Laser microphone

This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

A laser microphone is a pickup that uses laser beams to convert solid-state vibrations from targeted objects into electrical voltage pulses. Laser microphones can be used to measure structure-borne noise from objects with pinpoint accuracy and without contact. There is also the possibility of following conversations in a room using membranes that are vibrated by sound, for example window panes.

functionality

The solid-state vibrations of the targeted object are determined via the change in distance between the reflection point of the laser on the targeted object and the laser microphone. The distance measurement by laser can take place via a run-time measurement of laser pulses, via the phase position or via triangulation.

Runtime measurement

When measuring the transit time, a light pulse is emitted and the time between sending and receiving the reflected pulse is measured. The distance between the source and the object can be determined from the speed of light c, the refractive index and the signal transit time using the following formula . ${\displaystyle n}$ ${\displaystyle \Delta t}$ ${\displaystyle l}$

${\displaystyle l={\frac {c\cdot \Delta t}{2\cdot n}}}$

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

Measurement via the phase position

The phase shift of the reflected laser beam or its modulation in relation to the emitted beam is distance-dependent and can thus be used to determine the distance covered. If the laser frequency itself is used for superimposition, the device works like a laser interferometer . Laser interferometers do not measure absolute path lengths, but only the relative change when the target or a reference mirror is moved. When moving the mirror, the sum of the emitted and reflected beam is periodically modulated ( interference ). The advantage of these methods is the higher resolution compared to runtime methods, which can be achieved with less technical effort.

Laser triangulation

Principle of laser triangulation

With laser triangulation, a laser beam (with low requirements also the radiation of a light-emitting diode ) is focused on the measurement object and observed with a camera located next to it in the sensor , a spatially resolving photodiode or a CCD line. If the distance between the measuring object and the sensor changes, the angle at which the point of light is observed also changes and thus the position of its image on the photo receiver. The distance between the object and the laser projector is calculated from the change in position with the help of the angle functions .

An advantage of triangulation is the fact that it is purely trigonometric relationships. The measurement can therefore take place continuously and is therefore well suited for measuring distances on moving objects. However, the observation angle must be chosen to be sufficiently large to be able to measure the desired solid-state vibrations. This severely limits the range of this measurement method in practice.

The above scheme illustrates the relationships between the various distances. With the help of trigonometry it is possible to determine the distance from the measured distance : ${\displaystyle x-x_{0}}$ ${\displaystyle x'-x_{0}'}$

${\displaystyle \tan \delta ={\frac {x'-x_{0}'}{f}}\to \tan \alpha ={\frac {x_{0}}{D}}}$

${\displaystyle x=D\cdot \tan(\alpha +\delta )=D\cdot {\frac {\tan \alpha +\tan \delta }{1-\tan \alpha \cdot \tan \delta }}}$

${\displaystyle x=D\cdot {\frac {{\frac {x_{0}}{D}}+{\frac {x'-x_{0}'}{f}}}{1-{\frac {x_{0}}{D}}\cdot {\frac {x'-x_{0}'}{f}}}}}$

application

Laser microphones are mostly used to examine or monitor machines in which various parameters are to be recorded via structure-borne sound. For example, the degree of wear of rolling bearings can be inferred from structure-borne noise .

Due to the high precision of the distance measurement, in contrast to other sound transducers, recording with a laser microphone is also possible from a distance. For this reason, laser microphones are sometimes used by intelligence services and rarely by detectives . To follow conversations in a room, the laser beam is directed onto a membrane , which is deflected by the sound to be recorded. Usually window panes are used in homes or cars. The laser beam is weakly reflected by the window glass (approx. 4% at normal incidence). If the angle of incidence is not perpendicular, the laser beam is only reflected in the direction of the transmitter by microscopic unevenness and dust particles on the pane, which means that the intensity of the reflection is significantly lower than with perpendicular incidence. Window panes with multiple glazing can also reduce the reception quality through additional reflections and interference. This form of espionage can be warded off by blinds or shutters in front of the pane.

Although the laser microphone is not a microphone but a pickup, it still has the problem that vibrations (structure-borne noise) are measured on the laser microphone in exactly the same way as the structure-borne noise of the object to be measured. Therefore, the laser microphone must either be mechanically decoupled or interfering frequency ranges must be masked out using frequency filters.