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AES 24th International Conference on Multichannel Audio 1

Recording Concert Hall Acoustics For Posterity
Angelo Farina1, Regev Ayalon2
1 Industrial Engineering Dept., University of Parma, Italy
farina@unipr.it
2 K.S. Waves Inc., Tel Aviv, ISRAEL
regev@waves.com

2.5 B-format spatial criteria (Lateral Fractions)

The ISO 3382 standard defines two spatial descriptors derived by a B-format impulse response (more precisely, by the W and Y channels of a B-format 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 pressure-intensity index usually employed in applications of sound intensity measurement system (ISO9614).

Also for these B-format based parameters a special plugin was developed: its user interface is shown in the next figure.


Figure 14: Lateral Fraction parameters plugin

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 B-format 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 1-LF) 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.

 
Figure 15: Polar Plots of (1-LF) in Parma and Rome

It must be observed that employing (1-LF) 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 1-LF 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 B-format 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 (1-LF) for the two cases already reported on fig. 13 and 15:

Auditorium

IACC

1-LF

Parma

0.266

0.725

Rome

0.344

0.676

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 high-end impulse responses has shown how this standard, albeit having been updated in 1997, already requires substantial revision. In practice, three main topics require refinement:

  • The standard does not give proper indications for sweep-based measurements, nor discusses the issues which make the sweep method preferable to MLS (time invariance, non-linearity, clock mismatch tolerance, etc.)
     

  • Almost all parameters are said to be related to the ėacoustical energyî, but they are actually computed over the squared pressure. From a Bformat measurement, instead, the true values of active intensity and sound energy density are available. And it is well known how, in a partially reactive sound field, the true energetic parameters can differ significantly from the estimates based on the squared pressure.
     

  • The definition of the spatial parameters (either based on binaural or B-format impulse responses) assumes a specific orientation of the microphone, pointing to the sound source. This is meaningless in presence of multiple sources, or in rooms equipped with sound reinforcement systems. Also in case of a single point source, these parameters give contradictory results.

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