1	RECORDING CONCERT HALL ACOUSTICS FOR POSTERITY Angelo Farina ⁽¹⁾ – Regev Ayalon ⁽²⁾ ⁽¹⁾ Dipartimento di Ingegneria Industriale, Università di Parma, Via delle Scienze 181/A Parma, 43100 ITALIA HTTP://pcfarina.eng.unipr.it - mail: farina@unipr.it ⁽²⁾K.S. Waves Inc., Azrieli Center, Tel Aviv, ISRAEL HTTP://www.waves.com - mail:regev@waves.com
2	Background The title of this paper is exactly the same employed by Michael Gerzon in its JAES paper (Vol. 23, Number 7, 1975) He first proposed to collect impulse responses measured in famous theatres, with a microphone capable of capturing the complete spatial information This paper is consequently basically a tribute to M.Gerzon, who had foreseen most of the modern multichannel audio applications, including impulse response measurements and auralization obtained by convolution.
3	Goals The main goal is to measure an huge collection of impulse response in famous theatres, concert halls, cathedrals, etc. These impulse responses have two main uses: In case something happens to the original space (remember the case of La Fenice theater) they contain a detailed “acoustical photography” which is preserved for the posterity They can be used for studio sound processing, as artificial reverb and surround filters for today’s and tomorrow’s musical productions
4	Topics Description of the measurement technique Analysis of some acoustical parameters of the first theaters already measured Description of the processing methods to be employed for transforming the measured data in audible reconstructions of the original spaces Description of the usage of the measured data for studio processing and production
5	Sound propagation in rooms
6	Measurement process The desidered result is the linear impulse response of the acoustic propagation h(t). It can be recovered by knowing the test signal x(t) and the measured system output y(t). It is necessary to exclude the effect of the not-linear part K and of the background noise n(t).
7	Test signal: Log Sine Sweep x(t) is a sine signal, which frequency is variable exponentially with time, starting at f₁ and ending at f₂.
8	Deconvolution of Log Sine Sweep The “time reversal mirror” technique is emplyed: the system’s impulse response is obtained by convolving the measured signal y(t) with the time-reversal of the test signal x(-t). As the log sine sweep does not have a “white” spectrum, proper equalization is required
9	Test Signal – x(t)
10	Measured signal - y(t) The not-linear behaviour of the loudspeaker causes many harmonics to appear
11	Inverse Filter – z(t) The deconvolution of the system’s impulse response is obtained convolving the measured signal y(t) with the inverse filter z(t) [equalized, time-reversed x(t)]
12	Result of the deconvolution The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders
13	Measurement Setup The measurement method incorporates all the known techniques: Binaural B-format (1^st order Ambisonics) WFS (Wave Field Synthesis, circular array) ITU 5.1 surround (Williams MMA, OCT, INA, etc.) Binaural Room Scanning M. Poletti high-order virtual microphones This measurement setup has been named “Waves2003”, as it is being employed for the collection of impulse response to be employed together with the new convolution software being developed by KS Waves ltd.
14	“Waves2003” Measurement Parameters Test Signal: pre-equalized sweep
15	Transducers (sound source #1) Equalized, omnidirectional sound source: Dodechaedron for mid-high frequencies Subwoofer
16	Transducers (sound source #2) Genelec S30D reference studio monitor: Three-ways, active multi-amped, AES/EBU Frequency range 37 Hz – 44 kHz (+/- 3 dB)
17	Transducers (microphones) 3 types of microphones: Binaural dummy head (Neumann KU-100) 2 Cardioids in ORTF placement (Neumann K-140) B-Format 4 channels (Soundfield ST-250)
18	Other hardware equipment Rotating Table: Outline ET-1
19	Measurement procedure A single measurement session play backs 36 times the test signal, and simultaneusly record the 8 microphonic channels
20	Theatres measured
21	Uhara Hall, Kobe, Japan
22	Noh theater, Kobe, Japan
23	Kirishima Concert Hall, Japan
24	Kirishima Concert Hall, Japan
25	Greek Theater in Siracusa
26	Roman Theater in Taormina
27	Parma Auditorium, Italy
28	Rome Auditorium, 700 seats
29	Rome Auditorium, 1200 seats
30	Rome Auditorium, 2700 seats
31	Bergamo’s Cathedral, Italy
32	Teatro Valli, Reggio Emilia, Italy
33	Acoustical Parameters
34	Acoustical Parameters Strenght:
35	Analysis of spatial attributes
36	Polar diagrams of IACC and (1-LF)
37	Auralization by convolution The basic method consists in convolution of a dry signal with a set of impulse responses corresponding to the required output format for surround (2 to 24 channels). The convolution operation can nowadays be implemented very efficiently on a modern PC through an ancient algorithm (equally-partitioned FFT processing, Stockam 1966).
38	Auralization types Stereo (ORTF on 2 standard loudspeakers at +/- 30°) Rotation-tracking reproduction on headphones (Binaural Room Scanning) Full 3D Ambisonics 1^st order (decoding the B-format signal) ITU 5.1 (from different 5-mikes layouts) 2D Ambisonics 3^rd order (from Mark Poletti’s circular array microphone) Wave Field Synthesis (from the circular array of Soundfield microphones) Hybrid methods (Ambiophonics)
39	ORTF Stereo Playback occurs over a pair of loudspeakers, in the standard configuration at angles of +/- 30°, each being fed by the signal of the corresponding microphone
40	Binaural (Stereo Dipole) Reproduction occurs over 2 loudspeakers angled at +/- 10°, being fed through a “cross-talk cancellation” digital filtering system
41	Ambisonics 3D 1^st order Reproduction occurs over an array of 8-24 loudspeakers, through an Ambisonics decoder
42	ITU 5.1 surround Williams MMA
43	ITU 5.1 surround OCT
44	Virtual high-order microphones (M. Poletti) One of the two ORTF cardioid is employed, which samples 36 positions along a 100 mm-radius circumference
45	Wave Field Synthesis (WFS) Flow diagram of the process
46	Hybrid methods (Ambiophonics) Ambiophonics 3D (10 loudspeakers):
47	Conclusions Main advantages of the new measurement method “Waves 2003”: Almost all previously known measurement techniques are incorporated in a single, coherent approach The spatial informations are accurately sampled, making it possible to store, analyze and preserve these “3D acoustical photographies” of existing musical spaces for the posterity The impulse response are stored in many different formats, allowing for their usage for surround productions with today technlogies (ITU 5.1, 1st order Ambisonics) and future, more advanced methods (high order Ambisonics, WFS, Ambiophonics)