YoungKadafi

Taken from mixonline.com

Sound Absorbing Material - A Quick Guide

May 31, 2004 11:19 AM

(excerpted from Mix, August 1997, “Room Acoustics: Basic Principles of Reflection, Diffusion and Abroption,” by David R. Schwind)

All materials have some sound absorbing properties. Incident sound energy which is not absorbed must be reflected, transmitted or dissipated. A material’s sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. The coefficient can be viewed as a percentage of sound being absorbed, where 1.00 is complete absorption (100%) and 0.01 is minimal (1%).

Incident sound striking a room surface yields sound energy comprising reflected sound, absorbed sound and transmitted sound. Most good sound reflectors prevent sound transmission by forming a solid, impervious barrier. Conversely, most good sound absorbers readily transmit sound. Sound reflectors tend to be impervious and massive, while sound absorbers are generally porous, lightweight material. It is for this reason that sound transmitted between rooms is little affected by adding sound absorption to the wall surface.

There are three basic categories of sound absorbers: porous materials commonly formed of matted or spun fibers; panel (membrane) absorbers having an impervious surface mounted over an airspace; and resonators created by holes or slots connected to an enclosed volume of trapped air. The absorptivity of each type of sound absorber is dramatically (in some cases) influenced by the mounting method employed.

1) Porous absorbers: Common porous absorbers include carpet, draperies, spray-applied cellulose, aerated plaster, fibrous mineral wool and glass fiber, open-cell foam, and felted or cast porous ceiling tile. Generally, all of these materials allow air to flow into a cellular structure where sound energy is converted to heat. Porous absorbers are the most commonly used sound absorbing materials. Thickness plays an important role in sound absorption by porous materials. Fabric applied directly to a hard, massive substrate such as plaster or gypsum board does not make an efficient sound absorber due to the very thin layer of fiber. Thicker materials generally provide more bass sound absorption or damping.

2) Panel Absorbers: Typically, panel absorbers are non-rigid, non-porous materials which are placed over an airspace that vibrates in a flexural mode in response to sound pressure exerted by adjacent air molecules. Common panel (membrane) absorbers include thin wood paneling over framing, lightweight impervious ceilings and floors, glazing and other large surfaces capable of resonating in response to sound. Panel absorbers are usually most efficient at absorbing low frequencies. This fact has been learned repeatedly on orchestra platforms where thin wood paneling traps most of the bass sound, robbing the room of “warmth.”

3) Resonators: Resonators typically act to absorb sound in a narrow frequency range. Resonators include some perforated materials and materials that have openings (holes and slots). The classic example of a resonator is the Helmholtz resonator, which has the shape of a bottle. The resonant frequency is governed by the size of the opening, the length of the neck and the volume of air trapped in the chamber. Typically, perforated materials only absorb the mid-frequency range unless special care is taken in designing the facing to be as acoustically transparent as possible. Slots usually have a similar acoustic response. Long narrow slots can be used to absorb low frequencies. For this reason, long narrow air distribution slots in rooms for acoustic music production should be viewed with suspicion since the slots may absorb valuable low-frequency energy.


The more fibrous a material is the better the absorption; conversely denser materials are less absorptive.

Human ears respond to frequencies between 20Hz and 20,000Hz. The human is most sensitive to sounds between 1000 and 4000Hz.

Rockwool i guess is a good asorber
So is fiberglass. However i think fiberglass isnt good for auto's because moisture will rott the fiberglass. Im thinking if you put the fiberglass in a sealed bag, it would prevent this, however wouldnt that just reflect most of the sound?

heres a small list of specs

50mm(2" rockwool) gives the following absorption
500hz = 0.96
1k = 0.94
2k = 0.92
4k = 0.82

as you can see its most effective at 500hz and stays failry even through to around 3k before it tails off

The material (a high density slab)
500hz = 0.93
625hz = 0.99
800hz = 1.06
1000hz = 1.14
1250hz = 1.15
1600hz = 1.09
2000hz = 1.08
2500hz = 1.05
3150hz = 1.02
4000hz = 0.97
these figures illustrate the point killahertz made about the thickness of the material absorbing lower frequencies.

I guess the best sound absorbing comes from having a less dense, but thicker barrier like dry-cell foam. Im going to pad underneath my carpet with a less dense type foam and see if it works!! I guess that tar can be reasoned with




Modified by YoungKadafi at 10:57 PM 1/28/2007