unit i loudspeakers and microphones year/ec e52- ce/unit 1.pdf · 2017-09-30 · the amount of...

1-Department of ECE, RGCET

UNIT I

LOUDSPEAKERS AND MICROPHONES

Loudspeakers and Microphones

Crystal Loudspeaker, Dynamic Loudspeaker, Electrostatic loudspeaker, Permanent Magnet

Loudspeaker, Woofers and Tweeters - Microphone Characteristics, Crystal Microphone,

Carbon Microphones, Dynamic Microphones and Wireless Microphones.

1.1 LOUDSPEAKERS

A loudspeaker is a transducer which converts electrical signals of audio frequency into

sound waves of the same frequency. It is also called as output transducer or reverse

transducer.

A loud speaker's performance is determined by the following characteristics:

1.2 CHARACTERISTICS

Efficiency:

It is defined as the ratio of output sound power to the input audio (electrical power).

Its value depends on proper matching of the mechanical impedance with acoustical

impedance of the air volume being disturbed. (Some manufacturers quote the efficiency

in terms of sensitivity which is defined to be the input signal required to give a sound

pressure level of 0.1 Pa or 1 microbar at a distance of 1 metre from the loudspeaker.)

Noise:

The unwanted sound, not contained in the input signal but present in the output of a

loudspeaker is called noise produced by the loudspeaker (the mechanical parts may

vibrate at some resonant frequency, causing noise).

Signal-to-noise ratio or SNR of the system which is defined as ratio of signal output' to

the `output of noise in the absence of signal'.

Frequency Response:

It indicates the loudspeaker's response for the audible frequency range of sound. Ideally,

the response of a loudspeaker should be flat within ± 1 dB for the frequency range of 16

Hz to 20 kHz.

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The mass of the diaphragm assembly have high frequencies which are attenuated; and

due to series compliance, low frequencies are attenuated. Moreover, the movable

system may have some natural resonant frequency within the audible range and the

output at that frequency will be emphasized.

Distortion:

Any change in frequency, phase and amplitude complexion of the output sound as

compared to the input audio signal is called distortion.

Frequency and phase distortions may result due to mass and compliance effect.

Amplitude or non-linear distortion will result due to non-uniformity in the magnetic field in

which the coil moves.

Directivity:

It is the ratio of actual sound intensity at a point (in the direction of maximum intensity) to

the sound intensity that would have been available there, had the loudspeaker been

omnidirectional.

Power :

It is the maximum audio power (indicated in watts) for which it is designed. Power more

than the maximum will damage the speaker.

Impedance :

The input impedance of the loudspeaker is represented in ohms and is an important

parameter, as its matching with the impedance of source amplifier is necessary for the

optimum efficiency.

1.3 CRYSTAL LOUDSPEAKER

Fig 1.1 Crystal type speakers

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Rochelle-salt crystals have the property of becoming physically distorted when a voltage

is applied across two of their surfaces.

This property is the basis of the crystal type of speaker driver.

The crystal is clamped between two electrodes across which the audio frequency output

voltage is applied.

The crystal is also mechanically connected to a diaphragm. The deformations of the

crystal caused by the audio frequency signal across the electrodes cause the diaphragm

to vibrate and thus to produce sound output.

Crystal speakers have been impractical for reproduction of the full audio-frequency

range because the input impedance is almost completely capacitive. Thus it is difficult to

couple Power into them.

At high audio frequencies, the reactance becomes lower and the relative amount of

power smaller.

In the base range, stresses on the crystals are very great, and the crystals have been

known to crack under stresses.

1.4 DYNAMIC LOUDSPEAKER

To provide very strong magnetic field for high wattage speakers, an electromagnet is

used instead of a permanent magnet. . Its construction is shown in Fig. 1.2.

Fig. 1.2 Dynamic Loudspeaker

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Loudspeakers of more than 25 watts and up to a few hundred watts are of the

electrodynamic type.

The strong and steady magnetic field is produced by a large field coil wrapped around a

core.

The shape of the magnet is pot type with the south pole in the centre and the north pole

in the periphery.

The special shape of the core allows magnetic flux to remain concentrated in the annular

gap between pole pieces.

The voice coil is wound on fibre or aluminium (to keep it light in weight). It is placed in

the annular gap.

The audio signal from the amplifier's output transformer is applied to the voice coil. This

signal causes a varying magnetic field.

The resultant interaction between the magnetic fields (one due to electromagnet and the

other due to audio current in the voice coil) produces mechanical vibrations (motor

action) in the coil assembly , which correspond to the audio signals.

The vibrations of the coil are transmitted to the attached cone which create sound waves

in the air in the listeners' area, and hence radiate sound energy directly.

Advantages

Higher power can be obtained

Frequency response is better (40 Hz to 5000 Hz) Disadvantages

Power supply needed for field coil

Heavier weight for the same amount of magnetic field

1.5 ELECTROSTATIC CONDENSER/CAPACITOR) LOUDSPEAKERS

This type of speaker operates on the principle that a dc voltage between two parallel

metal plates causes these plates to attract or repel each other.

The amount of attraction or repulsion depends on the applied voltage l f one of the

plates is a flexible metal, it will bend.

But the amount of attraction and repulsion is not directly proportional to the voltage

applied

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For example, considering the movable and fixed plates of Fig1.3 with no voltage

applied. Now suppose we apply a slowly varying ac voltage to both plates.

As the voltage increases from zero the potential difference between the two plates also

increases.

Which in turn produces an increasing force of attraction between the plates, so that the

movable plate tends towards the fixed plate.

As the ac voltage decreases once more to zero, the attractive force decreases, and the

movable plate moves back to its original position.

But, the second half of the ac cycle, in the negative direction. All that this means to the

metal plate is that the positive and negative voltages have switched plates.

The attractive farce is still there, and it is still the same. So, we get another bend in the

movable plate on the negative half of the ac cycle.

Thus, for one full cycle of ac we have two bends in the movable plate, in effect a.

frequency doubling_ A 2 kHz signal would giant: us a 4 kHz note.

Fig. 1.3 Frequency doubling of an unpolarised loudspeaker

To overcome, frequency doubling, we polarise the speaker, that we apply a high voltage

(1,000 volts or so) as a son of de bias, (Fig. 1.4).

The voltage exerts a stead' attraction between the two plates, so that now-with no signal-

the movable plate is bent slightly toward the fixed plate.

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If 400 V audio signal is applied to the speaker then the positive half cycle of the signal

increases from zero the voltage between the plates rises from 1,000 V toward 1,400 V

and the movable plate bends from its original position toward the fixed plate.

As the ac passes its peak and returns to zero, the voltage between the plates drop, from

1,000 V to 600 V. Instead of moving again toward the fixed plate, the movable plate

moves farther away.

The bending of the movable plate identical to the ac swing and there is no frequency

doubling.

Fig. 1.4 Frequency doubling eliminated by DC Polarization

A detailed view of a modern electrostatic speaker is shown in Fig. 1.5.

The practical speaker of today uses push-pull, with a built-in step-up transformer to work

from the ordinary 8 ohm amplifier output tap.

The polarizing voltage is applied to the centre or movable plate through a resistor that

keeps the voltage stable during variations in the signal voltage.

The signal voltage is applied to the m o outside plates. Because the diaphragm is

centered between the two plates that attract it equally, there is no bending when there is

no signal.

Also, because of the push-pull action the diaphragm can move twice as for in response

to signal voltages for the same amount of compression of the dielectric material.

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The major weakness of electrostatic speaker requires the DC bias is that 1i to be much

larger than the applied audio signal. In practical speakers, 1,000 to 1,200 volts may be

used.

Further, when the bass frequency ranges, a great deal or power would he required to get

enough output.

To produce such power, the speaker area would have to be very large. So, even though

full range electrostatic speakers have been constructed.

In practical use electrostatic speakers have been mostly confined to frequencies above

1,000 Hz.

Fig. 1.5 Electrostatic Loudspeaker

The step-up transformer and the high voltage polarizing supply is usually built right into

the modem electrostatic.

Often the electrostatic unit and its matching woofer are sold together as a complete

system.

Some high class systems use electrostatics to reproduce the high frequencies. Koss

uses electrostatics on some of their stereo headphones.

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1.6 PERMENANT MAGNET LOUDSPEAKER

1.6.1 Principle:

The moving-coil loudspeaker works on the principle of interaction between a magnetic

field and current in the same way as an ac motor works.

A coil, called voice coil, is placed in a uniform magnetic field.

When the audio current passes through the voice coil, there is an interaction between

the magnetic field and the current, resulting in a force working on the movable coil.

This force is proportional to the audio current, and hence causes vibrator motion (moor

like action) in the coil, which makes a conical paper diaphragm to vibrate and produce

pressure variations in air, resulting in sound waves.

The force on the coil due to interaction between the current through coil and the

magnetic field is given as eqn 1 and 2

The required stiffness to restrain the motion. The spiders also keep the coil centered, so

that the cone moves forward and backward only.

Leads from the voice coil are cemented to the cone surface. From there, It is brought to

the terminals mounted on the metal frame or basket.

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Fig. 1.6 Permanent Magnet Loudspeaker

1.6.2 Functioning:

When audio current flows through the voice coil placed in a magnetic field, a force equal

to newtons act on the coil and moves it to and fro.

The paper cone attached to the coil also moves and causes compression and

rarefaction cycles in the air.

Thus, audio current is converted into sound waves. The equivalent circuit of the cone

speaker is shown in Fig. 1.7.

There are two transformations. One is electromechanical and the other is mechano-

acoustical.

The electromechanical transfer, represented by transformer Tm, transfers force

(produced by the source current in inductance L of the voice coil and the associated

resistance R to the movable mechanical parts (voice coil, diaphragm, springs and core).

Mass, compliance and friction of the moving parts are represented by Lm, Cm and Rm

which are analogous to inductance, capacitance and resistance, respectively.

At low audio frequencies, Lm is negligible and the output depends on the compliance,

Cm .

At high audio frequencies, Cm is negligible and the output depends on Lm. So the high-

frequency speakers (tweeters) are of low mass, and the low-frequency speakers

(woofers) arc of high compliance (large size).

Typical frequency response of a 20-cm sized cone-type loudspeaker is shown in Fig.

1.8.

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Fig. 1.7Equivalent circuit for cone type speaker

Fig. 1.8 Frequency response of 20 cm sized cone-type loudspeaker

1.6.3 Direct Radiating Type:

The whole paper in a cone-type loudspeaker acts as a diaphragm and causes pressure

variations direct in the listeners' area. Hence it is called `direct radiating type loudspeaker'.

1.6.4 Characteristics of the Cone-type Speaker:

Efficiency is quite low, about 5 percent only.

The poor efficiency is due to the fact that it acts as a direct radiator, and so there is complete

mismatch between the low acoustic load presented by the large volume of air and the high

mechanical load presented by the voice coil and cone assembly,Signal to noise Ratio .

It is 30 dB or better.Frequency Response It is restricted to mid-frequencies only.

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Frequency response drops at low and high audio frequencies for a typical loudspeaker.

However a massive loudspeaker (called woofer) for low frequencies and small size speaker

(called tweeter) for high frequencies can be designed.

3 dB frequency response of a typical speaker is from 60 Hz to 2000 Hz. Low-frequency woofer

speakers with baffles will give frequency response up to 30 Hz, High-frequency tweeters extend

the higher frequency response to 10 kHz or even higher.

Distortion Non-linearity due to non-uniformity in the magnetic flux density causes severe non

linear or amplitude distortion (up to about 10%).

Directivity Basically, the loudspeaker is Omni-directional. But baffles and enclosures modify the

directivity so that most of the power is in the front.

High audio frequencies are concentrated in a narrow cone about the axis of the radiator.

Impedance The effective impedance taking into account the mechanical and acoustical loads

varies from 2Ω to 32Ω .

The common impedances in commercial speakers are 4, 8 or 16Ω (200 to 300Ω impedances

are obtained in an electrodynamic type cone speaker).

Power handling Capacity Power range of speakers lies between a few milliwatts (for 2 cm

speaker) to about 25 watt for large size speakers. (Electrodynamic speakers can withstand a

few hundred watts of input power)

1.6.5 Advantages:

Small size

Low cost

Satisfactory frequency response

1.5.6 Disadvantages:

Poor efficiency

Very low and high frequencies are attenuated

1.5.7 Applications:

Radio receivers

TV receivers

1.7 WOOFERS

There are two types of low-frequency speaker, the commonly known woofer, and the

more recent addition the sub roofer.

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The latter is used for the reproduction of frequencies below those produced by the

woofer, and it is generally purchased as an odd on to an existing system,

The low-frequency speaker provides the bass of any hi-fi system.

Its sole purpose is to reproduce the low-frequency notes of the program source.

The prime requisite for low-frequency reproduction is a large diaphragm, the larger the

better.

The smallest diaphragm for any halfway decent woofer is 8 inches; for a subwoofer it is

12 inches.

In addition to large size, the diaphragm must be of fairly heavy construction. Light

diaphragms just can’t hold up under the vibrations encountered under the lower audio

ranges.

A woofer must be able to vibrate back and forth very easily. (i.e.) have high compliance.

One way to accomplish this is to have the diaphragm loosely connected to the frame.

The gasketing that holds the periphery of diaphragm to the frame/basket is fastened so

that it barely keeps the diaphragm from slipping loose, but no more shown in Fig. 1.9.

With this construction it takes less force to move the diaphragm any circular distances.

Rather than the loose suspension system, the cone is supported by a very flexible

material so that it can be moved very easily by the voice coil.

The suspension is tight but the sine wave at the diaphragm edge is made very flexible.

Fig. 1.9 Woofer

A woofer must also have a large voice coil to handle considerable heat.

The larger the voice coil, the more the current produced by the amplifier output circuit

and, therefore, the more the power the woofer can handle.

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Finally, a strong magnet can be of great help to move the heavy voice coil and cone

assembly too well. The better the woofer, the heavier the magnet assembly.

To sum up, a good woofer must have a large, heavy diaphragm, a strong magnet, high

compliance and a large voice coil.

1.8 TWEETERS

There are two main types of high frequency speakers; the well-known sweeter and the more

recent supertweeter. Supertweeters can be add-ons or they can be integral with the system.

Six basic high-frequency speakers (tweeter) exists

The cone is a physically disincentive version of the woofer.

The dome, so called because of its dome-shaped diaphragm.

The horn, so named because it is a horn.

The Heil air-motion transformer which uses the principle of lever in its operation, named aid r its

inventor, Dr. Oskar Heil.

High polymer molecular film tweeter, uses the piezoelectric effect for its principle of operation

(used exclusively by Pioneer)-

The electrostatic tweeter works on the principle of attraction or repulsion between two metal

plates.

1.8.1 CONE TYPE TWEETERS

Since tweeters must reproduce high-frequency notes, they roust resonate at high frequencies.

High resonant frequencies are obtained with light weight, stiffly supported mechanisms.

To make the diaphragm of a con type tweeter light, it must be small.

When the size and weight is reduced the diaphragm in turn, reduce the size of the ice coil also.

Luckily, high frequencies card only a comparatively small amount of electric power, therefore,

the small voice coil is not subjected to electrical overload.

Without exception, it is wound with light weight wire such as aluminium wire or ribbon.

The lightness of the moving system provided by aluminium makes the high frequency response

much better than if copper were used.

Cone type radiators tend to concentrate radiation of the high frequency components of a sound

in a narrow cone about the axis of the radiator.

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The degree of directivity of speaker is indicated by a directivity pattern in Fig.1.9. The axis of

the radiator is considered the reference line with an angle of zero degrees, Directivity pattern are

normally shown as a top view in the horizontal plane through the radiator axis.

A cone in free space should have the same pattern in a vertical plane,.

The line OA in Fig. 1.10 indicates by its length that the sound radiated along it is a maximum to

comparison to that in any other direction.

At an angle 450, the line OB is a measure of the relative sound intensity in that direction- Since

OR is only half as long as a listener along OB would listen only about half the volume compared

to what a person along OA. At angles near 900, the pattern indicates minimum (zero) radiation.

In any practical setup, such a zero area would not exist cause sound waves reach there by

reflection.

Fig. 1.10 Radiation pattern for a typical cone at one frequency

Because, directivity normally varies considerably with frequency, a complete diagram (Fig. 1.11)

must show separate patterns for each of at least several frequencies.

Fig.1.11 depicts variation of directivity with frequency for a 12-inch cone, assuming that the

speaker is mounted in an infinite baffle. The radiation pattern will be narrow at highs than at

lows.

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Fig. 1.11 Variation of directivity with frequency for a typical cone radiator

A single cone-type tweeter distributes high-Frequency sounds unevenly. It lobes the

higher frequencies directly out in front and tends to cause a drop of at the sides.

This effect can be overcome by arranging two or more cone tweeters as shown in

Fig.1.11. In this way, overlapping individual lobes from separate speakers cover the

listening area.

Fig. 1.12 Sound distribution of two tweeter array. Highs can be spread out by angling the speakers

1.8.2 DOME TYPE TWEETERS

Uniformly dispersed flat energy response begins with a speaker system's ability to rd i

ate sound at ail frequencies evenly in all directions.

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Even dispersion of sound energy means that the sound emanating from the program

source will be heard same by listeners in all parts of the room.

For low frequency sounds this problem of dispersion is not of practical consequence,

since they are very nearly omnidirectional.

The limiting factor for high-frequency sounds is that a speaker will begin to directional

when its circumference equals the wavelength of the frequency being reproduced.

Directionality increases the wavelength decreases with respect to the speaker’s

dimensions,

The laws of physics dictate the most direct approach to the problem of even dispersion

of high-frequency energy; the rivers used must he as small as possible.

Dome tweeters, Fig.1.13 are designed according to this principle in order to use these

physical laws to the listener's advantage.

Fig. 1.13 Dome type tweeter

1.8.3 HORN TYPE TWEETER

To obtain reasonable output from a loudspeaker, we must vibrate large amounts of air.

For this, usually a fairly large vibrating surfaces, such as the cones in woofers.

The larger the cone surface, the greater the output.But the tweeter's cone (diaphragm)

must be small to attain its high-frequency response.

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Thus only a small amount of air can be moved, reducing the output power.

The increase in acoustic output from any type of diaphragm , couples directly to a horn,

converting the system to a horn loaded one. Fig. 1.13 shows the relative difference in

size between the diaphragms of a cone-type tweeter and a horn-loaded one.

The driving force of the voice coil of the latter is distributed between the small mass of

the diaphragm and the mass of air in the horn.

Since air weights much less than paper or metal the overall load on the voice coil for the

same acoustic output as that of the cone type tweeter, can be greatly reduced, Also, for

the same electrical input, the output of the horn loaded system is greater.

A horn is a tube so flared (tapered) that the diameter incases from a small value at one

end called the throat to a large value at the other end called the mouth.

Horns, Fig. 1.14 have been used for centuries for increasing the radiation of the human

voice and musical instruments.

The horn does acoustically what the cone does mechanically. It couples the small voice

coil area to a large area or air.

In this way, the horn acts as an acoustic transformer and converts the relatively high

impedance at the throat and driver.

Fig. 1.14 Horn Type tweeters

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The horn is a fixed physical boundary for its enclosed column of air and does not vibrate

itself.

Acoustic energy fed to its throat must therefore be obtained from a vibrating diaphragm

which converts mechanical motion from the driver voice coil to acoustic energy.

Although the cone type radiator acts as both diaphragm and radiator and transducer,

from mechanical to acoustic energy, the horn acts only as a radiator, with both input and

output energy being acoustic.

MICROPHONES

Microphone is a transducer which converts sound pressure variations into electrical

signals of the same frequency and phase and of amplitudes in the same proportion as in

pressure variations.

The electrical signals in the audible range are called audio signals.(Microphone , in

short, is sometimes written as mic or mike).

A microphone is the first 1 ink in sound recording and systems. Audio signals can be

used to cross the barrier of time (by recording) and the barrier of distance (by radio

transmission)

1.9 CHARACTERISTICS OF MICROPHONE

The quality of a microphone is determined by the following characteristics:

Sensitivity

Signal--to-noise ratio

Frequency response

Distortion

Directivity

Output impedance

These characteristics are defined as under:

Sensitivity:

It is defined as output in millivolts (or in dB below 1 volt) for the sound pressure of 1 Pa

(or ID microbars at 1000 Hz.

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As the norm al level of speech provides a sound pressure of l microbar (or 0.1 Pa), the

sensitivity based on this criteria for 1 microbar pressure (or 0.1 Pa) level would be one-

tenth the value for 1 Pa pressure. (Some Manufacturers quote the sensitivity in terms of

dBm, i.e., power output in dB below 1 milliwatt.)

Signal-to-noise Ratio:

Some noise (called self-noise or thermal noise) is generated inside the microphone due

to resistance of the circuit, built-in transformer, etc.

It is represented in terms of the sound pressure level (SPL.) that would give the same

output as the noise output.

The output is measured by passing it through a weighting filter which accounts for the

reduced sensitivity of the ear at high and low audio frequencies. The acoustically

weighted output is represented i

Distortion Besides frequency distortion (uneven frequency response) described above,

there are two types of distortions in microphones, namely, non-linear distortion, and

phase distortion.

Non-linear Distortion:

This disorts the amplitude of the audio signal, which retilts iii production of such harmonics in

the output that are not present in the input sound.

For quality microphones. such distortion should be less the 5%. For high-fidelity sound systems,

distortion should not be more than 1%.

Phase Distortion:

This may cause change of phase relationship between different components of a complex

sound wave.

Phase distortion occurs when multiple microphones are used causing relative path difference

from the source of sound.

Directivity:

The directivity of a microphone is defined with the help of a polar diagram.

The angle for half-power points in a polar diagram represents directivity of a microphone, as

shown in Fig. 1.15.

Maximum power is in the axial direction of the microphone t wards source of sound. When the

microphone's axis deflects away from the source of sound, power output is reduced.

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1.10 CRYSTAL MICROPHONE

Fig 1.16 Crystal Microphone

Certain crystals such as rochelle salt and quartz possess the property of generating

small emf when subject to stress or strain.

This effect is utilised in what is known as the crystal microphone.

A thin finger shaped slice of crystal is secured at one end by means of a compliant

clamp, and the apex of a cone is made to bear against the other.Sound pressure waves

causes the cone to alternately, press against and bend the crystal slice and release it.

Thus the corresponding voltages are generated across the slice.A pair of contacts is

fixed to opposite surfaces to take off the signal.

An improvement is obtained if the single slice of crystal is replaced by two slices

cemented together.

Then, when the pressure is exerted, one slice is compressed and other is strecthed.

Thus equal and opposite voltages are produced which, being in series like the cells of a

car battery, give double the output.

Any nonlinearity which may arise due to different mechanical strains between pressure

and release is also thereby compensated. The double crystal unit is termed as bimorph.

With some of the better microphones the cone does not actuate the crystal directly but

through cantilever.

Another type of construction is the sound cell where several crystal elements are sealed

together, this also termed as multimorph.

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The crystal microphone is the type most widely used in lower cost installations.

The crystal microphone is normally non-directional although a pressure-gradient crystal

microphone which gives unidirectional response pattern is also being marketed.

1.11 CARBON MICROPHONE

1.11.1 Principle

When fine carbon granules enclosed in a ease are subjected to variations of pressure,

the resistance of the granules changes, When such a device of carbon granules is

connected in wiles with a load through a do supply.

The current through the load will vary in accordance with pressure variations on the

carbon granules.

Fig.1.17 Carbon microphone

.11.2 Construction

The construction of a carbon microphone is shown in Fig.1.20. Fine carbon granules are

enclosed between two metal plates.

The upper plate called diaphragm, is attached to a movable metal plate through a metal piston

or plunger.

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The lower metal plate is fixed and is insulated from the diaphragm. A protective cover with holes

is used to prefect the unit.

A battery is connected between two m Metal plates. When the load is connected, current flows

through the carbon granules and the load.

Path of the current passes from the +ve battery terminal through the fixed lower plate, the

resistance of carbon granules, movable metal plate, metal casing, rind output transformer, as

shown in Fig.

The purpose of the output transformer is to eliminate content of the microphone.

1.11.3 Functioning:

When sound waves strike the diaphragm, it moves to and fro.

During compression condition, it presses the carbon granules and during rarefaction, it

loosens them, when carbon granules are pressed, the resistance decreases and hence

the current through the circuit increases.

When carbon granules loosen, the resistance increases, decreasing the current through

the circuit. In the absence of sound, a steady current flows. Thus, sound waves

superimpose a varying current, or audio current on the steady dc current.

The net resistance of the carbon granules is given by Eq. 1.4.

where,

r - Net resistance in ohms. -

R0 - Steady resistance in ohms for no sound

δr - Variation of resistance due to sound pressure (it will have positive as well as negative value)

The development of a voltage V across a load resistance RL is illustrated in Fig. 1.21.

Fig.1.18 Equivalent circuit of carbon microphone

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If E is the battery voltage, RL – load resistance, R0+RL= constant for constant load say R, and

steady current I0 is the sound pressure variations .When sound-pressure variations cause a

change in resistance R and δr/ - them the instantaneous current, I is given by Eq. 1.5.

Equation (1.6), shows that the change in current, and hence the change in voltage across the

load is proportional to the change in resistance r) of the carbon granules, and hence

proportional to the pressure variations due to sound.

When pressure increases, δr decreases and so I increases and the output voltage across the

load increases,

1.11.4 Characteristics of a Carbon Microphone

The Sensitivity will be Very high.

The output of a carbon microphone is about 20 dB below 1V i.e., about 100 mV).

Signal-to-noise Ratio Poor random variation of resistance of carbon granules generates a

continuous hiss.

Frequency Response Carbon microphones have a frequency response of 200 to 5000 Hz, and

therefore are unsuitable for high fidelity work.

The resonance peak is at 2000 Hz and overall frequency bandwidth is usually tip to 5 kHz.

Distortion is high. The content is rich in harmonics unless variation in resistance (δr) is a very

small percentage of steady resistance R. Distort ion is of the order of 10%. Also, carbon

granules have a tendency to stick to each other which further increases the distortion.

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Directivity of carbon microphone is substantially omnidirectional. However, high frequency

response over 300 Hz falls beyond an angle of 400 from the front of the microphone. Output

Impedance It is about 100Ω

1.11.5 Other Features

It is mechanically very rigid

It is prone to moisture and heat

It is small in dimensions

Cost of the microphone is the lowest of all other microphones

1.11.6 Advantages

Very rugged

Small size

Very cheap

Good sensitivity

1.11.7 Disadvantages

High distortion

Limited frequency response

Not suitable for high fidelity work

1.11.8 Applications

Due to limited frequency range, it is useful only in telephones. It is also sometimes used

in portable radio communication sets.

1.12 DYNAMIC (MOVING-COIL) MICROPHONE

1.12.1 Basics:

The moving-coil microphone (also called dynamic microphone) uses the principle of

electromagnetic induction.

When sound pressure variations move a coil placed in a magnetic field, there is a

change of magnetic flux passing through the coil.

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An emf is, therefore, induced in the coil and this emf forms output of the microphone,

(Due to similarity in construction, a moving coil loudspeaker can also work as a moving-

coil microphone. The same unit is often used both as microphone and loudspeaker in

office intercom systems.)

1.12.2 Construction:

The main components of a moving-coil microphone are a magnet, diaphragm and coil.

These are shown in Fig. 1.19. The magnet is a permanent magnet of pot type with a central

pole piece (south pole) and the peripheral pole piece (north pole).

This type of magnet gives a uniform magnetic field in the gap between the pole pieces.

The diaphragm is a thin circular sheet of non-magnetic material and is of' light weight.

It is slightly domed for extra rigidity. It is fixed to the body of the magnet with the help of springs.

The springs provide compliance (equivalent to electrical capacitance) to the motion of a

diaphragm. The mass of the diaphragm and coil assembly provide inductive effect.

A protective cover (a metal grill) is used to save the delicate diaphragm and coil assembly from

being mishandled.

A silk cloth partition is used to separate the upper chamber from the lower chamber. A small

tube is used in the lower chamber to give access to the free atmosphere.

Fig.1.19 Dynamic Microphone

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Fig.1.20 Equivalent circuit for Dynamic microphone

The mass of the diaphragm restricts the high audio frequency output, and the stiffness

(capacitive reactance) caused by the springs' compliance, restricts the low audio

frequency output.

The electrical equivalent circuit for a morning coil microphone is shown in Fig.1.20

1.12.3 Principle of Working:

When sound waves strike the diaphragm, it moves and hence, the coil moves in and out in the

magnetic field.

This motion changes the flux through the coil, which results in an emf being induced in the coil

due to electromagnetic induction.

The value of this emf depends on the rate of change of flux and hence on the motion of the coil.

The displacement of the coil depends on the pressure of sound waves on the diaphragm. Thus,

it is a pressure microphone,

The induced voltage, e, across the coil of the microphone is given by Eq. 1.7.

If B is the flux, density in tesla (or Weber/m2) 1, the length of the coil in metres, v, the velocity of

the diaphragm (and hence coil) in m/s then.

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Thus, the induced ernf is directly proportional to velocity which, in turn, is proportional to

frequency of sound pressure variations.

1.12.4 Advantages

Good frequency response

Bidirectional behavior

Good transient response

It does not need any external bias for its working

1.12.5 Disadvantages

It is a delicate and expensive microphone

It can be easily damaged due to slight mishandling

Its sensitivity is low

1.12.8 Applications

Dramas

Music

Radio broadcast

Public address system

1.13 WIRELESS MICROPHONE

The ultimate in mobility is afforded by the wireless (radio microphone) because with this

there is no connecting cable and the user is free to move around over distance of

several hundred meters.

There are two basic types, one where the radio transmitter is contained within the casing

of the actual microphone, and the other which takes the form of a slim pocket unit about

the size of a wallet into which an ordinary microphone can be plugged.

The integral microphone/transmitter unit, Fig. 1.21 is rather larger than a normal gun

microphone as batteries must be accommodated as well as the transducer and

transmitter.

In order to obtain sufficient power for the transmitter, the batteries are at least 9V, but

the size limits the capacity.

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The average life is three to five hours, but rechargeable batteries are often fitted to

make the instrument more economic to run.

With the separate pocket transmitter a lavalier or tie-clip microphone can be used to give

complete freedom to the user.

The aerial takes the rum] of a short flexible lead which trails from the microphone. Usual

length is a quainter wavelength at the permitted frequencies of the carrier wave.

There are fifteen frequencies allocated for wireless microphones and all units work on

any one of these interference is no problem because or the short range, it being unlikely

that another user will be operating on the same frequency within about half a kilometer.

The frequencies are in four groups: firstly group with a wide bandwidth, 174.1, 174.5,

174.8 and 175.0 MHz.

The second group is of narrow bandwidth, the frequencies being 174.6. 174.675.

174.77, 174.885, and 175.020 MHz.

The third group is also or narrows bandwidth, being reserved for teaching deaf children

in schools: these are 173.4, 173.465, 173.545 and 173.64 MHz.

In addition, in certain circumstances, the frequencies of 174.6 and 174.95 MHz are

allocated for communication on work sites. Ali ordinary E-M receiver will not pick up

wireless microphone transmissions.

Fig. 1.21 (a) VHF Wireless Microphone

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The narrow bandwidth specification is for a deviation of +20 kHz and is suitable for most

speech applications.

The wide bandwidth allocations Aim- a deviation of +75 kHz and give the better quality

reproduction required by stage and cabaret artists.

The transmitter output power must not exceed 50 mW in the case of narrow band

transmitters and 10 mW with the wide band units.

Certain specifications also apply to the receiver. Signal to noise ratio must be better than

30 dB and selectivity such that a signal with a deviation of + 10 kHz, 70 kHz away from

the wanted signal in the case of narrow bard receiver, and with a deviation of +2.5 kHz,

at 200 kHz away, from the wanted signal in the case of wide band receiver will not

produce an increase of noise plus unwanted signal of more than 3 dB in the output.

An interfering signal of 3mV should not give a signal in the output greater than 10 dB

above noise level in the case of wideband receiver and 20 dB above with that of the

narrow hand grit.

It is possible for any receiver to generate and radiate a signal from the local oscillator

which is part of the superheterodyne circuit universally used. The specification stipulates

that cm) such signal radiated from the receiver’s aerial should not exceed 2,5 µW at any

frequency.

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unit i loudspeakers and microphones year/ec e52- ce/unit 1.pdf · 2017-09-30 · the amount of...

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