Pneumatic power - Acoustic characterization of exhaust silencers

In pneumatic power transmission systems, power is transmitted by pressurized air or gas in a circuit that enables the operation of simple or complex machines. Excessive noise levels associated with the use of compressed air are generally caused by the release of pressurized air into the atmosphere. The most common sources of noise are pneumatic exhausts and compressed air jets. Pneumatic exhausts are fitted to a wide range of machines and tools for the sole purpose of dissipating compressed air after it has done its job. Although some pneumatic exhausts operate continuously (e.g. those on hand-held grinding wheels), the majority work on an intermittent basis dictated by the machine or tool's operating cycle.

Noise levels from pneumatic exhausts are reduced mainly through the use of silencers installed on machines. In fact, the noise generated by air exhausts is due to the strong turbulence zones created in the jet of air leaving the orifice as a result of the large pressure difference with the ambient air. The function of silencers is precisely to make this transition smoother by reducing this pressure difference, but without generating too much back pressure upstream in the pneumatic system.

European directives on industrial noise, on the one hand, regulate the health risk aspects, including those related to noise, to be taken into account by manufacturers when designing a machine, and, on the other, set minimum requirements for improving the working environment, such as limit values for daily noise exposure. However, pneumatic silencers are not directly subject to the directives, and very little information is available on their acoustic performance.

This led to the need to provide silencer manufacturers with a common framework for assessing the sound pressure levels of pneumatic exhaust silencers, and ISO 20145 was born. Its aim is to guide machine manufacturers in their choice of pneumatic exhaust silencers, and to help assess operators' exposure to noise.

It should be noted that, when installing pneumatic silencers, attention must be paid to the effect they will have on the air flow, so as not to reduce the working capacity of the compressed air, due to the back pressure introduced. This is why, in conjunction with acoustic characterization, it is proposed that silencer flow characterization be carried out under the same experimental conditions on common measurement benches.

In the first part of this article, we review the regulatory context concerning acoustic emissions, as well as the technical background leading to the use of silencers. Then, for didactic purposes, the fundamentals of acoustics are recalled, along with a description of the various noises produced when compressed air...