ITS will participate in the design and realization of a semi-anechoic room for acoustic measurements, in the Paris region (France).
This will involve providing engineers and technicians, in a context of R&D tests, an environment allowing indoor reproduction of the acoustic propagation conditions usually encountered outdoors on a reflecting plane, that is to say (except at ground level) with negligible acoustic wave reflections (in a frequency band of interest chosen by users of the semi-anechoic room).
The context being that of the improvement of an existing test means, that it will therefore be necessary to take into account the existence of external limits (e.g. walls and roof of a building) that cannot be overcome with regard to the rear face of the absorbent lining to be installed on vertical walls and on the ceiling, making it useful, with a view to maximizing the interior volume of the room available for measurement, the use of an absorber of as little thickness as possible, and nevertheless having a sound absorption coefficient being sufficiently high (99% at normal incidence, as measured in a standing wave tube, aka Kundt’s tube, defining the cutoff frequency) over the entire frequency range for which the metrology is considered.
In such a context, absorbent wedges (pyramid-shaped pieces of fibrous material, cut from the mass or from assemblies, with a protective cover in some cases) whose acoustic performance is such that their cut-off frequency is (approximately) proportional to ¼ of the wavelength considered is not an ideal solution, since requiring a significant thickness (at the usual ambient temperature of a laboratory: of the order of almost one meter) if the lowest frequency of interest is close to 90 Hz (e.g. when the frequency band of 1/3 octave of central frequency 100 Hz is considered).
To obtain a high absorption coefficient at low frequency, as often required in a semi-anechoic room such as the one envisaged within the context of this project, it is then preferable to opt for the design of a multilayered absorber, including:
- a thin plate (e.g. in steel or aluminum) for which the mass density, the distance separating it (at the rear) from a rigid support (with an intermediate filling ), the dimensions as well as the fixing conditions determine the (low) frequency (in general: close to the lower limit of the frequency range of interest) of the acoustic absorption peak of the resonator thus formed
- a dissipative material  , located at the front (i.e. on which sound waves emitted by the noise source installed in the testing room are incident) having high acoustic absorption properties at medium and high frequencies, for what a relatively thin layer may be sufficient (its surface may then be flat, and including a perforated protection)
Thus, with a total thickness which is approximately equivalent to 1/3 of that of wedges of equivalent acoustic performance (see above), it is possible to obtain a lining being efficient in a wide frequency band, satisfying both the requirements of such a project in terms of sound absorption and the space constraints available for such building systems.
This has been proven on the occasion of the realization of a large number of acoustic measurement rooms (including: in aeroacoustic wind tunnels) for all kinds of industrial applications (e.g. in the automotive and space domain, or for electro-acoustic hardware), even with a level of requirements in terms of acoustics (e.g. for the limitation of the unfavorable deviations observable on site with respect to the sound decay curve in free field) higher than that set by the standards in force, e.g. when it comes to the consideration of very specific technical specifications for first-rate installations (worldwide) in the field of Research and Development (R & D) in acoustics.
It is thus this principle of broadband compact sound absorber which will base the realization of a semi-anechoic room allowing measurements of noise levels in specified locations around hardware under test e.g. as subject of the ISO 3744 standard Acoustics - Determination of sound power levels and sound energy levels of noise sources using sound pressure - Engineering methods for an essentially free field over a reflecting plane.
In this case, it will come to drones, for which a world market leader has sollicited ITS for the modernization of a test room (engineering studies, drawings preparation, construction and installation) in which the lighting will also be replaced, the integration of lamps specially designed to not degrade the acoustic quality of the measurement room forming part of the scope of the work, at the end of which an anechoicity of the best level will be obtained thanks to the experience of ITS and its commercial partners for such projects, whether it comes to upgrades of anechoic and semi-anechoic rooms (as in the present case) or to the construction of new rooms.
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