Sound in Compressor Installations

Compressor noise level is measured in a standardized fashion on a machine (in an acoustic free field e.g. outdoors, without walls or by means of a sound intensity scanning technique). When the compressor is installed in a room, the noise level is influenced by the properties of the room. The size of the room, the materials used for the walls and ceiling, and the presence of other equipment (and its potential noise levels) in the room all have significant impacts. Furthermore, the positioning of the compressor in the room also affects the noise level due to the setup and connection of pipes and other components. Sound radiating from compressed air pipes is frequently more problematic than the noise produced by the compressor itself and its power source. This can be due to vibration transferred mechanically to the pipes, often in combination with vibration transferred through the compressed air. It is therefore important to fit vibration insulators and even enclose sections of the pipe system using a combination of sound-absorbing material covered with a sealed insulation barrier.

There are five different ways to reduce sound. Sound insulation, sound absorption, vibration insulation, vibration dampening and dampening of the sound source. Sound insulation involves an acoustic barrier being placed between the sound source and the receiver. This means that only a part of the sound can be insulated, depending on the area of the barrier and its insulation characteristics. A heavier and larger barrier is more effective than a lighter, smaller barrier. Sound absorption involves the sound source being surrounded by light, porous absorbents attached to a barrier. Thicker absorbents are more effective than thinner absorbents and typical minimum densities are approx. 30 kg/m3 for open-cell polyurethane foam and approx. 150 kg/m3 for mineral wool. Vibration insulation is used to prevent the transfer of vibrations from one part of a structure to another. A common problem is the transfer of vibrations from a built-in machine to the surrounding sound insulation barrier or down to the floor. Steel springs, air springs, cork, plastic, and rubber are examples of materials used for vibration insulation. The choice of materials and their dimensioning is determined by the frequency of the vibration and by the demands of stability for the machine setup. Vibration dampening involves a structure being fitted with an external dampening surface composed of elastic material with a high hysteresis. When the dampening surface applied is sufficiently thick, the barrier wall will effectively be prevented from vibrating and, consequently, from emitting sound. Dampening of a sound source will often influence its operational behavior. It may give limited results, yet provide a viable solution in terms of cost.