Electronics Elements

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Thursday, November 02, 2017

Sound Transducers

Introduction

Sound is a generalized term given to acoustic waves which are a type of longitudinal waves that propagate by compression and decompression in adiabatic process. The frequency range of acoustic waves is between 1 Hz to tens of thousands of Hz. In this huge range, human can hear between 20 Hz to 20 K Hz.

Audio or sound transducers are of two types: input sensors or sound to electrical transducers and output actuators or electrical to sound transducers. The example of input sensor is a microphone and for output actuator is a loudspeaker.



Sound transducers can detect and transmit sound waves. If the frequency of the sound wave is very low, then they are called infra – sound. And if the frequency of sound wave is very high, then they are called ultra – sound. 

Sound

Sound and vibration are interconnected as sound is associated with mechanical vibration. Many sounds are caused by vibration of solids or gases. According to ANSI, sound is defined as “oscillation in pressure, stress, etc., propagated in a medium with internal forces or superposition of such propagated oscillation.” Sound wave is the waveform caused by a vibration.

 This waveform causes an identical vibration to be set up in any material affected by the sound wave. In order to transmit sound waves, a medium that can be vibrated is needed. A vibrating object or material compresses the surrounding air molecules and rarefies them. There is no transmission of sound waves through vacuum.

When sound is transmitted, it has three important wave parameters: velocity or speed, wavelength and frequency. These characteristics are similar to that of an electrical waveform.

 
The frequency and the wave shape of the sound are determined by the origin of the sound or the frequency and wave shape of the vibration that causes the sound.

 The velocity and wavelength of the sound are dependent on the medium that transmits the sound waves. The relationship among the three parameters velocity, wavelength and frequency is shown below.

Frequency (f) = Velocity (m/s) / Wavelength (λ)

The units of frequency are Hertz (Hz).


The velocity of sound in a given material depends on the density and elasticity of the material. Hence the velocity of sound is higher in solids and low in high pressure gases.

Objective measurement of sound waves makes use of intensity of receiving surface measured as the number of watts of sound energy per square meter. The ear has a non – linear response and the sensitivity vary with the frequency of sound. 

The frequency range over which sound can be detected by the human ear is between 20 Hz to 20 kHz. The response of the ear is maximum in the region of 2 kHz.

Microphone (Input Sound Transducer)

The audio or sound to electrical energy transducer is the microphone or simply called as mic. A microphone produces electrical analog signals that are proportional to the sound waves acting on its diaphragm. 



Microphones are classified by the type of electrical transducer they use. In addition to the transducer, microphone uses acoustic filters and passages whose shape and dimension modify the response of the overall system. The characteristics of a microphone are both electrical and acoustic. The sensitivity of a microphone is expressed as mV of electrical output per unit intensity of sound wave. 

 The impedance of microphone has the considerable importance. A microphone with high impedance has a high electrical output while the one with low impedance is associated with low output. 

The high impedance makes the microphone susceptible to hum pick up. The directionality of the microphone is also an important factor. If the microphone is used in sensing of the pressure of sound waves, then it is Omni – directional i.e. it picks up sound arriving from any direction. A microphone is directional if it responds to the velocity and direction of the sound wave.

The type of sound transducer does not necessarily determine the operating principle as pressure or velocity, but the construction of the microphone is the most important factor. Some of the most common types of microphones are: Carbon microphone, Moving Iron microphone, Moving Coil microphone, Ribbon microphone, piezoelectric microphone and electret capacitor microphone.

Carbon Microphone

The carbon microphone was the first type of microphone to be developed for usage in telephones. Now they are replaced by electret capacitor microphones. Carbon microphone uses granules of carbon held between a diaphragm and a backplate.

When the granules are compressed, the resistance between the diaphragm and the backplate drops considerably. The vibrations of the diaphragm, which are the result of the sound wave incident on it, can be converted into variations of resistance of granules. The microphone requires an external power supply as it does not generate voltage.

The main and only advantage of carbon microphone is that it produces an output that is huge by microphone standards.

The disadvantages include poor linearity, poor structure that causes multiple resonances in the audio range and high noise level as the resistance of the granules alter even in the absence of sound.

Moving Iron Microphone

Moving iron microphones are also called as Variable Reluctance Microphones. Moving iron microphone uses a powerful magnet. The magnetic circuit contains an armature made of soft iron, which in turn is connected to a diaphragm.
 As the armature moves, the magnetic reluctance of the circuit alters and this in turn change the total magnetic flux in the circuit. The magnetic circuit in this type of microphone makes the instrument heavier.

Moving Coil Microphone or Dynamic Microphone

Moving coil (Dynamic) microphones use a constant flux magnetic circuit. In this circuit, the electrical output is generated by moving a coil of wire in the circuit which is attached to a diaphragm. This whole arrangement is in capsule form which makes this a pressure operated microphone rather than velocity operated.

The coil moves in response to movement of the diaphragm as the sound waves hit the diaphragm. By applying Faraday’s Law of Electromagnetic Induction, a voltage is induced in the coil due to the movement of the coil in the magnetic field. 

Maximum output occurs when the coil reaches maximum velocity between the peaks of sound wave so the output is 900 out of phase with the sound.

The internal view of a Dynamic Microphone is shown below.

The range of the movement of the coil is very small as the size of the coil is small. Hence the linearity of moving coil type microphones is excellent. Due to the low impedance of the coil, the output is considerably low and hence amplification of the signal is required. The inductance of the coil in moving coil microphones is less and therefore they are less susceptible to hum pick up from mains. The construction of moving coil microphone resembles that of a loudspeaker in reverse. 

Ribbon Microphone

The principle of operation of a ribbon microphone is derived from moving coil microphone and the change is that the coil has been reduced to a strip of conducting ribbon. The signal is taken from the ends of the ribbon. An intense magnetic field is used so that the movement of the ribbon cut across the maximum possible magnetic flux is possible. This generates an output with its peak value at 900 out of phase to the sound wave.

The internal view of ribbon microphone is shown below.


Ribbon microphone is a velocity operated microphone. Ribbon microphones are used in situations where directional response is important. The main application of this type of microphone is in voice commentary in noisy surroundings.

The linearity of ribbon microphones is very good and its construction makes it inevitably a low output device. In order to raise the voltage level and the impedance level, ribbon microphones are usually equipped with transformer. Good quality ribbon microphones are expensive items. The directional qualities of this microphone are suitable for stereo broadcasting.

Piezoelectric Microphone

The advantage of Piezoelectric Microphone over other type of microphones is that it is not confined to use in air but can be bonded to solid and also immersed in a non – conducting liquid. Piezoelectric transducers can be used at ultrasonic frequencies and some are used in the high MHz region. 

Piezoelectric transducers consist of crystalline material. When the crystal is strained by sound waves, the ions of the crystal are displaced asymmetrical way. Originally, Rochelle Salt Crystal is used as crystalline material in piezoelectric microphones and this crystal is coupled to a diaphragm. 

The output voltage and impedance are high, but the linearity is poor. Now a day, synthetic crystals are used over natural crystals. Barium Titanate is the synthetic crystal used for frequencies up to hundreds of KHz.

The figure of piezoelectric microphone is shown below.

                     

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1 comment:

Kiran Jameel said...

I guess I am the only one who comes here to share my very own experience guess what? I am using my laptop for almost the past 2 years.
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