Patent GB-282469-A (GB)
Title: Improvements in apparatus for transmitting and recording sound
IPC: G11B7/00
Filed: 1926-07-03
Abstract: 282,469. Henley, J., and Flinn, H. V. July 3, 1926. Transmitters in which a sound-controlled light-beam ants on a photo-electric element.- Sound is transmitted or recorded by means of alterations in the optical constants i.e. refractive index and absorption coefficient, of the air, vapour &c, conveying the sound waves, without the use of any material vibrating diaphragm, the alterations being directly caused by the physical changes in the condition of the air &c. due to the passage of the sound waves and being utilised to modify a beam of light or other electromagnetic radiation so as to produce therein changes of refraction or absorption or both. The sound waves are preferably concentrated by a funnel A, Fig. 1, before passing into the unit B. which may have attached to it a silencing chamber C. One form of the unit B made of a rigid material is shown in Fig. 2, the portions LM, LN and PQ being transparent. Mirrors E, F are mounted on opposite faces of the unit, the mirror E' being capable of a fine vertical adjustment. A further mirror HO, pivoted about the edge O, is mounted below the mirror F. An intense parallel beam of light S is directed into the apparatus at oblique incidence through the window LN so that it falls on the mirror HO which is so adjusted that the beam is reflected to and fro between the mirrors E, F as shown. The mirror E is adjusted so that, when no sound is passing, approximately half the beam emerges in the direction MR, the remaining half being reflected through the opposite window PQ. The emergent beam R is received on a photo-electric cell or other light sensitive unit. Sound waves passing through the unit MVHQ in the direction XY divide the air or other medium into alternating bands of different, density i.e. condensations and rarefactions, the refractive index and absorption coefficient of which differ from one another. This results in corresponding periodic variations in the intensity and direction of the energized beam R, which, falling on the photo-electric cell, produces corresponding electric pulses. These currents can then be amplified, transmitted, recorded or dealt with in any other way desired. If desired, the second emergent beam T may be received on a second sensitive cell, the two cells being arranged in the arms of a Wheatstone circuit so as to produce an additive effect. The beam S instead of being uniform may be (a) graded in intensity from one edge to the other by passing it through a screen graded from transparency at one side to almost complete opacity at the other or (b) passed through a screen consisting of equally spaced transparent and opaque strips, the whole of the reflected beam being, in this case, received upon a second exactly similar screen mounted above the mirror E. which is adjusted so that approximately half the beam is passed to the light sensitive cell when no sound waves are passing. If desired, the opaque strips of the second strip may be silvered, the reflected beam being received on a second light sensitive cell. In a further modification, the box form of unit, Fig. 2, is replaced by a hollow cylinder, Fig. 5, silvered on the inside and provided with a window X at the forward end for admission of, the incident beam B. A pivoted mirror Y serves to adjust the path of the beam within the cylinder, the beam finally emerging through a second window. The Provisional Specification states that totally reflecting prisms may be employed so that the beam after travelling between the mirrors E, F is caused to repeat the process two, three or more times before emergence.