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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

3.7 Hybrid methods (Ambiophonics)

The Ambiophonics method is an hybrid solution, aimed to mask the defects of two basic systems: cross-talk cancelled reproduction of binaural material over closely-spaced loudspeakers (Stereo Dipole) and 3D surround driven by convolution of corresponding oriented virtual microphones.

The following figure shows a typical Ambiophonics array, (frontal stereo dipole, plus 8-loudspeakers surround rig).


Figure 22: Ambiophonics array

The theory for deriving the signals for these loudspeakers has been already presented in the previous chapters, and the assembly of the whole system has been thoroughly described in [5]. The only point which deserves discussion here is the fact that, in an Ambiophonics system, the Stereo-Dipole loudspeakers should provide only the direct sound and early reflection from the stage enclosure, whilst the other ģsurroundī loudspeakers should provide the late reflections and the reverb.

This means that the measured impulse responses need to be properly edited: the ORTF ones, which are employed for the Stereo Dipole, need to be cut smoothly just after the direct sound. On the other hand, the B-format impulse responses, from which the surround channels are derived, need to be deprived of the direct sound. The management of this editing is quite delicate, because, if it is done improperly, it can cause an improper merging between the two basic systems, or can introduce artificial delays which alter the temporal distance between the direct sound and the subsequent reverberation.

The final remark regards the selection of the impulse responses for driving the ģsurroundī array. In [5], these IRs were all derived from a single B.-format impulse response, simply employing Visual Virtual Microphone and pointing the virtual microphone in a direction corresponding to that of the corresponding loudspeaker. Now, the availability of many B-format impulse responses along a circle, makes it possible to select, for any ģsurroundī loudspeaker, not only the direction of the virtual microphone, but also a corresponding position of it along the circumference.

This ameliorates significantly the results, because this way the impulse responses are sampled in different positions, and are mutually incoherent. This avoids interference and artifacts due to the interaction of signals coming from many loudspeakers, all fed with strictly correlated signals.

4. CONCLUSIONS

This paper has described a new, advanced measurement technique, which allows for capturing the widest possible acoustical information inside an existing theatre. The method is based on the measurement of a huge number of impulse responses, by means of a rotating microphonic set-up.

From the set of data measured, it is possible to derive subsets of impulse responses suitable for the reproduction of the virtual acoustic space, following the currently available reproduction technologies. Referring in particular to the reproduction of the spatial properties of the sound field, it is noticeable that the measured data allow for the auralization of the results employing:

  • Standard stereo reproduction over a pair of loudspeakers;

  • Binaural reproduction over headphones, with head tracking;

  • Reproduction over closely-spaced loudspeakers (by means of cross-talk cancelling filters);

  • Ambisonics reproduction over a 2D or 3D regular array of loudspeakers

  • ITU 5.1 ģsurroundī reproduction conforming to ģstandardī microphonic setups (OCT, INA, etc.)

  • High directivity, multichannel reproduction by means of Mark Polettiķs circular-array method.

  • Wide-area auralization by means of the Wave Field Synthesis approach (WFS)

  • Any combination of the above methods, resulting in hybrid, higher level surround methods (Ambiophonics, Panorambiophonics and derivations).

Consequently, this method provides the best available approach for storing the acoustical properties of famous and valuable rooms, such as concert halls and theatres, and preserving them for the posterity. The resulting data can be used for audible reconstructions (auralization) by means of todayķs surround systems, without limiting the future usage by sticking to the limited reproduction technology currently available.

On the other hand, the measured sets of data can immediately be employed for high-quality processing of dry recordings, outperforming current ģartificialī reverberation and spatialization units, if employed together with a state-of-the art convolution software.

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