|The Science of Domestic Concert Hall Design|
by Ralph Glasgal
AES 24th International Conference on Multichannel Audio 1
Concert Hall Acoustics For Posterity
1.3 The microphones
Three different microphonic probes were employed:
All these microphones were installed over a rotating table, in such a way that the rotation center passed through the center of the dummy head, and through the point at the intersection of the axes of the two cardioids (which were mounted just above the dummy head). alternatively the Soundfield microphone was displaced exactly 1m from the rotation axis, in front of the dummy head.
The rotating table (Outline ET-1) was programmed for stopping each 10ƒ, and consequently along a complete rotation 36 discrete sets of impulse responses were measured at each position of the microphonic array. Fig. 6 and 7 show photographs of the microphone setup.
1.4 Computer and sound card
The measurement method required the usage of a topgrade sound card, equipped with 8 analog inputs at 24 bits / 96 kHz, incorporating digitally controlled mic preamplifiers (for ensuring accurate control of the input gain, and relative and absolute calibration of the recordings). At the moment, these requirements can only be fulfilled by external rack-mounted units, connected to the computer by means of a PCI card. This impeded the usage of any portable computer, and forced the choice of the only currently-available fanless PC, which stands out for its completely silent design: the Signum Data Futureclient.
The model employed for this research mounts ax 1.8 GHz P-IV processor, and is equipped with 512 Mbytes Ram and an high speed (7200 RPM) hard disk. This allows for faultless operation when recording 8 channels and playing 2 channels at 96 kHz, 24 bits.
The sound card chosen for the task is an Aadvark Pro- Q10. Fig. 8 shows a picture of the equipment, which is installed inside a couple of fly-cases for easy transportation.
1.5 Measurement method
CoolEditPro was employed for the playback of the test signals and the simultaneous recording of the 8 microphonic channels. The test signal was looped 36 times, corresponding to the 36 steps of the rotating table along a complete rotation.
The following picture shows a multi-track session, resulting from a measurement with the above-described approach.
Each measurement takes approximately 15 minutes (25s x 36 repetitions); after the measurement is complete, Another 10 minutes are required for storing all the waveforms on the hard disk (in 32-bits format, for preserving all the available dynamic range); during this time, the source and/or the microphonic array are displaced into another position.
1.6 Measured data
At the time of writing, 9 famous theaters were measured with the previously described method, as reported in the following table.
However the number of rooms being measured is increasing quickly, and it is planned to reach at least 30 different rooms in less than 6 months. The goal of this paper is not to present a comprehensive comparative study of the measured data, which will follow when the collection of impulse response responses is complete, and all the results are fully analyzed.
However, the next figure shows a set of 36 impulse responses measured in the Auditorium of Parma, for giving an idea about the format in which the data are stored: for each microphone pair (Neumann ORTF in this case) the 36 impulse responses measured during the microphone rotation are stored one after the other, and the sequence is saved as a 32-bits float WAV file.