To obtain favourable listening conditions in a large lecture hall or theatre, it is necessary to take into consideration several factors from the earliest planning stage. These are—volume of the hall in relation to the size of audience, distribution of sound over the audience area, reverberation time, diffusion of sound, ratio between the direct and the diffuse sound, and background noise level due to sources within and outside the hall. The optimum limits for these factors have been worked out empirically for different classes of halls in relation to their ultimate use. However, these limits have often to be modified in practice, particularly in the case of the various multipurpose halls being planned and constructed in India at present. These halls are required to serve several purposes such as drama, music, speech, dance etc., and the optimum conditions for the various uses are different and, sometimes conflicting. For example, the volume of the hall is primarily dictated by economic factors and maximum audience has to be accommodated in a given space. The designer is, therefore, faced with a situation where criteria evolved for certain requirements have to be applied to entirely different conditions, it is necessary that halls built with these modified criteria be examined acoustically to determine the extent to which the various modifications are successful. These results cannot be derived solely from laboratory measurements or model studies; it is essential to supplement the work with field study comprising of an analysis of the data on performance characteristics of a number of auditoria. The modifications can be treated as reasonably established only after a comprehensive study of a large number of auditoria. Recently a number of auditoria have been constructed in various parts of the country for drama, music and dance performances and, in general, to give a fillip to music and the performing arts. The authors have been associated with acoustical designing of a number of these halls. This paper describes one of these halls, viz. Tagore Theatre at Chandigarh (Fig 1).

The Tagore Theatre is a medium sized hall intended for drama, music, and dance performances as well as for lectures. The main design was prepared by Shri Aditya Prakash, Architect of the Capital Project, Chandigarh, while the acoustical features were designed in consultation with one of the authors.

The acoustical criteria mentioned earlier have been kept in view in these designs and the actual performance characteristics have been verified by a detailed acoustic analysis.

Design objectives

In the design of a hall, the volume per person of the audience and the floor area in relation to the seating capacity constitute important basic criteria. Normally, the optimum seating area per person is taken as 0.65 to 0.85 m² (7-9 Ft²). In the present case, it was decided to provide 0.75 m² (8 ft²) per person for the proposed seating capacity of about 600 persons, giving a floor area of about 450 m² (4800 ft²). The choice of volume on the other hand is determined in terms of the type of performance for which the hall is to be used. For musical concerts, a total volume corresponding to 7 m³ (250 ft³) per person is considered satisfactory, whereas for speech and dramatic performances a lower limit of 4.25 m³(150 ft³) is acceptable. In the present case, as the hall was intended for various purposes, a compromise value of about 6 m³ (200 ft³) per person was provided for and an average ceiling height of 6 m (20 ft) was planned. It was proposed to accommodate the audience on a single floor with adequate gradient and no balcony was incorporated in the plans. The object of adopting the volume mentioned above was to obtain the optimum compromise between reverberation and articulations characteristics. The reverberation time for music in a hall of this size is required to be between 1.4 and 1.6 seconds, while for speech and dramatics reverberation time of about 1 second is considered satisfactory. While a lower reverberation time is preferred for good articulation, a comparatively high reverberation time is necessary for building up a sufficient volume of sound in the hall for unaided speech without a sound reinforcement system. In view of these requirements, a value of about 1.2 seconds for reverberation time at 500 c/s for full audience, with a slight increase at lower frequencies was adopted as a compromise to satisfy all these needs. Further, no objectionable and distinct reflections were to be allowed to fall in the audience area and, at the same time, there was to be good distribution of sound in the room. It was also necessary to reinforce the direct sound by early reflections from suitable surfaces near the stage.

In a general-purpose hall, there were two considerations namely reverberation and early reflection conflict. For purposes of music, a higher level of reverberant sound and diffusion is desirable, whereas for clarity of speech general reverberation should be reduced and only the direct sound with an early reflection boost be allowed to reinforce the direct sound. Thus, these two factors had to be judiciously balanced. Further, the early reflections had to fall within the limits imposed by the Haas effect, i.e. 30-50 milli-seconds.

Size, shape and construction

In keeping with the general trend of architecture of all construction in Chandigarh, the theatre is designed in a novel manner, as shown in plan and section in Fig 2. The main hall is located on the upper floor of the building, the ground floor being occupied by the foyer, while the basement contains rooms for heavy machines and air-conditioning equipment. The comparatively small size and judicious proportions of the hall provide maximum intimacy between the stage and the audience – the seating area being all on one floor with an upward raking for good visibility and acoustics as seen in Fig 3. The hall plan is in the shape of tow linked squares with the stage area at the intersection of the loops, keeping the length of the hall to a minimum and thereby maintaining the required intimacy between the stage and the audience. Three partitions are provided to square off the three corners of the hall.

The main hall has a volume of 2,700 m³ (95,000 Ft³) while the stage provides an additional coupled volume of 850 m³ (30,000 Ft³). A canvas screen and draperies divide the acting area from the rear portion which consists of the property area and dressing rooms.

Surface treatment

In order to improve clarity and intelligibility. It was planned to augment the sound intensity in the audience area by early reflection surfaces which would supplement the direct sound. These surfaces had to be so arranged that the reflected sound would arrive within less than 50 milli-seconds of the direct sound. The surfaces used for this purpose are a wooden canopy over the proscenium opening and the two side partitions mentioned earlier, which are all made of thick plywood paneling on a suitable framework.

The roof is of cast concrete slab supported over a trussed framework. In order to provide a better diffusion of sound and to provide a novel architectural feature it was decided to eliminate the conventional flat false ceiling and to leave the trusses exposed, as seen in Fig 4. To adjust the reverberation time, the absorbing material had to be applied on the remaining side walls only. This constitutes a deviation from the usual practice of having most of the absorption on the ceiling which has been replaced by distributed absorption. The remaining absorption required is provided by the rear walls in the form of mineral wool blankets fixed on wood battens and faced with perforated plywood panels.

The low frequency absorption is usually provided by a false ceiling in the conventional halls. In the absence of a false ceiling in the present case, use has been made of resonant absorbers to provide low frequency absorption. These are in the shape of earthen pitchers of the required size embedded in bitumenised mud plaster, the whole forming the partition in the rear corner. The absorption values for these resonators have been estimated from laboratory measurements on similar small panels. These remains a large volume behind the cloth cyclorama which acts as a partly coupled enclosure and increases the reverberation of the stage shell. This space has, therefore, been treated with wood-wool fixed with an air gap on the walls and ceiling.

Reverberation time

The values given in Table 1 were used to compute the value of reverberation time of the hall as shown in Fig 5. The reverberation time was estimated to vary from about 1.2 seconds for a full audience to 1.4 seconds for a two-thirds full audience and 1.7 seconds for a one-third full audience. While the reverberation time of 1.2 seconds for a full audience agrees well with the required optimum value, the reverberation time for a smaller audience tends to be slightly higher. This is in large measure due to the fact that the reverberation time in a small hall of this size is controlled. Usually this variation with audience is largely compensated for by the use of heavily upholstered chairs. In the present case, however, it was desired, from other considerations to have cane chairs only with the result that change in audience size resulted in a noticeable variation in reverberation time. The measured values of reverberation time in the hall are shown in Fig 5. These represent the mean of hundreds of observations taken in various position in the hall. The measured values are in fairly good agreement with the predicted values. The slight difference between the predicted and measured values can be ascribed to the uncertainty in estimating the precise values of absorption by the resonant absorbers in situ, as also the behavior of large surface areas of the floor and ceiling broken up by non-absorbing but regularly spaced irregularities in the form of row of chairs and exposed trussed beams.

A study of the decay curves as shown in Fig 6 shows that there are no pronounced modes of oscillation in the hall.

In order to study the variation of reverberation time with frequency in greater detail, measurements were made over the frequency range 100 c/s to 8,000 c/s at intervals of 1/3 octave. The resulting curve of reverberation time against frequency shows a smooth variation of the same general nature as shown in Fig 5. It is thus seen that there is no selective absorption or pronounced resonate mode over the frequency range.

Sound distribution

The sound level distribution in the hall is fairly satisfactory, with a maximum variation of 8 dB between the rear and front listener positions, as shown in Fig 7.

Echoes, resonances, etc

It was felt that the novel shape of the hall might give rise to reflections causing flutter or echoes. To avoid this possibility, partitions have been provided at the corners and absorbent treatment given to the rear walls. To test the effectiveness of these measures a check was made using impulse tones of short duration. No objectionable echoes were detected.

Pronounced resonance modes in the audible frequency range are also detrimental to good listening conditions. Tests conducted to detect this possibility gave negative results.

Noise

The hall is situated within a large compound in a quite locality at a sufficient distance from the main road. The road does not carry much heavy traffic, nor are there any other nearby sources of noise. There is thus no problem of noise from outside the hall. The background noise level in the hall is less than 35 phons.

The air-conditioning plant for the hall is located in the basement below the hall. Although the plant has been floated in the usual manner for vibration isolation and the plant room treated with absorbents, the treatment at the time of measurements was not found to be adequate enough to prevent the noise from being transmitted to the hall through the floor slab and through the ducts with the result that the background noise level rises considerably.

Speech intensity

A simple estimate of the sound intensity created in a hall by an average speaker can be made, assuming a uniform sound distribution in the hall. Assuming a speaker to deliver 200 microwatts of sound energy, the sound intensity in the present hall with full audience would be of the order of 55 dB. In practice, this level may be raised to about 65 dB by a judicious use of early reflection surfaces. Although this level is quite adequate against a quite background, the intelligibility may not be adequate if the background noise level due to audience and air-conditioning system becomes comparable. In such cases, the use of a sound reinforcement system will be helpful.

Subjective appraisal

Listener opinion in the completed hall has been elicited to determine the acoustic performance of the hall on a subjective basis. For music and for dance performances with musical accompaniment, the hall has been found to be entirely satisfactory. For speeches and dramatic performances, the general opinion has been that clarity and intelligibility are good over most of the area. Some listeners would, however, prefer to have a sound reinforcement system to be installed to be able to hear low intensity sounds above the background noise level.

Conclusion

The theatre with its several novel and unconventional features provides for an audience of 600 persons with a volume of 6 m³ per person and a seating area of 0.75 m² per person. The diffusion and distribution is satisfactory with a reverberation time of 1.2 seconds at 500 c/s for full audience which is good compromise for music and speech. The characteristics, in general, are in conformity with the design calculations.