Room Acoustical Measurements

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Room Acoustical

Measurements

27.11.2003

Timo [email protected]

Outline

Background & Tradition How to describe room acoustics? An overview of measurement methods

Room Acoustical Measurements and Analysis Standards Equipment, Case study Measurement positions A glance at room acoustical parameters Theory vs. practice Requirements for data analysis Reporting results Accuracy of results

Examples and Case Studies Discussion

Background & Tradition

The acoustical properties of rooms and halls how to describe/compare/quantify? subjective/objective issues

a combined result of many factors Historical issues

traditionally acoustical design was based onprior experience, copying, and chance

musical styles followed the architecture

First scientific approach by Sabine in 1900 a century of discoveries followed, but current practice is still largely empirical

Measurement Methods

Sabine's reverberation measurements


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B&K Omnisource Type 4295 B&K OmniPower Sound Source Type 4296

B&K OmniPower Sound Source Type 4296

B&K Omnisource Type 4295

B&K OmniPower Sound Source Type 4296

B&K Omnisource Type 4295

Microphones omnidirectional (standard) figure-of-eight (LEF) dummy head or in-the-ear mics (IACC) directional multichannel probes


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Signal source noise generator impulse source

(pistol, spark gap, hand clap, balloon etc.)

computer for modern methods

Data storage, processing and analysis usually by means of a computer traditional methods include devices such as

noise generator sound level meter, filter bank

pen plotter Filters 1/3 and 1/1 octave band filters (standard)

Case Study:The IRMA measurement system

laptop pc

12

12-channel

microphone grid

binaural headomni source

(subwoofer unit)omni and

2 cardioids

3

2 2

1(2)

3

6

A/D

rackmountPC

computer

D/A


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The IRMA hardware

The IRMA Matlab software

GUI

File I/O

Devices

Stimulus

Acquisition

Filtering

IRMA setup structureMeasurement

Stimulus

Acquisition

IR calc.

IR postprocessing

Analysis

IR post-proc.

Filtering

RoomAcoustic

Parameters

Otherapplications

Otherfunctions

Plotting

Accessories

MeasurementPC

Multichannel soundcard

External units

Multichannel preamplifier

Multichannel AD converter

Remote operationfor field use

Windowssoundcardinterface

Measurement positions for concert halls

based on specifications published byA.C. Gade (1989)

3 source points on stage S1: soloist next to conductor (front left) S2: string section, violas/cellos (mid-right) S3: winds (far left, 2nd row)

57 audience receiver points: R1 3 stage receiver points

P1: solo oboist P2: string section, 1st/2nd violins (mid left) P3: winds (far right, 2nd row)

S3P1

P3

S2

S1

R1

l / 4

2b / 5 l / 2

b / 4

l / 5

b / 3

l / 2

b / 2

l / 2

2b / 5

R2

R3

R4

R5

P2

ORCHESTRA

PLATFORM

EVENTUAL

BALCONIES

STALLS

measurement position heights: sources and stage receivers: 1 m above stage audience receivers:

1.2 m above floor / 0.7 m above seat variances caused by the seat dip effect

omni source and microphones as in ISO 3382 frequency range: octave bands 1254000 Hz

halls are usually empty during measurements

music stands and chairs present on platform slight adjustment required for S-P pairs


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A Glance at Room Acoustical Parameters

Room acoustics are always field measurements are the results

reliable? repeatable? representative?

(If so, what do they represent?)

An overview of standardized room acoustical

parameters and factors affecting theirmeasurement follows

A Little Theory

An ideal closed acoustical space has:

a perfectly diffuse sound field

evenly spaced absorption negligible single room modes an exponential decay of sound vs. time no background noise

Standard room acoustical parametersare based on this ideal model.

A Little Practice(or: stepping into a muddy puddle)

A real closed acoustical space typically exhibits:

a mixed sound field of direct, reflected and

diffuse sound (strong temporal variance) unevenly spaced absorption (audience) strong single room modes at low frequencies multiple decays high levels of background noise

(demo)

(The Lumpy Road of)Data Analysis

In practice, 99% of measurement results need tobe analyzed automatically.

Robust methods are required!


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Response analysis in the IRMA systemResponse analysis

IR processing

detectnoise floor

locatedirect sound

Filtering

calculatedelays

calculatecoefficients

time-reversedata

filterdata

time-reversefiltered result

Room acousticparameters

calculate

truncatepre-delay

estimatedecay curve

iteratenoise-decaycrosspoint

truncatenoise tail

calculate decaycompensation

displayresults

Acquired response (ETC, Schroeder) Extracted decay (ETC, Schroeder)

0 100 200 300 400 500

-100

-80

-60

-40

-20

0

0 20 40 60 80 100 120 140 160

-100

-80

-60

-40

-20

0

0 100 200 300 400 500

-60

-50

-40

-30

-20

-10

0

0 20 40 60 80 100 120 140 160

-60

-50

-40

-30

-20

-10

0

Example: Energy-time curves and Schroeder plots of an IR

1. raw response 2. noise floor truncated 3. direct sound located

0 100 200 300 400 500

-100

-80

-60

-40

-20

0

0 50 100 150 200 250-100

-80

-60

-40

-20

0

0 20 40 60 80 100 120 140 160-100

-80

-60

-40

-20

0

0 100 200 300 400 500

-60

-50

-40

-30

-20

-10

0

0 50 100 150 200 250 300

-60

-50

-40

-30

-20

-10

0

0 20 40 60 80 100 120 140 160

-60

-50

-40

-30

-20

-10

0

Reverberation time T

denoted as Tor T60 determined from the decay curve

straight? monotonic? ideal?

background noise level: (15)3545 dB dynamic range required

T20: evaluated between [-5-25] dB T30: evaluated between [-5-35] dB T10 or EDT: Early Decay Time

evaluated between [0-10] dB


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Early-to-Late Energy Ratios C, D Clarity C:

( )( )

C p t dt p t dt

t

t

t

e

e

e

=

10

2

0

2lg

Definition D:

( )

( )D

p t dt

p t dt t

t

e

e

=

10

2

0

2

0

lg

te is the early time limit (50 or 80 ms)

Center Time TS

( )

( )T

t p t dt

p t dt S =

2

0

2

0

an alternative to Cand D avoids the discrete division of the IR

into early and late periods

Strength G

( )

( )G

p t dt

p t dt ref

=

10

2

0

2

0

lg

p(t) is the IR measured at a receiver point pref(t) is the reference IR, measured in

a free field 10 m from the source

requires calibrated levels and gain settings

Lateral Energy Fraction LEF

( )

( )LEF

p t dt

p t dt

fig

omni

=

8

2

0

2

0

calibration of omni and fig-8 capsules required one solution is to use sum L,R cardioids:

omni = L+R, fig-8 = L-R

squaring of responses affects fig-8 directivity

cardioid = cos(), hypercardioid = cos2

() true method (two omnis) not practicable


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Inter-Aural Cross-Correlation IACC

( )( ) ( )

( ) ( )IACF

p t p t dt

p t dt p t dt t t

l rt

t

lt

tr

t

t1 2

1

2

1

2

1

22 2,

= +

( ) IACC IACF t t t t 1 2 1 2 1 1, ,max= <


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temperature and relative humidity type and position of sound sources description of stimulus signals type, position and height of microphones

date, name of measurement organization

Accuracy of Results

The validity of any objective measurement should bequestioned for the following :

reliability? repeatability? representativity?

if so, what?

Evaluating typical systematic errors:

system loop-back compensation level calibration and alignment of channels avoiding filtering artifacts

Discussion and Conclusions

How to objectively qualify room acousticalproperties?

Standard parameters, novel approaches

Modern measurement methods offer goodaccuracy, but require a thorough understandingand evaluation of the underlying processes

Documents

Room Acoustical Measurements