British Kinematography (1950)

March, 1950 glover and mclaren : acoustics 81 The fact that the curve is flat indicates that the ceiling must be contributing very greatly in absorption, particularly in the lower frequency range. Were this not so, then the reverberation time at the lower frequencies would have risen to a much greater value. The steady fall in reverberation time at frequencies above 2,000 c/s is due partly to surface absorption and partly to that caused by the air. The next stage in the re-treatment consisted in laying a floor of oak strip 3 in. wide supported on battens, fixed to the concrete structural floor. The curve B shows the reverberation time after this operation. It will be seen that the floor contributes a very considerable degree of low frequency absorption. The addition of the wood panelling and a certain amount of lath-and-plaster work contributed a small amount of absorption, which is indicated in curve C. The wood panelling, although adding considerably less absorption than had been expected, plays a useful part in adding to the brightness of musical tone by giving strong first reflections. The porous absorbent was then added ; this consisted of wall panels and a frieze of 2-in. rock wool covered with perforated asbestos tiles, the perforations being about |-in. in diameter and spaced at about 1-in. centres. About one-fifth of the wall surface was treated in this manner. The effect is shown in curve D. The rise in reverberation time above 500 c/s was unexpected, and it is now considered that it is caused by the diameter, thickness and spacing of the holes in the asbestos tiling. To counteract this rise in reverberation time, a suitable quantity of J-in. felt, covered with cribble cloth which is a perforated American cloth, was. applied to the ceiling. The acoustical effect is shown in curve E. It will be noted that the thin felt had negligible absorption for frequencies below 500 c/s. The slight rise in reverberation time, which has a peak value at about 1,500 c/s, was found to be undesirable in practice, and this was corrected by hanging light casement cloth curtains over the rock wool panels. The final reverberation curve is shown in curve F. Owing to wartime conditions, this last curve was not taken until 1945, hence the differences from the previous curve may be due to other causes than the addition of curtains. The information given by this series of experiments and other information obtained from experimental work carried out in the past provided valuable practical data in the design of broadcasting studios. Shape and Profile of Studio So far little mention has been made concerning the shape of the studio or of the profile of its interior. There has been a great deal of discussion and argument on this subject, and all that can be reasonably said is that so far, there is no conclusive evidence that parallel walls are better than non-parallel walls, or that one type of breaking up of the wall surface is better than another. It can be said, rather facetiously, perhaps, that the continent of Europe and the United States of America are littered with the corpses of acoustical fads and fancies which have been tried and found wanting ! To take as an example the much vaunted polycylindrical treatment, which was so popular a few years back, in the form of a band shell. This type of treatment was used by Walt Disney for the recording of his fine film, ■' Fantasia," and it gave a very good result, but further experience in this type of treatment has shown that for general use too much diffusion of the sound was obtained. • In order to investigate the differences between plane parallel walls and those treated with hemi-cylindrical treatment, a graphical study was made by the Research Department of the British Broadcasting Corporation. In order to reduce the problem to its simplest terms, only two dimensions were considered and furthermore the study was confined to the first reflections only. The results showed that in the case of the plain walls there were four strong reflections which arrived to a listener at intervals after he heard the original