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Boundary conditionsBoundary 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 conditionssurround 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 frequencyas 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 frequencyimpedence 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 toolssoftware tools

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Reference measurements in the carReference 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

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Reference measurements in the carReference measurements in the car

Software

Aurora Plugins

Generate SweepGenerate Sweep

Convolution / DeconvolutionConvolution / Deconvolution

Impulse Response extractionImpulse Response extraction

Cross FunctionsCross Functions

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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)

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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.

1e

f

fln

Tf2sin)t(x 1

2

f

fln

T

t

1

2

1

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Test Signal – x(t)

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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)

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Measured signal - y(t)

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

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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)]

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Result of the deconvolution

The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders

1°2°3°

5°

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Maximum Lenght Sequence vs. Sweep

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In-situ measurement of the acoustical propertiesIn-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 probeMicroflown pressure-velocity probe

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In-situ measurement of the acoustical propertiesIn-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 frequencylow frequency

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

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In-situ measurement of the acoustical propertiesIn-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 techniquepressure-velocity probe technique

Input parametersInput parameters ResultsResults