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By Robert Fry

At the time he wrote this in fall 2001, Robert Fry was an exhibit developer at the Exploratorium in San Francisco, working in the Sound and Hearing group. Shortly after, he moved to Explora in Albuquerque, New Mexico. In addition to being an exhibit developer, Robert is a sculptor who works with sound.


Science centers are concerned with experiential learning, but the places we inhabit often provide an atmosphere of acoustic chaos. If visitors have to wade through a distracting cacophony, there's no guarantee that they'll be able to concentrate enough to absorb the content of exhibits. On the other hand, unexpected and unusual aural phenomena delight and surprise. People often mention boinks, pings, clangs, whooshes, wow-wows, and clack-clacks when they describe their visits to the Exploratorium. We don't want a library-like stillness in a science center—what we want is an acoustic environment that enhances the visitor experience without undermining it.

At the Exploratorium, we achieved a reduction in the ambient sound level by implementing a number of simple solutions that specifically address our noisiest exhibits. Among them are the following:

Motors


P
roblem:
The electric motor produces broadband sound, compiled from many frequencies. Exhibit furniture and hard surfaces can amplify and reflect annoying frequencies, and turn a motor into a rattling nuisance

Solutions:
Isolate the motor with a rubber mat, acoustic foam, or rubber isolators, which are widely available in different densities that absorb different vibrations. Balance the motor to mitigate high-energy vibrations.

Baffling

Problem:
Some exhibits generate noise because of the way they function, or because of how visitors interact with them.

Solutions:
Acoustic baffling can result in a marked reduction in the perceived loudness of a noisy event. One solution is to make graphic panels, walls, or furniture surfaces of acoustically transparent perforated steel or aluminum layered over acoustically absorptive foam or fiberglass. Baffling does not imply absolute containment. Booths, kiosks, and chambers deaden the ambient space between exhibit events.


Speakers

Problem:
Exhibits with sound are often built with speaker boxes aimed straight out toward the general space, so sound spills around the visitor. Also, speakers purchased off the shelf rarely have been designed to reduce rearward spill

Solutions:
Contain the acoustic spill in any direction that is not toward the visitor. If speakers are housed inside exhibit furniture, place absorptive material throughout the interior of the exhibit furniture. Be sure to test, because too much absorption can reduce sound quality.

Supplemental holes may be drilled into exhibit furniture or speaker boxes, creating what amount to "f holes" in a violin and directing sound energy where it's desired.

Sound bells and parabolic reflectors contain and focus speaker-produced sounds. However, it is difficult to focus low-frequency sound vibrations, and the sound produced by the most effective versions lacks the texture of warm, low tones. Supplemental base speakers filtered to drive only the lowest tones can help overcome this drawback. Surface-mounted base drivers also can transmit tactile frequencies without much audible noise; these can be mounted in a floor pad beneath the visitor while suspending a parabolic speaker overhead.

If a parabolic speaker is mounted over a tile or concrete floor, its purpose will be defeated. A rubber mat, carpet, or other soft matting can help. Porous rubbers work better than smooth, soft better than firm; unfortunately, the most absorptive are also typically the least durable.


Architecture

Problem:
Acoustically reflective surfaces—including concrete floors, metal ceilings, and large expanses of glass and wood—lengthen the amount of time it takes for a given noise event to decay.

Solutions:
Get at least one surface in the space to absorb sound waves. Treating the ceiling leaves you with most latitude for customizing the space. Treat walls only if the situation positively requires it, because acoustic panels are fragile, difficult to clean, and aesthetically bereft. Carpet is the most common acoustic absorber, but isn't always effective. Short-nap carpet absorbs noise at very short wavelengths (6000 Hz and above), but if it is not backed by another effective absorber it will likely reflect wavelengths longer than the depth of the fibers, especially if glued directly to substrate concrete.

Background

Acoustic transparency
The "transparency index" (or TI) is a measurement of the amount of sound energy that passes though a material at a given frequency or range of frequencies. Many perforated metals and plastics have a high TI. Usually, though not always, those sheets that have a relatively high percentage of open area are most transparent. The exception is when the holes are large and the surface between the holes becomes large with respect to wavelengths as well.

Acoustic absorption
Acoustically absorptive material is usually measured in NRC, the "noise reduction coefficient," a value between 0 and 1. A good absorber is likely to measure at .80 meaning 80 percent of the sound energy is attenuated. Perception Low frequency spill is generally perceived as much quieter than high. Most people find that frequencies in the 2500 to 4000Hz range are those to which we are most physically sensitive; low frequencies require much more power to be perceived as equally loud as high.

Noisy classrooms
In 1975 Arline Bronzaft and Dennis McCarthy published a groundbreaking study of schoolchildren in New York. This and other articles can be found at:

Noise Pollution Clearinghouse Noise Pollution Clearinghouse
Educator’s Cheapbook World Forum for Acoustic Ecology
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