GB2161341A - Laser microphone - Google Patents

Abstract

A microphone comprises a laser 1 whose beam passes through a chopping system 2 which also emits a reference frequency through connection 10. The beam strikes a vibrating surface 3 and (given the laser frequency is visible) appears as a spot of light 4. A lens 5 collects the image of the spot and focusses it onto a detector 6 to produce a varying electrical signal 7. The signal then passes through an amplifier, band-pass filter and a lock-in amplifier, into which also feeds the reference frequency. The signal then passes to outputs for either a cathode ray oscilloscope or a frequency spectrum analyser. Alternatively an audio amplifier and speaker is fitted. The signal can be digitalised for micro-processors and fitted with a chart recorder. <IMAGE>

Description

SPECIFICATION Laser microphone This invention relates to a laser microphone.

Microphones have been well developed for most applications of receiving compression waves of audio frequencies. Established designs of microphones use the compressional waves in a fluid medium to displace a material in an electrical or magnetic field or simply by moving a piezo-electric crystal. The laser microphone discussed in this paper relies on none of these principles.

The invention will now be described with reference to the accompanying drawing in which: Figure 1 shows a block diagram of the Detection Apparatus; Figure 2 shows a block diagram of the Electrical Apparatus.

Referring to the Drawings described in the Abstract on page 5; as shown in Figure 1, the invention uses a beam from a Laser 1 passing through a Chopping System 2 which also emits a Reference Frequency 10 into the Electrical Apparatus (shown in Figure 2). The beam then strikes a moving surface 3 and is seen as a Spot of light 4 (if the laser frequency is visible). The image of the Spot is collected by a Lens 5 and focussed onto a Detector 6; the Detector then produces a varying electrical signal 7. The signal is passed through a Unit containing an Amplifier and Band-pass Filter at 8 and it next passes through a Lock-in Amplifier 9 into which is fed the lead carrying the Reference Fre quency from the Chopping System.There is then produced a signal available to a number of outputs 11-16 providing facilities for inspection either by a Cathode Ray Oscilloscope 11 or by a Frequency Spectrum Analyser 12. Alternatively an Audio Am plifier and Speaker can be fitted 13. The signal can also be digitised for computer analysis 14 and rep resents an electrical variation of the amplitude of the laser light returning to the Detector. A Chart Recorder can be fitted 15 and outputs provided for other facilities 16.

A discriminating filter for the laser light fre quency can be placed in front of the Detector to re duce effect of ambient light on the moving surface.

The laser, detector and electrical equipment can be made portable with an independent power sup ply.

As the spot of laser light is only detected the laser microphone has distinct advantage over a conventional microphone; it is unnecessary for it to be indirect contact with the moving surface by any material medium, unlike a conventional micro phone that requires a fluid medium to transmit the compressional waves to the microphone. The spot can be rendered invisible by using an invisible laser frequency.

1. A laser microphone comprising a laser whose beam strikes a vibrating surface, the scat tered light then focussed onto a light sensitive detector such as a photodiode, and the resulting electrical signal analysed by audio, visual or digital techniques or any other technique.

2. A laser microphone as in Claim 1 wherein means is made to focus the image of the surface receiving the laser light onto the light sensitive detector by moving either the detector or lens system or part therein.

3. A laser microphone as in Claims 1 and 2 wherein means is made to split a sample of the laser beam onto a separate detector to Claim 2 for use of electrical analysis, particularly of reduction of noise produced by the laser.

4. A laser microphone as in Claims 1, 2 and 3 wherein provision is made for a filter to reduce frequencies other than that of the laser beam frequency.

5. A laser microphone as in Claims 1, 2, 3 and 4 wherein provision for a device to modulate the laser beam either by frequency or amplitude modulation is incorporated for purpose of reducing signal noise level.

6. A laser microphone as in Claims 1, 2, 3, 4 and 5 wherein provision is made for electronic analysis of the signal by established techniques such as phase locked amplification digital analysis or any other method intended to clarify and enhance the signal for analysis.

7. A laser microphone as in Claims 1, 2, 3, 4, 5 and 6 wherein a power supply is either externally or internally incorporated.

8. A laser microphone as in Claims 1, 2, 3, 4, 5, 6 and 7 substantially described herein with reference to Figures 1 and 2 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Laser microphone This invention relates to a laser microphone. Microphones have been well developed for most applications of receiving compression waves of audio frequencies. Established designs of microphones use the compressional waves in a fluid medium to displace a material in an electrical or magnetic field or simply by moving a piezo-electric crystal. The laser microphone discussed in this paper relies on none of these principles. The invention will now be described with reference to the accompanying drawing in which: Figure 1 shows a block diagram of the Detection Apparatus; Figure 2 shows a block diagram of the Electrical Apparatus. Referring to the Drawings described in the Abstract on page 5; as shown in Figure 1, the invention uses a beam from a Laser 1 passing through a Chopping System 2 which also emits a Reference Frequency 10 into the Electrical Apparatus (shown in Figure 2). The beam then strikes a moving surface 3 and is seen as a Spot of light 4 (if the laser frequency is visible). The image of the Spot is collected by a Lens 5 and focussed onto a Detector 6; the Detector then produces a varying electrical signal 7. The signal is passed through a Unit containing an Amplifier and Band-pass Filter at 8 and it next passes through a Lock-in Amplifier 9 into which is fed the lead carrying the Reference Fre quency from the Chopping System.There is then produced a signal available to a number of outputs 11-16 providing facilities for inspection either by a Cathode Ray Oscilloscope 11 or by a Frequency Spectrum Analyser 12. Alternatively an Audio Am plifier and Speaker can be fitted 13. The signal can also be digitised for computer analysis 14 and rep resents an electrical variation of the amplitude of the laser light returning to the Detector. A Chart Recorder can be fitted 15 and outputs provided for other facilities 16. A discriminating filter for the laser light fre quency can be placed in front of the Detector to re duce effect of ambient light on the moving surface. The laser, detector and electrical equipment can be made portable with an independent power sup ply. As the spot of laser light is only detected the laser microphone has distinct advantage over a conventional microphone; it is unnecessary for it to be indirect contact with the moving surface by any material medium, unlike a conventional micro phone that requires a fluid medium to transmit the compressional waves to the microphone. The spot can be rendered invisible by using an invisible laser frequency. CLAIMS

1. A laser microphone comprising a laser whose beam strikes a vibrating surface, the scat tered light then focussed onto a light sensitive detector such as a photodiode, and the resulting electrical signal analysed by audio, visual or digital techniques or any other technique.

2. A laser microphone as in Claim 1 wherein means is made to focus the image of the surface receiving the laser light onto the light sensitive detector by moving either the detector or lens system or part therein.

3. A laser microphone as in Claims 1 and 2 wherein means is made to split a sample of the laser beam onto a separate detector to Claim 2 for use of electrical analysis, particularly of reduction of noise produced by the laser.

4. A laser microphone as in Claims 1, 2 and 3 wherein provision is made for a filter to reduce frequencies other than that of the laser beam frequency.

5. A laser microphone as in Claims 1, 2, 3 and 4 wherein provision for a device to modulate the laser beam either by frequency or amplitude modulation is incorporated for purpose of reducing signal noise level.

6. A laser microphone as in Claims 1, 2, 3, 4 and 5 wherein provision is made for electronic analysis of the signal by established techniques such as phase locked amplification digital analysis or any other method intended to clarify and enhance the signal for analysis.

7. A laser microphone as in Claims 1, 2, 3, 4, 5 and 6 wherein a power supply is either externally or internally incorporated.

8. A laser microphone as in Claims 1, 2, 3, 4, 5, 6 and 7 substantially described herein with reference to Figures 1 and 2 of the accompanying drawing.