Principle of Focusing
Sound waves behave a lot like light waves, so some of the same principles that are used to focus or amplify light can be used to amplify sounds. A simple lens can be used to focus sunlight to a tiny spot that is so hot it can light a fire. Lenses in binoculars and telescopes catch light and focus it into a magnified image. Really large telescopes replace the optics of glass lenses with mirrors. A concave mirror catches a faint distant light signal over its entire surface and focuses it into a smaller, brighter image that is now visible. This is the same principle that makes a parabolic sound dish work.
Parabolic Sound Dish
A parabolic sound dish is like a telescope mirror--for sound. It collects a faint distant noise and focuses it onto a microphone. The focused sound is much more intense than the surrounding signal and can be broadcast or recorded. The parabolic shape ensures that all distant sound waves that strike the surface parallel to the central axis (the direction the dish is pointed) will be focused exactly on the microphone.
Considerations
Sound waves have much longer wavelengths than light waves, so the size of the parabolic sound dish will affect the quality of the sound, especially with longer wavelengths (deep bass and lower sounds). Larger dishes (~55cm or larger) somewhat compensate for this effect.
The size of the sound dish also determines how much it magnifies the sound. Larger dishes will collect more sound. A two-foot diameter dish will capture four times as much sound as a one-foot diameter dish--double the size and square the amplification.
For higher specificity and less noise, the focal length (the distance from the center of the dish to the point where the sound is focused) is important. A longer focal length gives a more specific and background-free signal.
Parabolic sound dishes have many uses. They are used to record bird songs and other wildlife sounds. Television crews use them to hear what players on the athletic field are saying. They are advertised heavily for use as covert listening devices.
Sound dishes also pick up and amplify unintended nearby sounds, so use caution when aiming the sound dish to avoid damaging the receiving electronics--or your ears.
Tags: sound dish, dish will, faint distant, focal length, Larger dishes, light waves