Crystal Loudspeaker Dynamic Loudspeaker, electrostatic loudspeaker, Permanent Magnet Loudspeaker, Woofers and Tweeters Microphone Characteristics, Crystal Microphone, Carbon Microphones, Dynamic Microphones and Wireless Microphones

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Loudspeakers and Microphones
Unit 1
Crystal Loudspeaker Dynamic Loudspeaker, electrostatic loudspeaker,
Permanent Magnet Loudspeaker, Woofers and Tweeters Microphone
Characteristics, Crystal Microphone, Carbon Microphones, Dynamic
Microphones and Wireless Microphones.
1 INTRODUCTION TO LOUDSPEAKER
There are a number of interrelated factors that must be considered in designing transducers for
converting electrical energy into acoustic energy. These include electroacoustic efficiency, uniformity
of frequency response, linearity of amplitude response, transient response, and power handling
capacity, size, durability and cost. An ideal loudspeaker:
 Would have an electroacoustic efficiency approaching 100 per cent.
 Would have an acoustic output response that is independent of frequency over the entire
audible range.
 Would introduce neither harmonic nor inter modulation distortion into its output.
 Would faithfully reproduce transients as well as steady input signals.
 Would be capable of producing a non-directional radiation pattern.
 Would be of as small a size as is possible considering the required acoustic output.
No single transducer has been designed that is capable of satisfying all the above requirements.
Out of many devices developed for the radiation of acoustic energy into air, the two most widely used
are the direct-radiator or dynamic loudspeaker and the horn loudspeaker. Both of these loudspeakers
utilize the electrodynamics coupling that exists between the motion of a vibrating surface, called the
cone or diaphragm and the current in a so called voice-coil. Additional types of electromechanical
coupling that are used for this purpose include electrostatic coupling in electrostatic loudspeakers and
electromagnetic coupling in telephone receivers.
The speaker system itself can be divided into three functional parts:
1. The electromagnetic part, consist of the voice coil and the field magnet. Audio frequency
electric current in the coil causes mechanical motion of the cone or diaphragm on which it is
mounted. This part is often referred to as the driver or motor of the system.
2. The mechanical part, on which the driving coil is usually mounted and which is set into
mechanical motion by the audio frequency electric current in the driving coil.
3. The acoustic part, which transmits the sound energy developed by the mechanical part of the
area served by the system in the most efficient and faithful manner possible. This takes the
form of a baffle or enclosure with a horn being a form of enclosure.
A complete understanding of the operation of the speaker systems requires a sufficient view of
the flow of acoustic energy from the output amplifier stage to the listener.

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There are three types of speakers in modern use are:
 Crystal
 Electrostatic (Condenser) And
 Electrodynamics (Or Simply Dynamic) Speakers.
1.1 SPEAKER PARTS:
MOTOR PARTS:
 Back Plate, Center Pole, and Front Plate – these three parts are usually made of iron or
a similar permeable material to form the magnet circuit with the magnet. The front plate and pole piece
form the "gap" of the magnet circuit. All three parts, along with the magnet and frame, serve to
dissipate heat away from the voice coil. Some drivers (usually small ones such as tweeters and
midrange drivers) include Ferro fluid in the gap to further cool the voice coil and provide resonance
damping.
 Magnet – provides a stationary magnetic field to oppose the alternating electromagnetic field
of the voice coil and thereby cause the attached cone to move inward and outward. Most drivers use a
ring shaped magnet that is made of a ferrous ceramic material.
 Screen, Vent – some drivers include a rear vent to prevent pressure from building behind the
cone in the magnet assembly and to provide cooling of the voice coil. A screen is usually provided to
prevent debris from entering through the vent.
 Voice Coil & Former – the voice coil is a coil of wire, usually copper or aluminum, through
which the electrical audio signal flows. The flowing current of the audio signal alternates, creating an
electromagnetic field which is opposed by the permanent magnetic field of the magnet circuit. This
causes the voice coil and diaphragm to move. Some drivers have two (dual) voice coils to provide

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various wiring options, including the ability of simultaneously connecting two different signals to the
driver. Voice coils can be "overhung" or "underhung" as shown below:
 An overhung voice coil is taller than the height of the gap while an underhung voice coil is
shorter than the height of the gap. Overhung voice coils are the most common. Underhung voice coils
can offer the advantage of a more linear motor strength (BL product) over their excursion range
(Xmax) but they are usually more expensive—especially when a large excursion is desired. Finally,
the voice coil is wound around the former which serves as a heat-resistant spool for the wire.
 Connection Terminal – provides a way to make an electrical connection to the voice coil. A
variety of terminal types are used, including simple push-on terminals or gold-plated 5-way binding
posts. The positive terminal should be labeled "+" or with a red dot. When the driver is wired so that a
positive signal flows to the positive terminal, the cone should move outward. If it moves backward, the
terminal labels are reversed. Drivers with dual voice coils will have two sets of terminals.
DIAPHRAGM PARTS:
 Cone – also called the "diaphragm", moves like a piston to pump air and create sound waves.
The mass of the moving parts (the cone, dust cap, voice coil and former) and the compliance of the
suspension (surround and spider) control the resonance (Fs) of the driver which in turn controls its
low-frequency response.
 Dust Cap– covers the hole in the center of the cone. This has several benefits: it reduces the
amount of dust and dirt that can get into the gap of the magnet, it reduces the leakage losses (QL)
through the driver, it adds strength to the cone while helping to maintain its shape and it can add mass
to the cone to help lower the driver‘s resonance (Fs). Some dust caps include a screen or vent to
allow airflow and aid cooling of the voice coil.
SUSPENSION PARTS:
 Spider, Surround– these two parts form the suspension of the driver. The suspension fulfills
several purposes: it centers (both axially and front-to-back) the voice coil in the gap of the magnet
circuit and it exerts a restoring force to keep it there, it limits the maximum mechanical excursion
(Xmech) of the diaphragm and voice coil, it determines the compliance (Cms and Vas) of the driver
and together with the mass of the moving parts determines the resonance (Fs) of the driver. Ideally,
the suspension should provide a linear restoring force on the diaphragm and voice coil over its full
range of excursion.
FRAME PARTS:
 Frame – also called the "basket" or "chassis", provides a rigid structure to which the driver
components are mounted. It must be made with a high degree of precision so that all of the driver
components will align properly. The frame can also aid the motor parts in dissipating heat away from
the voice coil. It is commonly made of stamped steel, cast aluminum or plastic.
 Gasket, Optional Gasket – most drivers include a front gasket to provide a smooth and flat
mounting surface. However, since most drivers are mounted using the back side of the mounting
flange, a rear (optional) gasket is often desired. The driver should have an airtight seal to the box.

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1.1.1 CRYSTAL LOUDSPEAKER
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 (
Fig.1.1)
Fig.1.1 Schematic representation for a crystal type speaker
 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 (Xc = 1/2π f C) and the relative amount of power smaller.
 In the bass range, stresses on the crystals are very great, and crystals have been known to
crack under stresses. Consequently,
 Crystal units have found some use in tweeters (the high-frequency portion of dual speaker units)
and rarely even in this application because their response is not linear.
1.1.2 ELECTRODYNAMIC LOUDSPEAKER
 There are two varieties of dynamic loudspeakers: electrodynamics and permanent magnet (PM)
speakers.
 Both works in exactly the same way, the difference is in their construction. The electrodynamics
speaker has a soft iron magnetic circuit, non-retentive of magnetism, around whose centre leg.
 A large, multilayer field coil is wound, as shown in below fig. When dc flows through this field
coil, it magnetizes the iron core.
 A magnetic flux field directly proportional to the strength of the current through the coil is thus set
up across the air gap.

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Fig 1.2 Dynamic speaker
 The iron core is not permanently magnetized; it stays magnetized only as long as current flows
through the field coil.
 Improvements in permanent magnet materials have made the electrodynamics speaker
practically obsolete, but some still exist in vintage radios.
 Note that these use the field coil as part of a choke filter in the power supply, a good example of
killing two birds with one stone.
 The electrodynamics speaker has disappeared completely, so far as hi-fi is concerned, the
permanent magnet speaker reigns supreme.
1.1.3 ELECTROSTATIC (CONDENSER/CAPACITOR) LOUDSPEAKER
An electrostatic loudspeaker (ESL) is a loudspeaker design in which sound is generated by
the force exerted on a membrane suspended in an electrostatic field. 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. If 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.
The speakers use a thin flat diaphragm usually consisting of a plastic sheet coated with a
conductive material such as graphite sandwiched between two electrically conductive grids, with a
small air gap between the diaphragm and grids. For low distortion operation, the diaphragm must
operate with a constant charge on its surface, rather than with a constant voltage. This is
accomplished by either or both of two techniques: the diaphragm's conductive coating is chosen and
applied in a manner to give it a very high surface resistivity, and/or a large value resistor is placed in
series between the EHT (Extra High Tension or Voltage) power supply and the diaphragm.
The diaphragm is usually made from a polyester film (thickness 2–20 µm) with exceptional
mechanical properties, such as PET film. By means of the conductive coating and an external high
voltage supply the diaphragm is held at a DC potential of several kilovolts with respect to the grids.

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The grids are driven by the audio signal; front and rear grids are driven in antiphrasis. As a result, a
uniform electrostatic field proportional to the audio signal is produced between both grids.
This causes a force to be exerted on the charged diaphragm, and its resulting movement drives
the air on either side of it. In virtually all electrostatic loudspeakers the diaphragm is driven by two
grids, one on either side, because the force exerted on the diaphragm by a single grid will be
unacceptably non-linear, thus causing harmonic distortion. Using grids on both sides cancels out
voltage dependent part of non-linearity but leaves charge (attractive force) dependent part. The result
is near complete absence of harmonic distortion. In one recent design, the diaphragm is driven with
the audio signal, with the static charge located on the grids.
The grids must be able to generate as uniform an electric field as possible, while still allowing
sound to pass through. Suitable grid constructions are therefore perforated metal sheets, a frame with
tensioned wire, wire rods, etc. To generate sufficient field strength, the audio signal on the grids must
be of high voltage. The electrostatic construction is in effect a capacitor, and current is only needed to
charge the capacitance created by the diaphragm and the stator plates (previous paragraphs referred
to as grids or electrodes). This type of speaker is therefore a high-impedance device. In contrast, a
modern electrodynamics cone loudspeaker is a low impedance device, with higher current
requirements. For example, consider the movable and fixed plates of below fig 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. (Fig 1.4). This, in turn,
produces an increasing force of attraction between the plates, so that the movable plate bends
towards the fixed plate.
Fig 1.3 Working principle of a typical electrostatic loudspeaker
As the ac voltage decreases once more to zero, the attractive force decreases, and the movable
plate moves back to its original position. However, now we have the second half of the ac cycle, in the
negative direction. All that this means to the metal plates is that the positive and negative voltages
have switched plates. The attractive force 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.

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Fig.1.4 Variation of input and output voltages
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 give us a 4 kHz note. The step-up transformer and the high voltage
polarizing supply is usually built right into the modern electrostatic. Often the electrostatic unit and its
matching woofer are sold together as a complete system.
Advantages:
1. levels of distortion one to two orders of magnitude lower than conventional cone drivers in a
box
2. the extremely light weight of the diaphragm which is driven across its whole surface
3. Exemplary frequency response (both in amplitude and phase) because the principle of
generating force and pressure is almost free from resonances unlike the more common
electrodynamics driver.
Disadvantages:
Typical disadvantages include sensitivity to ambient humidity levels and a lack of bass response,
due to phase cancellation from a lack of enclosure.
1.1.4 PERMANENT MAGNET LOUDSPEAKERS
The most popular type of loudspeaker today is the permanent magnet dynamic type. Because of
its comparative simplicity of construction and design, the precision that may be built into it, the ease
with which it is interfaced with other equipment, its easy adaptability to many different applications,
and its comparative freedom from electrical trouble, the dynamic loudspeaker has found acceptance in
all kinds of reproducing systems.
Construction:
1. Magnet: The magnet provides the fixed magnetic field against which the field from the coil
operates. The magnet is typically made from ferrite or powerful neodymium.

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2. Chassis / frame: The moving coil loudspeaker is generally built on a circular frame -
occasionally these may also be elliptical. The frame forms the basis for the loudspeaker and
provides its structure, although it does not perform any active part in the operation.
3. Diaphragm or cone: At the front of the moving coil loudspeaker there is a cone or diaphragm.
This transfers the vibrations of the moving coil to the air, presenting a large surface area. The
loudspeaker diaphragm can be made of fabric, plastic, paper, or lightweight metal. The outer
part of the diaphragm is fixed to the rim of the frame - the extremity of the diaphragm often
having some undulations to enable the main part to vibrate easily.
4. Diaphragm undulations: The diaphragm undulations enable the main part of the diaphragm to
move freely and in a linear fashion.
5. Dust cap (dome): This part of the moving coil loudspeaker protects the voice coil from dust and
dirt.
6. Voice coil & former: The moving coil is the key element of the loudspeaker. It takes the current
from the audio amplifier and the current flowing generates a magnetic field which interacts with
at from the permanent magnet and this creates a force which moves the coil and hence the
diaphragm to which it mechanically attached creating the sound waves.
7. Spider (suspension): This is a flexible, corrugated support that holds the voice coil in place,
while allowing it to move freely. It is not present on some lower end loudspeakers, but provides a
very useful support ensuing the coil is properly centered. Normally the space around the coil is
limited to enable the maximum efficiency to be maintained and the coil needs to be free from
touching the adjacent magnet. If it does touch, it causes a rasping sound to be heard as the
loudspeaker coil moves in and out.
Working
 It is found in the smallest pocket radios and is a major component of the most elaborate
theatre systems. Just about all hi-fi woofers are of the permanent magnet (PM) type. Exploded
view of the PM cone type speaker is given in Fig.1.5
 The cone (diaphragm) is energized by a moving coil. The woofer's magnetic field is supplied by
a permanently magnetized and highly magnetic alloy instead of the iron-cored coil used in
electrodynamics speakers.
 The PM speaker contains a very light coil of wire affixed to the diaphragm and located
concentrically around, within, or in front of the centre of the permanent magnet. The coil (voice
coil) is free to move in the field of the magnet. Electrical impulses, varying at an audio rate, are
applied to the voice coil by the amplifier.
 Because these impulses are constantly changing in amplitude and direction, a changing
magnetic field is set up in the voice coil. This field reacts with the constant field of the
permanent magnet. The result is that the voice coil moves further into the gap when the fields
are opposite and attract, and farther out of the gap when they are alike and repel.
 This causes an in-and-out movement of the diaphragm; consequently, we obtain sound waves
from electrical impulses. The speed at which the coil and diaphragm vibrate depends upon the
frequency of the impulses. The distance that the diaphragm moves in and out depends on their
amplitude.

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
Fig 1.5 Permanent magnet cone type speaker
1.1.5 WOOFERS
Woofers are made to handle the lower range of frequencies (sound waves) for a speaker
system, and there are a few different types, depending on your needs.
A woofer is a speaker that constructed to reproduce low frequencies. This type of speaker does
most of the work in reproducing the frequencies you hear, such as voices, most musical instruments,
and sound effects. Woofer driver designed to produce low frequency sounds, typically from 50 Hz up
to 1000 Hz. The most common design for a woofer is the electrodynamics driver, which typically uses
a stiff paper cone, driven by a voice coil surrounded by a magnetic field. Depending on the size of the
enclosure, a woofer can be as small as 4-inches in diameter or as large as 15-inches. Woofers with
6.5-to-8-inch diameters are common in floor standing speakers, while woofers with diameters in the 4
and 5-inch range are common in bookshelf speakers. The low-frequency speaker provides the basis
of any hi-fi system. The prime requisite for low-frequency reproduction is a large diaphragm, the larger
the better. In addition to large size, the diaphragm must be of 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 casketing 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 as shown in Fig.1.6. With this construction it takes less force (acoustical power in this
case) to move the diaphragm any particular distance (less power required for a given excursion).
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. The large speakers have more extended lows, the smaller ones
more extended highs. 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. 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.

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Fig.1.6 Suspension of woofer diaphragm
1.1.6 TYPE OF WOOFERS:
 Standard Woofer: A standard woofer produces frequencies from 20 Hz up to 2,000 Hz (2
kilohertz, or 2 kHz). The woofer is often characterized by its bass sound, which comes from the
lower frequency sine wave. You will typically see standard woofers as part of higher-end speakers
that contain either a woofer or tweeter (a setup known as a 2-way speaker) or a woofer, tweeter,
and mid-range speaker (a setup known as a 3-way speaker).
 Subwoofer: Subwoofers are only capable of producing tones lower than 200 Hz in consumer
systems. They are made up of one or more woofers, often mounted inside a wooden enclosure.
Although the human ear is only able to pick up a frequency as low as 12 Hz, subwoofers working at
lower frequencies can only be felt, if not heard. Subwoofers are the most common add-on to a
consumer speaker setup. They are typically placed in their own, isolated enclosure and provide the
low-level thump that you just cannot get with standard woofers.
 Midwoofer: Midwoofers land right in the middle of the ‗woofer‘ range, coming in from 200 Hz -
5 kHz. Having such a wide range of frequencies, this speaker will produce the best quality sound
from 500 Hz-2 kHz and start to deteriorate at either end of the spectrum.
 Rotary Woofer: A rotary woofer is a woofer-style loudspeaker that uses a coil‘s motion to
change the pitch of a set of fan blades, instead of using the cone shape. Since the audio amplifier
changes the pitch of the blades, the power required is much less than that of a conventional
subwoofer. They are also far superior at creating sounds well below 20 Hz, below the normal level
of human hearing, able to produce frequencies down to 0 Hz by compressing the air in a sealed
room.
1.1.7 TWEETERS
A tweeter is a specially designed speaker that is much not only much smaller than the woofer
but is tasked with only reproducing audio frequencies above a certain point, including, in some cases,
sounds that human ear cannot directly hear, but can sense.
Another reason that a tweeter is beneficial is that since high-frequencies are highly directional,
tweeters are designed to disperse high-frequency sounds into the room so that they are heard

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accurately. If the dispersion is too narrow, the listener has a limited amount of listening position
options. If the dispersion is too wide, the sense of direction of where the sound is coming from is lost.
There are two main types of high frequency speakers; the well-known tweeter and the more
recent super tweeter. Super tweeters can be add-ons or they can be integral with the system.
Types of Tweeters
 Cone- A smaller version of a standard speaker.
 Dome- The voice coil is attached to a dome that can be made of fabric or a compatible metal
(such as the one shown in the photo).
 Piezo- Instead of a voice coil and cone or dome, an electrical connection is applied to a
piezoelectric crystal, which in turn vibrates a diaphragm.
 Ribbon- Instead of a traditional diaphragm, a magnetic force is applied to a thin ribbon(s) to
create sound.
 Electrostatic- A thin diaphragm is suspended between two metal screens. The screens react
to an electrical signal in such a way that they become out-of-phase, thus alternately attracting
and repelling the suspended diaphragm, creating the needed vibration to create sound.
Cone type tweeters
Since tweeters must reproduce high-frequency notes, they must resonate at high frequencies.
High resonant frequencies are obtained with lightweight, stiffly supported mechanisms. To make the
diaphragm of a cone-type tweeter light, it must be small.
 When we reduce the size and weight of the diaphragm we must, in turn, reduce the size of the
voice coil also. Luckily, high frequencies carry 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 sound in
a narrow cone about the axis of the radiator. The degree of directivity of speaker is indicated by
a directivity pattern in Fig. The axis of the radiator is considered the reference line with an angle
of zero degrees. Directivity patterns 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 indicates by its length that the sound radiated along it is a maximum in comparison
to that in any other direction. At an angle 45°, the line OB is a measure of the relative sound
intensity in that direction. Since OB is only half as long as OA, a listener along OB would listen
only about half the volume compared to what a person along OA. At angles near 90°, the pattern
indicates minimum (zero) radiation.
 In any practical setup, such a zero area would not exist because sound waves reach there by
reflection. Because, directivity normally varies considerably with frequency, a complete diagram
must show separate patterns for each of at least several frequencies.
 Depicts variation of directivity with frequency for a 12-inch cone, assuming that the speaker is
mounted in an infinite baffle. Notice how much narrower the radiation pattern is at highs than at
lows

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Fig. 1.7 Radiation pattern for a typical cone at one frequency
1.1.8 DOME TYPE TWEETERS
Uniformly dispersed flat energy response begins with a speaker system's ability to radiate sound
at all frequencies evenly in all directions. 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 Omni directional.
The limiting factor for high-frequency sounds is that a speaker will begin to be directional when
its circumference equals the wavelength of the frequency being reproduced. Directionality increases
as 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 drivers used
must be as small as possible. Dome tweeters, are designed according to this principle in order to use
these physical laws to the listener's advantage.
Fig. 1.8 Dome type tweeters:

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1.1.9 HORN TYPE TWEETERS
 To obtain reasonable output from a loudspeaker, we must vibrate large amounts of air. For
this, we must usually have 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. Thus
only a small amount of air can be moved, reducing the output power. We can increase the
acoustic output from any type of diaphragm if we couple directly to a horn, converting the
system to a horn-loaded one.
Fig.1.9 horn-loaded tweeters
 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 increases from a small value at one end
called the throat to a large value at the other end called the mouth.
 Horns 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 of air. In this way, the horn acts as an acoustic
transformer and converts the relatively high impedance at the throat and driver.
 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.

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Piezo tweeter
A piezo (or piezo-electric) tweeter contains a piezoelectric crystal coupled to a mechanical
diaphragm. An audio signal is applied to the crystal, which responds by flexing in proportion to the
voltage applied across the crystal's surfaces, thus converting electrical energy into mechanical.
The conversion of electrical pulses to mechanical vibrations and the conversion of returned
mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element
is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa.
The active element is basically a piece of polarized material (i.e. some parts of the molecule are
positively charged, while other parts of the molecule are negatively charged) with electrodes attached
to two of its opposite faces.
When an electric field is applied across the material, the polarized molecules will align
themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure
of the material. This alignment of molecules will cause the material to change dimensions. This
phenomenon is known as electrostriction. In addition, a permanently polarized material such as quartz
(SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes
dimensions as a result of an imposed mechanical force. This phenomenon is known as the
piezoelectric effect.
Piezo-electric Speaker
1.2 MICROPHONE
INTRODUCTION
Microphones are an essential part of any audio recording system. The microphone picks up the
sound and converts it into electrical energy that can then be processed by electronic amplifiers and
audio processing systems. Microphones come in all shapes and sizes. Also different types of
microphone may use different technologies. These different types of microphone have different
properties, and therefore knowledge of the various forms of microphone will enable the best
microphone type to be chosen for a given application. A microphone is a device of the class called
transducers which converts sound waves in air into electrical waves of the same frequency and shape.
In the process of conversion, the microphone must make use of either the pressure of the air waves,
or the velocity at which the air moves. So, we have two types of microphones, the pressure-operated
types, and the velocity-operated types. All sound recording starts with the use of microphones.

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Professional recording engineers use microphones in large number, with the output from each
microphone being separately recorded on a wide strip of recording tape.
For example, 32 or more microphones are used to record a large orchestra. The placing of
each microphone, the amplitude of recording from each microphone, and the subsequent mixing of the
sound from each track to form two tracks of a stereo recording or four tracks of a discrete quad
recording are operations which require great skill and experience. In terms of their technology, most
microphones use electromagnetic induction (dynamic microphones), capacitance change (condenser
microphones) or piezoelectricity (crystal or ceramic microphones) to produce an electrical signal from
air pressure variations. Microphones produce very small output signal levels. Accordingly they need to
be connected to a preamplifier before the signal can be recorded or reproduced. A knowledge of the
different types of microphone also enables the way it is used to play to its strengths. Therefore, even if
you are not a techie, it helps to know a little about them to use them to their best.
Microphone parameters
When choosing a microphone for any given application, it is necessary to take account of the
various attributes, specifications, and performance parameters that it has.
Some of the key parameters for microphones include:
 Fidelity
 Sensitivity
 Frequency response
 Directional attributes
 Robustness
 Cost
 Convenience of use
 Appearance
The importance of the different microphone parameters will depend upon the application. Before
making a choice of any given microphone it is necessary to decide what is important.
Microphone features
When looking at the optimum microphone for any given application, there are many different
parameters and issues to consider. Some of the issues will include the following features:
Type of microphone: There are very many different types of microphone that are available. Each
type has its own characteristics and is best suited for particular applications
Directional characteristics: Microphones can have different sensitivity levels in different directions.
The microphone directional characteristics are important in making sure the microphone used can pick
up all the sounds that are needed. Accordingly the directional capabilities are of great importance and
microphones are often characterized by them
Diaphragm size: Microphones with different diaphragm sizes have different characteristics and are
therefore often used in different applications.

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Microphone history highlights
Microphone Characteristics: There are many types of microphones available. Each has certain
advantages and disadvantages. The choice of a microphone depends upon the type of material to be
reproduced, the placement of the microphone, whether it is to be used indoors or outdoors, the
frequency response desired, and a number of other factors.
The basic types of microphones, grouped according to their principle of operation are: carbon,
crystal, dynamic, ribbon and capacitor. Each of these has its own characteristics with respect to
a. output level,
b. frequency response,
c. output impedance and
d. Directivity.
These characteristics ultimately determine the particular type of microphone suitable for a given
application.
1) Output Level
The output level of a microphone governs the amount of amplification that must be available for
use with the microphone. The output level of microphones is usually given in dB preceded by a minus
sign. The minus sign means that the output level is so many dB below the reference level of 1 milliwatt
for a specified sound pressure.
The unit of sound pressure used for rating microphones is referred to as a bar. A bar is equal to
a sound pressure of 1 dyne per square centimeter. Speech provides sound pressures between 0.4
and 15 bars. For music the pressure ranges from 0.5 bars to 1250 bars. Microphones are rated in a
number of different ways, and this often causes confusion. If ratings are given in any manner other
than in bars, it is a good idea to convert their output level rating to dB below 1 milliwatt for a sound
pressure of 1 bar.
2) Frequency Response
 The frequency response of a microphone is a rating of the fidelity of relative output voltage
which results from sound waves of different frequencies. The simplest way to find a complete
picture of the frequency response characteristics of a microphone is to plot a curve of its output
voltage vs input frequency.
 Since good modern microphones are relatively flat over their range, it is often considered
sufficient to specify the range over which their output does not vary more than plus or minus 1
or 2 dB.
 For ordinary home high-fidelity use, a microphone frequency-response curve should be
reasonably flat between 40 and 10,000 Hz.
 With systems designed specifically for speech reinforcement, a lower limit of 150 Hz and an
upper limit of 5,000 Hz is entirely satisfactory. Where it is desired to reproduce music with the

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highest possible fidelity, the frequency response should be flat (within 2 dB) from about 40 to
15,000 Hz, Fig. 2.1 shows the response of several types of microphones.
3) Output Impedance
A microphone, like any other component with electrical inputs or outputs, has a value of
impedance. When a microphone is connected to an amplifier, a complete circuit is formed and electric
current flows whenever a sound causes the microphone to generate an electrical voltage.
 For most high quality microphones impedance is low, a few ohms ranging up to a hundred
ohms or so, but as little as a fraction of an ohm in a ribbon microphone. Only capacitor and
ceramic crystal microphones have high impedances.
 The importance of microphone impedance is not a matter of the precise value but of the ability
of the microphone and the recorder to be matched together.
 High impedance microphones must be connected into a recorder with high impedance input;
otherwise both the signal amplitude and the frequency range will be adversely affected.
4) Directivity
 Microphones do not respond equally to sound reaching them from all directions. Their
frequency response characteristics also vary, depending on the angle at which the sound
reaches them. A microphone may respond equally to all frequencies between 40 and 10,000
Hz
 When the sound is originating directly in front of it, while the high-frequency response falls off
rapidly as the sound originates further to either side. Where it is necessary to pick up sound
from all directions, the directional characteristics of all microphones are not suitable.
 The way in which a microphone responds to sounds coming from different directions is plotted
on a circular graph which is known as a polar diagram. The centre of the circle is the zero point
and concentric circles indicate successively higher levels of response as they move outward.
 The top of the circle is the front and the bottom the back of the microphones, and the straight
lines radiating from the centre denote the corresponding angles.
1.2.1 CRYSTAL MICROPHONES
 It is based on the piezoelectric effect, which produces a potential difference between the
different faces of the same crystals when these are subjected to mechanical pressure. The
crystals, which show these effects, are quartz, tourmaline, Rochelle salt, and ceramic.
Rochelle salt has a high piezoelectric effect but is susceptible to moisture. Quartz and
tourmaline have a low piezoelectric effect. Ceramic is most suitable for crystal microphones as
it is not susceptible to moisture and can also withstand high temperatures up to 1000 C.
Construction
The construction of a crystal microphone is shown in. The crystal is cut along certain planes
from a slice. Metal foil terminals are attached to the two-layer of foil to carry the voltage difference to
the output terminals.

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Crystal Microphone
Two thin layers of a crystal are properly cut and placed in a holder that is insulated and has an
air space between them. To increase the EMF, a large number of elements are combined. For
pushing the rod, an aluminum diaphragm is attached to the crystal surface. The whole unit is encased
in a protective case. There is a protective mesh cover over the diaphragm.
Applications
It is used the following purposes:
 Home recording systems
 Amateur communication
 Mobile communication
Crystal microphones
Certain crystals, such as rochelle salt and quartz possess the property of generating small
EMF'S when subject to stress or strain. This effect is utilized in what is known as the crystal
microphone. The construction of a crystal microphone is shown in Fig. 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 cause the cone to alternately press against and bend the
crystal slice and release it. Thus, 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 two slices cemented together replace the single slice of crystal.
Then, when pressure is exerted, one slice is compressed while the other is stretched. Thus
equal and opposite voltages are produced which, being in series like the cells of a car battery,
give double the output.
 Any non-linearity, which may arise due to the different mechanical strains between pressure and
release, is also thereby compensated. The double crystal unit is termed as bimorph. One of the
snags with this type of transducer is the mass, which must be moved by the sound pressure
acting on the cone.

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A typical crystal microphone
 This consists of the mass of the cone plus that of the crystal, or that part of it which is moved.
This restricts the frequency response to its upper end to around 10 kHz and limits the transient
response.
 In addition, there are resonances due to the cone and the crystal. With some of the better
microphones, the cone does not actuate the crystal directly but through a cantilever. According
to the dimensions involved, the effect of the mass of the crystal and the mechanical resistance
offered by its stiffness can be reduced but at the same time so also is the amplitude of the
transmitted vibrations
 Hence the signal output. Another type of construction is the sound cell where several crystal
elements are sealed together, this also being termed as multimorph. Here the cone is often
dispensed with, the sound pressure waves acting directly on the crystal.
 Output is lower with this arrangement, but the frequency response is better and also the cone
resonance is eliminated. There is no dc path through a crystal microphone, the crystal being an
insulator.
 Having the two electrical contacts on either side of the slice, the unit behaves as a capacitor. The
equivalent circuit, then, consists of a voltage source in series with a capacitor, Capacitance
values vary, but around 1,000 pF (0.00μF) is typical. This should be taken into account when
considering cable requirements.
1.2.2 CARBON MICROPHONES
When fine carbon granules enclosed in a case subjected to variations of pressure, the
resistance of the granules changes. When such a device of carbon granules is connected in series
with a load through a DC supply, the current through the load will vary in accordance with pressure
variations on the carbon granules.
For a telephone system important requirements are
1. the microphone shall be of convenient size;
2. capable of mass production at low cost while possessing high sensitivity to operate from a
simple battery;

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3. its performance must be stable and adequate to provide intelligible speech and articulation
and;
4. It need not necessarily include the higher harmonic frequencies for reproducing. On the
other hand, microphones used for purposes such as radio broadcasting are relatively few in
number and their cost is not a primary consideration:
 High fidelity reproduction up to about 10,000 Hz for natural speech and music transmission is
essential. An inset pattern of carbon granule microphone is in general use for telephone systems:
 The usual type is a self-contained and sealed microphone which can be readily and completely
removed from the telephone instrument. Its operation depends upon the variation in contact
resistance of the carbon granules when they are subjected to the pressure changes of sound
waves.
 It follows that this type of microphone does not produce an emf but functions by modulating the
current obtained from an external battery.
Carbon microphone in a telephone handset
The carbon microphone generates a continuous hiss. This hiss is due to small variations in
contact resistance, which take place between the carbon granules. With carbon microphones the
electrical output is not directly proportional to the sound input level. The practical effect of this non-
linear distortion is to produce harmonics of the lower speech frequencies and these harmonics tend to
mask higher frequencies normally present in the speech, resulting in loss of clarity or articulation.
The average output level of carbon microphone is of the order of –30 dB The best carbon
microphones have a frequency response of approximately 60 to 7,000 Hz. They are non-directional
although their high frequency response above 300 Hz usually falls off at angles exceeding 40 degrees
from the front of the microphone. When the maximum output level is required from a microphone, the
carbon microphone is often used. The frequency response characteristics of the carbon microphone
are poor and cannot be used for high-fidelity work.

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Construction
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. The lower metal plate is fixed
and is insulated from the diaphragm. A protective cover with holes is used to protect the unit.
Carbon Microphone
A battery is connected between two 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 plate, the resistance of carbon granules, movable metal plate, metal casing, and
output transformer. The purpose of the output transformer is to eliminate the DC content of the
microphone.
Advantage
 Durability, Robust microphone, especially when compared to other types as a result of few
and simple working parts.
Applications
 Due to the limited frequency range, it is useful only in telephones.
 It is also sometimes used in portable radio communication sets.
1.2.3 DYNAMIC MICROPHONE OR MOVING COIL MICROPHONE
The dynamic microphone or moving coil microphone is widely used for stage, musical and other
applications. The moving coil microphone or as it is more commonly called, the dynamic microphone
is one of the most widely used forms of freestanding microphones. It is widely used for vocals for
musical performances as well as for many other applications. The dynamic microphone is also simple
in its design and as a result good microphones offer good value for money.

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Fig.: Moving coil / dynamic microphone
Fig. 1.13 Directivity of a dynamic microphone
Basics of Dynamic microphone:
The dynamic or moving coil microphone relies on the fact that if a wire held within a magnetic
field is moved then an electric current is induced. This is the same effect as seen in an electric
generator and many other items. The dynamic microphone consists of a magnet, and a diaphragm to
which a coil is attached. The assembly is held in place by an outer casing and the coil can move freely
over the magnet.
Fig.: Construction of a moving coil / dynamic microphone

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As sound waves hit the diaphragm, this causes the coil to move backwards and forwards within
the magnetic field and as a result an electric current is induced in line with the incoming sound
vibrations.
Dynamic microphone features
The dynamic microphone has many advantages. It is very sturdy and can tolerate comparatively
rough handling. Dynamic microphones are also able to handle high sound levels without distorting –
this makes them useful for certain musical instruments. Also they do not require an internal
preamplifier like some types including the condenser microphone. Whilst the response of the dynamic
microphone is not bad, they often have a response peak around 2.5kHz or so. This is sometimes
described in the marketing literature as a presence effect. It emphasizes the ambient noise which
tends to be around this frequency. It also gives what is often termed a bright tone to the audio and this
is often liked in some situations where it enhances a musical instrument or lifts the vocals. Another
advantage of the response peak is that it can increase the intelligibility of speech under some
circumstances, although it can make lisps or other similar affects worse. In more expensive dynamic
microphones the peak is well damped, although in less expensive models the peak can be quite
significant. The overall frequency response of these microphones is good, although the inertia caused
by the coil can limit the top frequencies that can be handled.
Dynamic microphone summary
FEATURE DETAILS
Output impedance
Normally around 200Ω although 600Ω and sometimes even 50kΩ
versions are seen.
Basic transducer
impedance
Typically around 30Ω - transformers are used within the microphone to
transform to desired impedance.
Typical frequency
response
40 – 15 000 Hz – often have a peak around 2.5 kHz.
Typical applications Musical instruments, vocals for musical applications.
1.2.5 WIRELESS MICROPHONES
Wireless Microphones:
Wireless microphones work nearly the same as wired microphones. The one big difference
between the two: the typical ―wired‖ mic has a male XLR output connection and relies on a cable to
carry its signal to a mic input, while the wireless microphone relies on a radio transmitter to broadcast
its output signal to a receiver before being sent to a mic input.

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Fig.: Block diagram of wireless microphones
Wireless microphones work in tandem with wireless microphone systems. A wireless
microphone system is made up of the following 3 pieces:
1. Microphone.
2. Transmitter.
3. Receiver.
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 a 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, 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. The average life is three to five hours, but rechargeable batteries are often fitted to
make the instrument more economical to run.
Fig. 1.14 (a) VHF wireless microphone (b) VHF transmitter

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With the separate pocket transmitter a lavaliere or tie-clip microphone can be used to give
complete freedom to the user. The aerial takes the form of a short flexible lead which trails from the
microphone. Usual length is a quarter wavelengths at the permitted frequencies of the carrier wave. The
transmission is picked upon a special receiver, which tunes to the frequency used and demodulates the
signal delivering an audio frequency output, which can then be applied, to an amplifier or recorder in the
normal way. Practically all wireless microphone systems use FM (frequency modulation) and need
roughly 200 kHz bandwidth (to modulate within). In order to have this bandwidth, the radio frequencies
bands utilized for sending wireless microphone signals are typically either
There are fifteen frequencies allocated for wireless microphones and all units work on any one of
these. Interference is no problem because of 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 a 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 of narrow 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 174.65 and 174.95 MHz are allocated for
communication on work sites. An ordinary FM receiver will not pick up wireless microphone
transmissions.
Wireless microphone signals follow this basic path:
 The wireless microphone outputs an audio signal to its connected transmitter.
 The transmitter sends this audio signal wirelessly through radio waves.
 The receiver is tuned to receive these radio waves and audio signals.
 The receiver outputs the balanced audio signal via an XLR cable.
 The XLR cable carries the balanced audio signal to the mic input.
 From the basic descriptions above, that wireless mic systems simply replace the mic cable.
The advantages of wireless microphones include:
 Less/no cable (fewer trip hazards, and no potential tension/pulling between the microphone and
console).
 Increased mobility of the microphone (both for a performer and for passing the mic around to
various people).
 Cleaner signal since the audio doesn‘t travel through any length of cable.
The disadvantages of wireless microphones include:
 The transmitter‘s need for batteries (and the subsequent static noise many receivers output when
the transmitter dies or is turned off).
 The possibility for radio interference to intercept the signal