Sensing and Sensors: Acoustic Sensors
version 1.1
MediaRobotics Lab, January 2008
Background: sound waves
Sound waves are created by alternate compression and expansion of solids, liquids or
gases at certain frequencies.
Longitudinal mechanical waves: oscillation in the direction of wave propagation
'Sound' are longitudinal mechanical waves between 20 and 20khz, based only on our
own hearing abilities / limitations... Mechanical waves below 20hz. Are perceived by
dogs and called Infrasound by humans.
Check your hearing and your audio equipment here:
http://www.audiocheck.net/audiotests_frequencychecklow.php
References:
Fraden: Handbook of Modern Sensors
Drafts, Acoustic Wave Sensors
Buff, SAW Sensors
Cady. Piezoelectricity: An Introduction to the Theory and Applications
of Electromechanical Phenomena in Crystals.
The Speed of Sound
The speed of sound depends on the medium through which the waves are passing, and
is often quoted as a fundamental property of the material. In general, the speed of sound
is proportional to the square root of the ratio of the elastic modulus (stiffness) of the
medium to its density. Those physical properties and the speed of sound change with
ambient conditions.
For example, the speed of sound in gases depends on temperature. In air at sea level,
the speed of sound is approximately 343 m/s, in water 1482 m/s, and in steel about
5960 m/s (at 20 °C). The speed of sound is also slightly sensitive (a second-order effect)
to the sound amplitude, which means that there are nonlinear propagation effects, such
as the production of harmonics and mixed tones not present in the original sound.
http://en.wikipedia.org/wiki/Sound
Sound as an waveform can be described in terms of its energy and the
frequencies it can be decomposed into
sound wave of a human voice
in the time domain
Signals are converted from time or space domain to the frequency domain usually
through the Fourier transform. The Fourier transform(s) describe a decomposition of a
function in terms of a sum of sinusoidal functions (basis functions) of different
frequencies that can be recombined to obtain the original function.
The Fourier transform and its various derivatives form an important part of the art and
science of digital signal processing (more on this later in the course).
S(t) =
50mV . sin (2 pi 1000 t + pi/2) +
100mV . sin (2 pi 2000 t + 0 ) +
100mV . sin (2 pi 3000 t + 0 ) + ..... + ....
http://www.4p8.com/eric.brasseur/fouren.html
Human voice signal (5 seconds) and the corresponding frequency componets
http://en.wikipedia.org/wiki/Frequency_spectrum
Power spectrum of a human whistle versus a human (male) voice
Sound as a qualitative measure is often described as having the following components
"Music components":
* Pitch
* Timbre
* Harmonics
* Loudness
* Rhythm
"Sound envelope components":
* Attack
* Sustain
* Decay
The pitch of a sound is determined by the frequency of the sound.
* low (bass) - sounds of thunder and gunshots
* midrange - a telephone ringing
* high (treble) - small bells and cymbals
Timbre is that unique combination of fundamental frequency, harmonics, and overtones
that gives each voice, musical instrument, and sound effect its unique coloring and
character.
The harmonic of a wave is a component frequency of the signal that is an integer multiple
of the fundamental frequency.
1f
2f
3f
4f
440 Hz
880 Hz
1320 Hz
1760 Hz
fundamental frequency
first overtone
second overtone
third overtone
first harmonic
second harmonic
third harmonic
fourth harmonic
Rhythm is a recurring sound that alternates between strong and weak elements
Envelope of a sound
peak
loudness
[dB]
time [seconds]
attack
sustain
decay
Loudness, a subjective measure, is not equivalent to objective measures of sound
pressure such as decibels or intensity. Research suggests that the human auditory system
integrates intensity over a 600-1000 ms window.
The abstraction of loudness is sound intensity. Like several other physical properties (light
and noise) sound intensity is measured in decibel, a logarithmic scaling. The decibel scale
linearizes a physical value in which exponential changes of magnitude are perceived by
humans as being more or less linearly related; a doubling of actual intensity causes
perceived intensity to always increase by roughly the same amount, irrespective of the
original intensity level.
sound intensity is described by convention in Decibels : β=10 log10  P1/ P0
where the unit of β is the decibel (dB) and p0=10−12 W / m2 , the 'sound threshold'
Example: 30dB is the ratio between a base sound and a sound 1000 times more intensive
10 log10 1000W /1 W =30dB
Here some notable sound levels
Threshold of hearing
heavy traffic
Niagara Falls
threshold of pain
hydraulic press at 1m
0dB (β = 0)
80 dB
85 dB
120dB
130dB
Microphones
Microphone: acoustic sensors for air waves in the audible range
Hydrophone: acoustic sensor for liquid waves
microphone / hydrophone are pressure sensors with a wide dynamic range...
A microphone / hydrophone is a pressure transducer, adapted for the transduction of sound /
liquid waves.
All microphones / hydrophones have a moving diaphragm and a displacement tranducer that
converts this motion into an electric signal.
Microphones / hydrophones differ by :
sensitivity, direction characteristics, frequency bandwidth, dynamic range
condensor microphones / capacitive microphones
background: capacitance, charge and voltage across two conducting plates a distance d apart
area A
+
+
+
+
+
+
+
+
+
+
+
+
voltage V
-
distance d
+q
-q
V = q∗d /em∗e0∗A
em: material constant
e0: permitivity constant
−12C2
8.8542∗10
/ Nm
2
A capacitive microphone linearly converts a distance between plates into an electric voltage.
The device requires a source of electric charge (q) whose magnitude directly determines the
microphone sensitivity.
Many capacitive / condenser microphones are fabricated of silicon diaphragms that convert
the acoustic pressure of the sound wave into a (distance) displacement
Mechanical feedback:
improves the frequency
range of the microphone,
but reduces deflection ->
lower sensitivity
fiber-optic microphones
Preferable where capacitive measurements are impossible (inside a rocket engine)
Design: a single-mode temperature insensitive interferometer + reflective plate diaphragm.
The interferometer emits a laser beam that is used to detect the plate deflection which is
directly related to the acoustic pressure. The phase of the reflected light will vary and differ
from that of the (reflected reference light). Since both sensing and reference light travel in
the same light guide, they interfere resulting in light intensity modulation.
Such microphones can detect diaphragm movement in the order of 10−10 m
piezoelectric microphones
background: the piezoelectric effect
A piezoelectric crystal is a direct converter of mechanical stress to electric charge.
When compressed or pulled, a piezoelectric crystal will build up alternate charges on
opposite faces, thus acting like a capacitor with an applied voltage. A current
(piezoelectricity) can then be generated between the faces.
When subjected to an external voltage, the crystal will expand or contract accordingly.
1880 - 1882
The first experimental demonstration of a connection between macroscopic
piezoelectric phenomena and crystallographic structure was published in 1880 by
Pierre and Jacques Curie.
Their experiment consisted of a conclusive measurement of surface charges
appearing on specially prepared crystals (tourmaline, quartz, topaz, cane sugar {sic}
and Rochelle salt) subjected to mechanical stress. These results were obtained using
tinfoil, glue, wire, magnets and a jeweler's saw.
Other areas of scientific phenomenological experience that were noted around the
same time:
"contact electricity" (friction from static electricity)
"pyroelectricity" (electricity from crystals via heating)
http://www.designinfo.com/kistler/ref/tech_theory_text.htm
http://www.piezo.com/tech4history.html
http://www.piezomaterials.com/
http://www.nanophys.ethz.ch/members/baumgartner/Work/AFMBasics.html
Today piezoceramics are preferred as there specifications can be more
tightly controlled (and synthesized). Also, piezoceramics can operate up
to higher frequencies.
Typically, a piezoelectric disk with two electrodes serves as the input to a
high impedance amplifier. Incoming acoustic waves generate
mechanical stress in the disk and a corresponding piezoelectric current.
Electret microphones
An electret microphone is a permanently electrically polarized crystalline dielectric material. It
is an electrostatic transducer consisting of metalized electret and a backplate separated from
the diaphragm by an air gap.
Because the electret is permanently electrically polarized, there is an electric
field in the air gap. When an acoustic wave hits the device, the air gap is altered
(reduced):
V =s∗ds/e0 se∗s1
Fraden states (after a few derivations) that the sensitivity does not depend on the area
of the dielectric.
fr=1/ 2pi∗  po/ so∗M 
M: mass of membrane
po: atmospheric pressure
so: effective thickness of membrane
This frequency should be set such that it is larger than the highest frequency to
which the microphone is expected to properly respond.
Electret microphones do not require a DC bias voltage for operation.
Acoustic wave sensors
Acoustic wave sensors are so named because their detection mechanism is a
mechanical, or acoustic, wave. As the acoustic wave propagates through or on
the surface of the material, any changes to the characteristics of the propagation
path affect the velocity and/or amplitude of the wave. Changes in velocity can be
monitored by measuring the frequency or phase characteristics of the sensor and
can then be correlated to the corresponding physical quantity being measured.
Virtually all acoustic wave devices and sensors use a piezoelectric material to
generate the acoustic wave. Piezoelectricity refers to the production of electrical
charges by the imposition of mechanical stress. The phenomenon is reciprocal.
Applying an appropriate electrical field to a piezoelectric material creates a
mechanical stress. Piezoelectric acoustic wave sensors apply an oscillating
electric field to create a mechanical wave, which propagates through the
substrate and is then converted back to an electric field for measurement.
Among the piezoelectic substrate materials that can be used for acoustic wave
sensors and devices, the most common are quartz (SiO2), lithium tantalate
(LiTaO3), and, to a lesser degree, lithium niobate (LiNbO3). An interesting
property of quartz is that it is possible to select the temperature dependence of
the material by the cut angle and the wave propagation direction.
The advantage of using acoustic
waves (vs electromagnetic waves) is
the slow speed of propagation (5
orders of magnitude slower). For the
same frequency, therefore, the
wavelength of the elastic wave is
100,000 times shorter than the
corresponding
electromagnetic
shortwave.
This allows for the fabrication of very
small sensors with frequencies into
the gigahertz range with very fast
response times.
Solid state acoustic detectors have
the electric circuit coupled to the
mechanical structure where the
waves propagate.
The sensor generally has two
(piezoelectric) transducers at each
end. One at the transmitting end
(generator) and one at the receiving
end (receiver) where the wave is
converted into an electric signal.
A typical acoustic wave device consists of two sets of interdigital transducers. One
transducer converts electric field energy into mechanical wave energy; the other converts
the mechanical energy back into an electric field.
Influence on SAW sensors
SOURCE:
W. Buff, SAW SENSORS FOR DIRECT AND REMOTE MEASUREMENT