Transcript Sound recording for scientific purposes a tutorial on

A tutorial on acoustic measurements
for the non-technician
Svante Granqvist
Royal Institute of Technology (KTH)
Dept of Speech Music and Hearing (TMH)
Stockholm, Sweden
[email protected]
Today’s topics
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Sound and microphones
Room acoustics
Calibration
Recommendations
www.speech.kth.se/~svante/pevoc5
Conclusions
• Use omni-directional electret or condenser
microphones whenever possible
– Do not use directed (e.g. cardioid) microphones unless
you really need the directivity
• Especially not close to the speaker
– Avoid dynamic microphones
• Place the microphone within the reverberation
radius of the room
• Keep noise level low
• Establish a routine for level calibration
www.speech.kth.se/~svante/pevoc5
What is sound?
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Demo of sound field
Sound pressure (pascals, Pa)
Sound pressure level, SPL (decibels, dB)
Particle velocity (metres per second, m/s)
Particle velocity level? Rarely!
www.speech.kth.se/~svante/pevoc5
Sound pressure
• Simple relation to sound intensity
• Our ears are mainly pressure sensitive
• Simple relation to distance (~1/r)
– Doubled distance => halved SP <=> SPL: -6dB
• Pressure has no direction
• Pressure sensitive microphones are
omni-directional (no directivity)
• So: how do they make directed
microphones?
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Particle velocity
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Particle velocity has a direction
So it can be used to create directivity!
Particle velocity only => figure of eight
Mainly sensitive in two directions
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Cardioid
• Particle velocity and sound pressure
combined => cardioid
• Mainly sensitive in one direction
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Directivity
• Omni-directional (SP only)
• Directed (involves particle velocity)
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Figure of eight
Cardioid
Super-cardioid
Other special directivity patterns
• Great!
...or is it?
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Directed microphones
• We are primarily interested in sound
pressure
• ...but also measure particle velocity
• ...then PV and SP have to be proportional to
one another!
• Are they?
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NO!
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Particle velocity
• Particle velocity is proportional to sound pressure,
but only in the far field (~1/r)
• In the close field, it differs! (~1/r2)
• The limit between far and close field depends on
frequency
D is ta n c e d e p e n d e n c e o f p re s s u re a n d ve lo c ity fie ld s
130
125
120
L evel [d B ]
115
110
SPL
105
PVL @ 100 H z
100
PVL @ 1000 H z
95
90
85
80
0
0 .2
0 .4
0 .6
0 .8
1
1 .2
D is ta n c e [m ]
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1 .4
1 .6
1 .8
2
Particle velocity
• Particle velocity exhibits a bass lift in the
close field
– proximity effect
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Proximity effect (cardioid mic)
F re q u e n c y a n d d is ta n c e d e p e n d e n c e o f
s o u n d p re s s u re a n d c a rd io id o u tp u t
130
SPL @ 30 cm
125
SPL @ 5 cm
C a rd io id @ 3 0 c m
L evel [d B ]
120
C a rd io id @ 5 c m
115
110
105
100
95
90
10
100
1000
F re q u e n c y [H z ]
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10000
Proximity effect
• Demo
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Manufacturers’ data sheets
Cardioid
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Manufacturers’ data sheets
Cardioid
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Manufacturers’ data sheets
Cardioid
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Manufacturers’ data sheets
Omni-directional
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Manufacturers’ data sheets
• Frequency responses are mostly measured
in the far field, even for microphones that
obviously are intended to be mounted in the
close field
• You have to add the proximity effect for
directed microphones to those curves!
www.speech.kth.se/~svante/pevoc5
Proximity effect (cardioid mic)
F re q u e n c y a n d d is ta n c e d e p e n d e n c e o f
s o u n d p re s s u re a n d c a rd io id o u tp u t
130
SPL @ 30 cm
125
SPL @ 5 cm
C a rd io id @ 3 0 c m
L evel [d B ]
120
C a rd io id @ 5 c m
115
110
105
100
95
90
10
100
1000
F re q u e n c y [H z ]
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10000
Manufacturers’ data sheets
Cardioid
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Manufacturers’ data sheets
Cardioid
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Manufacturers’ data sheets
Cardioid
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Manufacturers’ data sheets
Omni-directional
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Demo
• Proximity effect:
– Hear the bass lift from the directed microphone
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OK, point taken, he doesn’t like
directed microphones
• But then, why are there so many directed
microphones out there?
• Music industry, broadcasting, stage use etc:
– A bass boost of a few dBs does not matter much or
might even be desired (sound ”better”)
– Noise supression may be more important than a flat
frequency response
• Most recordings do not have a scientific purpose
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Transducer type
• Electret/condenser
– Can easily be made to have flat response
– Cheap electret microphones (< €30 ) can be of
sufficient quality
– Requires battery/power supply
– Sensitivity may decrease towards end of battery life
• Dynamic
– Difficult to acheive a flat response
– Good dynamic microphones are expensive
– Rarely purely pressure sensitive (even though datasheet may say so)
– No need for battery/power supply
www.speech.kth.se/~svante/pevoc5
Bottom line...
• Use omni-directional, electret/condenser
microphones for scientific purposes!
• Make sure batteries are fresh or use some
other type of power supply
www.speech.kth.se/~svante/pevoc5
Room acoustics
• In a room sound originates from:
– the sound source, directly
– or from reflections at the walls
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Room acoustics
105
100
S P L [d B ]
95
D iffu se
D ire ct
90
Sum
85
80
75
70
0
1
2
3
P o s itio n [m ]
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4
5
Room acoustics
• Reverberation radius, rr
– The distance where reflected and direct sound
are equally loud
• Less absorbtion => stronger reflections =>
smaller rr
www.speech.kth.se/~svante/pevoc5
Room acoustics
105
100
S P L [d B ]
95
D iffu se
D ire ct
90
Sum
85
80
75
70
0
1
2
3
P o s itio n [m ]
www.speech.kth.se/~svante/pevoc5
4
5
Room acoustics
• Reverberation radius
– At what distance is the direct sound as loud as
the sound that has been reflected from the walls
– Typical value 4 – 0.5 meters
• Reverberation time
– How long does it take for the sound level to
drop by 60 dB?
– Typical value 0.5 – 4 seconds
www.speech.kth.se/~svante/pevoc5
rr  0 . 056  V T
Room acoustics
• How to measure Reverberation time/radius
– Several ways, one would be to record a ”bang”
and see at what rate the sound level drops
– The time for a 60dB drop corresponds to
reverberation time
– Calculate reverberation radius from this time:
rr  0 . 056  V T
www.speech.kth.se/~svante/pevoc5
Bottom line...
• Within the reverberation radius, conditions are
similar to free field
• Outside, reflections from the walls dominate the
sound
• So, put the microphone (well) within the
reverberation radius!
105
100
S P L [d B ]
95
D iffu se
D ire ct
90
Sum
85
80
75
70
0
1
2
3
4
5
P o s itio n [m ]
www.speech.kth.se/~svante/pevoc5
Level calibration
• Most common method:
– Record a signal with a known level
i.e. a calibration tone
– By relating the level of the calibration tone to
the levels of the signals of interest, absolute
calibration is acheived
www.speech.kth.se/~svante/pevoc5
Calibration file, example
Calibration tone
”The level was 89 dB”
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Calibration
Calibrator
• Procedure:
– Mount and start the calibrator (2-10
seconds)
– Unmount calibrator and say the level of the
calibrator
• Advantages:
– Stable calibration tone
– No sensitivity to room acoustics or
surrounding noise
• Disadvantage:
– Calibrator that fits the microphone required
• Important that the seal is tight!
www.speech.kth.se/~svante/pevoc5
Calibration
Loudspeaker + SPL meter
• Procedure:
– Beep at 1kHz ~ 80 dB (2-10 seconds)
– say the level as read on the level meter
• Advantages:
– Stable calibration tone
• Disadvantage:
– Loudspeaker + signal source reqiured
– Some sensitivity to room and surrounding noise
www.speech.kth.se/~svante/pevoc5
Calibration
Voice + SPL meter
• Procedure:
– Sustain /a/ ~80 dB (5-10 seconds)
– say the level as read on the level meter
• Advantages:
– No loudspeaker required
– Calibration signal (voice) has approximately the same
spectrum as the signals of interest
• Disadvantages:
– Hard to keep the level of the /a/ stable
– Some sensitivity to room and surrounding noise
www.speech.kth.se/~svante/pevoc5
Calibration
Voice + SPL meter
• Procedure
– Sustain /a/ ~80 dB (5-10 seconds)
– say the level as read on the level meter
• Advantages:
– No loudspeaker required
– Calibration signal (voice) has approximately the same spectrum as
the signals of interest
– Automatic compensation for microphone distance
• Disadvantage:
– Hard to keep the level of the /a/ stable
– Only valid for this particular distance
– Some sensitivity to room and surrounding noise
• dB meter should be within rr
www.speech.kth.se/~svante/pevoc5
Calibration,
directed microphones ?
• Only in the far field (>30
cm), but still within rr
• Only for rough estimation
of SPL
• Never use SPL calibrators!
• ”Don’t try this at home”
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>30cm
>30cm
Distance compensation
• Sound pressure drops as ~1/r
• Re-calculate SPL to appear as recorded at a
different distance, e.g. record at d2=5 cm,
but report at d1=30 cm.
• Only for omni-directional microphones!
• Formula:
 d2 

L 1  L 2  20  lg 

d
 1 
www.speech.kth.se/~svante/pevoc5
Bottom line...
• Establish a routine for calibrations
• Don’t calibrate directed microphones
• Report SPL at 30 cm
– Compensated or actual
• Beware of the ”mixer” on most PC
soundcards
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Recommendations
microphone and room acoustics
• Depend on
– the purpose of recording
– the recording environment
• Noise
• Room acoustics
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Example of purposes:
SPL
Spectrum
F0
Inverse filtering
HNR
Perceptual evaluation
SPL
• Omni-directional electret/condenser microphone
• If noisy environment:
– Try to attenuate the noise
– Shorten microphone distance (10 cm to the side of the
mouth)
• Avoid directed microphones for this purpose!
• Put the microphone well within the reverberation
radius of the room (~rr/2)
• Re-calculate or calibrate for 30 cm
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Spectral properties
(spectrogram)
• Omni-directional
electret/condenser microphone
• If noisy environment:
– Try to attenuate the noise
– Shorten microphone distance
(5-10cm to the side of the
mouth)
• If background noise still is a
problem a directed microphone
can be used, but beware of the
proximity effect and keep
microphone distance constant!
• Put microphone well within rr
www.speech.kth.se/~svante/pevoc5
Spectral properties
(LTAS, H1-H2, line spectra)
• Omni-directional
electret/condenser microphone
• If noisy environment:
– Try to attenuate the noise
– Shorten microphone distance
(5-10cm to the side of the
mouth)
• Do not use a directed
microphone
• Put microphone well within rr
• Pay attention to reflective
surfaces such as windows,
manuscripts etc.
Added proximity effect, cardioid at 5 cm
www.speech.kth.se/~svante/pevoc5
F0, jitter/shimmer
• Any decent microphone is OK, since periodicity is
independent of frequency response
• If noisy environment:
– Try to attenuate the noise
– Shorten microphone distance (5-10 cm)
– Use a directed microphone
• Check if F0 algorithm is affected by a bass lift!
www.speech.kth.se/~svante/pevoc5
Inverse filtering
• Omni-directional electret/condenser
microphone flower < 10 Hz
• Reduce background noise as much
as possible
• Never use a directed microphone
• Microphone distance 5-10 cm
– Within rr/10
• Pay attention to reflective surfaces
such as windows, manuscripts etc.
• Anechoic chamber is preferred
Addition of reflection to the direct signal
www.speech.kth.se/~svante/pevoc5
Harmonics-to-noise ratio
• Omni-directional electret/condenser microphone
• Background noise must be lower than voice noise
• Microphone distance 5-50 cm
– Well within rr
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Perceptual evaluation
• Omni-directional electret/condenser microphone
• Reduce background noise as much as possible
• Microphone distance 5-50 cm
– Well within rr
www.speech.kth.se/~svante/pevoc5
But you never know...
• For example, the first intention may be to
only extract F0
• It might turn out, after the recordings are
made, that the recorded material would be
suitable for some other measurement, like
SPL
• Therefore, do it ”right” from the start!
www.speech.kth.se/~svante/pevoc5
Conclusions
• Use omni-directional electret or condenser
microphones whenever possible
– Do not use directed (e.g. cardioid) microphones unless
you really need the directivity
• Especially not close to the speaker
– Avoid dynamic microphones
• Place the microphone within the reverberation
radius of the room
• Keep noise level low
• Establish a routine for level calibration
www.speech.kth.se/~svante/pevoc5
These were my recommendations
• You may find reasons to not follow them
• But they better be good... 
www.speech.kth.se/~svante/pevoc5
Questions?
This presentation is available on the web
www.speech.kth.se/~svante/pevoc5
[email protected]