Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, June, 1930 Page 23 cay, and the reverberation time in terms of power level and its time of decay, the key to this problem is the relation between the rates of decay of loudness and of power level. This relation has been worked out in the Bell Laboratories for different loudnesses and frequencies by Dr. Fletcher and his associates. At 1000 cycles, and indeed in the range between 700 and 4000 cycles, the two are identical ; at lower and higher frequencies the loudness decays more rapidly than the power level. If it is specified that the calculation shall apply to tones of equivalent loudness, whatever their frequency, equating the new parameter to its optimum constant is the same as asserting that the loudness of these tones should all decay at the same rate under the best conditions. Thence from Dr. Fletcher's work the most desirable rates of decay of power, and thus the most desirable reverberation times, can be calculated for all frequencies (Fig. 2). The well-known formula of W. C. Sabine indicates that the reverberation time is inversely proportional to the number of absorption units in the room. If this formula is accepted, the optimum absorption-frequency characteristic for an auditorium of any volume can be readily figured from Fig. 3. The ideal absorption-frequency c h aracteristic of an auditorium a pp roximates the c haracteristic (broken line) of an audience. IOO 90 SO Z 70 g t 60 a. o ^"^ ^ J / / / / X m < > Ij 30 £ 20 10 o / D O O FREQUENCY O o o o o o* the curve of optimum reverberation times against frequencies for that volume. If the room is to be panelled predominantly with a single material, the absorption-frequency characteristic of the best material for the purpose can thus be determined, to guide the search for the proper substance. In Fig. 3 appear the optimum absorptions at all frequencies, relative to that at 1000 cycles. The calculation of these absorptions affords an opportunity for a further check, rough but interesting, of the validity of the theory that the new parameter should have a certain constant value under ideal acoustic conditions. This check arises from the observation, often made, that an auditorium which when empty has an excessive reverberation time frequently obtains excellent acoustic properties when filled with an audience. Since under these conditions the audience is the major absorbant, its absorption-frequency characteristic should approximate the optimum. The characteristic of the average audience has been figured by W. C. Sabine ; to its plot in Fig. 3, the optimum calculated from the new theory shows an encouragingly close correspondence. Supported by the confidence in it which these experimental checks give, the theory is being applied to the solution of other important problems in acoustic engineering. With its assistance, the construction and treatment of auditoriums may be placed on a more exact basis. NEW MUSICAL INSTRUMENT ENABLES PIANO NOTES TO BE SUSTAINED ANEW device for the sustaining and intensification of the tone of a piano, known as the CreaTone and invented by Simon Cooper, Brooklyn scientist and engineer, was demonstrated recently at the Wanamaker Auditorium, New York. Unlike the tone-sustaining device of John Hays Hammond, Jr., introduced in November, 1925, Mr. Cooper's invention is electrical. Its principle is the prolongation of the vibrations of the piano strings by electromagnets operated by a current obtainable from a battery of the type used for radio apparatus. The magnets, operated through a small switch over the keyboard, are set in action by the vibrations of the strings struck, and recommunicate these vibrations to the strings. Intensification or diminution of the tone is obtained through the pedals. The sound continues as long as the keys are pressed. As demonstrated, the Crea-Tone was successful both in prolonging the sound of a desired note, and varying its intensity, with ability to increase it to a notable degree. In some cases an intensified note acquired a sound somewhat suggesting that of an organ tone. As there seemed to be an interval, though a very slight one, before the effect of the device on a given note became fully apparent, it seemed best adapted to music of a sustained type. THOUGHTS ON GRANDEUR* By Chas. Reese Heavy machinery usually brings to mind power houses with giant turbines and gigantic steam engines or locomotives with heavily loaded freight trains behind. Such thoughts are distinctly foreign to the projectionist, but the old order has changed with the coming of Grandeur. We were accustomed to small sweet running machines which made so little noise in operation that they might easily be mistaken for a grandfather's clock, and were capable of producing acceptable results when illuminated by an arc consuming from 35 to 90 amperes. We made a broad jump indeed from these ideal small machines to the modern Grandeur projector which by comparison to the old order is of real locomotive proportions and consumes from 190 to 200 amperes. The original equipment of the Fox Circle Theatre consisted of two standard projectors, four spotlights and one Brenkert effect machine. Direct cur *A. P. S. Loudspeaker, April, 1930. rent for this equipment was supplied by two 120-amp. m. g. sets which were run parallel. In order to make room for the Grandeur machines (three of them) it was necessary to remove entirely the effect machines and two spotlights and to re-locate the two standard projectors and the remaining two spotlights. To supply sufficient direct current for the Grandeur machines it was necessary to install a 300-ampere generator. This additional current supply leaves the original motor generator sets for emergency supply. After the installation was completed the projection room resembled a corner of the Baldwin Locomotive Works. It was suggested that the projectionists be supplied with a set of road maps, so that they might find their way around through the maze of equipment more easily. Naturally, considerable rehearsing was necessary to smooth out the wrinkles that are always present in any equipment so completely revolutionary as Grandeur. If new film in standard projectors gets noisy, then new film in Grandeur resembles thunder. As far as we have gone we have found Grandeur presentation very interesting indeed, and predict for it a tremendous success in the amusement field.