sound sensors

Sound transducers : 

Sound transducers Eng . Amgad younis Eng . Walaa Mohamed

introduction : 

introduction The sound wave is the waveform caused by a vibration and which in turn cause an identical vibration to be setup in any material affected by the sound wave . Wave parameters are : velocity (speed) , wavelength and frequency .

Slide 3: 

The measurement of sound wave in two ways : Intensity : measure as the number of watts of sound energy /square meter of receiving surface Wave quantity : pressure amplitude or displacement amplitude . The frequency range of the sound detected by the human ear is limited to range of about 20Hz to 20kHz The sound transducers are not necessarily constrained to these frequency limits, and some can be used with infrasound (very low frequencies) or with ultrasound (very high frequencies)

Slide 4: 

The effect of a sound wave on a material that it strikes is to vibrate the material, and so every part of the material will be accelerated ..... an electrical output can be obtained from an accelerometer connected to the material ....... so it is the same form for acceleration and velocity sensors and the main difference are the ways of use . There are three main types of sound transducers : Audio to electrical transducers : microphone . Electrical to audio transducers : earphones/loudspeakers . two way transducers : ultrasound .

Slide 5: 

The sound to electrical energy transducers is the microphone ., and the microphone types are Classified by the type of electrical transducer they use . Microphones can also be identified by their directional properties, that is, how well they pick up sound from various directions

Slide 7: 

Principle : uses loosely packed granules of carbon held between a diaphragm and a backplate . When the granules are compressed the resistance between diaphragm and backplate drops and the vibration in diaphragm converted into variation in granules resistance

specification : 

specification 1- doesnot generate voltage so it require external supply 2- it provide an o/p that is large by microphone standards with o/ps of 1 V peak-to-peak . 3 - very poor linearity . 4- has multiple resonance in the audio range and high noise level (the resistance of the granules changes in a random way even no sound presence) . application : telephony because of high o/p at a time when no amplification is possible .

Slide 9: 

The principle : the magnetic reluctance of the circuit changes as the armature moved where the diaphragm attached, and so changes with the magnetic flux in the magnetic circuit ,then this produce emf in the coil proportional to the magnetic flux and the diaphragm movement .

specification : 

specification 1- Linearity : reasonable for small movement amplitude of the armature . Very poor for large amplitude . Can be improved by appropriate shaping of the armature and carful attenuation to its path of vibration 2- high max o/p : 50mV . 3- high o/p impedance (hundereds) . 4- very sensitive to external changes on the magnetic flux , can be reduced by shielding the magnetic circuit 5- heavier than other types due to the magnetic circuit .

Slide 11: 

Principle : use constant flux magnetic circuit where by moving a small coil of wire in the magnetic circuit the electrical o/p is generated (coil attached to a diaphragm)

Slide 12: 

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.

specification : 

specification 1- excellent Linearity due to small movement range of the coil 2- low o/p . 3- low impedance . specification Element: Neodymium dynamicimpedance: 200 ohmspolar pattern: Hypercardioidsignal to noise ratio: 0.00db, 1 kHz at 1 pafrequency response: 70 - 16,000 Hzopen circuit sensitivity: -56 db (1.5 mV) re 1V at 1 pa

Slide 14: 

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.

specification : 

1- Wide frequency range 2- Very low sensitivity . 3- o/p very low and need amplifier . 4- Highest sound quality . 5- Excellent linearity . 6- Very expensive . 7- Velocity operated . specification

Slide 16: 

characteristic of certain crystalline minerals: when subjected to a mechanical force, the crystals became electrically polarized. Tension and compression generated voltages of opposite polarity, and in proportion to the applied force. if one of these voltage-generating crystals was exposed to an electric field it lengthened or shortened according to the polarity of the field, and in proportion to the strength of the field. These behaviors were labeled the piezoelectric effect and the inverse piezoelectric effect, respectively, from the Greek word piezein, meaning to press or squeeze

Slide 17: 

Principle : 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 diaphragm deflects the crystal. The charges are proportional to the amount of deformation and disappear when the stress on the crystal disappears.

specification : 

1- crystal material : - Rochelle salt because of its high output, but it was sensitive to moisture and somewhat fragile. - ceramic materials such as barium titanate and lead zirconate. 2- The electric output of crystal microphones is comparatively large . 3- but the frequency response is not comparable to a good dynamic transducer. 4- Impedance: 1 Meg ohm or more. (This unit could be destroyed with a bit of humidity or high temperature) 5- pressure operated . Application : cassette recorder . specification

Slide 19: 

Principle : the diaphragm is mounted close to, but not touching, a rigid backplate. 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 proportional to the displacement of the diaphragm.

specification : 

1- very linerar 2- o/p depend on the spacing between plates , if small spacing introduce high voltages . 3- Extremely wide frequency. 4- Impedance: very high . 5- pressure operated . 6- very good quality of audio signals . Application : cassette recorder . specification

Slide 21: 

Element: condenserphantom power requirements: 9 - 52V, 2 ma typicalpolar pattern: Hypercardioidbattery type: 1.5V AA/UM3frequency response: 30 - 20,000 Hzopen circuit sensitivity: phantom: -38 db (12.5 mV) re 1V at 1 pa, battery: re 1V at 1 paswitch: flat/roll-offimpedance: phantom - 500 ohmslow frequency roll-off: 180 Hz, 12 db per octavemaximum 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

Slide 22: 

Electret Condenser Transducers The electret class of microphones is 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.

specification : 

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 Element type: Electret condenserfrequency response: 40 - 18,000 Hzpolar pattern: half-Supercardioidimpedance: 150 ohmssensitivity: 22 mV/pa GLM-100 electret- specification

Slide 24: 

Microphones Characteristics Comparison:

Slide 25: 

Earphones are the opposite acts of microphone for which it convert the electrical signal to a vibration of the diaphragm of the transducer used which is a sound .

Slide 28: 

- 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.

Slide 32: 

Ultrasound echo sounders operate of the following principle : A short pulse of ultrasound is transmitted by the PZT Transducer in the direction of the object that needs to be located. The waves will be reflected by the object and can be picked up by the receiver. Types of ultrasound transducers : 1- magnetostrictive . 2- piezoelectric .

Ultrasound applications : 

Ultrasound applications

The end : 

The end