Every year in Toulouse (south western France), the equipment manufacturer, Liebherr-Aerospace, carries out around fifteen tests in an unusual place, called an anechoic chamber. In this bunker, completely independent of the building in which it is located, cones are fixed to the walls. A means of capturing the majority of sound waves, and therefore noise ; here the teams measure the real noise generated by an air-conditioning pack (A.C.M), for example.
As Liebherr-Aerospace’s, Development Testing Manager, Mireille Lemasson explains "Aircraft manufacturers call for noise standards, both on the outside of the aircraft, of 77 dB (A), and also between 65 to 75 dB (A), to meet aircraft cabin interior acoustic standards.’’
To measure the noise level of the equipment, per se, the pack must be positioned inside the chamber. The temperature is reduced to -40 °C ( -40 °F), and then increased to 80 °C (176 °F), in oder to simulate the operating conditions to which the equipment will be subjected.
However, to measure the noise of the equipment’s outlet air, the air-conditioning pack is installed outside the anechoic chamber, in this room. Once in operation, the outlet air is then released into the awaiting chamber, in which they have a large number of microphones, waiting to measure the exact acoustic level emanating from their equipment’s air outlet.
Around one week’s testing may be required for each piece of developmental equipment. If the noise level meets the regulatory standards, it can be marketed, otherwise, corrective measures, such as the addition of silencers, similar to those found on car exhaust pipes, should be undertaken.