Accordial

Technical Definition

Absorption

Absorption is the dissipation of energy as it passes through a material. Sound energy is transformed into heat as the material resists deformation. Absorption coefficients are used to describe the amount of energy which is not reflected by the material; technically this describes both absorbed and transmitted energy. Sound absorption coefficients (µ) are usually in the range from 0 (perfect reflector) to 1 (perfect absorber).

Absorption Classes

BS EN ISO 11654:1997 ‘Acoustics-Sound Absorbers for use in Buildings-Rating of Sound Absorption’ defines absorption ‘classes’ to materials based on the measured absorption performance across the 250Hz to 4KHz frequency range. The weighted sound absorption µw are categorised as;

  • Values of µw of 0.90, 0.95 & 1.00 are Class A
  • Values of µw of 0.80 & 0.85 are Class B
  • Values of µw of 0.60, 0.65, 0.70 & 0.75 are Class C
  • Values of µw of 0.30, 0.35, 0.40, 0.45, 0.50 & 0.55 are Class D
  • Values of µw of 0.15, 0.20 & 0.25 are Class E
  • Values of µw of less than 0.15 are not classified
  • There are two potential problems with acoustics in schools. The first is reflected sound and the need to increase the amount that is absorbed. The second relates to sound transmission and the structural ability of a room or space to either contain or prevent the ingress of sound intrusion.
     

Sound Absorption

Classrooms, assembly halls and gymnasiums are, of necessity, constructed with hard, robust surfaces to withstand the rigours of school life. This may afford a degree of decorative and structural longevity, but it provides very poor acoustics. This is due to sound impacting upon the hard surfaces and being reflected back into the room in a new direction, causing a build up of reverberant sound that mixes with direct sound. The result is an acoustic muddle that makes communication and listening difficult.

Solution

The first step is to assess the existing reverberation time (RT) for the space. This is the time taken in seconds for the average sound energy created within the space, to decrease to one millionth of its original steady level, after the source has ceased ie. the time taken for a 60dB decrease to take place. The RT can be measured directly by charting the decay of sound following a gunshot or balloon burst, or it can be estimated from the following: R = 0.16V ÷ S`a where R is the reverberation time in seconds, V is the room volume in m³, S is the surface area of the space in m² and `a is the average room absorption coefficient.

What is the absorption coefficient?

This is the amount of sound energy that can be absorbed by a surface or material. This is expressed in values between 0 and 1. The former absorbs none of the sound incident upon it and the latter will absorb all of the sound. Our computer models consider the full sound frequency spectrum for surfaces and materials as the absorption coefficient will vary over the sound spectrum. Sound absorption is generally poor at low frequency but usually much better at high frequency.

 

How do we determine the correct RT?

Building Bulletin 93 makes this very easy as they recommend reverberation times. For example classrooms should generally be <0.8s, assembly halls 0.8 - 1.2s, sports halls <1.5s and music classrooms <1.0s. With this information we can readily determine how much sound absorption needs to be introduced to comply with BB93. Careful consideration must be given at this point to the selection and positioning of acoustic products within the space, to provide a satisfactory acoustic solution that is also practical and aesthetically pleasing. Solutions will vary dependent upon many factors and the products usually proposed are sound absorbers or acoustic curtains.

Sound Transmission

Problems relating to the unwanted transmission of sound can be attributed to any number of potential areas, such as lightweight wall structures, hollow core doors, single glazed windows or 'flanking paths' via suspended ceilings or timber floors.

School Acoustics

There are two potential problems with acoustics in schools. The first is reflected sound and the need to increase the amount that is absorbed. The second relates to sound transmission and the structural ability of a room or space to either contain or prevent the ingress of sound intrusion.