Acoustic mirror

Concave mirror microphones are used for direction determination ( localization ) of noise. They are constructed, in principle, such as parabolic antennas, wherein the detector is replaced by a microphone. They are used, often in aeroacoustic wind tunnels with open path. Usually this case, the sound emission of road vehicles or aircraft, is under investigation. In addition, there are often small, hand-held systems are used for example for interception.

Operation

For parabolic microphone is positioned so that it lies at the intersection of the reflected parallel rays incident sound. If you want to increase the location accuracy, the use of an ellipsoidal mirror is advantageous. When the microphone is placed in an ellipsoidal focal point directly in front of the mirror, in the other the measurement point on the object to be examined. If the measured distance is not too small, and the parabolic mirror can be focused on a measuring point by the microphone is positioned accordingly. The parabolic mirror shown in the image can be focused, for example, by increasing the distance from the microphone of the mirror surface by 50 millimeters to a sound source at a distance of 7 meters. Further enlargements of the microphone distance from the mirror surface allow in this case measuring distances down to about four feet.

The signal gain of a concave mirror opposite a microphone omni-directional microphone is frequency dependent. The smallest gains take place in the lower frequency range. Larger gains can be there achieved with larger mirror diameters. The spatial resolution is better, the larger the mirror is used.

Some systems are equipped with a plurality, in a plane closely spaced microphones arranged such that each of these microphones is focused on a different measurement point on the measurement object. This allows the simultaneous measurement of a total surface area, in that the concave mirror needs to be implemented without microphone.

History

The first concave mirror microphones were military developments. From about 1916 to 1930 developed, for example, on the English coast several concrete constructions that served to can already hear enemy air ships and aircraft before they were in sight. The best known among them on the former base of the Royal Air Force Denge in Dungeness and Hythe in Kent can still be seen today. Other systems are for example in Sunderland, Redcar, Kilnsea in Holderness, to find at Selsey Bill at Selsey and Malta.

Also, structures that do not focused sound beams in a point, but similar to a stethoscope act by multiple reflections have been used for military purposes. Such systems for air defense were, for example, in Japan (Japanese Kriegstuba ) and the Wehrmacht ( as " ring direction listener RRH " ) is used. From the United States are similar arrangements - but in a smaller version - known that should serve for locating fog horns of ships ( " Topophone " by Prof. Alfred M. Mayer 1880 ). Most of these systems have been intercepted directly and contained no microphones.

A hand-held microphone concave mirror.

Concave mirror microphones in Denge

4.5 meter high concrete hollow mirror microphone at Kilnsea Grange, East Yorkshire, UK. Before the opening of the tube for attachment of the microphone is visible.