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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