Research Department. REPORT No. B.014 9th October, 1936. o J Serial No. 1936/13 Work carried out by Drawing Nos. B.014.l A. B. Howe. toB.014.l2. J. McLaren. A. L. Newman. ACOUSTICS OF MAIDA VALE sruDIOS. SU1~. This report describes the considerations leading to the design of the various studios at Maida Vale. An epitome specification is given in each case. The results of reverberation measurements in each studio are discussed in their relation to the acoustical treatment and to the practical results obtained. Studios 2 and 3, and 4 and 5 form two pairs designed for the purpose of experiment relating to the acoustical effect of variations in studio form. The results of the experiment are given and possible explanations discussed. Modifications to the studios now in hand are considered. The report is of an interim character •. INTRODUCTION. Maida Vale studios were intended to provide accommodation supplementary to that available in Broadcast.ing House, catering particularly for the musical side of the programmes. In the final BBC R & 0 111111111111111111111111111111111111111111111111111111111111 300008835 R -------j ,/ -2- . scheme, five studios were provided. No. 1 is a large orchestral studio capable of accommodating a full symphony orchestra. Nos. 2 and 3 are orchestral studios equivalent, in a general way, to the Concert Hall in Broadcasting House. Nos. 4 and 5 are general purpose music studios roughly equivalent to studio BA in Broadcasting House. The studios are practically independent structures built inside the shell of a disused skating rink. All are built of thick bri.ckwork and have independent roofs below the main roof of the building. This method of construction was designed in part so as to provide the maximum sound inSUlation between the various studios. Additional sound insulation from street noises is provided, much as in \ Broadcasting House, by offices, recording rooms etc., which abut directly on the street. In each case the actual studio roof or ceiling is supported by steel t~sses or girders, and consists of lath and plaster on heavy wooden joists. The upper side of the joists is covered with 1" boarding and the space between them packed with sawdust 'and shavings. This type of ceiling was specified with the object of reducing both structura) resonance and sound interference. The· foregoing remarks apply to all studios alike. The characteristics of the individual studios will now be considered separately. -3- STUDIO No. 1. This studio was intended to supersede "No.10 studiol1 and to accorrrrnodate the full BBC symphony orchestra of 119 perfo:!;,mers for rehearsal purposes, and even occasionally for transmission. A volume of about 250,000 cubic feet was therefore required. Although adequate floor area could be obtained there was some limitation as regards height, owing to the existing roof of the building and to the undesirability of excavating, In order to get the maxlimwn possible height, the usual restriction regarding the use of a flat ceiling was relaxed, and the design shovm in Fig. 1 was adopted. The average height was about 28'-6", the overalllength of the studio 125 ft. and the width 72 ft. The actual volume of the studiO, allowance being made for gallery, beams etc., is 230,000 cubic feet. Acoustical Treatment. At the time a decision was made as to the acoustical treatment, -i" building board cemented to a hard rigid surface was still believed to hav~ a sensibly flat absorption-frequency characteristic with a value of absorption coefficient of about 0.25. As precautions had been taken to avoid structural resonance, it was thought that an ideal type of reverberation-frequency characteristic would be the result of the use of this material. The usual limits of reverberation time in BBC practice, for a studio of the size in question, are 1.9 and 2.4 seconds. A value -4- of 2.1 seconds was actually adopted as the basis of the calculation of the treatment. The following is an outline of the treatment. The floor was covered entirely with building board and carpet with an underfelt, the area being 7920 sq. ft. 6500 sq. ft. of board were also ap~lied to the wallS, representing an area of about 85% of the total wall surface and leaving a plaster dado four feet high. The ceiling was untreated. Tvvelve heavily upholstered setliBes were also included as part of the acoustical treatment. Reverberation Measurements. The reverberation-frequency characteristic of the studio was determined in the usual manner. In view of the possibility of sound concentration effects due to the shape of the roof, two series of measurements were made. For one series the microphone was suspended at about 8 feet from the floor and for the other at about 4 feet, the usual height in our reverberation measurements. This procedure arose from the fact that in preliminary experience in the practical use of the studio, which had been gained before it was possible to make reverberation measurements, a relatively high microphone position had been found to reduce the bass-heaviness which had been experienced. Five microphone positions were used for each series. In addition to these measurements, decay curves were plotted -5- for a ce~tral microp4one position, by measuring the time for various values of'decay from 10 to 40 or 45 db., according to ground noise conditions. These measurements confirmed the legitimacy of the usual method of extrapolation from a value of about 30 db., and this was done for the principal series of measurements. The results of the reverberation measurements are shown in Table I and are plotted in Figs. 2 and 3(a). Fig. 2 shows the curves for high and low microphones respectively, and Fig. 3(a) the mean of the two series. It was not feasible to obtain a value of 62.5 cycles for the high position owing to the low sound intensity obtainable, and the reading for the low position must be accepted with reserve. The curve below 125 cycles is therefore dotted in. Conclusions from Reverberation Measurements. Considering first the curve of Fig. 3(a), it is eVident that the studio is too live for frequencies below 1000 cycles per second, and that the reverberation time falls off somewhat too rapidly for the higher frequencies. The latter fact is readily explained in terms of the relatively large proportion of carpeted building board used in the treatment. This feature was, of course, foreseen and was unavoidable on account of the proportions of the studio, on the assumption that carpet was tQbe used for the floor. It was considered to be acceptable in practice. The high reverberation at low frequencies was not understood at the time the curve was determined, since precautions had been taken -6- to avoid structural resonance in walls and ceiling. It is readily explained, however, in terms of the now known absorptive properties of building board; in fact, were it not for a certain amount of absorption provided by the ceiling, considerably longer reverberation for frequencies below 250 cycles would occur. Referring now to the two curves shown in Fig. 2, confirmation is obtained of the theory that concentration of sound due to the shape of the ceiling occurs for frequencies below about 400 cycles. The method of reverberation measurement used is such that if the initial sound intenSity before cutting off the source is non-uniform, a high initial intensity tends to give a low value of measured reverberation til'OO and vice versa. Hence ·a higher j.ntensity, at low frequencies, occurs near the floor than at, for example, a height of 8 feet. This result was also obtained in the practical use of the studiO, a high microphone position being found to reduce the effect of bass. It was also supported by ripple tank experiments. In the latter experiments the assumption was made that pure reflection, as in optics, occurs from the ceiling surfaces for sound waves of the higher frequencies, but that for low frequencies the ceiling behaves as a cylindrical surface, 'in accordance with accepted theory. Models were made representing these two conditions. For the case representing low frequency behaviour, it was -'7- easily seen that sound originating from a source near the floor, after bei~g reflected twice from the ceiling and once from the floor was brought to a focus just above floor level. For the high frequency case, a much reduced focussing effect was observed, owing to the scattering effect of the various ceiling surfaces. Another factor tending to reduce the effect of the roof for the higher frequenCies is the considerably increased absorption for the single reflection from the floor. The focussing effect produces a reflected component of high intensity which gives interference effects with the direct ray from the source and causes the undesirable effects inseparable from standing wave patterns, as well as l!bass-heaviness Y1 • The conclusion is that concave ceilings must be avoided in future in studio construction, even if they are composed of large plane surfaces. A similar effect has been noticed in the Birmingham studiO, which has a similar ceiling. Effect of the Studio in Practice. The actual use of the studio for orchestral work was attended with fair success so far as the result heard via the microphone was concerned. Some observers considered the effect to be extremely good; others thought it was too reverberant, particularly at low frequenCies. Difficulties of balance, poor string tone and excessive bass existed, however, only partially solved by the use of a high microphone and other expedients such as a relatively -8- close teclmique. Volume control was difficult for passages involving heavy bass, owing partly to the interference effects and partly to the high general bass intensity due to insufficient low frequency absorption~ .