Sound absorption

The sound absorption property of a material expresses the ratio of incident sound and absorbed sound. The efficiency of sound absorption is expressed by the term sound absorption coefficient a at a specific frequency. It should be noted that the sound absorption of a material can vary depending on how it is installed and the depth of any air space behind it.



a is a ratio between 0 & 1.

a =Absorbed sound energy / Incident sound energy

All sound absorbed
a =1 ( 100% efficiency )

No sound absorbed
a =0( 0% efficiency ).


The above figure gives some examples of the sound absorption of common materials.
Sound absorption can be measured and classified in many different ways. The following information explains some of the most popular classifications:



as
as is the sound absorption coefficient at 1/3 octave band centre frequencies, and is an exact value and is measured in accordance with ISO 354. as is the most detailed method of recording sound absorption and forms the basis for other more useable and compact building acoustic sound absorption classifications.


ap
ap is the practical sound absorption coefficient used for reverberation time estimates in building acoustics. ap is quoted at octave band centre frequencies and is the average of the three 1/3 octave values centred on the quoted octave band centre frequency. It is calculated in accordance with ISO 11654 from as values and is a more practical but nonetheless accurate value.


aw
aw is a single figure weighted sound absorption coefficient. It is calculated in accordance with ISO 11654 using ap values and comparing them with a reference curve. The reference curve is shifted so that the unfavourable deviations are <=0.10. the value for aw is recorded as the value of the reference curve at 500 hz. it is a useful single figure classification, but it has the draw back that the reference curve drops between 2000 hz and 4000 hz, and it is at around 4000 hz that the human ear is most sensitive and high sound absorption is required in many applications.



Absorption classes
Absorption classes A to E are another classification method described in the International Standard ISO 11654. The ap values are compared to a series of fixed reference curves. However, due to the wide range between each reference curve, these classes are an unprecise way of defining sound absorption requirements.


NRC-Value
The Noise Reduction Coefficient (NRC) calculated in accordance with ASTM C423 is a mathematic average of as values at the frequencies: 250 - 500 - 1000 and 2000 Hz. Due to the fact that this method does not include the 4000 Hz value which is critical to ear sensitivity and speech intelligibility, and gives equal weighting across the frequency range, it is an unsuitable method of defining sound absorption requirements. The illustration shows the sound absorption of two materials that have the same NRC but have completely different acoustic characteristics.

Rockfon recommend that the most appropriate way of defining ceiling sound absorption requirements is to state the required ap values at specific frequencies or the aw value.