Understanding Directional Audio Technology in Museum Exhibits

Anyone concerned with exhibit audio for museums or similar exhibit areas appreciates the value of concentrating sound on a limited target audience without disturbing others nearby. Sound focusing devices allow more exhibits per unit area and create a personalized sound field in front of each display.

It’s safe to say that none of the focusing devices available today can create sound at only one spot and absolutely nothing elsewhere. All have a certain amount of spillover beyond the target area. This at first seems disappointing, but spillover can be beneficial because it gives a hint that the display actually carries a soundtrack. Ideally, the sound should be a low indistinct murmur that suddenly “lights up” when one enters the target zone. Some museums have decals placed on the floor with a logo or symbol that hints: “stand here”. When sound is focused, guiding the audience to the right spot is not a trivial issue

Hemispherical reflectors produce a defined focal point that is more intense than that of the parabolic reflector. Source to listener distance becomes critical, but distance can be altered by mechanically adjusting the position of the speaker within the dome.
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Heterodyne emitters can produce very narrow coverage angles of only a few degrees. They do this by emitting an intense modulated ultrasonic beam that can be demodulated in mid-air when the beam strikes an object.

Needless to say, the aiming of a heterodyne emitter becomes an important issue. Once the beam strikes a solid object, the resulting sound spreads out just like any other sound source. Unintended reflections of the sound beam may also be a problem. Serious questions have been raised about the safety of the intense ultrasonic sound level.

An array, as its name implies, is an arrangement of many identical transducers (usually high quality 2″ speakers) all playing the same signal.A steered array, by strictly controlling the launch time from each element, causes all elements to act together in concert at the target location whereas at any other location sounds arrive totally out of step with each other.

The precise term for this effect is correlation. In the target area, the correlation is high, outside that location the correlation is low. Typical steered arrays can give a reduction in volume of 75% as one steps from the target zone to the untargeted area just a few feet away. By varying the design of the array and the timing of the speakers, different patterns and focal lengths may be obtained. Note that a steered array does not bend the sound waves, nor does it restrict the coverage angle of the individual elements. Instead, it performs a time alignment of many sources for a chosen point in space.

Ordinary loudspeakers project a 90 degree or greater cone of sound. This is obviously much too wide if there are other exhibits nearby. If this is the only option, one can at least take advantage of the proximity law that states that sound volume increases by 6 dB every time speaker to listener distance is halved. By locating the speaker as close as possible to the listener’s head, sound volume can be decreased commensurately, thereby decreasing the sound level in the spillover area.

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