Sensing and Sensors: Acoustic Sensors version 1.1 MediaRobotics Lab, January 2008

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1 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: 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.

2 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.

3 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

4 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 )

5 Human voice signal (5 seconds) and the corresponding frequency componets

6 Power spectrum of a human whistle versus a human (male) voice

7 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

8 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 440 Hz fundamental frequency first harmonic 2f 880 Hz first overtone second harmonic 3f 1320 Hz second overtone third harmonic 4f 1760 Hz third overtone fourth harmonic Rhythm is a recurring sound that alternates between strong and weak elements

9 Envelope of a sound loudness [db] peak time [seconds] attack sustain decay

10 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 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 log 10 P1/ P0 where the unit of β is the decibel (db) and p0=10 12 W /m 2, the 'sound threshold' Example: 30dB is the ratio between a base sound and a sound 1000 times more intensive 10log W /1 W =30dB Here some notable sound levels Threshold of hearing 0dB (β = 0) heavy traffic 80 db Niagara Falls 85 db threshold of pain 120dB hydraulic press at 1m 130dB

11 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

12 condensor microphones / capacitive microphones background: capacitance, charge and voltage across two conducting plates a distance d apart area A q voltage V distance d q V = q d / em e0 A em: material constant e0: permitivity constant C2 / Nm 2

13 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

14 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 m

15 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 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)

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19 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.

20 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.

21 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.

22 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.

23 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.

24 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

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