1	La misura della risposta all'impulso per la caratterizzazione di sistemi acustici e vibrazionali
2	Time Line
3	The Past
4	Starting point: room impulse response
5	Traditional measurement methods Pulsive sources: ballons, blank pistol
6	Example of a pulsive impulse response
7	Loudspeaker as sound source A loudspeaker is fed with a special test signal x(t), while a microphone records the room response A proper deconvolution technique is required for retrieving the impulse response h(t) from the recorded signal y(t)
8	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).
9	Electroacoustical methods Different types of test signals have been developed, providing good immunity to background noise and easy deconvolution of the impulse response: MLS (Maximum Lenght Sequence, pseudo-random white noise) TDS (Time Delay Spectrometry, which basically is simply a linear sine sweep, also known in Japan as “stretched pulse” and in Europe as “chirp”) ESS (Exponential Sine Sweep) Each of these test signals can be employed with different deconvolution techniques, resulting in a number of “different” measurement methods Due to theoretical and practical considerations, the preference is nowadays generally oriented for the usage of ESS with not-circular deconvolution
10	The first MLS apparatus - MLSSA MLSSA was the first apparatus for measuring impulse responses with MLS
11	More recently - the CLIO system The Italian-made CLIO system has superseded MLSSA for most low-cost electroacoustics applications (measurement of loudspeakers, quality control)
12	The first TDS apparatus - TEF Techron TEF 10 was the first apparatus for measuring impulse responses with TDS Subsequent versions (TEF 20, TEF 25) also support MLS
13	Theory of MLS method X(t) is a periodic binary signal obtained with a suitable shift-register, configured for maximum lenght of the period.
14	MLS deconvolution The re-recorded signal y(i) is cross-correlated with the excitation signal thanks to a fast Hadamard transform. The result is the required impulse response h(i), if the system was linear and time-invariant
15	MLS example
16	MLS example
17	Example of a MLS impulse response
18	The Present
19	Today’s Hardware: PC and audio interface
20	Hardware: loudspeaker & microphone
21	The first ESS system - AURORA
22	Exponential Sine Sweep method x(t) is a band-limited sinusoidal sweep signal, which frequency is varied exponentially with time, starting at f₁ and ending at f₂.
23	Test Signal – x(t)
24	Measured signal - y(t) The not-linear behaviour of the loudspeaker causes many harmonics to appear
25	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)]
26	Deconvolution of Exponential Sine Sweep The “time reversal mirror” technique is employed: 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
27	Deconvolution = rotation of the sonograph Convolving with the inverse filter rotates the time-log(f) plane counter clockwise
28	Result of the deconvolution The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders
29	IR Selection After the sequence of impulse responses has been obtained, it is possible to select and insulate just one of them:
30	Maximum Length Sequence vs. Exp. Sine Sweep
31	Example of an ESS impulse response
32	Post processing of impulse responses A special plugin has been developed for the computation of STI according to IEC-EN 60268-16:2003
33	The STI Method
34	MTF from Impulse Response It is possible to derive the MTF values from a single impulse response measurement:
35	Background noise If the background noise is superposed to the impulse response, the previous method already takes care of it, and the MTF values are measured correctly However, in some cases, it is advisable to perform a noise-free measurement of the IR, and then insert the effect of the noise with the following expression:
36	Post processing of impulse responses A special plugin has been developed for performing analysis of acoustical parameters according to ISO-3382
37	The new AQT plugin for Audition The new module is still under development and will allow for very fast computation of the AQT (Dynamic Frequency Response) curve from within Adobe Audition
38	Distortion measurements A headphone was driven with a 1 V RMS signal, causing severe distortion in the small loudspeaker. The measurement was made placing the headphone on a dummy head. Measurements: ESS and traditional sine at 1 kHz
39	Distortion measurements Comparison between: traditional distortion measurement with fixed-frequency sine (the black histogram) the new exponential sweep (the 4 narrow, coloured lines)
40	Spatial analysis by directive impulse responses The initial approach was to use directive microphones for gathering some information about the spatial properties of the sound field “as perceived by the listener” Two apparently different approaches emerged: binaural dummy heads and pressure-velocity microphones:
41	IACC “objective” spatial parameter It was attempted to “quantify” the “spatiality” of a room by means of “objective” parameters, based on 2-channels impulse responses measured with directive microphones The most famous “spatial” parameter is IACC (Inter Aural Cross Correlation), based on binaural IR measurements
42	Lateral Fraction (LF) spatial parameter Another “spatial” parameter is the Lateral Fraction LF This is defined from a 2-channels impulse response, the first channel is a standard omni microphone, the second channel is a “figure-of-eight” microphone:
43	Are binaural measurents reproducible? Experiment performed in anechoic room - same loudspeaker, same source and receiver positions, 5 binaural dummy heads
44	Are IACC measurents reproducible? Diffuse field - huge difference among the 4 dummy heads
45	Are LF measurents reproducible? Experiment performed in the Auditorium of Parma - same loudspeaker, same source and receiver positions, 4 pressure-velocity microphones
46	Are LF measurents reproducible? At 25 m distance, the scatter is really big
47	3D Impulse Response (Gerzon, 1975)
48	3D extension of the pressure-velocity measurements The Soundfield microphone allows for simultaneous measurements of the omnidirectional pressure and of the three cartesian components of particle velocity (figure-of-8 patterns)
49	Directivity of transducers
50	A-format microphone arrays Today several alternatives to Soundfield microphones do exists. All of them are providing “raw” signals from the 4 capsules, and the conversion from these signals (A-format) to the standard Ambisonic signals (B-format) is performed digitally by means of software running on the computer
51	The Waves project (2003) The original idea of Michael Gerzon was finally put in practice in 2003, thanks to the Israeli-based company WAVES More than 50 theatres all around the world were measured, capturing 3D IRs (4-channels B-format with a Soundfield microphone) The measurments did also include binaural impulse responses, and a circular-array of microphone positions More details on WWW.ACOUSTICS.NET
52	The Ciresa project (2005) Measurements of the vibrations and radiated sound from wood panels Mapping of harmonic tables by means on an XY scanner Pressure measured by means of a linear microphone array Velocity measured by means of a laser vibrometer
53	Conclusions The sine sweep method revealed to be systematically superior to the MLS & TDS methods for measuring electroacoustical impulse responses The ESS method also allows for measurement of not-linear devices and to assess harmonic distortion Current limitation for spatial analysis in room acoustis is due to transducers (loudspeakers and microphones) A new generation of loudspeakers and microphones, made of massive arrays, is under development. The “harmonic orders” impulse responses obtained by the exponential sine sweep method can be used for not-linear convolution, which yields more realistic auralization