Acoustic testing is an important phase in the development of many products, whether to determine their sound power level or their sound pressure level at specified locations (to meet certain limits in this area or to develop a particular sound e.g. telephone ringing, door closing noise, acoustic discretion of moving vehicles).
A specific measurement environment is then necessary for the associated acoustic Research and Development (R&D) work, with in general (as main objectives):
- a background noise level low enough not to disturb the measurements
- anechoicity of the walls of the test room to avoid unwanted sound reflections
In some contexts, other features are sought for the test environment:
- the absence of sound propagation towards the outside, when testing very noisy equipment (e.g. test benches for high-power thermal or electric engines adjacent to each other or located in a working area where a certain concentration is required to carry out different tasks)
- temperature control, when the noise emissions of the tested equipment depend on climatic conditions e.g. for Heating Ventilation Air Conditioning - HVAC - devices
In all these contexts, ITS offers acoustic testing infrastructures that combine the different functionalities required, meeting the needs of the most demanding users.
ITS will participate in the construction of an anechoic chamber for carrying out acoustic tests on electrical equipment in the aera of Bordeaux (France, Nouvelle Aquitaine region).
Based on a concept that has been the subject of standardization work over the course of similar projects in different sectors of activity (e.g. telephony, electroacoustic equipment, drones to name just the latest projects in France), it will involve installing a factory-prefabricated, dismountable, large-sized acoustic enclosure, which will both limit the transmission of noise through its walls and provide the required anechoicity conditions inside. .
The envelope will be made using modular industrial soundproofing panels without sound absorbing face, in steel (with a sound reduction index close to 50 dB at high frequencies), and an acoustic doorsets (pivoting) will allow access for pedestrians and equipment under test; unlike other constructions envisaged in a soundproofing context (machine enclosures, test benches and studios), there will be no glazing for the translucent parts (which could provide residual safety in the event of accidental breakage): acoustic window sets would certainly ensure the continuity of sound insulation (i.e. without weakness in limiting noise transmission, their sound reduction index being comparable to that of solid construction elements) but their presence would harm the anechoicity of the test room (the sound absorption coefficient of glass is low, particularly at medium and high frequencies, even though a value of 99 % is required above the cut-off frequency). The floor (reinforced to allow movement and the installation of heavy loads) will be detached from its support and sufficiently open so as not to compromise the effectiveness of the sound-absorbing lining ; cable passage and air renewal will be carried out using ventilation silencers .
Among the sound aborbing linings for acoustic laboratories marketed by ITS (cf. https://www.its-acoustique.fr/fr/isolation-insonorisation/revetement-absorbant-laboratoire-acoustique) it is the concept of a compact broadband absorber that will be selected for the intended application, constituting - as is often the case - the best choice of technology:
- its sound absorption coefficient is very close to 100% from the frequency of 100 Hz
- its thickness (0.25 m or 0.35 m for current uses) is low compared to that which would be required for a lining with conventional sound absorbing wedges (a quarter of the wavelength corresponding to the cut-off frequency i.e. 0.85 m considering 340 m/s for the speed of sound)
- its reaction to fire is good (the components are generally non-flammable, and can - in case of specific need - be non-combustible)
This comes to a patented multi-layer acoustic structure, which has proven itself in a number of contexts (semi-anechoic and anechoic rooms, aeroacoustic wind tunnels) including (from back to front):
- a dissipative material (mineral wool, polyester wool or foam) whose vibroacoustic behavior is similar to that of a spring, attached to the support constituted by the envelope of the testing room
- a metal plate: combined with the layer located at the back, it constitutes a resonator, allowing acoustic absorption at low frequencies (in red in the photo below)
- a dissipative material (mineral wool, polyester wool or foam): allowing sound absorption in medium and high frequencies
- a perforated protection
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Broadband Compact Absorber for anechoic room) |
Thus, as their name indicates, compact broadband absorbers for anechoic rooms allow the achievement of perfectly free acoustic field conditions:
- in a volume as large as 2.5 m3, by means of a construction whose overall volume (including the envelope) does not exceed 36 m3 (which is nothing compared to that of a room equipped with conventional sound absorbing wedges)
- from 100 Hz to 12.5 KHz (1/3 octave frequencies) and even below and beyond
As for the on site acoustic performance, it is known in advance, having been verified by an independent body during the construction of an identical anechoic chamber. In particular, the specifications of the international standard ISO 3745 Acoustics - Determination of sound power levels and sound energy levels emitted by noise sources from sound pressure - will be met; the deviations of the measured sound pressure levels from those estimated using the distance inverse square law, obtained in accordance with ISO 26101:2017, 5.1 (but excluding 5.1.6), will not exceed the values given in the table below:
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|
Third octave band central frequency (Hz) |
Allowed deviations (dB) |
|
Anechoïc |
≤ 630 |
±1,5 |
|
Semi-anechoïc |
≤ 630 |
±2,5 |
There is no doubt that this anechoic chamber will for a long time offer its users an environment of choice for acoustic metrology allowing perfect control of the sound emissions of the materials tested.