1	Advanced techniques for measuring and reproducing spatial sound properties of auditoria Angelo Farina ⁽¹⁾and Lamberto Tronchin ⁽²⁾ ⁽¹⁾ Dipartimento di Ingegneria Industriale, Università di Parma, Via delle Scienze 181/A Parma, 43100 ITALY HTTP://pcfarina.eng.unipr.it - mail: [email protected] ⁽²⁾DIENCA - CIARM, Università di Bologna, Viale Risorgimento 2 Bologna, 40136 ITALY HTTP://www.ciarm.ing.unibo.it - mail: [email protected]
2	Topics This paper is a tribute to M. Gerzon, who had foreseen 3D impulse response measurements and 3D Auralization obtained by convolution. The advantages (and disadavantages) of employing measured IRs Comparison between Auralizations based on calculated and measured IRs (e.g. Theatre “La Fenice”, Venice) Different approaches to 3D Auralization
3	Concept (Gerzon, 1975)
4	The measurements of IRs: In case something happens to the original space (e.g.: 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 Several configurations of listening rooms could be developed, by means of multichannel auralization, for subjective tests
5	Theatre la Fenice, Venice The first theatre was realised in 1792 by Gian Antonio Selva, after the burning of Teatro San Benedetto In December 1836 the theatre burned down again and was rebuilt by G. and T. Meduna the year after The theatre was closed in 1995 for maintainance; it had to open again in February 1, 1996, but it burned two days before (January 29, 1996) A few weeks before the fire, L.Tronchin measured binaural impulse responses
6	Impulse Responses of La Fenice In 27 positions a series of binaural impulse responses (with gun shots) was recorded Each recording is consequently a stereo file at 16 bits, 48 kHz During measurements the room was perfectly fitted, whilst the stage was empty (no scenery)
7	Example n. 1 – La Fenice Dry music Convolution with experimental I.R. (pt. 12) Convolution with simulated IR
8	Example n. 2 – Paris vs ...la petit Paris Citè de la Musique, Parigi
9	Advanced IR capture and rendering ( project) Description of the measurement technique Analysis of some acoustical parameters of some theaters 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, musical production and for scientific Auralization tests
10	Sound propagation in rooms
11	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).
12	Test signal: Log Sine Sweep x(t) is a sine signal, which frequency is varied exponentially with time, starting at f₁ and ending at f₂.
13	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
14	Test Signal – x(t)
15	Measured signal - y(t) The not-linear behaviour of the loudspeaker causes many harmonics to appear
16	Inverse Filter – z(t) The deconvolution of the IR is obtained convolving the measured signal y(t) with the inverse filter z(t) [equalized, time-reversed x(t)]
17	Result of the deconvolution The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders
18	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 Any multichannel auralization systems nowadays available is supported
19	Measurement Parameters Test Signal: pre-equalized sweep
20	Transducers (sound source #1) Equalized, omnidirectional sound source: Dodechaedron for mid-high frequencies One-way Subwoofer (<120 Hz)
21	Transducers (sound source #2) Genelec S30D reference studio monitor: Three-ways, active multi-amped, AES/EBU Frequency range 37 Hz – 44 kHz (+/- 3 dB)
22	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)
23	Other hardware equipment Rotating Table: Outline ET-1
24	Measurement procedure A single measurement session play backs 36 times the test signal, and simultaneusly record the 8 microphonic channels
25	Theatres measured
26	Uhara Hall, Kobe, Japan
27	Noh theater, Kobe, Japan
28	Kirishima Concert Hall, Japan
29	Kirishima Concert Hall, Japan
30	Greek Theater in Siracusa
31	Roman Theater in Taormina
32	Parma Auditorium, Italy
33	Rome Auditorium, 700 seats
34	Rome Auditorium, 1200 seats
35	Rome Auditorium, 2700 seats
36	Bergamo’s Cathedral, Italy
37	Teatro Valli, Reggio Emilia, Italy
38	Sydney Opera House
39	Sydney Opera House – opera theatre
40	Sydney Opera House – concert hall
41	Sydney Opera House – the studio
42	Teatro Regio in Parma (Italy)
43	Acoustical Parameters (ISO 3382)
44	Acoustical Parameters (ISO 3382) Strength:
45	Analysis of spatial attributes
46	Polar diagrams of IACC and (1-LF)
47	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).
48	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)
49	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
50	Binaural (Stereo Dipole) Reproduction occurs over 2 loudspeakers angled at +/- 10°, being fed through a “cross-talk cancellation” digital filtering system
51	Binaural (Stereo Dipole#2)
52	Binaural (Stereo Dipole#3)
53	Binaural (Dual Stereo Dipole)
54	Binaural (Dual Stereo Dipole#2)
55	Ambisonics 3D 1^st order Reproduction occurs over an array of 8-24 loudspeakers, through an Ambisonics decoder
56	Rooms for Ambisonics 3D 1^st order
57	ITU 5.1 surround Williams MMA
58	ITU 5.1 surround OCT
59	Virtual high-order microphones (M. Poletti) One of the two ORTF cardioid is employed, which samples 36 positions along a 110 mm-radius circumference
60	Wave Field Synthesis (WFS) Flow diagram of the process
61	Hybrid methods (Ambiophonics) Ambiophonics 3D (10 loudspeakers):
62	Hybrid methods (Ambiophonics#2)
63	Conclusions The spatial informations have to be accurately sampled, making it possible to store, analyze and preserve these “3D acoustical photographies” of existing musical spaces for the posterity Many different kinds of impulse-response measurements are required for different 3D auralization methods: a proper set-up should include all different approaches Once the impulse responses are stored in suitable multichannel formats, they become available for surround productions with today technologies (ITU 5.1, 1st order Ambisonics) and future, more advanced methods (high order Ambisonics, WFS, Ambiophonics) The only point which requires substantial enhancements: sound sources used for IR measurements
64	Future enhancements Sound source for realistic emulation of an human singer
65	Future enhancements Omnidirectional sound source with enhanced power frequency response
66	Acknowledgements This research was started thanks to the support of Waves, Tel Aviv, Israel (www.waves.com) For years 2004 and 2005 the research is also supported by the Italian Ministry for the University and Research (MIUR) During 2004 a new web site will be started: www.acoustics.net This will provide free access to the whole data-base, and it will be possible to add contributions.