In 1997, David R. Schwind wrote an in-depth article in Mix Magazine on the basic principles of room acoustics. The article wasn’t very practical, but it did shed some light on why certain materials have the effect of sound absorption, reflection and diffusion. Reading the article, the following basic concept tends to be reinforced:

Sound Absorption

All building materials have some acoustical properties in that they will all absorb, reflect or transmit sound striking them. Conventionally speaking, acoustical materials are those materials designed and used for the purpose of absorbing sound that might otherwise be reflected. Sound absorption is defined, as the incident sound that strikes a material that is not reflected back.

If you want to get really technical, here are some absorption coefficients.

The absorption coefficient is measured in sabins (explained below). The scale of sabins is from 0 to 1, 0 being completely opaque to that sound at that frequency, and 1 being completely transparent (as if there were nothing there to stop the sound.)Take the building material Brick (unglazed) from the table below. Notice how at 125 Hz brick is not as transparent in absorbing sound waves at that frequency as say a porous concrete block.

The concept of absorption  coefficient or attenuation coefficient is widely used in acoustics for characterizing particle size distribution. A common unit in this contexts is inverse metres, and the most common symbol is the Greek letter α. It is also used in acoustics for quantifying how well a wall in a building absorbs sound. Wallace Sabine was a pioneer of this concept. A unit named in his honor is the sabin: the absorption by a 1-square-metre (11 sq ft) slab of perfectly-absorptive material (the same amount of sound loss as if there were a 1-square-metre window).

Note that the sabin is not a unit of attenuation coefficient; rather, it is the unit of a related quantity.

A small linear attenuation coefficient indicates that the material in question is less transparent, while a larger values indicate greater degrees of opacity. The linear attenuation coefficient is dependent upon the type of material and the energy of the radiation.

This is a table of some materials measured in Sabine at certain Frequencies

Source: http://www.gsu.edu

To read the article from Mix, scroll down.

(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”