The Science of Domestic Concert Hall Design  
by Ralph Glasgal 




AES 24th International Conference on Multichannel Audio 1
Recording
Concert Hall Acoustics For Posterity 2.5 Bformat spatial criteria (Lateral Fractions) The ISO 3382 standard defines two spatial descriptors derived by a Bformat impulse response (more precisely, by the W and Y channels of a Bformat impulse response), called respectively LF and LFC. LF is the ratio between the early lateral sound and the omnidirectional sound:
For the application of the above formula to the measurement with a Soundfield microphone, it must be noted that the X axis should be horizontal and pointing towards the sound source, the Y axis is horizontal and orthogonal to X pointing in the direction of the left ear, and the Z axis is pointing to the ceiling. Furthermore, it is necessary to compensate for the fact that the W channel (omni) has a gain 3 dB lower than X, Y and Z. The second parameter, LFC, is defined by:
In this case the numerator equals the Sound Intensity, whilst the denominator equals the squared RMS sound pressure. In substance, LFC is a parameter quite close to the definition of the pressureintensity index usually employed in applications of sound intensity measurement system (ISO9614). Also for these Bformat based parameters a special plugin was developed: its user interface is shown in the next figure.
It must be noted that the plugin also computes the Jordanís LE (Lateral Efficiency) parameter [13], which definition resembles LF, but with a starting time limit for the integral at numerator equal to 25ms instead of 5ms. As the Soundfield microphone can be ėvirtually rotatedî around its axis, it is easy, from a single Bformat impulse response, to compute a complete polar plot of LF. But the microphone was not simply rotated, it was displaced along a circumference with 1m radius. So, taking for each microphone position the radial orientation of the microphone, it is also possible to build a modified polar plot, which shows the variation of LF (or 1LF) along the circumferential path described by the microphone. The following picture shows these polar plots for the same two rooms already analyzed with the IACC. It must be observed that employing (1LF) the parameter has the same polarity as IACC, so the polar plots of Fig. 15 are directly comparable to those of fig. 13. Also in this case it is quite evident how the sound field is much more diffuse in the Parma Auditorium, whilst in Rome Auditorium the sound is more polarized. In the second, furthermore, there is a small angular sector where LF is almost unitary (and consequently 1LF is zero). Analyzing the results, shows little significance for the parameter LFC (which is always very little, independent of the room and of the orientation of the probe) and the weak dependence on the orientation of the probe of LE. LF is confirmed to be the more sensitive parameter based on Bformat impulse responses, although it is also clear how the ranking of the spatial impression based on LF does not necessarily correspond with the ranking based on IACC. The following table compares the values of IACC and (1LF) for the two cases already reported on fig. 13 and 15:
From the above table, looking at IACC Parma seems to have greater spatial impression than Rome, whilst looking at LF the opposite judgment is obtained. This means that the information about sound diffusion derived from these two descriptors can be misleading, and that the true evaluation of the two rooms actually characterized by a more enveloping soundfield cannot be derived just by the parameters computed pointing the microphones towards the sound source, but instead requires one to analyze the variation of the spatial parameters when the microphones are rotated in all directions. The subjective listening experience of the authors clearly indicates, in the above two cases, that the Parma Auditorium is significantly more diffuse than the ėsala 1200î of the Rome Auditorium, and the same conclusion appears evident when comparing the polar plots, both in fig. 13 and in fig.15. 2.6 Criticism of ISO3382 parameters Applying the ISO 3382 parameters to these highend impulse responses has shown how this standard, albeit having been updated in 1997, already requires substantial revision. In practice, three main topics require refinement:
<< Previous Page  Next Page >> Article Pages
1  2
 3  4 
5  6 
7  8
 9 