Projection engineering (Jan-Dec 1931)

SEPTEMBER, 1931 Sound projection practice Page 15 Part THE Sound Committee, in preparing this report, has confined itself mainly to a consideration of the status of present-day practices in sound recording and reproducing. Some study has also been given to the possibilities of standardization as well as to those items which might well be investigated further. Good Practices Recommend Where a practice has proved itself worthy of usage it is the plan of the committee to recommend it for standardization to the Standards Committee of the society. It is recognized that in an art so comparatively young as sound recording and reproducing, care must be taken against premature attempts at standardization. The progress of development is so rapid and the technic of recording and reproduction is undergoing adjustment so quickly that premature attempts at standardization might prove a hindrance rather than a help. On the other hand, the moment any phase of the art becomes stabilized, it should be presented at once as a matter worthy of standardization throughcut the industry. In this report the committee intends to show a cross-sectional view of the newer and more important phases of sound recording and reproducing. It is not intended that the material presented here shall encroach upon the activities of the Progress Committee although there may unavoidably be some slight duplication. Suggestions For Betterment Neither the Committee nor the So ciety now has facilities to carry on investigations, but it can recommend what is of importance for further progress in the art. The Committee, therefore, feels it may be of considerable service in presenting to the Society and to the industry matters on which work should be done. Some of the items which have been suggested to the Committee as worthy of consideration have already received sufficient study to permit the formation of definite recommendations. In these cases, arguments for and against are presented and the Committee's conclusions submitted. Status Report Directional Sound Detectors. A directional sound detector comprises a device in which the efficiency of response is a function of the angle between the direction of incident sound and a reference axis in the system which coincides with the direction from which it is desired to receive the sound. In general, there are two principles used in directional sound detectors, one amplifies the sounds desired by concentrating them and the other avoids or suppresses the unwanted sounds. Horn and reflector types employ both principles. The ribbon microphone and absorptive baffle make use of only the second. Horns. Horns have been long used in conjunction with various types of sound reception apparatus, but have not been used for high-quality pickup due to the difficulty of obtaining a good frequency characteristic in spite of the apparent efficiency of this type of unit. Reflectors. The use of reflectors for the reception and focusing of sound is well known. In order to receive sound pressure variations over a wide frequency range, it is necessary to use a reflector having large dimensions. Within practical limits of size, a reflector is likely to have a characteristic which will be better at the high end of the frequency scale than at the low end, although compensation for this effect can be applied. Combination of Horn and Reflector. It is possible to combine the horn and reflector principles in a device which has a fairly good resultant frequency response. The directional properties, however, as limited by the design of a horn and a reflector, may not be uniform with frequency. The directional characteristics of Report of the Sound Committee of the Society of Motion Picture Engineers, Presented at the Hollywood Meeting, June, 1931. H. B. Santee is chairman of this committee. H. B. SANTEE Sound Committee, S. M. P. E. these devices have been found useful in eliminating undesired sounds and noises, particularly where the sound which it is desired to pick up is weak. The effectiveness has been greater for outdoor work where there is no reflected sound than for use in studios where reverberation is encountered. Ribbon Microphone. A properly designed ribbon microphone may be made very directional. Its directional characteristic is practically independent of frequency because of its dimensions, and, by virtue of its directional effect, increases the distance from which acceptable sound may be picked up, in spite of the fact that it receives a relatively small amount of energy due to its size. It is also particularly effective in reducing unwanted sounds, such as camera noises and the like. Absorptive Baffle. It has been found possible to design an absorptive baffle for a microphone in such a way that any sound coming from a direction not included in the throat angle of this absorptive structure will reach the diaphragm at a very much reduced intensity. This structure, while fairly large in dimensions in order to obtain the necessary absorption, is not dependent entirely upon the wavelength of the lowest frequency for its minimum dimension since the wave front remains practically undisturbed. This arrangement, of course, is no more efficient than the microphone would be without the absorptive device, but its sharp selectivity of the direction from which it effectively receives sound makes it appear promising. Camera Silencing Devices Silencing of cameras became necessary with the advent of talking pictures.