Page 140 - Master Handbook of Acoustics

FIGURE 6-6 A parabolic surface can focus sound precisely at a focal point or, the converse, a sound
source placed at the focal point can produce plane, parallel wavefronts. In this case, the source is an
ultrasonic Galton whistle driven by compressed air.

Plane waves striking such a reflector are brought to a focus at the focal point. Conversely, sound
emitted at the focal point of the parabolic reflector generates plane wavefronts. For example, the
parabola of Fig. 6-6 is used as a directional sound source with a small, ultrasonic Galton whistle
pointed inward at the focal point. Standing waves are produced by reflections from a heavy glass
plate. In a Galton whistle, the force exerted by the vibration of the air particles on either side of a

node is sufficient to hold slivers of cork in levitation.