Misura della risposta all’impulso
Misura della risposta all’impulso
Schema di misura della risposta all’impulso
Schema del processo di misura
• Si desidera misurare la risposta impulsiva lineare h(t). Essa puo’ essere ricavata dalla conoscenza del segnale di test x(t) e del segnale
Not-linear, time variant
system K[x(t)]
Noise n(t)
input x(t)
+
output y(t) linear systemw(t)h(t) distorted signal
w(t)
Tempo di Riverberazione dalla Risposta Impulsiva
• Integrale inverso di Schroeder.
• E’ possibile
ricostruire sia la curva di
“carico” sia la curva di
“scarico”
integrando in avanti o
all’indietro la risposta
all’impulso.
A
Bg t d t
20
' '
g t d t
2' '
g t d t
t 2
0
' '
La Risposta Impulsiva
I metodi tradizionali (veri impulsi)
I metodi tradizionali (veri impulsi)
Metodi tradizionali
Palloncini
• Il diametro influenza la risposta alle basse frequenze
Esempio di risposta impulsiva (pistola)
Il “clappatore”
Il “clappatore”
Il “clappatore”
Il “clappatore”
Il metodo MLS Il metodo MLS
(Maximum Lenght Sequence)
(Maximum Lenght Sequence)
The first MLS apparatus - MLSSA
More recently - the CLIO system
Il metodo MLS
• x(t) è un segnale periodico binario,
ottenuto mediante uno
“shift-register”, configurato per la
massima lunghezza del periodo di ripetizione
1 2
L N
N stages
XOR k stages
x’(n)
Deconvoluzione MLS
• Il segnale misurato y(i) è cross-correlato con il segnale di test x(i) mediante una trasformata veloce di Hadamard. Se il sistema in prova è lineare e tempo-invariante, il risultato è la risposta impulsiva h(i)
y 1 M
L
h 1 ~
In cui M è la matrice di Hadamard trasposta, ottenuta permutando la sequenza MLS originaria m(i)
i j 2 mod L 1
m )
j ,i (
M ~
Esempio di misura MLS
Portable PC with 4- channels sound board Original Room
SoundField Microphone
B-format 4- channels signal
(WXYZ)
Measurement of B-format Impulse Responses
MLS excitation signal
Esempio di misura MLS
Example of a MLS impulse response
Il metodo ESS (exponential sine sweep)
Il metodo ESS (exponential sine sweep)
Edirol FA-101 Firewire sound
card:
10 in / 10 out 24 bit, 192 kHz ASIO and WDM
Today’s Hardware: PC and audio interface
Hardware: loudspeaker & microphone
Dodechaedron loudspeaker
Soundfield
microphone
Aurora Plugins Generate MLS
Deconvolve MLS Generate Sweep Deconvolve Sweep Convolution
Kirkeby Inverse Filter Speech Transm. Index
The first ESS system - AURORA
• Aurora was the first measurement system based on standard sound cards
Il metodo Log Sine Sweep
• x(t) è un segnale sinusoidale a frequenza
variabile, con variazione esponenziale della frequenza nel tempo.
1 e
ln sin T
) t (
x 1
ln 2
T t
1
2
1
Il metodo Log Sine Sweep
• La metodica di deconvoluzione della risposta all’impulso è semplice:
supponiamo di realizzare un filtro inverso z(t) tale che:
) ideale impulso
( ) t ( )
t ( z )
t (
x
• Se ora applichiamo tale filtro inverso al risultato della misura y(t), che altro non è che la convoluzione di x(t) con la risposta all’impulso dell’ambiente, h(t),
otteniamo:
) t ( h )
t ( )
t ( h )
t ( x )
t ( z )
t (
y
•
Test Signal – x(t)
Stop Stop
Measured signal - y(t)
Stop Stop
Inverse Filter – z(t)
Stop Stop
Deconvoluzione del Log Sine Sweep
• Viene usata la tecnica del “time reversal mirror”, cioè la convoluzione del segnale misurato con lo stesso
segnale di test, temporalmente invertito. Se il
contenuto spettrale del segnale non è piatto, occorre
una opportuna ri-equalizzazione del risultato.
Deconvoluzione = rotazione del sonogramma
Linear
Linear
2 2
ndndorder order
Risultato della deconvoluzione
1°
2°
5° 3°
IR Selection
• After the sequence of impulse responses has been
obtained, it is possible to select and insulate just
one of them:
Example of an ESS impulse response
Esperimento di comparazione
Test comparativo fra diverse tecniche di misurazione Organizzato dalla AES Italian Section
(Bergamo’s Workshop 1999, 27/28 aprile 1999)
Sperimentatore Sistema di misura - Metodo Altoparlante Microfono Angelo Farina Aurora (synchronous measurement
on PC+Layla) – MLS
Dodechaedron (Look Line D1)
Soundfield MKV + binaural (Ambassador) Angelo Farina Aurora (synchronous measurement
on PC+Layla) – log sweep
Dodechaedron (Look Line D1)
Soundfield MKV + binaural (Ambassador) Angelo Farina MLSSA board – MLS Dodechaedron
(Look Line D1)
Soundfield channel W A. Ricciardi MLSSA board – MLS Directional,
custom-made
Stage Accompany omnidirectional
Walter Conti Techron TEF 20 – MLS & TDS Directional, custom-made
B&K Omnidirectional Nicola Prodi Aurora (asynchronous playback &
record through a Tascam DA38 recorder) – log sweep
Dodechaedron (Look Line D- 300)
Soundfield ST250 +
binaural (Neumann KU-
100)
Apparecchi
Ambassador pre-amp Soundfield pre-amp
Soundfield Microphone Ambassador Dummy Head
Risultati
Informazioni estraibili dalla Risposta Impulsiva
Alcuni Descrittori Acustici
I parametri acustici (ISO 3382)
Tempo di Riverberazione Iniziale (EDT):
estrapolato da 0 a -10 dB
Tempo di riverberazione T 10 : estrapolato da -5 a -15 dB
Tempo di riverberazione T 20 : estrapolato da -5 a -25 dB
Tempo di riverberazione T 30 :
estrapolato da -5 a -35 dB
I parametri acustici (ISO 3382)
ms 80
2 ms 80
0 2
80
dτ τ
p
dτ τ
p lg
10 C
Indice di chiarezza C 80 (musica sinfonica):
Indice di chiarezza C 50 (parlato):
Valore ottimale = +/- 1 dB
ms 50
2 ms 50
0 2 50
dτ τ
p
dτ τ
p lg
10
C
I parametri acustici (ISO 3382)
Tempo baricentrico T S :
2 0
2 s
d p
T
100 d
p
d p
D
0 2 ms 50
0 2
Indice di definizione D:
I parametri acustici (ISO 3382)
• Strenght: G SPL L w 31 dB
d t h
d h
d t h
h t
s 2 d 2
s d
IACC:
SPL a 10 m
I parametri acustici (ISO 3382)
80 ms
2 ms
80 ms 5
W
Y h d
h
LFC: LFC
ms
ms W ms
ms Y
d h
d h
LF 80
0
2 80
5
2
Lateral Fraction:
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:
Binaural Binaural microphone (left) microphone (left)
and and
Pressure-velocity
Pressure-velocity
microphone (right)
microphone (right)
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
LeftLeft
Right Right
80 ms 80 ms
p pLL(())
p pRR(())
d t p
p
ms 80
R
L
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:
Figure Figure
of 8of 8 OmniOmni
ms 80
ms 5
82
d
h LF
hhoo(())
hh88(())
Are binaural measurents reproducible?
• Experiment performed in anechoic room - same loudspeaker, same
source and receiver positions, 5 binaural dummy heads
Are IACC measurents reproducible?
• Diffuse field - huge difference among the 4 dummy heads
IACCe - random incidence
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
IACCe B&K4100
Cortex Head Neumann
Are LF measurents reproducible?
• Experiment performed in the Auditorium of Parma - same
loudspeaker, same source and receiver positions, 4 pressure-
velocity microphones
Are LF measurents reproducible?
• At 25 m distance, the scatter is really big
Comparison LF - measure 2 - 25m distance
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
LF
Schoeps Neumann Soundfield B&K
Il plug-in Aurora Acoustical Parameters
The STI Method
The STI method is based on the MTF concept: a carrier signal (one-octave-band-filtered noise) is amplitude
modulated at a given modulation frequency with 100%
modulation depth. At the receiver, the modulation depth
E ne rg y E nv el op e
Acoustic system
1/F 1/F
time time
The STI Method – MTF matrix
The STI method is based on the MTF concept: a carrier signal (one-octave-band-filtered noise) is amplitude
modulated at a given modulation frequency with 100%
Octave-band STI value, computed by the average of MTF values
Overall weight STI (male)
MTF from Impulse Response
• It is possible to derive the MTF values from a single impulse response measurement:
2 0
2
exp 2
) (
d h
d F
j h
F m
f f
To compute each value of m(F) from the impulse response h(t), an octave-band filter is first applied to the impulse response, in order to select the carrier’s frequency band f.
Then m(F) is obtained with the formula
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:
10 10
1 ) 1 (
' )
( F m F L
noiseL
signalm
Post processing of impulse responses
• A special plugin has been
developed for the computation
of STI according to IEC-EN
60268-16:2003
Transducers: mouth simulator
• The mouth simulator was built inside a wooden
dummy head, employing low-cost parts. Its
compliance with the ITU recommendation was
confirmed by means of
Directivity measurements
• In both cases, the anechoic directivity measurements were performed employing a
rotating table, directly
synchronized with the sound board employed for measuring the impulse response. The
Aurora software generates the
required pulses on the right
channel, which cause the
rotating board to advance.
Directivity of the mouth simulator
The amplitude varies
smoothly, and respects the directivity mandated by
ITU recommendation.
Nevertheless, the sound is
heavily coloured, as shown
by the horizontal stripes in
the lower plot.
125 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°
-15°
Directivity of the mouth simulator
250 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30° -15° 500 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°
-15° 1000 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
+135°
+150°
+165°
±180°
-165°
-150°
-135°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15°
2000 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15° 4000 Hz
-35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15° 8000 Hz
-45 -35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15° 16000 Hz
-65-55 -45 -35 -25 -15 -5
5+0° +15°
+30°
+45°
+60°
+75°
+90°
+105°
+120°
-120°
-105°
-90°
-75°
-60°
-45°
-30°-15°
Directivity of a real human (I. Bork, PTB)
125 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 140 160150 170 180 200190 210 220 230 240 250 260 270 280 290
300 310
320 330340 350
250 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 140 160150 170 180 200190 210 220 230 240 250 260 270 280 290
300 310
320 330340 350
500 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 120 130 140 160150 170 180 200190 210 220 230 240 250 260 270 280 290
300 310
320 330340 350
1000 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 250
260 270 280 290
300 310
320 330340 350
2000 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 250
260 270 280 290
300 310
320 330340 350
4000 Hz
-35 -25 -15 -5 5 0
10 20 30
40 50
60 70
80 90 100 110 250
260 270 280 290
300 310
320 330340 350
Equalization of the mouth simulator
• The spectrum of the emitted test signal
should correspond to the prescriptions of ITU T- P50 Recommendation.
• The overall SPL should
be 67 dB(A) at 1m, on
axis, for STI standard
measurements
Equalization of the mouth simulator
• The MLS signal is
prefiltered, so that the frequency response,
measured at 1m in front of the mouth, complies with the IEC spectrum.
• The filtering is performed by means of the grahic
equalizer incorporated in
3D Impulse Response (Gerzon, 1975)
Portable PC with 4- channels sound board Original Room
Sound Source
SoundField Microphone
B-format 4-channels signal (WXYZ) Measurement of B-format
Impulse Responses
MLS or sweep excitation signal
Convolution of dry signals with the B-format Impulse
Responses Sound Source
Mono Mic.
B-format Imp. Resp.
of the original room
B-format 4-channels signal (WXYZ) Convolver
Ambisonics decoder
Speaker array in the
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)
Directivity of transducers
Soundfield ST-250 microphone
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
30
60
90
120
150
180 210
240 270
300 330
125 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
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300 330
250 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
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300 330
500 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
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300 330
1000 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
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270 300
330
2000 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
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330
4000 Hz
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
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330
8000 Hz
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16000 Hz
