Laser microphone

A laser microphone is a pickup that converts laser beams by means of solid-state vibrations of targeted objects into electrical voltage pulses. Laser microphones can be used to measure body sound of objects precisely and without contact. It is also possible, by means of sound over oscillated membranes, such as windows, to track calls of a room.

Operation

The solid-state oscillations of the observed object are determined by the change in distance between the point of reflection of the laser on the targeted object and the laser microphone. The distance measurement by laser can be done via a runtime measurement of laser pulses on the phase position or triangulation.

Transit time measurement

In the transit time measurement, a light pulse is emitted and the time between transmission and reception of the reflected pulse is measured. From the speed of light c, the refractive index and the signal delay of the distance between source and object can be determined using the following formula.

In order to measure the vibrations caused by impact sound very small distance changes, a highly precise time measurement is required. However, the requirement for the measurement time can be reduced by methods are used in which the laser beam itself is frequency-modulated or is modulated with a high frequency signal.

Measurement of the phase angle

The phase shift of the reflected laser beam or the modulation with respect to the emitted beam is dependent on distance and can be used to determine the distance traveled. If the laser frequency itself is used to overlay, the unit operates as a laser interferometer. Laser interferometers do not measure absolute path lengths but only the relative change in displacement of the target and a reference mirror. When moving the mirror, the sum of ausgesandtem and reflected beam is periodically modulated ( interference). The advantage of these methods is compared with runtime process higher resolution, which can be realized with less metrological effort.

Laser triangulation

In laser triangulation, a laser beam ( with low requirements, the radiation of a light-emitting diode ) onto the object to be measured and observed with a camera, a position-sensitive photodiode or a CCD line sensor located in the next. Changing the distance of the object from the sensor, also the angle at which the light spot is observed, and thus the position of its image on the photoreceptor. Changes From the position change, the distance of the object is calculated from the laser projector using the trigonometric functions.

One advantage of triangulation is the fact that it is purely trigonometric relationships. The measurement can therefore be continuous and is thus ideally suited for distance measurement on moving objects. However, the observation angle must be chosen sufficiently large to be able to measure the desired solid-state oscillations. This restricts the range of this measurement method in practice a strong.

The above scheme illustrates the relations between the different distances. With the aid of trigonometry, it is possible to determine the distance from the measured distance:

Application

Laser microphones are usually used in the investigation or monitoring of machines in which various parameters are to be recognized over the body sound. For example, it can be concluded on the degree of wear of rolling bearings on the structure-borne sound.

Due to the high precision of the distance measurement, the recording is possible in contrast to other transducers by a laser microphone from a distance. For this reason, laser microphones are sometimes also used by intelligence services and infrequently by detectives. For monitoring of conversations in a room, the laser beam is directed onto a membrane, which is deflected by the recorded sound. Usually, windows of homes or cars will be used. The laser beam is reflected from the window glass low ( about 4 % at normal incidence ). Non-perpendicular angle of incidence, the laser beam is reflected only by microscopic unevenness and dust particles on the disk in the direction of the transmitter, whereby the intensity of the reflection is much lower than at normal incidence. Even more -glazed windows can reduce by additional reflections and interference reception quality. With blinds or shutters in front of the disc, this form of espionage can be averted.

Although the laser microphone is not a microphone, but a pickup, it still has the problem that vibrations (acoustic emission ) at the laser microphone can be measured just as the body sound of the object to be measured. Therefore, the laser microphone either mechanically decoupled or disturbing frequency ranges over frequency filter needs to be hidden.