In an acoustic test and measurement room, pieces of foam or fibrous material (e.g. mineral wool - rock, glass - or polyester), cut from the mass or obtained from assemblies (of plates), with in some case a support and protection cover (even when there is a perforated sheet at the front) often constitutes an anechoic (i.e. avoiding the reflections of acoustic waves on the surfaces they cover: walls, ceiling, even in somes cases: ground flloring) lining.
In relation to products that can be quite different in how they are made, in shape and in frequency performance - there is usually (at least) this in common among these conventional absorbers:
- they are usually designated by the term "wedgel" (regardless of their effective geometry, almost always: angular)
- the lower limit of the frequency domain (often called: cut-off frequency) from which their sound absorption coefficient reaches or exceeds 99%, which is necessary to obtain, inside a room, an acoustic field as free as outdoors - in an unlimited space - (which is precisely the purpose of the implementation of a sound-absorbing lining in a room dedicated to research and development - R&D - in acoustics) is very dependent on their depth e.g.  0.825 m for a cut-off frequency of 100 Hz
This characteristic of their performance poses a problem in different contexts, with regard to the thickness of conventional absorbers which would be required to obtain a given cut-off frequency (which must be counted twice when considering the dimensions usable in practice in the experimental chamber between parallel walls having a given spacing):
- where, taking into account the distance required on the one hand between the noise source under test and the microphones, and on the other hand between the microphones and the sound-absorbing coating, it would only allow measurements for a hardware with a size that would be too small compared to that of the hardwarel to be tested indeed
- when it would require a distance between walls greater than what is possible
This is why, whether it is a project to build a new acoustic testing and measurement room, or to retrofit an existing test facility (and then: there is no not uncommon for the search for a lower cut-off frequency to be topical, with respect to what the problem of available space is further increased), an "assymetrically structured absorber"constitutes a particularly suitable sound-absorbing lining since it allows (for cut-off frequency) obtaining:
- 125 Hz for a thickness of 0.53 m
- 100 Hz for a thickness of 0.65 m
- 80 Hz for a thickness of 0.78 m
The asymmetrically structured absorbers marketed by ITS (available in 3 thicknesses, as mentioned above) are machined from blocks of non-flammable, light-coloured foam (it is possible to add, at the front, a perforated metal protection).
As their name suggests, asymmetrically structured absorbers have an angular corrugation, i.e. a form of regularity in the variation of the thickness of the material, which is different from that - less sophisticated - of conventional wedges.
Such a choice of asymmetrically structured absorbers is appropriate in all cases where an absorbent lining with both a very good absorption factor and a thickness less than that of a conventional wedge is required, at a reasonable supply and installation cost: for acoustic testing and measurement rooms such as anechoic or semi-anechoic rooms with which universities, engineering schools and acoustics laboratories are often equipped..
ITS masters all aspects of the choice of an anechoic lining "asymmetrically structured absorber" for an acoustic testing and measurement room.
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