Journal of the Society of Motion Picture Engineers (1930-1949)

170 D. G. BLATTNER AND L. G. BOSTWICK [j. s. M. p. B. baffle speakers in close proximity, and design considerations have been outside the scope of this paper, but such factors do not greatly influence the case and an advantage to one type of speaker is offset by other advantages to the other type. It, therefore, does not seem unreasonable to conclude in a very general way that for theater in«:allations general considerations favor the use of horn type loud speakers. DISCUSSION MR. PALMER: Can Mr. Blattner tell us anything about the condenser type of speaker and its relative advantages? MR. BLATTNER: Dr. Kranz has done a considerable amount of work on the electrostatic type of loud speaker and I am sure will be able to describe its outstanding characteristics better than I. DR. KRANZ: I can't give you any quantitative, comparative data on this point. Mr. Blattner has described in his paper a serious limitation in the baffle type of speaker resulting in a low sound power capacity at low frequencies. So far as the electrostatic speaker is concerned this limitation does not apply. MR. CRABTREE: Do I understand that one watt is sufficient for a theater of 500-seat capacity? Is that amount of power capable of reproducing with any degree of realism, for instance, a bass drum, or a bass horn, or a falling building? MR. BLATTNER: Let me repeat for Mr. Crabtree's benefit that the ordinate of the curve Fig. 2 is the acoustic power output required from the loud speaker. So far as we are able to determine the curve shows the acoustic power actually delivered in theaters today. In discussing a previous paper someone in this audience complained that the theater managers are "blowing their customers out of their seats." In general I believe that voices are reproduced at least as loud as the original. As for incidental noise effects, I doubt if it is practical or even necessary to reproduce the noise of a falling building as loud as the original. MR. KELLOGG: I think one of the first calculations of the efficiency of a cone in a baffle was in a paper by Chester W. Rice and myself published in the A. I.E. E. in 1925, based on formulas given in Rayleigh's "Theory of Sound." The efficiency came out about three per cent, which is the figure Mr. Blattner gave. I didn't know of that figure being challenged until Mr. E. D. Cook, who was with the General Electric Company at that time, when making careful measurements by means of the motional impedance of the cones came out with a figure of 9 per cent. It was a surprise to me, and I cannot vouch for that figure, but I think 3 per cent is low. The 3 per cent estimate was based on the assumption of a perfect plunger in a baffle. In most of our units, the cabinet seems to build up the response in the bass range, above that obtained with a baffle. In the high frequency range the efficiency is raised by the fact that the cone cannot function as a perfect plunger, but it breaks up into resonance and under these conditions is more efficient than a simple, ideal plunger. On the other hand, the tests that Mr. Cook made with the horns indicated that while they might give a theoretical efficiency of 25 or 30 per cent in the mid range, there was a loss difficult to explain in the high frequency range, and these factors offset the large predicted difference in efficiency between cones and horns. Further measurements are needed, but