Electroacoustic Electroacoustic sources sources (loudspeakers) (loudspeakers)

(1)

Electroacoustic Electroacoustic

sources sources

(loudspeakers)

(2)

Loudspeaker as sound source Loudspeaker as sound source

 A loudspeaker is fed with a special test signal A loudspeaker is fed with a special test signal x(t), while a microphone records the room

x(t), while a microphone records the room response

response

 A proper deconvolution technique is required for A proper deconvolution technique is required for retrieving the impulse response h(t) from the

retrieving the impulse response h(t) from the recorded signal y(t)

recorded signal y(t)

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

Portable PC with additional sound card Room

Microphone

Output signal y Measurement of Room

Impulse Response

MLS test signal x

Loudspeaker

(3)

Loudspeakers

 Equalized, omnidirectional sound source: Equalized, omnidirectional sound source:

► Dodechaedron for mid-high frequencies

► One-way Subwoofer (<120 Hz)

Dodech. LookLine D200

60.0 70.0 80.0 90.0 100.0 110.0 120.0

25 31.5 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000

Frequency (Hz)

Sound Power Level (dB)

Unequalized Equalized

Lw,tot = 94.8 dB Lw,tot = 106.9 dB

(4)

Directivity of loudspeakers

LookLine D-200 dodechaedron LookLine D-200 dodechaedron

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(5)

LookLine D-300 dodechaedron LookLine D-300 dodechaedron

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Directivity of loudspeakers

(6)

Omnisonic 1000 dodechaedron Omnisonic 1000 dodechaedron

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Directivity of loudspeakers

(7)

 Genelec S30D reference studio monitor: Genelec S30D reference studio monitor:

► Three-ways, active multi-amped, AES/EBU

► Frequency range 37 Hz – 44 kHz (+/- 3 dB)

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Directivity of loudspeakers

(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 Electroacoustical methods

 Different types of test signals have been developed, Different types of test signals have been developed,

providing good immunity to background noise and easy providing good immunity to background noise and easy deconvolution of the impulse response:

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 Each of these test signals can be employed with different deconvolution techniques, resulting in a number of

deconvolution techniques, resulting in a number of

“different” measurement methods

 Due to theoretical and practical considerations, the Due to theoretical and practical considerations, the

preference is nowadays generally oriented for the usage of preference is nowadays generally oriented for the usage of ESS with not-circular deconvolution

ESS with not-circular deconvolution

(10)

The first MLS apparatus - MLSSA The first MLS apparatus - MLSSA

 MLSSA was the first apparatus for measuring impulse responses with MLS MLSSA was the first apparatus for measuring impulse responses with MLS

(11)

More recently - the CLIO system More recently - the CLIO system

 The Italian-made CLIO system has superseded MLSSA for most electroacoustics The Italian-made CLIO system has superseded MLSSA for most electroacoustics applications (measurement of loudspeakers, quality control)

applications (measurement of loudspeakers, quality control)

(12)

The first TDS apparatus - TEF The first TDS apparatus - TEF

 Techron TEF 10 was the first apparatus for measuring impulse responses Techron TEF 10 was the first apparatus for measuring impulse responses with TDS

with TDS

 Subsequent versions (TEF 20, TEF 25) also support MLS Subsequent versions (TEF 20, TEF 25) also support MLS

(13)

Edirol FA-101 Firewire sound

card:

10 in / 10 out 24 bit, 192 kHz ASIO and WDM

Today’s Hardware: PC and audio interface

(14)

Hardware: loudspeaker & microphone Hardware: loudspeaker & microphone

Dodechaedron loudspeaker

Soundfield

microphone

(15)

Aurora Plugins

Generate MLS Generate MLS Deconvolve MLS Deconvolve MLS Generate Sweep Generate Sweep Deconvolve Sweep Deconvolve Sweep Convolution

Convolution

Kirkeby Inverse Filter Kirkeby Inverse Filter Speech Transm. Index Speech Transm. Index

The first ESS system - AURORA

 Aurora was the first measurement system based on standard sound cards Aurora was the first measurement system based on standard sound cards and employing the Exponential Sine Sweep method (1998)

and employing the Exponential Sine Sweep method (1998)

 It also works with traditional TDS and MLS methods, so the comparison It also works with traditional TDS and MLS methods, so the comparison can be made employing exactly the same hardware

can be made employing exactly the same hardware

(16)

MLS method MLS method

 X(t) is a periodic binary X(t) is a periodic binary signal obtained with a signal obtained with a suitable shift-register, suitable shift-register, configured for

configured for

maximum lenght of the maximum lenght of the period.

period.

N stages

XOR k stages

x’(n)

1 2

L  ^N 

(17)

MLS deconvolution MLS deconvolution

 The re-recorded signal y(i) is cross-correlated with the The re-recorded signal y(i) is cross-correlated with the

excitation signal thanks to a fast Hadamard transform. The excitation signal thanks to a fast Hadamard transform. The

result is the required impulse response h(i), if the system result is the required impulse response h(i), if the system

was linear and time-invariant was linear and time-invariant

y 1 M

L

h 1  ^~ 

 

 Where M is the Hadamard matrix, obtained by permutation of the original MLS sequence m(i)

 

 ⁱ ^j ² ^mod ^L  ¹

m )

j ,i (

M ~    

(18)

MLS measurement procedure MLS measurement procedure

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

(19)

MLS measurement procedure MLS measurement procedure

play play

record

(20)

Example of a MLS impulse response

(21)

Exponential Sine Sweep method

 x(t) is a band-limited sinusoidal sweep signal,

which frequency is varied exponentially with time, starting at f ₁ and ending at f ₂ .

 





 







 







 









 

 









  ^ ^

 



 

1 e

f ln f

T f

sin 2 )

t (

x ¹

2 f ln f T

t

1

2

1

(22)

Test Signal – x(t)

(23)

Measured signal - y(t)

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

(24)

Inverse Filter – z(t)

The deconvolution of the IR is obtained convolving the measured signal

(25)

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

Test Signal x(t) Inverse Filter z(t)

(26)

Deconvolution = sonogram rotation Deconvolution = sonogram rotation

 Convolution with inverse filter rotates the time-frequency Convolution with inverse filter rotates the time-frequency plane counter-clockwise

plane counter-clockwise

Linear

2 2 ^nd ^nd order order

(27)

Result of the deconvolution

The last impulse response is the linear one, the preceding

2° 1°

5° 3°

(28)

IR Selection IR Selection

 After the sequence of impulse responses has been obtained, it is possible to After the sequence of impulse responses has been obtained, it is possible to select and extract just one of them (the 1°-order - Linear in this example):