British Kinematography (1950)

82 BRITISH KINEMATOGRAPHY Vol. 16, NO. 3 sound, and in the latter case, there was a very large number of low-intensity reflections arriving after a strong direct sound. Neither effect is entirely desirable, the latter tending to provide a colourless studio without character, whilst the former one which may be harsh and " echoey." The answer probably is to effect a compromise between these two extremes. Such a design was carried out just after the war in the retreatment of another of the Maida Vale musical studios, No. 5, whose volume is of 26,000 cu. ft. in which the upper surface of the long walls has been broken up by vertical architectural features and in the short walls by horizontal features. The lower portions of the walls up to door height are panelled in half-inch plywood. The acoustical treatment is carried out in Acousti-Celotex tiles, and has been graduated so that the maximum area of absorbent tile is at the end of the studio remote from the players. The wall space not occupied by the tiles is filled up with quarter-inch plywood mounted on battens at about one-foot centres. This panelling provides resonant absorption in the region between 200 and 300 c/s. The reverberation curve is very flat, having a mean reverberation time of approximately 0.8 seconds. It has proved itself very satisfactory in practice, when used for the purpose for which it was designed, namely for small musical combinations of up to about 20 players. DISCUSSION Mr. Baynham Honri : Years ago the B.B.C. experimented with a wallpaper on top of felt as a treatment for studios. Mr. W. S. Bland : In broadcast work or gramophone recording, people listen at home ; in film work we project sound into a large place with reverberation time of a very high order. The problems in designing a recording enclosure for the original recording are quite different. Mr. McLaren : It is the practice of the B.B.C. to try to have monitoring rooms and control cubicles the same size and with the same conditions as in an ordinary living room. Measurements have been made in people's houses and the reverberation period is of the order of half a second. Dr. L. E. C. Hughes : Since I did some research on the magnitude of the disturbance of the steady sound-field in 1928, I have always advocated the quasi-optical examination of proposed structures for the conservation of sound-power. Strong reflections should be eliminated and there should be no parallel walls. Such acoustic conditions, and particularly low reverberation times, conflict with the requirements of artistes. The engineer in placing his microphone must make the best compromise ; the answer at one time was the " live-end dead-end " studio, the live-end being shaped so that the artistes get strong short-delayed reflections, while the dead-end is so treated that there are no strong reflections arriving at the microphone from any non-axial direction. The polycylindrical surfaces now in fashion disperse the reflected sound-waves through increased solid angles, and also effectively bring local reflections nearer the surface for the benefit of "contact" among a group of artistes. Unfortunately, the acoustic refinements suitable for music conflict with those for speech, necessitating compromises which degrade both. The correctly designed kinema is really an extension of the loudspeaker loading behind the screen. All other schemes are makeshifts, but sometimes they can be good acoustically. Mr Glover : Long before the diffuse cylinders were discussed I had to find an expedient in dealing with the De La Warr pavilion at Bexhill. The acoustic reflection from the flat surface of the ceiling follows optical laws and the acoustical image is as far behind the reflector as the original source is in front of it. In order to avoid that I put in a number of small domes over the ceiling. Instead of getting reflection from a second source each little dome acted as a real image would in optics, and we had a series of separate sources at the ceiling level rather than some hundred feet behind it. Mr. Ross : On the measurement of reverberation time, was a pure or warble tone used and at what frequency ? Mr. Glover : You can use a warble tone with advantage with a modulation of ±10 per cent, and at 5-10 c/s. Mr. McLaren : The reverberation times of studio and listening room are not necessarily additive — one curve sometimes masks the other. Dr. L. E. C. Hughes : The equivalent reverberation of two reverberations in series is a difficult calculation and is not continuous.* Solutions are possible when the two reverberations are equal, or when either is more than 1.5 times the other. * E. A. Johnson, /. Maths and Physics, M.I.T., 9, 1930.