Page 13 - Winter 2011

Fig. 1. Room impulse response measured on the main floor of an opera house with a loudspeaker sound source in the pit.
current parameters that tend to limit and constrain our view of concert hall acoustics.
Imperfections of the “perfect” omnidirectional sound source
The vast majority of acoustics parameters used by room acousticians are based on the use of omnidirectional sound sources. An omnidirectional source is understandably attrac- tive from a theoretical point of view, since it should most eas- ily allow different spaces to be compared. After all, if two dif- ferent rooms are excited by the same sound source emitting equally in all directions, then should this not clearly reveal the aural differences between the two halls? This is not nec- essarily so...
Musical instruments are generally highly directional,
with their sound radiation patterns varying with frequency in
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complicated ways. Loudspeakers used in amplified perform-
ances are not omnidirectional by design. Listeners perceive aural phenomena in concert halls that are influenced strong- ly by the sound directivity of the source. For example, a rear wall echo is a simple acoustical phenomenon that can be experienced even by casual listeners in an auditorium, and is, at times, considered to be a room acoustical defect. Rear wall echoes are most noticeable with highly directional sound sources, such as a trumpet or directional loudspeaker aimed at the rear wall. If measured with an omnidirectional sound source, however, an echo may not be especially noticeable— even to a discerning eye studying the energy decay curve— simply because there is not enough sound energy concentrat- ed into the direction of interest.
The dodecahedral loudspeaker, or “dodec,” commonly used for room acoustics measurements is not truly omnidi- rectional across its usable frequency range. These measure- ment loudspeakers begin to exhibit pronounced lobing above approximately 1 kHz,6 and the influence of this directivity on measured room impulse responses and parameters is not well understood. This also calls into question how easily high fre- quency data taken in different halls can be properly com- pared, since a slight re-aiming of the loudspeaker might sig- nificantly change the results.
While we would not argue that dodecs should be elimi- nated completely from the room acoustician’s toolbox, we do propose that our toolbox should be expanded to include other types of sound sources so that conclusions drawn from them might have closer connections with the acoustical phe-
positions on the main floor. We began by listening to music in the space during a rehearsal to become familiar with the space as a whole, and to understand in particular the com- plaints on the main floor. We noted a strong upward shift in the aural image of the pit orchestra. The strength of this upward shift was caused by a long radius dome covering most of the ceiling in front of the balcony. Clearly this was one of the sources of instrumental/vocal imbalance. Were there others?
After the rehearsal, we placed an ILG fan4 in the pit. With this venerable, yet underutilized, constant sound-power source we were able to quickly identify those areas of the main floor that received strong reflections from portions of the concave ceiling overhead. In this case, the reflections were clearly audible without any special listening device, and did not require “golden ears” to detect. Stagehands and administrators joined us in the exploration and easily could hear the effects of the focusing geometry. With a loudspeak- er sound source in the pit, we then measured the room impulse response illustrated in Fig. 1.
Figure 1 shows the direct sound arriving at 0 ms, but at a low level since there is no line-of-sight to the sound source placed in the pit. The “interesting” feature of this particular impulse response is the strong single reflection at about 83 ms followed by a cluster of slightly-less-strong reflections that all arrive between 83 and 100 ms. These arrival times correspond to path lengths from the elliptical dome ceiling in this hall. The combination of these focused ceiling reflections is the loudest sound heard at this position, and the measure- ment clearly demonstrated the upward image dislocation and balance issues that we had experienced during the rehearsal.
The subjective experience caused us to make the objec- tive measurement, but this phenomenon might have been missed if our measurements had been based only on param- eters averaged over many seat positions or if the impulse response had been integrated over time to calculate an ener- gy-based parameter. The concentration of energy in this time region was not about total loudness or clarity, but rather about confused clarity, displacement, and imbalance, none of which are addressed by typical parameters!
Most importantly, this approach gives primacy to actual- ly listening in concert halls, and if implemented more widely in the room acoustics community, we hope will result in new parameters that relate more closely to the acoustical prob- lems and issues encountered in concert halls, rather than our
In Search of a New Paradigm 9