Acoustical
measurements
Iiro Jantunen
Nokia Research Center
19.4.2006
S-108.4010 Licentiate course in
measurement science and
technology
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Contents
• Principles of acoustics
• Acoustics measurements
• Microphone
• Sound pressure level
measurements
• Sound intensity
measurements
• Calibration
• SoundField
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Principles of acoustics
• Sound waves in gas or liquid
• No shear forces
→ no transverse waves
→ purely longitudinal waves
• Audible sound range 20 Hz – 20 kHz
• Fully described by 3 variables
• Pressure
• Particle velocity
• Density
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Wave equations of sound
• Euler’s equation
• Newton’s 2nd law (F=ma)
applied to fluid
• Continuity equation
• Bringing extra air to a volume
increases density
• State equation
• Relates pressure changes to
density
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Acoustical measurements.ppt / 2006-04-19 / IJ
Wave equation of sound
• Previous wave equations
used pressure, density and
particle velocity
• Eliminating density and
particle velocity the wave
equation of sound is
obtained
• Two basic solutions:
• Plane wave
• Spherical wave
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Acoustical measurements.ppt / 2006-04-19 / IJ
Free field acoustics
• Sound propagates to all
directions without diffraction,
reflection or absorption
• Spherical waves
• In principle, infinite, empty
space without reflections
• In practice, anechoic
chamber, with near 100%
absorptive walls
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Acoustical measurements.ppt / 2006-04-19 / IJ
Free field microphone
• Intended to measure the sound
pressure as it existed before the
microphone was introduced
• Microphone pointed to source
• Microphone tip causes an increase
in sound pressure
• Taken care of by internal
acoustical damping to achieve flat
frequency response
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Diffuse field – random incidence
microphone
• Sound reflects from many
directions → sound comes to
microphone from every direction
• In practice achieved in a
reverberation room with 100%
reflective and unparallel walls
• Microphone diffracts the sound
waves from different directions in
different ways
• Combined influence depends on
directional distribution of sound
waves
• Standard distribution based on
statistical considerations used for
random incidence microphone
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Acoustical measurements.ppt / 2006-04-19 / IJ
Closed coupler
• Chamber with small
dimensions compared to
sound wavelength
• Special case:
standing wave tube
• Diameter smaller than sound
wavelength
• Source at the end
• Possible to calculate the
sound field
• Used in calibration
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Acoustical measurements.ppt / 2006-04-19 / IJ
• Used in microphone
calibration
Pressure microphone
• Measuring the actual
pressure on a wall
• Typically used in closed
coupler for calibration
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Microphone directionality
• Directionality indicates the sensitiveness of a microphone to
sound coming from different directions
• No microphone is perfectly omnidirectional
• Cardioid or hypercardioid commonly used to record vocals
• Most ribbon microphones are bi-directional
• Shotgun directionality used outdoors for TV/film production
and wildlife recordings
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Acoustical measurements.ppt / 2006-04-19 / IJ
Parabolic microphone
• Parabolic reflector used to
collect sound waves to
microphone
• Very directional
• For eavesdropping in e.g.
spying
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Acoustical measurements.ppt / 2006-04-19 / IJ
Microphone transducers
• Condenser microphones
• Electret capacitor microphones
• Dynamic microphones
• Ribbon microphones
• Carbon microphones
• Piezoelectric microphones
• Laser microphones
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Acoustical measurements.ppt / 2006-04-19 / IJ
Condenser microphone
• Diaphragm and backplate
form a plate capacitor
• Charge kept constant
→ voltage varies as
pressure actuates the
diaphragm
• External voltage supply or
pre-charged diaphragm
• Acoustical performance
determined by physical
dimensions
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Condenser microphone – cont
• The larger the diaphragm, the
more sensitive the microphone
• Upper limit is defined by
diaphragm touching the backplate
• The smaller the microphone, the
greater the frequency range
• Increasing tension extends range
but decreases sensitivity
• Optimum size of a measurement
microphone is (up to 20 kHz) is
about 12.6 mm (1/2’’)
• Damping effect of air reduced by
drilling holes in the backplate
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Electret microphone
• Invented at Bell Labs in 1962 by
Gerhard Sessler and Jim West
• Diaphragm permanently polarized
the same way as permanent
magnets magnetized (electrostatic
magnet)
• Once considered low price and low
quality
• Now most common microphone
type
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Acoustical measurements.ppt / 2006-04-19 / IJ
Dynamic microphone
• A movable coil is attached to the
diaphragm
• An unmovable magnet produces a
magnetic field
• Moving diaphragm moves the coil
in the magnetic field, inducing a
measurable current
• Exactly same principle as in
loudspeakers, only reversed
• Poor low-frequency response
→ reduces handling noise
• Robust, relatively inexpensive and
resistant to moisture
→ widely used on-stage
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Acoustical measurements.ppt / 2006-04-19 / IJ
Ribbon microphones
• Revolutionized recording and
broadcast industry in the 30’s
• Special type of dynamic
microphones
• Thin metal ribbon between poles
of magnet
• Voltage output typically low
compared to normal dynamic
microphones
• Bidirectional
• Very sensitive and accurate
• Generally delicate and expensive
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Acoustical measurements.ppt / 2006-04-19 / IJ
Carbon microphones
• Invented by David Hughes in 1878
• Very important in the history of
telephone
• Sound pressure (AP) presses the
diaphragm (2) to a bed of carbon
granules (1). Contact resistance
depends on the pressure
→ resitance R changes
• Also an amplifier
• Extremely low-quality sound
reproduction
• Very limited frequency range
• Very robust
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Piezo microphones
• Piezoelectric material
• Diaphragm moves the armature to
bend piezoelectric crystal over a
fulcrum
• Small size, cheap, low quality
• Have replaced carbon
microphones
• Often used as
• contact microphones to sound
instruments
• underwater or other unusual
environments
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Acoustical measurements.ppt / 2006-04-19 / IJ
Laser microphones
• Window of a room acting as
diaphragm
• Reading with laser beam reflected
from the window
• Two laser beams for common
mode rejection of large window
movements and path disturbances
• For eavesdropping
• Works best with one-glass
windows
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Acoustical measurements.ppt / 2006-04-19 / IJ
Sound level measurements
• Measurement of sound pressure filtered by
• frequency (A-weighting)
• time-domain (RMS)
• Mimics response of human ear to noise
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Human hearing frequency response
A-weighting curve
For subjective responses in special cases there are B-, C- and D-weighting curves
•very high or low level
•special noise, e.g., of aircraft
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Sound level measurements
• IEC International Standard 651 ”Sound Level Meters”
• Tolerances per frequency band defined for 4 classes
of accuracy
• Type 0: precision laboratory use
• Type 1: general purpose
• Type 2: low price
• Type 3: not used in practice (too wide tolerances)
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Acoustical measurements.ppt / 2006-04-19 / IJ
Sound intensity measurements
ISO Standard 3745 “Acoustics —
Determination of sound power
levels of noise sources —
Precision method for anechoic
and semi-anechoic rooms”
no.
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Acoustical measurements.ppt / 2006-04-19 / IJ
x/r
y/r
z/r
1
-0.99
0
0.15
2
0.5
-0.86
0.15
3
0.5
0.86
0.15
4
-0.45
0.77
0.45
5
-0.45
-0.77
0.45
6
0.89
0
0.45
7
0.33
0.57
0.75
8
-0.66
0
0.75
9
0.33
-0.57
0.75
10
0
0
1.00
Two-microphone probe
• Measures the sound intensity in
two directions
• Pressure is mean of the two
measured pressures
• Air particle velocity calculated from
the two pressures
• All intensity is in radial direction,
no intensity in perpendicular
• Powerful tool to locate noise
sources
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Calibration techniques
• Reciprocity calibration method
• Comparison or substitution methods
• Pistonphone (closed coupler)
• Sound pressure calibrator
• Electrostatic actuation
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Reciprocity calibration method
• Microphone can be used as
a loudspeaker
• Very accurate
• Three test microphones
measured against each
other alternating the function
• Requires well-controlled
environment
• As a result a set of 3
equations with microphone
sensitivities as unknowns
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
• Rather tedious
• Seldom used in practical
situations
Comparison/substitution methods
• Microphone measured related to a
reference microphone
• Comparison method: microphone and
reference at the same time
• Substitution method: microphone put in the
lace of the reference
• Sound source stability
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
Pistonphone
• Closed coupler
• Well-defined sound pressure
level
• Relatively simple
mechanically, very stable
• Used often as the sound
source in
comparison/subsitution
calibration
• Accuracy around 0.1 dB
• Depends on
• Volume of the coupler
• Volume displacement
• Barometric pressure
• Humidity
• Heat dissipation
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Acoustical measurements.ppt / 2006-04-19 / IJ
Sound pressure calibrator
• Small, self-contained
• Comparison calibrator
• Closed coupler
• Small loudspeaker produces
single-frequency signal
• Reference microphone gives
feedback signal
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
• Well-defined, provided that
reference microphone and
feedback gain are stable
• For field-calibration of
microphones
• Normally not for laboratory
calibrations
Electrostatic calibration
• Direct use of electrostatic
actuator to drive the
diaphragm
• 800 V DC
• 50-150 V AC signal
• Generally used to measure
frequency response of
microphones
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ
• Widely used as a convenient
and accurate test method
• For production and final
calibration of measurement
microphones
SoundField microphone
• 3D view of the sound with a single
device
• 4-channel measurement of sound:
B-format
• The spatial pattern can be decided
later
• Mono, stereo, 5.1, …
• Fairly expensive, but replaces
effectively a system of many
microphones
• http://www.soundfield.com
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© 2006 Nokia
Acoustical measurements.ppt / 2006-04-19 / IJ