Page 14 - Winter 2011

nomena we experience as listeners. One needs only to walk around a dodec while it is emitting pink noise to realize that its distribution pattern is a bit like an acoustical mirror ball!
Recent measurements we have made with small direc-
tional loudspeakers have proven to be useful for document-
ing particular reflection paths and highlighting surfaces of
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interest. It is especially interesting to note that the reverber-
ation time is largely independent of source directivity, while other parameters that depend heavily on the direct sound and early reflection sequence (e.g., Early Decay Time, Early/late Energy Balance, Loudness) vary strongly with source type. Perhaps this characteristic could be exploited to develop a set of “directivity dependent parameters” that would more closely resemble those experienced by listeners in a hall. Other recent work has explored the possibility of using multichannel loudspeaker arrays to simulate the sound
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Why do we squeeze our frequency range of interest?
No pianist would be content if asked not to venture lower in frequency than the octave below middle C (i.e., funda- mental frequency of approximately 125 Hz) on the keyboard, just as no recording engineer would consider limiting musi- cal recording to frequencies below 4 kHz! (See Fig. 2.) Why then, have we in the room acoustics community limited our frequency domain to the octave bands between 125 Hz and 4 kHz?
The answer probably lies in the limitations of the equip- ment historically used for room acoustics measurement. The typical dodec, probably not coincidentally, has a usable fre- quency range between about 100 Hz and 5 kHz. A large por- tion of the available room acoustics data has been collected
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using this sound source. Another major factor is that labora-
tories used for sound absorption measurements are not typi- cally qualified at lower and higher frequencies,9 and so we do not have reliable data for building materials outside the stan- dard frequency range.
Despite this, loudspeaker technology with significantly wider frequency bandwidth is readily available today. Technology will hopefully result in a greater proliferation of
data measured in concert halls that extends below the 125 Hz octave band.
What is bass, really?
It is apparent from Fig. 2 that many orchestral instru- ments have important sound energy below 100 Hz. The low frequency fundamentals of pianos, double basses, contrabas- soons, bass saxophones and bass clarinets, tympani, and bass drums all occur in this frequency region. One of the most frequent complaints heard in concert halls is that these instruments sound weak, and that the hall therefore has “poor bass response.” In the amplified music realm, sub- woofers are used to reinforce or reproduce sound energy between approximately 30 and 100 Hz. Concertgoers and mix engineers alike know this as sound that is felt as much as it is heard.
Based on these considerations, we suggest that room acousticians have largely neglected “real” bass, and any parameter or measurement that does not take into account energy below 100 Hz has little hope of being a proper meas- ure of bass response.
In our consulting practice and listening experience in concert halls throughout the world, we have observed the important role that heavy building materials play in provid- ing deep, rich, palpable bass. Old concert halls often have strong bass response not because they are old, but because they were constructed with heavy masonry walls that reflect long wavelength sounds efficiently.
As an example, London’s Royal Festival Hall was origi- nally designed with an interior lining of thin wood paneling
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The bass response improvements to Royal Festival Hall required removal of all the thin wood paneling from the upper and lower side walls and then infilling the voids with massive materials to form a bond back to the heavy concrete and brick structural walls of the original building. The other low frequency improvement was the replacement of the 35
mm shredded-wood/plaster ceiling with solid 100 mm thick plaster. Reverberation time data for this hall are shown in Fig. 3.
In Fig. 3, it is clear that low fre- quency reverberation times increased significantly after the renovation, and the improvement to bass response is profoundly appreciated.
While the reverberation data for the 63 Hz octave band in Fig. 3 seems to correspond well with our subjective impressions in this instance, we sug- gest that reverberation times may not ultimately be the best measure of bass response. Sound levels at low frequen- cies (i.e., below 100 Hz), or perhaps
The aim of this work has been to develop sound sources that excite a room in a more
realistic way than the dodec.
directivity of musical ensembles.
to provide low and low-mid frequency sound absorption. The resulting reverberation time spectrum in this hall unfor- tunately lead to complaints of poor bass response by musi- cians, listeners, and critics. Correction of this issue was a major factor leading to the acoustics renovation completed in 2007.
Fig. 2. Comparison of frequency ranges.
10 Acoustics Today, January 2011