Sound Absorption : Definitions, Terms, Units and Measurement.

Sound Absorption : Sound Energy is absorbed by 'acoustically soft' materials that Sound Waves encounter, as opposed to being Reflected by 'acoustically hard' materials. An open window is an example of 100% sound absorption i.e. no reflection, whereas bathrooms have 'hard' reflective surfaces and therefore very low sound absorption properties

Atmospheric Absorption : over short distances (100 metres) atmospheric absorption can be ignored. At greater differences it can be significant particularly at frequencies above 1 kHz. It is temperature and humidity dependent.

Equivalent Sound Absorption Area : the area of a perfect absorber or soft furnishings, etc., required to produce the required reverberation time in a room.

Impedance Tube : a method for measuring Acoustic Absorption Coefficients by means of
Standing Waves in a tube.

Noise Reduction Coefficient : NRC : a single-number rating system used to compare the sound-absorbing characteristics of building materials. A measurement of the acoustical absorption performance of a material, calculated by averaging its Sound Absorption Coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest multiple of 0.05.

Reference Sound Absorption : A0 : When measuring sound insulation between rooms is common practice to Normalize the measured levels to a Reference Sound Absorption A0, of 10 m²

Room Absorption : sum of
Sabin Absorption due to objects and surfaces in a room, and due to dissipation of energy in the medium within the room.

see also : Reference Sound Absorption above

Room Absorption Coefficient : ratio of Sabine Absorption of a surface to the area of the surface.

A square metre of 100% absorbing material has a value of 1 metric sabin. An example of this would be a 1 m² open window. One square foot of 100% absorbing material has a value of 1 imperial sabin.

Sabin Absorption : defined by the Sabine Reverberation time equation. In a reverberant room of volume V, Speed of Sound c, and decay rate d, the Sabin Absorption is A = 0.921 Vd / c.

Note : the unit is the sabin, not the sabine. Wallace Sabine developed the Formulae

Sabine : Wallace Clement : American pioneer in architectural acoustics. He derived an expression for the duration T of the residual sound to decay below the audible intensity. The associated absorption unit was named after him - sabin without the e.

Sabine Equation : in 1898 W C Sabine also came up with the formulae relating
Reverberation Time with Sound Absorption and Room Volume.

T = 0.161 V/A

where :
V = room volume in m^{3}
A = α x S = equivalent absorption surface or area in m^{2}
α = Absorbent Coefficient or attenuation coefficient
T = RT60 = Reverberation Time in s, seconds
S = absorbing surface in m^{2}

It follows if you know the Reverberation Time you can calculate the A, Absorption Coefficient and vice-versa.

Sound Absorption : property of materials and objects to convert sound energy to heat either by propagation in a medium or when sound strikes the boundary between two media.

Sound Absorption is the product of Sound Absorption Coefficient and Surface Area of a material. The total Sound Absorption units are sabins.

Sound Absorption Class : Classification of sound absorbers into Sound Absorption Classes A-E, according to BS EN ISO 11654, including frequencies 200-5000 Hz..

Sound Absorption Coefficient : the fraction of
Sound Energy absorbed by a material. It is expressed as a value between 1.0, perfect absorption (no reflection) and 0, zero absorption (total reflection).

The value varies with frequency and angle of incidence, determined experimentally - see sabin and Sabine's equation.

A Sound Absorption Coefficient of 1 = 100% absorption - for example an open window.

Sound Absorption Loss : that part of the transmission loss due to the dissipation or conversion of sound energy, either within the medium or attendant upon a reflection.

Standing Wave Tube : a method for measuring Sound Absorption Coefficients by means of the Standing Waves in a tube.