What are Sound-Absorbing Materials?

Sound absorption refers to the process where sound waves are absorbed by a porous material, dispersing and converting energy into vibrations, and eventually transforming into heat. This characteristic is present in “sound-absorbing materials,” such as glass wool, rock wool, urethane, and perforated metal. Adding an air layer behind the material can enhance its absorption effect.

Glass wool, made from fine glass fibers, provides stable sound absorption, while rock wool, made from fibers of molten rock and slag, has a rougher feel due to its less uniform fiber thickness and length.

On the other hand, materials like Styrofoam are thermal insulators, not sound absorbers. Products with surface coatings or board-like structures may also be marketed as sound absorbers, but they lack the porous structure required for true sound absorption. (Strictly speaking, Styrofoam, metal plates, or concrete absorb only negligible amounts of sound.)

The effectiveness of sound-absorbing materials is often expressed as an “absorption coefficient,” which compares materials under identical measurement conditions rather than indicating intrinsic performance.

Common Misconception #1

There is no single “correct method” for measuring sound absorption because different materials require different methods. For example, thick sound-absorbing wedges used in anechoic chambers may yield inaccurate results if measured in a reverberation room, as the “surface effect” can inflate the absorption coefficient. Unfortunately, some companies exploit this discrepancy to make their sound-absorbing products appear superior. Without disclosure of measurement methods, users cannot access accurate information.

Common Misconception #2

Even manufacturers may misunderstand the nature of sound absorption. For example, comparing the absorption coefficient of one’s own material measured by the “reverberation room method” against another product measured by the “normal incidence method” is misleading. Accurate comparison requires measurements under identical conditions and methods; otherwise, discrepancies arise due to differences in equipment.

Comparison: Glass Wool vs. Rock Wool

Reference: Asahi Fiber Glass Co., Ltd. catalog

Glass wool features long, uniform fibers that are highly durable and provide consistent performance, while rock wool, with varying fiber thickness and length, feels rougher to the touch.

Key Points

– Sound-absorbing materials must be porous, allowing sound waves to penetrate and disperse.
– The thicker the sound-absorbing material, the greater the effect.
– Thermal insulation materials are distinct from sound absorbers.
– High-frequency sounds are easier to absorb, while low frequencies are harder to manage.
– Some metallic sound absorbers exist, but metal reflects sound and is unsuitable for anechoic chambers.
– Displaying sound absorption rates above 0.99 is generally unnecessary.
– A 3dB(A) reduction in sound halves the sound energy.
– While materials like foam or curtains absorb some sound, classifying them as sound absorbers is a decision left to manufacturers.