Projection engineering (Jan 1932-Mar 1933)

APRIL, 1932 Page 19 distance of 20-30 feet and return it at such volume that the hard-of-hearing person can hear his own voice in the phones. This is a prime necessity in teaching speech, since the pupil must hear his own voice as well as that of the instructor in order to be certain that he has imitated the sounds correctly. This necessity imposes a certain restriction on the sensitivity of the microphone, which must not have a tendency to feed back from the phones, even though the phones are within ten feet of it. Teaching Equipment The most useful general design of teaching equipment employs a flattopped desk of the kneehole type. Into the center of the forward edge of the desk is built the microphone head, with the diaphragm upwards in a horizontal position. Carbon microphones positively can not be used for this work, as with the high amplification necessary it is impossible to get rid of hiss and distortion. Immediately under the head, and inside the desk is the amplifier, with a grid lead from the head of the microphone well shielded and not more than five or six inches long. Another necessity in this equipment is the incorporation of a phonograph for the purposes of ear stimulation. This can very well be arranged in a drawer to the right of the kneehole, with a drawer for records underneath it. It is a peculiar fact that a deafened child will often pretend to hear when it can not. For this reason it is as well to incorporate a cutoff switch which can be operated by the knee of the teacher. A visible switch can not be used since the child will see it operated and act accordingly. Similarly the operation of the switch must be perfectly silent so that there is no indication of when it is used. The controls on this instrument can conveniently be mounted close together on a panel on the front of the phonograph drawer. They should consist of an on-off switch, phono-mike switch, Fig. 1. Teaching set for the hard of. hearing. PHONE PLUG . HERE MICROPHONE POSITION PHONO. DRAWER THE following are the answers to technical questions for projectionists, Nos. 39 to 48 of the series, which were presented in the March issue of Projection Engineering: 39. .00075 inch high, .070 of an inch wide. 40. The one on the left, facing fader, is for use in event the fader becomes inoperative, when this switch is thrown either to the left or right to close these respective circuits it introduces resistance, which is equal to the number nine fader setting. The switch on the right hand side of phono volume control — the phonograph being cut into the last two stages of the amplifier — and some designers prefer to incorporate an amplifier volume control, although, as each phone has to be controlled separately at the students' desks, this hardly seems necessary. As it is sometimes necessary for the teacher to examine a child's hearing separately, it is advisable to incorporate a phone plug at the end of the desk, and mount the volume control for this on the panel. It is rather important that the volume controls on the students' desks be matched as closely as possible so that comparisons can be made in the improvement of the hearing. It is also an advantage to have the calibrated knobs securely attached to the spindles so that the children can not twist them on their shafts and so confuse the readings. Single headphones are always used for this work. In Private Clubs Any one producing equipment such as this will also find a ready market in private clubs having a number of hardof-hearing members. In such cases the equipment can be mounted in a smaller reading desk such as used by lecturers, and the floors of one or more rooms wired for easy installation by the building electrician. Finally, a word about costs. In the A A A Answers to Technical Questions the fader, is for use when three projectors are installed. 41. A storage battery neither stores nor generates electricity, what is done is to start a chemical action. 42. Amplification and volume are not the same thing. Amplification is the increase of signal amperage or voltage or both. Volume is the result in power received through the amplifier. 43. The fader in the RCA circuit is used to switch sound from one projector to the other, while the fader in the WE circuit besides being used for this purpose also within certain limits controls the volume. (Designed by Henry L. Williams) atre installations it is usual to supply only six or seven phones for each twelve wired positions. A prominent manufacturer of motion picture equipment supplies seven phones and 12 positions wired into the projection amplifier, installed for $250. A manufacturer of the telephone type units supplies the same quantity of apparatus for $195 and leaves the installation to the theatre electricians. The writer has built and installed condenser microphones in "legitimate" theatres with seven phones and 12 positions for a net cost of $150 for which the usual list price is $500. A similar price can be obtained very easily for club installations and the net costs are about the same. The usual price for single phones with the plug boxes is $15 each, although one maker asks $20. The actual net costs for good quality instruments is around $3.40, without volume controls. The actual cost for the desk teaching equipment should not exceed $170 plus the phones. The selling prices of this equipment have always been high due to lack of competition, the desks selling for around $900 upwards. A list price of around $500, however, allows a fair profit, taking into consideration the amount of attention and instruction in use necessary. It will therefore be seen that the market for these types of equipment offers far greater possibilities than the attention it has received would indicate. 44. Yes. The RCA employ such a motor on some of their installations. These motors are 220 volt 60 cycle 3 phase. 45. Both systems can be used on RCA Photophone without any changes in the equipment. In fact, all 35 mm. film carrying sound may be used. 46. To hold the speed of the projection motor to as near a uniform speed as possible. 47. This is an attachment used to "filter" out the scratch noises in the reproduction of sound. 48. The speed is decreased when the field current is strengthened and increased as the field current is weakened.