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168 D. G. BLATTNER AND L. G. BOSTWICK [j. s. M. P. E.
use of an amplifier of 1.84 watts output capacity for use with horn type speakers 25 per cent efficient, or 18.4 watts capacity for use with good baffle speakers. Assuming that the horn type speaker and the baffle type speaker each are capable of handling a maximum sinusoidal electrical power of 6 watts (a value that seems to have some experimental backing), it will be clear that in the first case a single horn type speaker is capable of handling the total output required of its amplifier but we will assume that two such speakers would be used in order to obtain satisfactory distribution. In the second case it will be observed that three baffle speakers are required to handle the power output required from their amplifier. Therefore for an economical balance in cost of equipment the three baffle speakers should cost less than the two horn type speakers by an amount equal to the difference in cost of the two amplifiers.
Now let us consider the relative costs of the two types of amplifiers. On the above basis as to efficiency and power capacity of the speakers the amplifier for the baffle speaker installation should have 10 decibels more gain and ten times the power capacity of the amplifier for the horn installation. An increase in gain of 10 decibels if effected at low power capacity is not particularly expensive, but where the gain is already considerable and the circuits are otherwise complicated this increase in gain may increase the cost of design and installation to insure satisfactory operation. From the standpoint of power capacity the increase required necessitates the use of higher power vacuum tubes and current supply apparatus which requires larger amounts of iron and higher voltage insulation in transformers and choke coils and more costly mountings. In view of these factors and the large powers required in theaters, it might be assumed that the relative cost of two amplifiers having the same features would not be greater than the ratio of their power capacities, nor perhaps less than the square root of their capacities. Assuming the relative cost to be the latter figure as an approximation it will be clear that since the power capacities are in the ratio of 10 : 1 the cost of the baffle speaker amplifier will be approximately three times that of the horn speaker amplifier.
For the small theater installation it, therefore, appears that baffle type speakers can be justified only if three such speakers are procurable at a cost less than that of two horn type speakers by twice the cost of the horn speaker amplifier.
Large Theater Installations. — The situation in large theaters is even