Page 48 - Occupational Health & Safety, November/December 2019

HEARING PROTECTION
conveying or sorting methods. Similarly, some machine surfaces such as lightweight sheet metal safety guards can act like a loudspeaker and efficiently turn the silent vibrations of a machine into high level sound, via panel radiation. If those light- weight solid panels can be replaced with perforated panels, they become inefficient at moving air, and thus do not convert vi- bratory energy into acoustic energy.
Often, there are no quieter alterna- tive methods available to accomplish the production task, particularly in the cases of machining methods, or the intricate processing steps in the production of pharmaceuticals, petrochemicals, micro- components, etc.
In the few cases where it is feasible to reduce the at-source noise generation, the solution needs to be a collaborative effort between the process engineering team and the noise consultant. And, in many cases, reducing the noise generated at the source is just not practicable.
Reverberation Control
with Acoustical Absorption
Reducing reverberation is another method that is, unfortunately, only effective in a relatively small number of cases. Never- theless, it is important to understand the benefits and limits of using acoustically ab- sorptive materials to reduce reverberation.
When sound is released into any en- closed space, such as a room or the inte- rior of a machine enclosure, those sound waves will continue to travel around inside the space until they are absorbed through successive reflections off of the interior sur- faces. If the interior surfaces are primarily reflective, the acoustic energy will accumu- late and the resulting sound levels will be greater—sometimes much greater—than if the same amount of sound were released into an unenclosed space or one with ad- equate acoustical absorption.
When this build-up of sound occurs in a room, it is termed “reverberation.” If the surface finishes in a room do not provide ad- equate acoustical absorption, the reverbera- tion can be excessive, which can adversely elevate the sound levels within. Similarly, if there is little or no acoustical absorption inside a machine enclosure, the interior sound levels will escalate until the point at which the amount of sound transmitted through the enclosure walls approaches the
amount of sound emitted from the sources within, such that the enclosure will provide minimal sound containment.
Therefore, acoustical absorption is an important element in noise control. If an enclosure around a machine is to serve as an acoustical enclosure, it must include some interior acoustical absorption to dis- sipate the contained sound; otherwise, it will be ineffective.
While acoustical absorption is impor- tant in controlling reverberation, there is often already an adequate degree of natu- rally occurring acoustical absorption in many rooms such as room furnishings, ceiling tiles, and occupants themselves— which may already afford appreciable ab- sorption. In those cases, there is minimal incremental benefit from adding additional absorption, such as suspended acoustical baffles, fiberglass or foam wall panels, or spray-on fibrous ceiling insulation.
Moreover, it is important to understand that the total sound level impinging on a worker’s ear is composed of two parts—the direct sound and the reverberant sound. The direct portion of the sound is affected only by the loudness of the source(s), the distance between the source and the work- er, and anything obstructing the travel of sound from the source to the worker. The reverberant sound is affected by the geom- etry of the room and the amount of acous- tical absorption.
At workstations close to a sound source, the direct component of the sound is usu- ally much greater than the reverberant component, such that a reduction of rever- berant sound will have little to no effect on the total sound level. Thus, in a majority of cases, controlling reverberation will only reduce the sound levels in areas far away from the production equipment—typically locations where workers are rarely present. Experience shows that the use of absorptive treatments to reduce room reverberation is rarely an acoustically effective or cost-effec- tive solution in the workplace.
Blocking or Containing
and Dissipating the Sound
By a considerable margin, the noise con- trol measures with the widest applicability in the industrial workplace environment is engineered noise control hardware. There are many types, but in essence they are all examples of blocking the sound, or con-
taining and absorbing it. The following sec- tions cover the most common, tried-and- true noise control methods.
Noise Barriers and Acoustical Enclo- sures. Noise barriers, usually in the form of a noise wall or other obstruction, block the sound, creating an “acoustical shad- ow” for a protected area. In contrast to a full acoustical enclosure, a noise barrier is typically open on top, or on one or more sides. Although noise barriers are effective outdoors, they usually have minimal ben- efit indoors because of reverberation or discrete acoustical reflections (e.g., from the ceiling).
Some amount of sound always diffracts over the top or around the open sides of the barrier. Therefore, full acoustical en- closures are generally the most common and effective noise control measure in the manufacturing environment.
An acoustical enclosure functions by effectively containing the sound and then dissipating it by absorption. In order to contain the sound, the enclosure walls must be air-impervious and have sufficient mass (depending on the magnitude and frequency of the sound being contained).
Any openings in the enclosure—e.g., to allow ingress and egress of product or ventilation/cooling air—must be fitted with silencers or acoustically lined passages. Otherwise, any joints, gaps or cracks must be sealed air-tight. Even a small unsilenced gap or opening can dramatically degrade the containment of the sound.
In some cases, items of equipment tar- geted for noise control already have inte- gral enclosures for safety or product qual- ity reasons. However, because of openings or gaps, and absence of interior acoustical absorption, they do not naturally afford appreciable acoustical benefit. Sometimes these enclosures can be upgraded to func- tion as effective noise control measures by sealing gaps, providing interior absorption, and devising silenced openings for ingress/ egress of product materials and cooling air.
Acoustical Lagging. For pipes and ves- sels, acoustical “lagging” (wrapping) is a common noise control method, as it can be less cumbersome than a full acoustical enclosure. In essence, acoustical lagging is simply an acoustical enclosure that is sup- ported on the surface of the noise source itself. It consists of one or more layers of fibrous or porous material, potentially one
44 Occupational Health & Safety | NOVEMBER/DECEMBER 2019
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