A tutorial on acoustic measurements for the non-technician Svante Granqvist Royal Institute of Technology (KTH) Dept of Speech Music and Hearing (TMH)

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]

www.speech.kth.se/~svante/pevoc5

Today’s topics

• Sound and microphones

• Room acoustics

• Calibration

• Recommendations


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


What is sound?

• 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!


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?


Particle velocity

• Particle velocity has a direction

• So it can be used to create directivity!

• Particle velocity only => figure of eight

• Mainly sensitive in two directions


Cardioid

• Particle velocity and sound pressure combined => cardioid

• Mainly sensitive in one direction


Directivity

• Omni-directional (SP only)• Directed (involves particle velocity)

– Figure of eight– Cardioid– Super-cardioid– Other special directivity patterns

• Great! ...or is it?


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?


NO!


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

Distance dependence of pressure and velocity fields

80

85

90

95

100

105

110

115

120

125

130

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Distance [m]

Le

ve

l [d

B]

SPLPVL @100 HzPVL @1000 Hz


Particle velocity

• Particle velocity exhibits a bass lift in the close field – proximity effect


Proximity effect (cardioid mic)Frequency and distance dependence of

sound pressure and cardioid output

90

95

100

105

110

115

120

125

130

10 100 1000 10000

Frequency [Hz]

Le

ve

l [d

B]

SPL @ 30 cmSPL @ 5 cmCardioid @ 30 cmCardioid @ 5 cm


Proximity effect

• Demo


Manufacturers’ data sheets

Cardioid



Cardioid



Cardioid



Omni-directional



• 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!


Proximity effect (cardioid mic)Frequency and distance dependence of

sound pressure and cardioid output

90

95

100

105

110

115

120

125

130

10 100 1000 10000

Frequency [Hz]

Le

ve

l [d

B]

SPL @ 30 cmSPL @ 5 cmCardioid @ 30 cmCardioid @ 5 cm



Cardioid



Cardioid



Cardioid



Omni-directional


Demo

• Proximity effect:– Hear the bass lift from the directed microphone


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


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 data-

sheet may say so)– No need for battery/power supply


Bottom line...

• Use omni-directional, electret/condenser microphones for scientific purposes!

• Make sure batteries are fresh or use some other type of power supply


Room acoustics

• In a room sound originates from:– the sound source, directly– or from reflections at the walls


Room acoustics

70

75

80

85

90

95

100

105

0 1 2 3 4 5

Position [m]

SP

L [

dB

]

DiffuseDirectSum


Room acoustics

• Reverberation radius, rr

– The distance where reflected and direct sound are equally loud

• Less absorbtion => stronger reflections => smaller rr


Room acoustics

70

75

80

85

90

95

100

105

0 1 2 3 4 5

Position [m]

SP

L [

dB

]

DiffuseDirectSum


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

TVrr 056.0


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:

TVrr 056.0


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!

70

75

80

85

90

95

100

105

0 1 2 3 4 5

Position [m]

SP

L [

dB

]

DiffuseDirectSum


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


Calibration file, example

Calibration tone ”The level was 89 dB”


CalibrationCalibrator

• 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!


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


• 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

Calibration Voice + SPL meter


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


Calibration, directed microphones ?

>30cm

>30cm

• 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”


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:

1

221 lg20

d

dLL


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


Recommendationsmicrophone and room acoustics

• Depend on – the purpose of recording– the recording environment

• Noise

• Room acoustics Example of purposes:SPLSpectrumF0Inverse filteringHNRPerceptual 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


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


Spectral properties (LTAS, H1-H2, line spectra)


• 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


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!


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


Harmonics-to-noise ratio


• Background noise must be lower than voice noise

• Microphone distance 5-50 cm– Well within rr


Perceptual evaluation


• Reduce background noise as much as possible

• Microphone distance 5-50 cm– Well within rr


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!


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


These were my recommendations

• You may find reasons to not follow them

• But they better be good...

Questions?

This presentation is available on the webwww.speech.kth.se/~svante/pevoc5

[email protected]

Documents

A tutorial on acoustic measurements for the non-technician Svante Granqvist Royal Institute of Technology (KTH) Dept of Speech Music and Hearing (TMH)