# Sound Absorption : Definitions, Terms, Units, Measurements..

### property possessed by materials and objects of converting 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 units are Sabins.

**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.

**Attenuation Coefficient** another name for Sound Absorption Coefficient.

**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. Also known as a **Standing Wave 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 it 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.

**Sabin**

### a unit of Sound Absorption of a 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 : the unit is the sabin, not the sabine. Wallace Sabine developed the Formulae

**Sabine** : Wallace Clement Sabine an 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 absorption coefficient and vice-versa.

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

### quotient of Sabine absorption of a surface by the area of the surface

● Note : With αi as the Sabine absorption coefficient of the i-th surface whose area is Si, the Sabine absorption attributed to the surface is Ai = Siαi.

**Sound Absorption Coefficient** is also known as **Sabine Sound Absorption Coefficient**

**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.

**Standing Wave Tube** a method for measuring **Sound Absorption Coefficients** by means of the standing waves in a tube.