Download Directivity of ribbon microphone

CHAPTER 2:
MICROPHONES
 INTRODUCTION:
• Basically any audio and video
systems deals with the microphones
and the loudspeakers, as these are
the basic transducers required in any
audio and video systems, for the
conversion of acoustic signal into
electrical and vice-versa.
2.1 CHARACTERISTICS OF
MICROPHONES:
 Microphone is a transducer which converts sounds
pressure variations into electrical signals of the same
frequency and phase and of amplitudes in the same
proportion as in pressure variations:
 Quality of a microphone is determined by the
following characteristics:
• Sensitivity
• Signal to noise ratio
• Frequency response
• Non-linear distortion
• Directivity
• Output impedance
 Sensitivity:
 It is defined as output in milli volts (or in dB
below 1 volt) for the sound pressure of 1 microbar (or 0.1 pa) at 1000 Hz.
 Signal to noise ratio:
 Some noise (called self noise) is generated
inside the microphone due to resistance of the
circuit, built-in transformer,etc.
 Instead of quoting noise alone, manufacturers quote
signal to noise ratio.
 It is defined to be the ratio in dB of the output
( with SPL of 1µ bar ) to the output in the absence of
sound.
 Frequency response:
 The frequency response of a microphone is defined by
the bandwidth of audio frequencies in the output of
microphone within +1 dB of the output at 1000 Hz.
 Although the complete audible frequency range of
sound waves is 16 to 20000 Hz, a microphone which
gives flat response for frequencies 40 to 15000 Hz is
considered good for high fidelity audio systems.
 Mass of the vibrating system and its compliance are
equivalent to electrical inductance and capacitance,
respectively.
 Mass causes attenuation at high and compliance at
low audio frequencies.
 Distortion:
♦ Besides frequency distortion (frequency response)
described above,there are two type of distortions in
microphones,namely ,1. Nonlinear distortion , 2. Phase
distortion.
1. Non-linear distortion:
♦ Non-linear distortion distorts the amplitude of the
audio signal,which results in production of such
harmonics in the output that are not present in the
input sound.
2. Phase distortion:
► Phase distortion 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:
 Directivity is defined as the ratio of actual output
when placed in a direction of maximum response to
the output which an omni-directional microphone in
the same direction would have given, keeping the
intensity of sound constant.
 Directivity is given by,
D = E / Eo
Where, E= actual output in the direction
of maximum output.
Eo = output in that direction had
the microphone been omnidirectional.
 When represented in dB, directivity would be 20 log
d.
 Microphone with the following directivities are used
in in practice:
1.Omni directional
2. Figure of eight directivity, and
3.Cardioid or heart shaped directivity.
OMNI-DIRECTIONAL
Mic
BI-DIRECTIONAL
CARDIOID OR
FIGURE OF EIGHT
HEART SHAPED
Mic
Mic
(A)
(B)
(C)
FIG 2.3: TYPICAL DIRECTIVITY PATTERNS OF MICROPHONES
(A)PATTERN DUE TO ALL PRESSURE TYPE MICROPHONES
(B) PATTERN DUE TO RIBBON MICROPHONE
(C) PATTERN DUE TO SERIES COMBINATION OF PRESSURE TYPE
MICROPHONE WITH RIBBON MICROPHONE.
 All pressure microphones
directional pattern.
have
basically
omni-
 Cardioid pattern is obtained by combining an
omnidirectional microphone with a figure of eight
pattern microphone in series.
Output impedance:
 A microphone has an output impedance which is
represented in ohms.
 This is an important parameter which is used to
determine which type of matching transformer would
be needed to transfer the power efficiently from
microphone to the transmission line and then to the
amplifier.
2.2 REQUISITES OF A GOOD
MICROPHONE :
 A good microphone should have high sensitivity, high
signal to noise ratio, flat frequency response over most
of the audible frequency range, natural resonant
frequency outside the audible range, and very low
distortion.
 The directivity of the microphone should be such as
to meet the requirement of application.
 From the point of view of impedance,
microphones are divided into three categories:
the
1. Very low impedance (from a fraction of an Ω to about
50 Ω )
2. Medium impedance (100-600 Ω )
3. High impedance (750 k Ω and higher)
 Ribbon and moving coil microphones are low
impedance microphone and need a step-up transformer
to increase the impedance looking into the circuit.
 Crystal and condenser microphones are high
impedance microphones, needing an emitter follower
amplifier for matching.
 Carbon
microphone
microphones.
are
medium
impedance
2.3 MOVING COIL MICROPHONE:
• The moving coil microphone (also called dynamic
microphone) uses the principle of electro magnetic
induction.
• When sound pressure variations move a coil placed in
the magnetic field, there is a change of magnetic flux
passing through the coil
• An e.m.f. is, therefore, induced in the coil and this
e.m.f. forms output of the microphone.
• Due to similarity in construction, a moving coil
loudspeaker can also work as moving coil microphone.
• The same unit is often used both as microphone and
loudspeaker in office intercom systems.
Construction:
the main components of a moving
coil microphone are magnet, diaphragm and coil. These
are shown in fig. 2.4.
• 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 poles.
• Diaphragm is of non magnetic material and is light
weight. It is fixed to the body of magnet with the help
of springs.
• The springs provide compliance (equivalent to
electrical capacitance) to the motion of diaphragm
when sound waves strike it. Mass of the diaphragm
and coil assembly provide inductive effect.
• Coil is wound on a cardboard cylinder which is
attached to the diaphragm.
• A protective cover 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
atmosphere.
• 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. Electrical
equivalent circuit is shown in the figure.
Functioning:
 When sound waves strike the diaphragm, it moves
and hence, coil moves in and out in the magnetic field.
This motion changes the flux through the coil which
results in e.m.f. being induced in the coil due to
electromagnetic induction.
 The value of e.m.f. depends on the rate of change of
flux and hence on the motion of the coil.
 This displacement of the coil depends on the
pressure of sound waves on the diaphragm. Thus it is
a pressure microphone.
 The induced voltage, e,
microphone is given by eq.2.2.
across
the
coil
of
e=dΦ
dt
=B * Change of area per second
=B * length of conductor * distance
moved per second
=B * l * v
where,
B= flux density in tesla (or weber
per sq. m)
l=length of coil in metre
v=velocity of diaphragm (and hence
coil) in m/sec.
Other features:
1. As magnet is quite heavy, it can be encased in a
heavy protective case, and hence, it is very robust.
2. It does not need external bias.
3. It can be spoken into from a distance of about 25 cm.
4. Its cost is not high. It is lower in cost than ribbon and
condenser microphones, but higher than crystal and
carbon microphones.
Applications:
• Moving coil microphone is the most
microphone.
widely used
• It is suitable for use in public address systems and
broadcast studios.
• When it is coupled with ribbon microphone in series,
its cardioid pattern makes it useful for dramas and
concerts.
2.4
RIBBON MICROPHONE:
 In moving coil microphone, high frequency response
is poor because of the mass of the diaphragm
(including coil assembly).
 Mechanical
mass
is
equivalent
to
electrical
inductance, and this attenuates higher frequencies.
 Hence, light aluminium ribbon is used in place of
diaphragm and coil assembly.
 This ribbon acts as conductor (placed in a magnetic
field) as well as the diaphragm.
 Such a microphone is called ribbon microphone.
CONSTRUCTION:
• The main parts of a ribbon microphone, shown in fig.
2.7, are: 1. Permanent magnet, and 2. Ribbon
conductor.
• The permanent magnet is a specially designed magnet
with extended pole pieces. It provides strong magnetic
field.
• The ribbon is a light aluminium foil.
• The main feature is lightness of the ribbon, which is
only 0.2 mg in weight, a few microns thick and about
3mm wide.
• It is suspended in the magnetic field of the permanent
magnet and the stiffness of suspension is small.
 WORKING PRINCIPLE:
 When the ribbon (an electric conductor), placed in a
magnetic field, is made to move at right angles to the
magnetic field by the force of sound pressure, there is a
change of magnetic flux through the ribbon conductor
and e.m.f. Is induced across the ribbon.
 In the ribbon microphone, the driving mechanical
force is proportional to the difference of the pressure
acting on two sides of the ribbon, or the particle
velocity of sound waves. Hence it is also called
pressure gradient or the velocity microphone.
 Characteristics
microphone:
of
ribbon
• Frequency response of ribbon microphone:
In a microphone, high frequency signal is attenuated
due to inductive effect of mass of the moving system,
and low frequency signal is attenuated due to low
compliance (mechanical capacitance) or high stiffness
(capacitive reactance). In ribbon microphone, mass of
the system is small and hence mechanical inductance is
low resulting in good response for high audio
frequencies.
Directivity of ribbon microphone:
As force of sound can be applied to the ribbon from
the front as well as the rear side, this type of
microphone is bi-directional and its polar pattern is
figure of eight .
 Other features:
1) The ribbon ,being very light is delicate and can be
easily damaged by dropping. Hence it requires careful
handling.
2) A d.C. Current through ribbon from a battery of a
meter can strain the ribbon. Hence, resistance or
continuity of ribbon should not be measured with a
meter. It should be measured with visual inspection.
3) It is sensitive to the movement of air near it and to
the shock vibrations,it is also sensitive to breathing. So
should be kept far from the sourse of sound.
4) It has excellent transient response.
5) It does not need external bias.
6) Its cost is high.
 Applications:
• It is very suitable for dramas.
• Its figure of eight polar diagram allows actors to talk
face to face which is more natural than standing side
by side.
Cardiod microphones:
• When a ribbon microphone and a moving coil
microphone both are mounted in the same housing,
connecting them in series, a cardioid (or heart
shaped) pattern results.
• Such microphones are suitable for orchestras and
court scenes in dramas where a large number of
persons are present.
2.5 CRYSTAL MICROPHONE:
• A crystal microphone is based on the principle of
‘piezo electric effect’, which is defined as:
• “Difference of potential between the opposite
faces of some crystals is produced when these
are subjected to mechanical pressure”.
• The crystals which show this effect are quartz,
tourmaline, rochelle salt and ceramic.
• Ceramic is most suitable for crystal microphones as it
is not susceptible to moisture and can also withstand
high temperatures upto 100 0c.
• When pressure is applied to the crystal, it deforms
and momentary displacement of charge takes place
within the crystal structure.
• This creates a potential difference between two
surfaces of the crystal.
SPRING
SPRING
DIAPHRAGM
PUSHROD
BIMORPHIC
CRYSTAL UNIT
OUTPUT
TERMINALS
INSULATING SUPPORT
FIG 2.10 CRYSTAL MICROPHONE
 CONSTRUCTION:
 A typical crystal microphone is shown
The crystal is cut along certain planes to
Metallic foil electrodes are attached to the
to carry the potential difference to
terminals.
in fig. 2.10.
form a slice.
two surfaces
the output
Two thin crystal slices suitably cut are placed in an
insulating holder with an air space between them.
 A large number of such elements are combined to
increase the e.m.f.
 The whole unit is encased in a protective case.
 FUNCTIONING:
 When there is a sound wave of compression, it
compresses the crystal.
 In case of rarefaction, converse takes place and the
crystal is extented and is under tension.
 Due to this compression and extention, a varying
potential difference is generated which is proportional
to the mechanical pressure applied to the crystal by the
sound waves. It is therefore pressure microphone.
OTHER FEATURES:
1. Not as rugged as moving coil one, but more rugged
than ribbon type.
2. Unlike moving coil microphone and ribbon
microphone, it has no frequency discrimination with
direction.
3. It does not need a bias supply.
4. It should not be exposed to direct sun light for long
time.
5. Its cost is low.
 APPLICATIONS:

It is used for the following purposes
1. Home recording systems
2. Amateur communication
3. Mobile communication
2.6 CONDENSER (OR CAPACITOR)
MICROPHONE:
Principle:
• When capacitance of a capacitor changes, the charge
on the capacitor tends to remain the same, and
hence voltage changes in accordance with equ.,
V = Q/C
where v = voltage across c
Q = charge in coulombs
C = capacitance in farads
• Equation shows that if c increases v will decrease
and if c decreases, v will increase.
• Diaphragm of the microphone acts as one plate of
condenser.
• The other plate, called back plate, is fixed.
• When sound pressure moves the diaphragm
in, the capacitance increases and vice-versa.
• The change in capacitance results in change
in voltage (a condenser microphone is a
pressure microphone).
Construction:
• A condenser microphone is shown in fig. 2.11.
• It consists of a light weight metal diaphragm
(generally aluminium) which is suspended above a
fixed metal back plate.
• A fixed dc voltage of about 50 to 100 volts is applied
between the backplate and the movable plate.
Functioning:
• When sound waves strike the diaphragm, it moves.
• During compression waves it moves towards the fixed
back plate and increases capacitance
• During rarefaction, it moves away from the back plate
and decreases capacitance.
• The change in capacitance changes the d.C. Voltage
across capacitor plates.
• The net voltage vt at any instant t is given by eq.,
Vt = v d.C+ va
vt = instantneous voltage
Vdc =d.C. Bias applied to the condenser
plates (it is about 50 to 100 volts).
va = Audio signal corresponding to the
sound
pressure variations
• The output of a condenser microphone is quit low
and hence a high gain amplifier is built inside the
microphone housing.
Other features:
1. It needs external D.C. Bias supply. Typical voltage
required is 100 volts D.C.
2. It is delicate because of the narrow separation between
the moving plate (diaphragm) and the fixed back plate.
3. It can not withstand excessive heat. Moisture is also
harmful to condenser microphone.
4. It is costly because of the necessity of D.C. Bias.
Applications:
1. It is used as standard microphone for
calibrating other microphones.
2. It is used in sound level meters ( db
meters ).
3. It is used in professional high fidelity
recording.
2.7
ELECTRET MICROPHONE:
• External D.C. Bias in condenser microphone makes it
costly and unsuitable for field work.
• The electret microphone is also a condenser
microphone, but it has built-in charge. The insulating
( dielectric ) material used is teflon.
• Insulating materials can trap large quantity of fixed
charge and can retain it indefinitely.
• The back plate of the microphone is coated with a
thin layer of teflon.
• This thin layer is charged negatively at the time of
manufacturing.
• This negative charge remains trapped for long period
• The –ve charge on the diaphragm and +ve charge on the teflon
establish an electric field across the gap of the capacitor plates.
The charge results in a terminal voltage.
• When the capacitance changes due to sound pressure, the charge
tends to remain constant and hence the terminal voltage changes.
• It has same characteristics as condensor microphone except that it
does not need extenal bias and is less costly.
• As the electret microphone is cheap, has good frequency response
,is rugged, and does not need bias supply, it is used in small p.A.
System for clubs and small halls, to keep the cost low. It is used in
sound level meters and in wireless microphones.
• It being very light is also used as tie clip microphone.
2.8 CARBON MICROPHONE:
Principle:
• When fine carbon granules enclosed in a case are
subjected to variations of pressure, the resistance of
granules changes.
• When such device of carbon granules is connected in
series a load through a dc supply, the current
through the load will vary in accordance with
pressure variations on the carbon granules.
 CONSTRUCTION:
The construction of a carbon microphone is shown in
fig.
• Fine carbon granules are enclosed between two metal
plates.
•
• The upper plate is attached to a movable metal
diaphragm through metal piston or plunger. The lower
metal plate is fixed and is insulated from the
diaphragm.
• A battery is connected between two metal plates.
• When load is connected, current flows through the
carbon granules and load.
• The output is obtained through a transformer to
eliminete D.C. Content of the microphone output.
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 circuit.
 In the absence of sound , a steady current flows
through the circuit.
 Thus sound waves superimpose a varying current,
or audio current on the steady dc current.
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
microphone.
Applications:
• Due to limited frequency range, it is useful
only in telephones.
• It is also sometimes used in portable radio
communication sets.
2.10 SPECIAL MICROPHONE:
Lavalier microphone:
 A small transducer suspended on the chest by means
of a chord around the neck is called lavalier microphone.
 It is a small moving coil type, specially designed to
work as lavalier microphone.
 It has applicability where mobility is necessary for
example, for a lecture.
Tie-clip microphone:
 It is an electret type tiny microphone which can be
cliped on a tie, or any other convenient part of the
clothing.
 An external amplifier made on a tiny chip of silicon is
used inside the microphone.
 Even with tiny amplifier and its cell, it is very light.
Radio(wireless) microphone:
 It uses a small VHF transmitter, so that cable from
amplifier to microphone is not needed.
 The signal is received by a VHF receiver placed at a
suitable distance, then amplified and fed to the
loudspeakers.
 It is useful in sports for oath taking ceremony.
Noise cancelling microphone:
 In this type of microphone two matched transducing elements are
used.
These are mounted a few inches apart in the microphone housing
and are connected together in the opposite phase.
 Sound which originates a few inches away from the microphone will
actuate both transducers equally. The output being in opposite phase
will cancel.
 Sounds which originate within a few inches will affect the nearer
element more and hence the two outputs will be unequal and so, will
not completely cancel each other, resulting in signal output.
The speaker has to use the microphone very close to his lips.
Such microphones are suitable for use in noisy environments like
sports meet, etc.