1	Measurement and 5.0 rendering of spatial impulse responses of rooms Angelo Farina Dipartimento di Ingegneria Industriale, Università di Parma, Via delle Scienze 181/A Parma, 43100 ITALY HTTP://pcfarina.eng.unipr.it - mail: [email protected]
2	Topics This presentation is a tribute to M. Gerzon, who had foreseen 3D impulse response measurements and 3D Auralization obtained by convolution. Comparison between Auralizations based on calculated and measured IRs (e.g. Theatre “La Fenice”, Venice) The advantages (and disadavantages) of employing measured IRs Possible approaches to Auralization over ITU 5.0 “surround” systems
3	Concept (Gerzon, 1975)
4	Why do we measure these 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 (5.1) and tomorrow’s musical productions Auralization in special listening rooms can be performed 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, 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	Numerical simulation vs. measurement
8	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
9	Sound propagation in rooms
10	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).
11	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.
12	Test Signal – x(t)
13	Measured signal - y(t) The not-linear behaviour of the loudspeaker causes many harmonics to appear
14	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
15	Inverse Filter – z(t) The deconvolution of the IR is obtained convolving the measured signal y(t) with the inverse filter z(t)
16	Result of the deconvolution The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders
17	Maximum Lenght Sequence vs. Sweep
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	Transducers (sound source #1) Equalized, omnidirectional sound source: Dodechaedron for mid-high frequencies One-way Subwoofer (<120 Hz)
20	Transducers (sound source #2) Genelec S30D reference studio monitor: Three-ways, active multi-amped, AES/EBU Frequency range 37 Hz – 44 kHz (+/- 3 dB)
21	Directivity of transducers LookLine D200 dodechaedron
22	Directivity of transducers
23	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)
24	Directivity of transducers
25	Directivity of transducers
26	Other hardware equipment Rotating Table: - Outline ET-1
27	Measurement procedure A single measurement session play backs 36 times the test signal, and simultaneusly record the 8 microphonic channels
28	Theatres measured
29	Uhara Hall, Kobe, Japan
30	Noh theater, Kobe, Japan
31	Kirishima Concert Hall, Japan
32	Kirishima Concert Hall, Japan
33	Greek Theater in Siracusa
34	Roman Theater in Taormina
35	Parma Auditorium, Italy
36	Rome Auditorium, 700 seats
37	Rome Auditorium, 1200 seats
38	Rome Auditorium, 2700 seats
39	Bergamo’s Cathedral, Italy
40	Teatro Valli, Reggio Emilia, Italy
41	Sydney Opera House
42	Sydney Opera House – opera theatre
43	Sydney Opera House – concert hall
44	Sydney Opera House – the studio
45	Teatro Regio in Parma (Italy)
46	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).
47	Auralization types Stereo (ORTF on 2 standard loudspeakers at +/- 30°) Rotation-tracking reproduction on headphones (Binaural Room Scanning) Stereo Dipole (cross-talk cancellation) Full 3D Ambisonics 1^st order (decoding the B-format signal) ITU 5.1 “surround sound” systems 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)
48	Approach # 1 – separate rendering for each recorded track (source) Each of the dry recordings represents a source in a different position, so it must be separately convolved with its own set of impulse responses
49	Approach # 2 – the recorded tracks are first panned to 5.0, then the “room” is added The “room effect” is a global filtering applied to a 5.0 “dry mix” of several tracks
50	Full Auralization vs. Reverb In “full auralization” also the direct sound comes from the measured IRs
51	Software tools Gerzonic’s Ambisonics decoders (Emigrator, DecoPro)
52	Approach # 1 – single source rendering – choice of 1x5 filter set Ambisonics (1° order) from a single B-format impulse response SIRR according to Ville Pulkki (sound intensity analysis of a single B-format IR) 5 “virtual mikes” from 5 different B-format impulse responses 5 selected Neumann cardioids (future) – 5°-order Ambisonics from the whole set of cardioid impulse responses
53	Approach # 2 – the recorded tracks are first panned to 5.0, then the “room” is added
54	Approach # 2 – the recorded tracks are first panned to 5.0, then the “room” is added
55	Conclusions It is now possible to sample accurately the spatial room impulse response, making it possible to store, analyze and preserve a “3D acoustical photography” We are still learning what is the best way to render these sets of impulse responses over a standard 5.0 (or 5.1) setup The only point which requires substantial enhancement: sound sources (loudspeakers) used for IR measurements
56	Future enhancements Sound source for realistic emulation of an human singer
57	Future enhancements Omnidirectional sound source with enhanced power & frequency response
58	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) The following software tools were provided free: Adobe Audition, Gerzonic Decopro