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Of all the markets for perforated metals, acoustical applications have seen the most dramatic growth in the last few decades. There is every reason to expect a further surge of growth in this area in the near future, to match that of the general economy. Much of the new growth in the acoustical market will come from original equipment manufacturers (OEM) and from architectural firms. OEM's will find that they must reduce the noise of their products to meet consumer demands, and the architects (including highway, airport, and rapid transit designers) are already designing noise control into their projects; these applications will proliferate as commercial building and government construction pick up.

In order to take full advantage of this potential market development, it is important to ensure that the designers of noise control applications give full consideration to the use of perforated materials, and to present a convincing argument that perforated materials are often the best alternative in noise control programs.

The best way of doing this is to present up-to-date, factual information on the acoustical applications of perforated materials and to illustrate these uses with enough practical examples to help specifiers gain a sense of confidence in recommending the right material for the right application, without feeling intimidated by the technical aspects of the design.

It is the purpose of this booklet to provide the necessary technical information in an easy-to-use style, and to provide helpful hints in the choice of perforated metals, so that professionals can recommend these materials to their clients with pride and confidence.

How Perforated Metals Are Used in Acoustical Applications

Figure 1. As an example of the first application, the curved surfaces above the stage of the Orpheum Theatre, Vancouver, B.C., are made of finely perforated metal sheet, not of plaster as they appear. The perforations allow the sound to pass through and to reflect back into the hall at desired locations, from specially designed surfaces behind the perforated metal.

There are three principal acoustical applications for perforated metals:

1. As a Facing for Something Else

Here the perforated metal is used as a protective or decorative covering for some special acoustical material; that material may be designed either to absorb sound or to reflect or scatter sound in a special way. It is this special material that does the actual acoustical work, so the purpose of the perforated metal in such applications is to "disappear" acoustically: that is, it must be so trans parent that the sound waves can pass right through it to encounter the acoustical treatment that lies behind. Our design goal in this case is to choose the perforated metal for greatest sound transparency, for sounds of all frequencies.

2. In Tuned Resonant Sound Absorbers
Sometimes, however, we may wish to absorb sound very selectively, only in a certain band of frequencies but not at frequencies lying above and below that band. For this purpose we design a so-called Resonant Sound Absorber. Here, the perforated metal, instead of disappearing, takes an active part in tuning the absorber, that is, in determining which frequencies of sound are absorbed.

3. As Airflow Diffusers
In the acoustical treatment of certain specialized aerodynamic test facilities, such as wind tunnels, perforated metals are often used to break up the turbulence in airflows. This last application is both highly specialized and highly technical. Moreover, it does not represent a significant portion of the market; therefore, the rest of this book will be concerned entirely with the first two applications.

The main text of this book is intended for readers with no special technical background. It is divided into two parts. The first part deals with the principles of noise control treatments using perforated metals; the second part deals with typical applications. Readers who want more technical detail will find it in the Appendices.

Appendix D also includes worksheets that may be photocopied, filled out and included in the job files for individual projects.

Noise Control With Sound Absorptive Treatments Using Perforated Metals

Noise control measures are often applied in order to quiet noisy equipment. We treat various household and office appliances to make them acceptable to the user because excessive noise is annoying; we treat heavy industrial equipment so that it will comply with current OSHA regulations that limit the noise exposure of workers so as to protect them from hearing damage.

In 1970, the Occupational Safety and Health Act (OSHA) set limits on the levels of noise to which workers may be exposed in their work environments. This regulation requires that industries monitor the noise in all worker locations, and, where this noise exceeds the permissible limits, it must be abated by any feasible noise control measures, or by administrative methods such as limiting the employees' exposure time. If such noise control procedures turn out not to be feasible, then hearing protection must be provided for the workers.

One effective and commonly used approach is to CONTAIN the noise by providing an enclosure around the noisy equipment. This approach can work very well, so long as we attend to one very important matter: we must provide sound-absorptive treatment inside the enclosure, to soak up as much sound as possible. This step is necessary because the first thing that happens when we enclose a noise source is that the noise, which can no longer escape, builds up inside the enclosure to levels that are higher than they were without the enclosure. Providing the sound absorptive treatment inside the enclosure prevents this undesirable noise build-up and allows the enclosure to get on with its job of attenuating the noise to acceptable levels outside the enclosure.

In some cases, the "user" is actually inside the enclosure with the noise source, as in a road traffic tunnel. The application of sound-absorptive materials on the walls and ceiling of the tunnel prevents a serious build-up of tire and motor noise, which otherwise could distress and confuse the drivers.

In all such sound absorptive treatments we must take care to match the acoustical performance of the treatment to the frequency range in which the equipment generates the greatest amount of noise. And this is where treatments using perforated metal come in!