Basic Noise Control

Sound Absorption and Reverberation

One method for controlling the sound level within a room is through dissipation of the sound energy in absorptive materials. Sound is absorbed when a portion of the sound energy striking a surface is not reflected, but passes into the material and is converted into heat energy. Generally, higher frequencies are more easily absorbed than low frequencies. Materials that are good absorbers permit sound to pass through them relatively easily; this is why sound absorbers are generally not good sound barriers. They reduce the level of noise inside an enclosure, because while the sound waves are being reflected from the surfaces in the room, they interact with the sound-absorbing materials and lose some energy each time.

However, it requires large thicknesses or many passes for the sound energy to be significantly reduced. It is important to understand the difference between sound-absorption and sound transmission loss. Materials that prevent the passage of sound are usually solid, fairly heavy and non-porous. A good sound absorber is 50 mm of mineral wool; a good sound barrier is 150 mm of poured concrete.

Sound-absorptive materials are used to reduce the level of steady sound in a room, from a machine for example, and to reduce the reverberance. The reverberance or liveness of a room is usually characterized by its reverberation time, the time it would take the energy of an abruptly stopped sound source to decay through a range of 60 dB. Just as sound-absorption varies with frequency, so too does reverberation time. No single figure rating is used to summarize reverberation time information, but the value at 500 Hz is often used for this purpose.

Resilient Mountings

Most machines generate vibration, which can be transferred to the structure of a building through the machine supports. The vibration level can be reduced by balancing the machines to reduce the forces at the source. Transmission into the building structure can be reduced by using resilient, anti-vibration mountings between the machine and the supporting structure. Common materials used as vibration isolators are rubber, cork, visco-elastic polymers, various types of steel springs, and resilient pads. The transmission of vibrational energy depends on the resonance frequency of the machine on the resilient supports and on the energy damping in the system.

Combination of Techniques

Three important measures are used in almost all noise control work:

If resilient supports were omitted, the vibrational energy from a machine would pass into the building structure with very little attenuation, to radiate as sound elsewhere. If a solid enclosure were not there, the sound would pass through the sound-absorbing material with very little attenuation. If the sound-absorbing material were omitted, the reverberant sound inside the enclosure would build up and the energy loss through the enclosure would be much less than expected. All three elements should be present.

The correct application of these three measures is the keystone of good acoustic design and noise control. The enclosure can be a box housing a printer, a large room containing several noisy machines or a living room in an apartment, but the principles remain the same. Good sound control incorporates resilient supports or connections, sound-absorbing material and leak-free sound barriers.

The extent to which these measures are necessary will depend on a number of factors. If quiet machines are selected for use in a building and are placed as far as possible from areas where quiet is required, fewer noise control procedures will be necessary. It is not always possible, of course, to control the sources of noise in a building, since sometimes the occupants are the sources.

Noise control is an involved and fascinating subject where there is always something to learn. Much of the principles outlined here are also applicable to engine, machine, motor or fan noise, where through logical principles and intelligent material selection there is an optimum solution to be found for almost every acoustic challenge.

Siderise has the expertise and experience to find solutions to your challenges, please email us with your enquiry.