In The Name Of God

Acoustic Transducers

Fateme Mohandespour

Amirkabir University Of Technology

Autumn2003

Overview

l Sound to voltage transducers

· Carbon transducers

· Dynamic transducers

· Condenser transducers

· Piezo transducer

l Voltage to sound transducers

· Piezo transducers

· Dynamic transducers

l Ultrasound Technology

· Introducing ultrasound

· Ultrasound transducers

· Ultrasound applications

Sound to Voltage Transducers

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. In fact, this is how the first telephones were constructed, and many phones to this day still use the idea. Here is an ancient model of carbon transducers:T-17 W/PL-68

These types of transducers are not widely used because of:

Poor frequency response
Bad signal-to-noise ratio
Need Power supply

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.

Dynamic microphones are renowned for:

Their ruggedness and reliability
They are capable of smooth, extended response
Can withstand extremely high sound levels without damage or excessive distortion
They need no batteries or external power supplies
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.

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.

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.

The down side is that it is easy to overload and distorts if placed too close to a high intensity sound source.

If the mic is going to be exposed to "severe environments" such as outdoors, fellowship halls, etc., then the dynamic mic is probably the best choice. Conversely, if the mic is going to be used in controlled environments, then the condenser mic will best meet that need.

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 - 500 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

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.

· 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.

Piezo Transducers

In 1880, Jacques and Pierre Curie discovered an unusual 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. Subsequently, the converse of this relationship was confirmed: 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. More about piezoelectricity

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.

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!

Piezo Hydrophone

Designed to handle the high sound pressure levels and the high static ambient pressure in water and other fluids, 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.

Piezo optimization circuit:

Microphones Characteristics Comparison:

Microphones can also be identified by their directional properties, that is, how well they pick up sound from various directions. More about polar patterns.

Suppose you have set up several microphones to pick up sound sources. Each sound source has its own close-placed mic. You are mixing the mic signals through a mixer. Sound from a single source arrives at each microphone at a different time. So, a mic that is distant from the source is picking up the source with a delay, which causes variable phase shift vs. frequency. When you combine the close and distant mic signals in your mixer, certain frequencies cancel out due to phase interference, creating a "comb-filter" effect. The frequency response of a comb filter has a series of peaks and dips (see figure below.) This response often gives a thin, hollow, filtered tone quality. This problem can be minimized or eliminated by following the 3:1 rule: Separate the mics by at least 3 times the mic-to-source distance. This creates a level difference of at least 9 dB between microphones, which reduces the comb-filter dips to an inaudible 1 dB or less. More info.

In this figure you see a microphone, which is plated due to decrease the effect of reflecting sound.

More info

Want to learn more about microphone theory?

A brief guide to mic (crown magnetics)

Microphone theory

Acoustic MEMS

Akustica's microphone chips combine MEMS microphones with software and microelectronics onto a single, standard CMOS chip.

For more information see the manufacturer web page.

Smart Sensors:

Acoustic emission technology detects the sound of the wheel touching the part with accuracy of less than one micron.

More info

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.

More info

Additional Information:

Microphone Preamps

Crown products

Microphone calibration system

Cmos compatible piezoelectric microphones

Microphones available from crown (technical info)

Voltage to Sound Transducers

Piezo Transducers:

A piezo-electric transducer has a mechanically resonant structure, because of the mass and stiffness of the piezo-electric material. Due to the piezo-electric effect these mechanical properties manifest themselves as electrical equivalent properties. A coil, capacitor and a resistor represent the mechanical mass, stiffnes and damping. The electrical equivalent model looks a lot like the well-known model for a crystal, which is a series circuit of a coil (L), a capacitor (C) and a resistor (R), which all are paralleled by a another capacitor (Cp).

Some peizo electric materials have resonant frequency between 1.2-41 KHZ, and some are over 300MHZ.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.

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

Piezo transducers are usually specified for a single frequency to work on, for example 40 kHz.

Underwater piezo speaker

Piezo Transducers Applications

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.

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.

800/32/din specifications

Receiver type: dynamic 36mm speaker
impedance: 32 ohm +/-15% at 1khz
frequency range: (1mw input) 150hz - 10khz
input power: 10mw (max. 100mw)

More Magnetic buzzers

Additional Information:

Omegapiezo

Piezo and magnetic buzzers

Manufacturers of piezo buzzers

Manufacturers of electromagnetic buzzers

Smart materials for silent alarms

More buzzers

Piezo MEMS

Sound devices

Ultrasound

Introduction

Ultrasound is a technique for study hard to reach areas by using inaudible high frequency sound waves, first used in world war2 in finding submerged objects.

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. For economy, one transducer is normally used for both transmitting and receiving. The system is Electronically switched between transmitting and receiving

Functions.

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.

Ultrasound transducers Ultrasound microphones

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.

Industrial Applications

More about ultrasound receiver 1, 2

Additional Information:

Ultrasound echo sounder info

Some notes on ultrasound microphones

Medical Doppler ultrasound

Micro controlled ultrasound

A brief about Iran mic industry:

Although the idea is very simple and is easy to implement, some companies like Echo Chang import the permanent magnet and the diaphragm for dynamics (usually from China) and assemble them. They just assemble. Other types are widely imported from almost anywhere.

The end