Noise Control; Passive and Active
By James P. Cowan
Unwanted sound – noise, that is – can generally be controlled at one of three points: at its source, at the listener, or in the path between the source and the listener. The first two are typically more invasive than the last, but also more effective.
It is preferable to control noise at the source, such as by maintaining equipment, relocating noise sources, removing unnecessary noise sources, using quiet models, or redesigning noisy equipment. But these remedial actions are not always possible. Similarly, noise control at the listener is often impractical because it means enclosing the listener or requiring him/her to wear hearing protection.
Most often, then, noise control is restricted to intervening in the path of the sound source to the listener. The most common techniques for mediation involve: enclosures, barriers, HVAC noise control, absorption, isolation, sound cancellation, and sound masking.
PHYSICAL STRUCTURES
Enclosures. Properly designed enclosures can significantly reduce noise levels (from very loud to barely audible). To be effective, an enclosure must completely surround the noise source, leaving no air gaps. (Think of waterproofing: if water can leak through a partition, so can noise.) An enclosure with any side open is only a sound barrier, not an enclosure, and is considerably less effective in reducing noise (15 decibels compared with up to 70 decibels).
Enclosures should be isolated from floors or any structural members of a building. An enclosure covering only the sides and top of a noise source that sits on the floor or ground, for example, is compromised in several respects. The sides of the enclosure are unlikely to provide a perfect seal with the ground, resulting in air gaps. In addition, vibrations carry along the ground or floor if the source is in direct contact with it. To reduce these vibrations, one must isolate the source from the ground or floor using tuned springs (appropriate for the source) or pads.
Multilayering can add more than 20 decibels in noise reduction to a homogeneous enclosure. (By comparison, doubling the mass of an enclosure would only provide a reduction of 6 decibels in the same space, while risking excessive weight.) Adding sound absorptive material can further reduce noise when used on the inside of a multi-layered enclosure. But an enclosure should not be comprised of such material alone; its purpose is to control reflections within spaces, not to control sound transmission through walls and out of spaces.
Barriers. In contrast to the noise reduction of enclosures, a barrier is limited in effectiveness by diffraction, or the way sound waves surmount obstacles. The compromising effect of this phenomenon is particularly pronounced when the wave-length of the sound is comparable to or greater than the dimensions of the barrier.
A barrier may be further compromised by reflective ceilings, which facilitate sound travel. Therefore, when noise barriers are used indoors, an absorptive ceiling should be installed above them. It is also important to have no air spaces within or under the barriers, since this will reduce their already limited effectiveness.
PASSIVE NOISE CONTROL
HVAC Noise Control. Some noise sources such as motors require ventilation. If an enclosure is used to control noise, it would be self-defeating to cancel its effect by simply making an opening in it. Rather, ventilation systems should be developed to minimize noise transmission.
Airflow through ductwork with sharp bends and small grill openings can be noisy, especially at high speeds. In addition, fans in HVAC systems can generate tones related to their rotational speeds. To control these types of noise, the system should be designed to minimize flow turbulence. Also, ‘silencers’ can be installed to abate the fan noise, although these must be designed individually.
Finally, ducts can be lined with absorptive materials. The purpose of absorptive treatment is to reduce reverberation, or the amplification of sound from repeated reflections within an enclosed space.
Absorption. The ability of different materials to absorb sound can be measured (in sabins, or absorption coefficient times surface area). In a room, absorptive treatment can reduce noise levels up to 10 decibels. (Bear in mind, though, that absorptive treatment is only effective for reducing reverberation, not transmission of sound between rooms.)
Isolation. Mechanical equipment generates vibrations which can travel through a building’s structural members to affect remote locations within a building. It is therefore prudent to isolate any heavy equipment from the structural members of buildings. This can be accomplished by mounting the equipment on springs, pads, or inertia blocks.
Selecting specific isolating equipment should be left to a specialist trained in vibration analysis, who tunes each vibration isolation device to a specific frequency range. If not matched properly with the treated equipment, the devices can amplify the vibrations and cause more of a problem than would have occurred without any treatment.
ACTIVE NOISE CONTROL
Sound Cancellation. Unlike the noise control methods discussed so far, active noise control involves electronically altering the character of the sound wave in order to reduce its level. In sound cancellation, a microphone measures the noise and a processor generates a mirror image (180 [degree] out of phase) of its source. This mirror image is then broadcast in the path of the original sound. Depending on the circumstance, the new sound can cancel enough of the original signal to reduce noise levels up to 40 decibels.
Although sound cancellation is a very powerful noise control tool, it is only practical in confined environments where tonal frequencies are below 500 Hertz (cycles per second). Ventilation ducts are ideal candidates for active noise control because they are enclosed, with the dominant noise often consisting of low-frequency pure tones (associated with the fan characteristics).
Masking. In buildings where background sound levels are low, it is possible to alleviate a noise problem by adding a more pleasant sound to the environment. A good example is open-plan offices where people can hear activities in other offices and areas. The new sound has the effect of covering up the noise or making it less noticeable. (An important caution is that the system be set to operate at not too loud a level or with a harsh frequency response.)
Many people think of masking as ‘white noise,’ which is characterized by an equal amount of energy in all audible frequencies. More often, however, masking systems stress lower frequency sounds, such as the noise produced by a normal HVAC unit. For example, loudspeakers emitting an HVAC-like sound might be placed between dropped ceilings and structural ceilings.
In fact, any desirable sound can provide masking. Of nature’s many soothing sounds, running water is the most popular. Fountains, in fact, have been found to provide both acoustical and aesthetic enhancements to residential and commercial environments.
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This article was written by James P. Cowan in Archi-Tech, Vol.3, Num. 2. Published in the Spring of 2000. The author is a board-certified Noise Control Engineer in Massachusetts, working for Acentech Inc.
Fred J. Becker, Architect
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