Patent GB-248174-A (GB)
Title: Improvements in sound recording and reproducing apparatus particularly applicable for testing audibility
Filed: 1925-02-26
Abstract: 248,174. Western Electric Co., Ltd., (Western Electric Co., Inc.). Feb. 26, 1925. Ear instruments and hearing appliances.-In testing audibility, recorded sound is converted into electric current fluctuations which are attenuated to an adjustable extent, represented by a scale, before passing to a telephone receiver for reception by the individual to be tested, or alternatively, sounds are converted into current fluctuations which are attenuated to an adjustable extent, represented by a scale, before recording in different positions on a record, direct reproduction from the various parts of the record being then employed as the test. According to the invention the attenuating circuit comprises a plurality of resistances in series and a plurality of resistances in shunt. Fig. 1 shows a record 4 acting on an electromagnet reproducer 5 such as described in Specification 231,410, [Class 40 (ii), Phonographs &c.], so as to produce a fluctuating current in an attenuator circuit comprising series resistances 8, several shunt resistances 52, 53, 54 - -, a movable contact 9 and a telephone receiver 10. The intensity of sound given out by the receiver 10 varies with the position of the contact 9 along the resistance 8, and this is indicated on some suitable scale over which an index on the contact 9 moves. A suitable record is made by recording groups of numbers, each group for example consisting of three numbers each with three digits, spoken with uniform intensity and quality. In the intervals between the groups a percentage or test number is recorded according to which the position of the contact 9 must be adjusted in making the test. The arrangement can be housed in a portable case. Alternatively, a record may be produced with sounds, attenuated in accordance with a scale, recorded thereon, thereby dispensing with the use of the attenuating device for each individual test. Such a record is made by speaking uniformly in a microphone, the circuit of which is coupled through a transformer, an amplifying valve, and another transformer with an attenuator circuit similar to that shown in Fig. 1, which circuit actuates an electromagnetic recording tool to cut out a record which will reproduce the sound attenuated in accordance with the position of the movable contact of the attenuator during the reproduction. A scale is then prepared for the record to indicate the radial distance of the different degrees of attenuation, by means of which direct reproduction of the record can be employed in making the test. Fig. 3 shows an arrangement whereby sounds of different pitch or frequency can be recorded on a record to be used for test purposes. A thermionic valve 21 is employed as a generator, its anode circuit being supplied with current through an inductance 26 and resistance 27 and being connected through the primary coil of a transformer 22 and a condenser 29 with the grid of an amplifying valve 30. The condenser 23 and the secondary coil feeds back to the grid of the oscillator. A reactance coil 28 is inserted in the anode circuit. The anode circuit of the amplifying valve 30, supplied with current through an inductance 32 and resistance 33, contains a condenser 34 and a filter 35 of known construction and adjustable to pass any desired frequency or band of frequencies to the grid of another amplifier 46 the anode circuit of which includes the primary coil of a transformer 47. The secondary coil of this transformer is in an attenuator circuit similar to that shown in Fig. 1 which circuit includes an electromagnetic recorder 19 for recording the various frequencies set up by the generator. These frequencies may be recorded with attenuation when the record is to be used with the arrangement shown in Fig. 1, or each frequency may be recorded with various degrees of attenuation when the record is to be reproduced direct for the test.