|The Science of Domestic Concert Hall Design|
by Ralph Glasgal
AES 24th International Conference on Multichannel Audio 1
Concert Hall Acoustics For Posterity
2 EXTRACTION OF OBJECTIVE ACOUSTICAL PARAMETERS
Basically, the computation of objective acoustical parameters is based on the ISO 3382-1997 standard. Most parameters are computed from an impulse response captured with an omnidirectional microphone, which is substantially the channel W of the Soundfield microphone, at the initial position (0 degrees).
However, the spatial parameters require processing stereo impulse responses: consequently, also the binaural and the WY pair had to be processed.
This research is devoted mainly to capturing and analyzing the spatial properties of the sound field, with the goal of creating realistic multichannel surround reconstructions: consequently the greater effort was reserved for the analysis of the spatial parameters.
The highly innovative result made available from the new measurement technique is the possibility to measure and display polar plots of the spatial acoustical parameters, showing their variation along with the rotation of the receiver.
2.1 Reverberation time
The W channel of the B-format impulse response is employed (omnidirectional). The impulse response is first backward-integrated, following the Schroeder method, and applying the noise-removal allowed by the ISO 3382 standard.
Then the reverberation time T30 is computed, by means of a linear regression over the decay curve in the range between ˝5 and ˝35 dB below the steady-state level before the decay. It must be noted that usually these impulse responses are so clean and noiseless that it would be possible to measure directly the T60 (in the range ˝5 to ˝65 dB), but the ISO3382-1997 standard does not allow for this (it was written when measurement of impulse responses with such high dynamic range was very difficult to obtain).
Fig. 11 shows a typical plot of the impulse response and of the backward-integrated decay curve obtained in one of the theaters objects of this research.
The picture shows that the total integrated sound pressure level is approximately 90 dB above the steady background noise present after the impulse response is finished.
2.2 Monophonic temporal criteria
Although the reverberation time is the most important criterion for evaluating the acoustical behaviour of a room, it is often advisable to get a better insight about the fine temporal distribution of the acoustical energy. For this goal, the ISO 3382 standard suggests the usage of 4 temporal-monoaural criteria: C50, C80, D, Ts.
C50 is the Clarity over 50ms, evaluated by applying the following formula over the measured omnidirectional pressure impulse response, and starting from the arrival time of the direct sound:
C80 is similar, but the time boundary is moved from 50 ms to 80 ms. Usually C50 is considered more representative of the clarity of speech, whilst C80 is more relevant for assessing clarity of the instrumental music.
D is somewhat similar to C50, but it is expressed in % instead of in dB, following this equation:
Finally, the Center Time Ts is defined as:
Which has the advantage of avoiding a steep separation between the ýearlyţ and ýlateţ energy, inherent in the definition of C and D.
The computation of all the above parameters, and of the reverberation time, is made thanks to a proper plugin, developed with the goal of automatizing the computation of the ISO 3382 Acoustical Parameters.
Fig. 11 shows the userÝs interface of this plugin.
2.3 Absolute and relative sound pressure level
As the acoustical power of the sound source was carefully
calibrated thanks to the anechoic-room measurements, and having care of keeping
track of the gain applied in the microphone preamplifiers, it is possible to
know with reasonable accuracy
Furthermore, as the deconvolution of all the impulse responses of a given theater is done employing the same rescaling factor, the displayed amplitude of the impulse responses preserves a relative scaling. The difference between the absolute SPL and the radiated sound power level Lw allows for the computation of a very relevant acoustical parameter, the Strength G:
The corrective factor of +31 dB derives by the definition of G, which refers to the difference between the measured SPL inside the room and the theoretical SPL measured in free field, at a distance of 10m from the source.
2.4 Binaural spatial criteria (IACC)
Following AndoÝs theory , the basic binaural parameter is the Inter Aural Cross Correlation (IACC), defined as the maximum value of the Normalized Cross Correlation function:
Other related parameters are tIACC and wIACC, defined respectively as the delay (in ms) of the maximum value of the normalized cross correlation function, and as the width of the peak (at 10% of the maximum) in ms.
A special plugin was created for measuring the IACCbased parameters. This plugin also computes the time delay gap between direct sound and first reflection, and the Tsub (subsequent reverberation time), conforming to the AndoÝs theory. Fig. 12 shows the userÝs interface of this plugin.
Traditionally, this measurement is performed when the binaural dummy head is pointed directly towards the sound source. In this case, however, the head is pointed in 36 different directions, with 10â steps. Consequently, 36 values of IACC are obtained, and it is possible to create a polar plot of IACC.
The availability of these polar plots is new, and it is yet to be evaluated what information can be extracted from them. What immediately appeared, however, is that rooms with almost the same value of the ýstandardţ IACC can have quite different polar plots, showing that the ýsurroundţ properties of the room are not completely described by the old-style, single-valued ýstandardţ IACC.
This is proven by the comparison of the polar plots reported in Fig. 13, which refers to the Auditorium of Parma vs. the Auditorium of Rome. In the latter, the sound appears to be more strongly ýpolarizedţ, whilst in the Auditorium of Parma it is more diffuse.