Boundary conditions
 The external surface of the solid model need to be
surround by proper boundary conditions
 The sound sources (loudspeakers) were modeled
as areas where the normal acceleration is known
as a function of frequency
 The internal surfaces of the car are modeled as
sourfaces of known complex acoustical
impedence as a function of frequency
 Some surfaces were modeled as rigid walls
(glass, steel parts not covered by sound
absorbing materials)
 The values of acceleration and impedance were
measured “in situ” thanks to novel hardware and
software tools
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 1
Reference measurements in the car
Hardware: PC and audio interface
Edirol FA-101
Firewire sound
card:
10 in / 10 out
24 bit, 192 kHz
ASIO and WMA
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 2
Reference measurements in the car
Software
Aurora Plugins
Generate Sweep
Convolution / Deconvolution
Impulse Response extraction
Cross Functions
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 3
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).
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
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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.


t  f2 




ln  


2    f1  T
T  f1 

x ( t )  sin
e
 1
  f2  



 ln   

f
  1

25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 5
Test Signal – x(t)
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 6
Deconvolution of Log 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
Test Signal x(t)
25.01.2007 Angelo Farina
Inverse Filter z(t)
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Measured signal - y(t)
The not-linear behaviour of the loudspeaker causes many
harmonics to appear
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 8
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)]
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 9
Result of the deconvolution
2°
5°
1°
3°
The last impulse response is the linear one, the preceding
are the harmonics distortion products of various orders
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 10
Maximum Lenght Sequence vs. Sweep
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
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In-situ measurement of the acoustical properties
 The measurement of the acoustical impedance is performed employing a
Microflown pressure-velocity probe
25.01.2007 Angelo Farina
UNIPR / ASK Industries | All Rights Reserved | Confidential
| Page 12
In-situ measurement of the acoustical properties
 The probe needs to be calibrated for proper gain and phase matching at
low frequency
Calibration over a reflecting surface
25.01.2007 Angelo Farina
Free-Field calibration
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In-situ measurement of the acoustical properties
 A specific software (Aurora plugin) has been developed for speeding up
both calibration and measurement of the acoustical properties with the
new pressure-velocity probe technique
Input parameters
25.01.2007 Angelo Farina
Results
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| Page 14