Work

When sound waves strike an absorber, typically made of porous material like fiberglass, mineral wool, or open-cell foam, they enter the material’s structure. Inside, air particles vibrate, and friction within the fibers converts the motion into heat, dissipating the sound energy.

This process, called viscous damping, is highly effective for mid and high frequencies, where wavelengths are shorter and more easily disrupted.

The Challenge of Low-Frequency Absorption

Low-frequency waves (below ~250Hz) have long wavelengths, from several feet to several meters, and much more energy. This makes them harder to absorb for two main reasons:

1. Wavelength Size
To effectively absorb a frequency, the absorber's depth should be roughly ¼ of that wavelength. For example:

• An 80Hz wave is about 4.3 meters long.
• To absorb it effectively, you’d need over 1 meter of absorber depth, impractical in most spaces.

This increases in size as the frequency decreases. To properly treat 20 Hz in a room, you are looking at an absorption depth of over 4 meters.

2. Particle Velocity vs Pressure
Porous absorbers work best where the air particle velocity is high. However, low frequencies typically exhibit low velocity near room boundaries, such as walls. As a result, absorbers placed flat against the wall are far less effective at reducing bass energy

Why Air Gaps Matter

Placing absorbers a few inches off the wall increases their efficiency. It is a cost-effective way to get better performance from acoustic treatment without needing additional materials.

• The air gap acts like an extension of the absorber's depth, improving performance at longer wavelengths.
• Even a 10-20cm space behind a 10cm thick panel can dramatically increase low-frequency absorption, without needing to build thicker traps.

This is a simple and powerful technique to improve performance without adding additional material cost.

Implementation

Many rooms benefit greatly from the proper placement and planning of acoustic absorbers. Use tools like Room EQ Wizard (REW) to identify what your specific room needs, or consult with a local acoustician for tailored advice.