Download Velocity-dependent microphones

Microphones
In this chapter we will learn:
 what input and output transducers do
 what a microphone does and how to recognize its symbol
 to recall four important properties of a microphone
 to recall the main features of moving coil, ribbon, capacitor, crystal, carbon and
wireless microphones (including impedances and output voltages)
Transducers
Transducers change energy from one form to another. Those in which electrical energy is the
input or output will be considered; they enable electronic systems to communicate with the
outside world.
A transducer must he matched to the system with which it is designed to work. Sometimes
this means ensuring there is maximum voltage transfer between transducer and system, at
other times we have to transfer maximum power. In all cases, the impedance of the transducer
is a critical factor (rather than its resistance since many transducers have inductance or
capacitance and we are generally dealing with a.c.).
About microphones
A microphone changes sound into electrical energy, its symbol is
and there are
four important properties of a microphone:

The impedance.

The output voltage.
 The frequency response.
Ideally this should cover the audio frequency range from 20 Hz or so to about 20 kHz, and
the sensitivity should be the same for the full range, i.e. sounds of the same intensity but
different frequency should produce the same output.
 The directional sensitivity.
A microphone may be more sensitive, i.e. give a bigger output, for sound coming from one or
more directions than from others. This is shown by polar diagrams like those in the figures
below a, b, c for uni-, bi-, and omni- directional microphones respectively. In these, the length
of the line from the origin 0 to any point P on the diagram is a measure of the sensitivity in the
direction OP.
Microphones fall into three main groups: velocity-dependent, amplitude-dependent and
carbon.
Velocity-dependent microphones
The output voltage of these depends on the velocity of vibration of the moving parts, which in
turn depends on the sound received.
Moving-coil or dynamic type
This is a popular type because of its good quality reproduction, robustness and reasonable
cost. One is shown in the figure below. It consists of a small coil of many turns of fine wire
wound on a tube (the former) which is attached to a light disc (the diaphragm) as shown
below.
When sound strikes the diaphragm, the coil moves in and out of the circular gap between the
poles of a strong permanent magnet. Electromagnetic induction occurs and the alternating
e.m.f. induced in the coil (typically 1 to 10 mV) has the same frequency as the sound.
A moving-coil microphone has an impedance of 200 to 600 . Uni- and near-omnidirectional models are available.
Ribbon type
The moving part is a thin corrugated ribbon of aluminium foil suspended between the poles of
a powerful magnet as shown below. Sound falling on either side of the ribbon makes it vibrate
at the same frequency and a small e.m.f. is induced in it.
Ribbon microphones are bidirectional and have a very low impedance (e.g. 0.2 ). Their
performance is similar to that of the rnoving-coil type but they can be damaged by blasts of
air.
Amplitude-dependent microphones
In these types the output depends on the amount by which the sound waves displace the
diaphragm from its rest position, i.e. on the amplitude of the motion.
Capacitor (or condenser) type
This is used in broadcasting studios, for public address systems and for concerts where the
highest quality is necessary. It consists of two capacitor plates, A and B. Plate B is fixed while
the metal foil disc A acts as the movable diaphragm. Sound waves make the diaphragm
vibrate and as its distance from B varies, the capacitance changes. A small battery in the
microphone causes the resulting charging and discharging current to produce a varying p.d.
across a resistor. This p.d. provides the input to a small amplifier in the stem of the
microphone.
Capacitor microphones, like the tie (clip-on) model above, may be uni- or omnidirectional and
have impedances of the order of 1 k.
Crystal type
The action of a crystal microphone depends on the piezoelectric effect. In this, an electric
charge and so also a p.d. is developed across opposite faces of a slice of a crystal when it is
bent, because of the displacement of ions. The effect is given by natural crystals such as
Rochelle salt (sodium potassium tartrate) and quartz and by synthetic ceramic crystals of lead
zirconium titanate. When sound waves set the diaphragm vibrating, the crystal bends to and
fro and an alternating p.d. is produced between metal electrodes deposited on two of its faces.
The crystal microphone has a high output (e.g. 100 mV), is usually omnidirectional and has a
very high impedance (several megohms). Its performance is not as good as the previous types
but it is inexpensive and much used in cassette recorders.
Carbon microphone
This is used in telephone handsets but it is not a high quality device. The principle of the latest
type is shown below:
Sound makes the diaphragm vibrate and vary the pressure on the carbon granules between the
movable carbon dome, which is attached to the diaphragm, and the fixed carbon cup at the
back. An increase of pressure squeezes the granules together and reduces their electrical
resistance. Reduced pressure increases the resistance. Direct current through the microphone
from a supply (of 50 V at the telephone exchange) varies accordingly to give a varying d.c.
which can be regarded as a steady d.c. plus a.c. having the frequency of the sound.
Current variations represent information in a carbon microphone, whereas in other types, e.g.
crystal, the representation is in the form of a varying voltage. In the first case a current
analogue is produced, in the second it is a voltage analogue.
Wireless microphones
This type incorporates a small radio transmitter which sends the sound falling on the
microphone as a frequency modulated (FM) v.h.f. signal up to a distance of about 100 metres.
Its short aerial (e.g. 6 cm) transmits to any normal FM receiver and allows the
performer/lecturer/public speaker greater freedom of movement since it has no long trailing
cables attached. It is battery operated and usually tunable over the range 88 to 108 MHZ.
Questions:
What are transducers ?
What is the difference between VELOCITY-dependent and AMPLITUDE-dependent
microphones ?
Which are the four most important properties of microphones and what do they mean ?
What are the main features of:
moving coil,
ribbon,
capacitor,
crystal,
carbon and
wireless microphones ?
New words in order of appearance
transducer
to recognize
property
feature
Wandler, Umformer
(wieder-) erkennen
Eigenschaft
Merkmal, Charakteristikum
moving coil,
ribbon,
capacitor,
crystal,
carbon and
wireless microphones
Tauchspul-, (Lautsprecher)
Band-,
Kondensator-,
Kristall-,
Kohle- und
drahtlose Mikrofone
to enable
to match
to ensure
voltage transfer
frequency response
to cover
es möglich machen
an-, zusammenpassen
sicherstellen, dafür sorgen, dass ..
Spannungsübertragung
Frequenzverhalten, Frequenzgang
abdecken