Visit of the ONERA
The French aerospace lab
We had the chance to visit the acoustic laboratory of the ONERA (National Office of Study and Aerospace Research). Two acoustic chambers are available : an anechoic room and a reverberant one. In order to do research, studies or for pedagogic purposes, ONERA is using these two facilities.
The anechoic chamber
This is an experimental room designed to completely absorb reflections of sound waves. Anechoic chambers are mainly used in the context of acoustics to minimize the reflections of a room.
The one we visited is quite big with the following dimensions:
- Length: 8 meters
- Width: 4 meters
- Height: 4.5 meters
- Opening: 2.10 x 1.20 m
The cut-off frequency is about 90 Hz and the temperature is stabilized from 15 to 25 ° C to ensure the repeatability of measurements.
This room is equipped with an anti-vibration system to ensure that measures are not distorted by the external environment. This system is quite rare; there are only 10 like this in
This chamber is quite old, the sound insulation is about a meter thick and made of rock wool which is the equivalent of glass wool today. It is shaped with pyramids, as you see on the picrture, to improve the absorption of a wide range of sound waves frequencies. The small cavities included in the rock wool and the big prisms both improve the capacity of the room to absorb all the sounds in three dimensions.
Hence, this room can be used to test unknown acoustic sources.
For example, to measure the noise emitted by an engine, we place it in the middle of the chamber and set up calibrated sensors in strategic locations to get the best measurment. Therefore, they only measure the noise emitted by the engine and not the echo or background noise from the outside. Doing those measures, we can characterize the device and see whether it is standard.
The reverberant chamber
Unlike the anechoic chamber, this one is designed so that walls, ceiling and ground are unparallel to avoid fundamental acoustic modes. Walls are built with concrete and internal hard coat, covered by an epoxy painting. This room is coupled with the soundproofing one.
The one we visited has the following dimensions:
- Volume: 45 m3
- Wall surface: 78 m2
- Opening of 1.20 x 1.50 m2
The global reverberation time is 7 seconds.
This room is made so that the acoustic pressure is uniform inside the chamber. The interest is to get the absorption of the tested material. Indeed, by doing a measurement of echoes with an empty room and comparing it to the same measure with an object in the room, we can find the sound absorption of this object. However, this method is very old (1895) and includes mismeasurments.
