Acoustic Insulation - Some Details

Acoustic insulation (another term for sound proofing) is the method of reducing sound pressure with respect to a specific sound source and receiver. There are several ways in doing this. Some use noise barriers to reflect or absorb the energy of the sound waves. Others use damping structures (sound baffles) and still others use anti-noise generators.

The system is called by many names (noise control, acoustic quieting, noise mitigation, etc.) but the whole aim is to limit unwanted noise. Each of these methods can suppress indirect sound waves by transforming them into reflections that cause echoes and resonances that cause reverberations.

There are about four other forms of sound suppressants.

Distance

This is a natural phenomenon. When sound waves spread out, the energy density decreases. With the increase of distance between the sound source and the recipient, sound diminishes or the intensity is reduced.   

In a normal setting, this would include a point source and point receptor. The intensity of the sound waves will be dependent on the distance between the two. This is according to their inverse square of the distance from the source.

Damping

Technically, this is the term used in reducing the resonance of sound in a room by way of absorption or redirection (reflection or diffusion). The absorption will lower the resonance of the sound in a room while the redirection makes unwanted sound harmless by reducing its coherence.

Damping reduces the acoustic insulation resonance in the air or mechanical resonance in the structure of the room itself, or by the things inside the room. (Cork walls, thick carpets and curtains can reduce the resonance of sound in a room, though not actually eliminate it because sound mainly travels by air.)

Absorption

When sound is absorbed by an object, there is a small conversion of a part of the sound energy into a small amount of heat in the intervening or absorbing material. This reduces the sound intensity as it is being transmitted.

There are some ways where a material can absorb sound. This sound absorbing material will be determined by the frequency distribution of noise to be absorbed and the acoustic absorption profile needed.

Porous absorbers

Basically, these absorbers are open cell rubber foams or melamine sponges that can absorb noise friction within the cell structure. The use of dense materials typically used for insulation absorbs the frequencies of the sound due to the porous nature of the material and less likely to be reflected.

The absorption profile of porous open cell foam is determined by many factors. These would include the cell size, the porosity of the material, tortuosity, material thickness and material density.

Sound transfer

The vibrations or sound transfers from one room to another happen through mechanical means. The vibration passes through bricks, wood, and all the solid structural elements. When it meets another element like the wall, ceiling, floor or window, all will act as a sounding board.

Acoustic insulation (with the use of foams, and other sound absorbent materials) is a concrete show on how sound is stopped by a mechanical device.