Projection engineering (Jan 1932-Mar 1933)

Page 18 Sound transmission the hard-of-hearing f or By Henry L. Williams llililllilllllll Herein, Mr. Williams clearly outlines the requirements in a large engineering and sales field, offering an outlet for sound amplification equipment WITH the beginning of 1932, two large national organizations are inaugurating an intensive campaign which has for its purpose the installation of hearing devices in churches, theatres, auditoriums and other public meeting places. Back of this drive is a recognition of the fact that better than 8 per cent of the adult population of the United States have some hearing defect which makes it impossible for them to hear distinctly, normal speech. This drive, by its very nature, is also serving to call attention to the problem of educating hard-of-hearing children by means of speech amplifying equipment— a field which is at the moment somewhat of a monopoly in so far as the manufacture of teaching equipment is concerned. Why this should be so it is impossible to surmise, unless it be that the opportunity is not realized nor the potential value of the market appreciated. However, although the deaf schools form a large and attractive market in this respect, the equipment they require is highly specialized and demands an individual brand of salesmanship involving considerable study of the characteristics of the defective ear and the psychology of the deaf child. On the other hand, the picking up of speech from short distances and bringing it, free of extraneous noise and undistorted, to an earphone presents comparatively few difficulties when once a few simple factors are known. The problem is of course a little more involved when it is desired to cover an entire theatre stage, but a 30-foot stage can be taken care of by a single microphone if the apparatus is carefully designed and engineered. First, let us consider the characteristics of the defective ear. In practically all cases of partially lost hearing the greatest loss is in the higher sound frequencies. This means that in repro duction these frequencies should be accentuated. In other words, the amplifier, if such is used, should have a rising characteristic. It also means that there is little need to reproduce frequencies below 200 cycles, which is a big help as we shall see. It has been found that for all practical purposes a range of 200 to 5,000 cycles for music and 200 to 3,500 cycles for speech is sufficient. Seat Telephones in Theatres We need only consider the talking picture theatre here briefly. These installations often consist of phones wired into the loudspeaker circuit, or may utilize a telephone type transmitter hung in front of the horns. The only objection to these arrangements is that there is no frequency discriminating compensation, and all that is done is to bridge the distance between the listener and the loudspeakers, or, in other words, to give them extra volume. In all such cases it is necessary to have individual volume controls at the seat, and it is preferable to use the stick or lorgnette type of phone because many deafened people are sensitive and do not care to be seen with the very obvious headband. A third method which gives far more desirable results is to use a high grade microphone for the pickup, coupled to an amplifier having the characteristics already mentioned. This is the type of equipment which has been used with great success for the speaking stage. For this work the writer has used condenser microphones exclusively, and such microphones have been entirely a-c. operated. It is this a-c. operation of the condenser microphone which has opened up the whole field of amplification for the deafened and makes it now possible for any competent engineering firm to cater to this valuable market. PROJECTION ENGINEERING Amplifiers By careful design of the amplifier, using ordinary radio tubes, including the power pentodes, it is possible with double filtering, and careful selection of values, to produce an amplifier which will reproduce all frequencies between 200 and 5,000 without a trace of hum or distortion that is apparent to the normal ear, when used with a condenser microphone. The use of a-c. also has the advantage of economizing space and rendering the equipment extremely portable, which is a vital factor in teaching units. In the theatre, then, the condenser head with its single or twostage amplifier is built into the "foots," and carefully insulated with sponge rubber or felt against stage vibration. This insulation is important since otherwise footfalls will come through like thunder. The main amplifier can of course be stowed away below stage and a single pair of wires run to each row of seats where the phones are connected in parallel. It is good practice to put one pair of phones to each two seats, and to distribute them throughout the house. With a good condenser head it will be found that speech is picked up at good volume from all parts of the stage, even when an actor commits the unpardonable sin of turning away from the audience. For stages of more than 30feet width it may be necessary to use two microphones and incorporate an efficient mixer, care being taken not to separate microphones by more than ten or fifteen feet; otherwise the difference in pickup will result in a slight echo effect. As regards phone impedance, it is usual to employ receivers of around 100-ohms, putting, say, 12 in parallel and using an 8-ohm line. With this arrangement it will be found that ordinary potentiometer type volume controls can be used, and the variation in impedances which results will not have any appreciable effect on the quality or volume of any one phone. Seat Fixtures Undoubtedly the best arrangement is to have a jack box fastened under the arm of the seat, so that the customer can procure the phone at the box-office, usually leaving a $2 deposit, and plug it in. It is a matter of choice more or less as to whether the volume control is attached to the arm of the seat or built in the lorgnette handle; the latter, however, is generally the most convenient for the user, while the seat position helps the impedance problem somewhat. To turn now to the more intricate design of teaching equipment, there is one feature alone which will determine the success or failure of the instrument. That is the capability of the microphone to pick up speech with clarity from a