|The Science of Domestic Concert Hall Design|
by Ralph Glasgal
AES 24th International Conference on Multichannel Audio 1
Concert Hall Acoustics For Posterity
The title of this paper is the same as a famous contribution given by Michael Gerzon on the JAES Vol. 23, Number 7 pp. 569 (1975) . After more than 25 years the problem is still open, particularly about the optimal technique for capturing the "spatial" characteristics of the sound inside an existing theatre. A novel technique is presented here, which is compatible with all the known surround formats.
When the famous and renowned Gran Teatro La Fenice in Venice burned during the night of 29 January 1996, one of the best sounding opera houses in the world suddenly disappeared. Its sonic behaviour, however, was at least partially saved, because several acoustical measurements had been performed just two months before, employing the binaural impulse response technique .
The availability of these binaural impulse responses was very relevant during the design of the reconstruction of the theatre, and demonstrated the importance of recording and storing the acoustics of concert halls for posterity.
M.Gerzon  first proposed to start a systematic collection of 3D impulse responses measured in ancient theatres and concert halls, to assess their acoustical behaviour and preserve it for posterity. His proposal found sympathetic response only very recently, with the publication of the "Charta of Ferrara"  and the birth of an international group of researchers who agreed on the experimental methodology for collecting these measurements .
Only a small number of theatres have yielded a complete three-dimensional impulse response characterization up till now.
Nevertheless, the techniques proposed for recording "3D" impulse responses, containing both temporal and spatial information, are actually being criticized for then employing this measured data in surround reproduction, through the auralization technique (convolution).
In fact, the two currently employed methods (Binaural measurements with a dummy head facing the sound source, and B-format measurement employing a Soundfield microphone) are both unsuitable for effective high-quality reproduction over "standard" multichannel reproduction systems (ITU 5.1). Other "alternative" loudspeaker arrays have been developed (based on cross-talk cancellation for the reproduction of binaural material, and on Ambisonics-like decoding for the reproduction of B-format material). In some cases, these two techniques can be coupled together, for a better 3D reproduction (Ambiophonics, ).
Recently, a completely alternative, 2.5-D technique was proposed, based on the Wave Field Synthesis theory (WFS) and the usage of a Soundfield microphone moved around on a rotating boom . Also this technique, however, is unsuitable for direct employment of the measured impulse responses over a standard surround setup.
In this paper a new measurement method is proposed, which incorporates all the previously known measurement techniques in a single, coherent approach: three different microphones are mounted on a rotating boom (a binaural dummy head, a pair of cardioids in ORTF configuration, and a Soundfield microphone), and a set of impulse responses are measured at each angular position. Fig. 1 shows a schematic of this microphone setup.
The results of this set of measurements are compatible with the already proposed methods for measurements in concert halls (binaural, B-format and WFS), but add the possibility to derive "standard surround" formats such as OCT and INA, and open the possibility to employ even the Binaural Room Scanning method  or the Poletti high-order circular microphones .
The paper describes the details of the implementation of the new measurement technique, and provides the first experimental results obtained by measurements performed in several halls.