Acoustic Mirrors

Acoustic concrete mirrors were indeed an early form of radar and some of them can still be seen today. They were strung along the south east coast of England to detect the approach of hostile aircraft during the 1920s and 1930s.

The pre-radar acoustic mirrors 'collected' sound in the same way as the satellite dishes we see today collect radio waves. Sound reflects off surfaces in the same way light bounces off a mirror or shiny surface. That is to say, the angle the ray makes to the so-called normal is the same for both the incident and reflected rays. Because the reflecting surface is curved each ray is incident at a slightly different angle and therefore reflects at a slightly different angle. As a result, the rays will be 'focused' at a point where they all intersect. This focal point will occur at different places, depending on the direction the incident rays come from. In a satellite dish we might use today, the wave 'collector' is fixed to the dish, and we alter the direction the dish point in until the focal point of the rays lands on the collector. However, with a 20ft concrete mirror it is not so easy to change the direction it is pointing in. Instead the sound collector was moved until it matched the focal point. The reflection of the waves allows sounds to be heard at greater distances - about 8km further than with the unaided human ear. Also, the direction that the sound was coming from could be calculated.

Acoustic mirrors are still used today for sound source location in wind tunnels and for listening to conversations. However, today they are formed using an array of microphones and the directional beamforming is achieved using signal processing.

Reference

K. Nagakura, "Method of Analyzing the Wind Tunnel Test Data Measured with Directional Microphone Systems", Quarterly Report of RTRI, Vol. 42, No. 2 pp.104-109 (2001).

See also: Acoustics.

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Subjects: Noise & Vibration


Weblinks:
Sound Mirrors - Andrew Grantham Acoustic early warning systems, spectacular remnants of a dead-end technology on the British coast.