Boundary conditions Boundary conditions

Boundary conditions Boundary conditions

 The external surface of the solid model need to be The external surface of the solid model need to be surround by proper boundary conditions

surround by proper boundary conditions

 The sound sources (loudspeakers) were modeled The sound sources (loudspeakers) were modeled as areas where the normal acceleration is known as areas where the normal acceleration is known

as a function of frequency as a function of frequency

 The internal surfaces of the car are modeled as The internal surfaces of the car are modeled as sourfaces of known complex acoustical

sourfaces of known complex acoustical impedence as a function of frequency impedence as a function of frequency

 Some surfaces were modeled as rigid walls Some surfaces were modeled as rigid walls (glass, steel parts not covered by sound

(glass, steel parts not covered by sound absorbing materials)

absorbing materials)

 The values of acceleration and impedance were The values of acceleration and impedance were measured “in situ” thanks to novel hardware and measured “in situ” thanks to novel hardware and

software tools

software tools

Reference measurements in the car 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

Reference measurements in the car Reference measurements in the car

Software

Aurora Plugins

Generate Sweep Generate Sweep

Convolution / Deconvolution Convolution / Deconvolution Impulse Response extraction Impulse Response extraction Cross Functions

Cross Functions

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).

Not-linear, time variant

system K[x(t)]

Noise n(t)

input x(t)

+ output y(t) linear system

w(t)h(t) distorted signal

w(t)

Test signal: Log Sine Sweep

x(t) is a sine signal, which frequency is varied exponentially with time, starting at f

and ending at f

.

 

 





 

 







 







 









 

 











  ^{ }

 



 

1 e

f ln f

T f

sin 2 )

t (

x ¹

2

f ln f T

t

1

2

1

Test Signal – x(t)

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) Inverse Filter z(t)

Measured signal - y(t)

 The not-linear behaviour of the loudspeaker causes many The not-linear behaviour of the loudspeaker causes many harmonics to appear

harmonics to appear

Inverse Filter – z(t)

The deconvolution of the IR is obtained convolving the measured The deconvolution of the IR is obtained convolving the measured signal y(t) with the inverse filter z(t) [equalized, time-reversed x(t)]

signal y(t) with the inverse filter z(t) [equalized, time-reversed x(t)]

Result of the deconvolution

The last impulse response is the linear one, the preceding

2° 1°

5° 3°

Maximum Lenght Sequence vs. Sweep

In-situ measurement of the acoustical properties In-situ measurement of the acoustical properties

 The measurement of the acoustical impedance is performed employing a The measurement of the acoustical impedance is performed employing a Microflown pressure-velocity probe

Microflown pressure-velocity probe

In-situ measurement of the acoustical properties In-situ measurement of the acoustical properties

 The probe needs to be calibrated for proper gain and phase matching at The probe needs to be calibrated for proper gain and phase matching at low frequency

low frequency

Calibration over a reflecting surface

Calibration over a reflecting surface Free-Field calibration Free-Field calibration

In-situ measurement of the acoustical properties In-situ measurement of the acoustical properties

 A specific software (Aurora plugin) has been developed for speeding up A specific software (Aurora plugin) has been developed for speeding up both calibration and measurement of the acoustical properties with the new both calibration and measurement of the acoustical properties with the new pressure-velocity probe technique

pressure-velocity probe technique