An acoustic telescope or long range microphone focusses sound. The film clip at right shows an example: a convex dish made to focus microwaves is here used to focus sound on a microphone. It is the sound analogue of the Newtonian or reflecting telescope for light. This page supports the multimedia tutorial Geometrical Optics
Schematic of a Newtonian or reflecting telescope (for light)
This schematic from Newtonian telescopes shows how a parabolic mirror focusses rays of light, with a plane mirror and lens usually added for the convenience of the viewer, whose head is thus kept out of the optical path. The parabolic mirror, whose diameter might range from a few centimetres to several metres, is very much larger than the wavelength of light. Nevertheless, as we shall see in a later chapter, the resolving power of the telescope is usually limited by the diameter of the mirror because of diffraction.
If the parallel rays all come from the same very, very distant point, they converge at the focus at the same time.
A film clip demonstrating our acoustic telescope
Similarly, the resolving power and focussing of an acoustic telescope depends on the ratio of the reflector diameter to the wavelength. Because of diffraction and interference, the concept of rays and focussing only works if dimensions are much greater than a wavelength. At 340 Hz, the wavelength is one metre, so this telescope only works for frequencies rather higher than that. The information that identifies vowels is mainly in the range 300-2000 Hz (see the Human Sound), so this telescope does not work well for vowels. For consonants, however, much of the information is carried at higher frequencies. Language is highly redundant: we can lose much of the encoding information without losing the meaning. Hence it is easily possible to understand the voice in the film clip above. The voice sounds unnatural, of course, because it is missing the low frequencies: as though one had 'turned the treble knob up and the bass knob down' (see filters).