Acoustic Transducers

Transcription

1 Acoustic Transducers Fateme Mohandespour Amirkabir University of Technology Autumn2003

2 Overview Sound to voltage transducers Voltage to sound transducers Ultrasound as an instance References 2

3 Sound to Voltage Transducers Carbon transducers Dynamic transducers Condenser transducers Piezo transducer 3

4 Carbon Transducers The simplest type of modern microphone is the CARBON microphone, used in telephones. This microphone consists of a metallic cup filled with carbon granules. A movable metallic diaphragm mounted in contact with the granules covers the open end of the cup. Sound waves vibrate the diaphragm, varying the pressure on the carbon granules. The electrical resistance of the carbon granules changes with the varying pressure, causing the current in the circuit to change according to the vibrations of the diaphragm. 4

5 Carbon Transducer These type of transducers are not widely used because of : Poor frequency response Bad signal-to-noise ratios need Power supply Fig.1-1 5

6 Dynamic Transducers In the magneto-dynamic, commonly called dynamic transducer, sound waves cause movement of a thin metallic diaphragm and an attached coil of wire. According to the faraday's electromagnetism law, motion of the coil within the field causes current to flow. The principles are the same as those that produce electricity at the utility company, realized in a pocket-sized scale. The amount of current is determined by the speed of that motion. This kind of microphone is known as velocity sensitive. 6

7 Dynamic Transducers Fig.1-2 7

8 Dynamic Transducers Dynamic microphones are renowned for : Can withstand extremely high sound levels without damage or excessive distortion They need no batteries or external power supplies Their ruggedness and reliability They are capable of smooth, extended response Can be easily "tailored in response for special applications Reasonable care will maintain their performance for many years The disadvantage is that it doesn't respond well to extreme frequencies. 8

9 Dynamic Transducers Element: Neodymium dynamic impedance: 200 ohms polar pattern: Hypercardioid signal to noise ratio: 0.00db, 1 khz at 1 pa frequency response: 70-16,000 Hz open circuit sensitivity: -56 db (1.5 mv) re 1V at 1 pa Fig.1-3 9

10 Ribbon Transducers It is a form of dynamic, with a thin metallic ribbon (which serves as both voice coil and diaphragm) suspended between the poles of a magnetic circuit. While it is capable of excellent performance, the ribbon element must be protected against high acoustic pressures, since it is relatively fragile. Wide frequency range Very sensitive to physical abuse. Fig

11 Condenser Transducers In a condenser microphone, the diaphragm is mounted close to, but not touching, a rigid backplate. (The plate may or may not have holes in it.) A battery is connected to both pieces of metal, which produces an electrical potential, or charge, between them. As the distance changes, the diaphragm moves in response to sound and the current flows in the wire.the amount of current is essentially proportional to the displacement of the diaphragm. Extremely wide frequency. Good pick up sensitivity. Condenser microphones are easy to miniaturize, while dynamic microphones cannot be miniaturized. The down side is that it is easy to overload and distorts if placed too close to a high intensity sound source. 11

12 Condenser Transducers Fig.1-6 Element: condenser phantom power requirements: 9-52V, 2 ma typical polar pattern: Hypercardioid battery type: 1.5V AA/UM3 frequency response: 30-20,000 Hz open circuit sensitivity: phantom: -38 db (12.5 mv) re 1V at 1 pa, battery: re 1V at 1 pa switch: flat/roll-off impedance: phantom ohms low frequency roll-off: 180 Hz, 12 db per octave maximum input sound level: phantom- 130 db SPL, 1 khz at 1% T.H.D., Battery- db SPL, 1 khz at 1% T.H.D. Signal to noise ratio: 70.00db, 1 khz at 1 pa 12

13 Electret Condenser Transducers The electret class of microphones are condenser microphones which use a permanently polarized electret material for their diaphragms, thus avoiding the necessity for the biasing DC voltage required for the conventional condenser. They are the typical microphones on portable tape recorders. Fig.1-7(b) Fig.1-7(a) 13

14 Electret Condenser Transducers Very uniform frequency response. Ability to respond with clarity to transient sounds. No external power supply is needed(although an FET impedance matching circuit is typically required, powered by a small low-voltage battery in the microphone itself.) Very inexpensively The low mass of the diaphragm permits extended high frequency performance,while the nature of the design also insures outstanding low frequency response. Element type: Electret condenser frequency response: 40-18,000 Hz polar pattern: half-supercardioid impedance: 150 ohms sensitivity: 22 mv/pa Fig

15 Electret Condenser Transducers GLM-100 electret-condenser type omnidirectional Powered by 12 to 48V phantom power. Smooth frequency response from 20 Hz to 20 khz Uniform off-axis response Sensitivity : 3.2 mv/pa maximum SPL for 3% THD shall be 150 db Fig

16 Piezo Transducers The crystal microphone uses a thin strip of piezoelectric material attached to a diaphragm. The two sides of the crystal acquire opposite charges when the crystal is deflected by the diaphragm. The charges are proportional to the amount of deformation and disappear when the stress on the crystal disappears. Fig

17 Piezo Transducers Early crystal microphones used Rochelle salt because of its high output, but it was sensitive to moisture and somewhat fragile. Later microphones used ceramic materials such as barium titanate and lead zirconate. The electric output of crystal microphones is comparatively large, but the frequency response is not comparable to a good dynamic transducer. Impedance: 1 meg ohm or more. This unit could be destroyed with a bit of humidity! 17

18 Piezo Hydrophone Designed to handle the high sound pressure levels and the high static ambient pressure in water and other fluids,the the 8011 hydrophone uses a piezoelectric sensing element, which is frequency compensated to match the special acoustic conditions under water. The output is electronically balanced and offers more than 100db dynamic range. It is the ultimate choice for professional sound recordings in water or under other extreme conditions where conventional microphones would be adversely affected Fig hydrophone 18

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20 Characteristics Comparison Frequency response Electrets Impedance Piezo Dynamic Carbon Piezo Electrets Carbon Fig Direction patterns 20

21 Characteristics Comparison Fig.1-13 Fig.1-14 Fig

22 Acoustic MEMS Akustica's microphone chips combine MEMS microphones with software and microelectronics onto a single, standard CMOS chip. Fig.1-16 Fig

23 Smart Sensors Detect, track, and classify ground/air vehicles - 4 aperture, 5 mic array, DSP - Hand emplace or air deploy w/ optional parachute - Self mapping via GPS - Separate long haul and short haul data radios Fig.1-18 Acoustic emission technology detects the sound of the wheel touching the part with accuracy of less than one micron. Fig.1-20 Fig.1-21 Fig

24 Voltage to Sound Transducers Piezo transducers Dynamic transducers 24

25 Piezo Transducers The working component in an audible sound transducer usually is a thin disc of piezoelectric ceramic bonded to a similarly thin metal membrane. when it is excited at low frequency, a piezoceramic material vibrates; At high frequencies it also produces sound, as a transducer does. The resonant frequency of the ceramic is too high to produce an audible tone by itself, so a metal plate must be attached that vibrates with the contraction and expansion of the piezoceramic. 25

26 Piezo Transducers Both audible and silent alerts can be generated from the same source by exciting it with two different frequencies. Fig

27 Piezo Transducers Applications Receiver/microphone Piezoelectric vibrator Ultra-sound transmitter Ultrasound receiver Piezoelectric oscillator Clock/watch Computer Transmitter/handy-phone Piezoelectric transformer Liquid crystal display Piezoelectric optical modulator Beam scanner Semi-conductor device 27

28 Dynamic Transducers The speaker is essentially the same as the dynamic microphones except the current carrying the sound to be recovered is fed trough the coil, which creates a moving magnetic field around the coil. The interaction between the magnetic field about the coil and the field of the permanent magnetic cause the diaphragm to move back and forth. The motion of the diaphragm creates the original sound wave. Additionally implying an ac voltage to a piezoelectric material makes the piezo vibrating by the same frequency. Fig

29 Dynamic Transducers In fact, many intercom systems use small speakers (with lightweight cones) as both speaker and microphone, by simply switching the same transducer from one end of the amplifier to the other. Fig

30 Dynamic Transducers 800/32/din specifications Receiver type: dynamic 36mm speaker impedance: 32 ohm +/-15% at 1khz S.P.L: 99 db +/- 4 db at 1khz frequency range: (1mw input) 150hz - 10khz input power: 10mw (max. 100mw) Fig.2-4 Fig

31 Suppliers And so on 31

32 Ultrasound Introducing ultrasound Ultrasound transducers Ultrasound applications 32

33 Ultrasound Principle:a short pulse of ultrasound is transmitted by the PZT transducer a booster and a sonotrode (meant to increase vibratory speed). The waves reflected by the object, can be picked up by the receiver. For economy, one transducer is normally used for both transmitting and receiving. The system is electronically switched between transmitting and receiving functions.(fig 3-1) Fig

34 Ultrasound The interval between transmitting and receiving pulses provides a measure of the distance to the object and can be shown on a display.this operating principle requires a minimum distance between transducer and object.the transducer cannot receive properly before the vibrations caused by the transmission have subsided.the ultrasound pulse should therefore be as short as possible, which calls for a high operating frequency and a large bandwidth.a high operating frequency also means that the transducer can be smaller and more compact. In practice, an echo sounder operating at 200 khz will have A minimum detection distance of 0.2 m in air. Modern ultrasound devices, like those used in the medical field,operate at several megahertz.these can detect down to the millimeter region, but because of much higher attenuation at this frequency, their maximum range is limited. 34

35 Medical Applications The technique is now widely used in virtually every branch of medicine. it is used to : Sonograph Cardiology Retinal problems Heat joints Relieving arthritic joint pain Lithotripsy, in which shock waves break up kidney stones, eliminating the need for surgery Is noninvasive Involves no radiation Avoids the possible hazards such as bleeding, infection, or reactions to chemicals of other diagnostic methods. 35

36 Industrial Applications Positioning sensing Acoustic power and intensity measurement Non-destructive material evaluation Concrete tube inspection Bubble and particle detection Flow metering (speckle tracker) Leakage testing Level metering Acoustic microscopy 36

37 Ultrasound Receiver Fig.3-2 Fig

38 The End 38