Absorbing Material

Materials with a high sound absorption coefficient are usually porous.

Unlike soundproofing materials, sound absorbent materials aren't dense, they're permeable. Soundwaves penetrate the surface of these materials, and flow into the fibrous or cellular structure that it's comprised of.

It's important to remember that energy can never be created or destroyed, it can only be transformed. Porous absorbers will convert incident sound energy into heat energy through frictional and viscous resistance in the fribrous or cellular structure of the material.

The amount of heat that soundwaves generate is minimal; less than 1/1,000,000 of a watt. When porous sound absorbers are used, only a small portion of the sound energy is reflected back into the space.

Porous sound absorbers are most effective for mid-range frequencies or treble tones. They tend to have less effect towards lower frequencies and have minimal effect with bass.

Common examples of porous sound absorbers include mineral wool, carpets, fibreboards, insulation blankets, and certain forms of foam plastic.

A membrane or plate absorber is an air impervious, non-rigid, non-porous material that's placed over an airspace. When sound energy is applied to the absorber it causes the oscillating system (mass of the front panel and the spring formed by trapped air) to transform into mechanical energy.

They also will reflect higher frequency sounds. Other forms of soundproofing and absorbing will need to be applied to counter-act this added effect.

Common examples of membrane absorbers are wood or hardboard paneling, suspended plaster ceilings, windows, wood doors, gypsum boards, and wood floors.

These types of sound absorbers are typically only used when you need to combat sound in a narrow, yet defined frequency range. They're used to focus on issues related to bass frequencies.

These types of absorbers work based on sound pressure. It's essentially a mass (front wall or diaphragm) vibrating against a spring (the air inside the resonant absorber). By changing either the mass or the spring's stiffness, you can adjust for resonant frequency.

They work in a similar manner to that of a membrane absorber. Meaning they consist of a mechanical oscillation system with a solid plate and tight air space.

An example of a resonance absorber would be a bottle of Coca-cola. However, a more practical example would be layers of perforated plasterboard or perforated metal corrugated sheets. Where the perforations are the bottle's neck and the space behind the sheets are the bottle's container.