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 centerwhat 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:
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
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
Some exhibits generate noise because of the way they function,
or because of how visitors interact with them.
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.
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
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
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.
Acoustically reflective surfacesincluding concrete
floors, metal ceilings, and large expanses of glass and
woodlengthen the amount of time it takes for a given
noise event to decay.
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
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.
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.
In 1975 Arline Bronzaft and Dennis McCarthy published a
groundbreaking study of schoolchildren in New York. This
and other articles can be found at:
Forum for Acoustic Ecology