Concave mirror microphone
Concave mirror microphones are used to find the direction ( localization ) of sounds. In principle, they are constructed like parabolic antennas , with the detector being replaced by a microphone . They are often used in aeroacoustic wind tunnels with an open measuring section. In most cases, the sound radiation from road vehicles or aircraft is examined. In addition, there are mostly smaller, hand-held systems that are used, for example, for eavesdropping measures.
functionality
For parabolic mirrors , the microphone is positioned in such a way that it lies at the intersection of the reflected parallel incident sound beams. If you want to increase the positioning accuracy, the use of an ellipsoidal mirror is advantageous. In the case of an ellipsoid mirror, the microphone is in the focal point directly in front of the mirror, in the other the measuring point on the object to be examined. If the measuring distance is not too small, parabolic mirrors can also be focused on a measuring point by positioning the microphone accordingly. The parabolic mirror shown in the picture with a diameter of 1.2 m can be focused on a sound source at a distance of 7 meters by increasing the microphone distance from the mirror surface by 50 millimeters. In this case, further increases in the microphone distance from the mirror surface allow measuring distances down to about four meters.
The signal amplification of a concave mirror microphone compared to a microphone with an omnidirectional characteristic is frequency dependent. The lowest gains occur in the lower frequency range . Larger reinforcements can only be achieved there with larger mirror diameters. The spatial resolution is also better, the larger the mirror used.
Some systems are equipped with several microphones arranged close together on one level, so that each of these microphones is focused on a different measuring point on the measurement object. This enables the simultaneous measurement of an entire area without having to move the concave mirror microphone.
history
The first concave mirror microphones were military developments. From around 1916 to 1930, for example, several concrete structures were built on the English coast, which served to be able to hear enemy airships and aircraft before they were in sight. The most famous of them at the former Royal Air Force base Denge near Dungeness and in Hythe in Kent can still be viewed today. Other systems can be found, for example, in Sunderland , Redcar , Kilnsea in Holderness , on Selsey Bill near Selsey and on Malta .
Constructions that did not focus the sound beams in one point, but acted like an ear tube through multiple reflections, were also used in the military. Such systems for air defense were, for example, also in use in Japan ( Japanese war tuba ) and by the German Wehrmacht (as "ring direction receiver RRH"). Similar arrangements - but in a smaller version - are known from the USA , which were intended to be used to locate foghorns on ships ("Topophone" by Prof. Alfred M. Mayer from 1880). Most of these systems, however, were monitored directly and did not contain microphones.
Hand-held concave mirror microphone.
Concave mirror microphones in Denge
4.5 meter high concrete concave mirror microphone at Kilnsea Grange, East Yorkshire, UK. The tube for fastening the microphone can be seen in front of the opening.
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literature
- F.-R. Penny; H. Stiewitt; B. Binder: Sound Source Location and Discrimination from Background Noise in Wind-Tunnel Tests . International Congress on Instrumentation in Aerospace Simulation Facilities (ICIASF '75), 22. – 24. September 1975, Ottawa, pp. 301-310
- F.-R. Penny; JH Jones; GA Wilhold: Measurement of the Distribution of Sound Source Intensities in Turbulent Jets . Prog. Astron. Aeron., Vol. 37 (1975), pp. 79-92
- F.-R. Grosche: Distribution of Sound Source Intensities in Subsonic and Supersonic Jets , AGARD CP 131, 1974
- F.-R. Grosche: Application Possibilities of Acoustic Mirrors for Noise Source Localization . Proceedings of the DGLR workshop “Aeroacoustics of Cars”, 16. – 17. November 1992, German-Dutch wind tunnel, Emmeloord
- Martin Helfer: Assessment of concave mirror microphones for sound source location . Conference proceedings "Acoustics and Aerodynamics of the Motor Vehicle", 1. – 2. February 1994, Haus der Technik eV, Essen
- Andreas Zeibig; Matthias Lippmann; Dietmar Richter: Acoustic concave mirror measuring system with a multi-microphone arrangement . Proceedings of the DAGA '05, Munich, pp. 141–142, 2005
- Martin Helfer: Concave mirror microphones . Workshop "Measurement and Analysis Technology in Vehicle Acoustics ", FKFS , Stuttgart, 9. – 10. October 2007
