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This tutorial explains how balanced audio systems work. It is suitable for
people who have a basic understanding of audio cables and connectors, as well
as simple wave interactions (such as how waves from different sources interfere
with each other). If you don't understand these things, take our introduction to audio tutorial
first.
Balanced audio is a method of minimizing unwanted noise from interference in
audio cables. The idea is that any interference picked up in a balanced cable
is eliminated at the point where the cable plugs into a sound mixer or other
equipment.
Balanced audio works on the principle that two identical
signals which are opposite polarities (often erroneously called "out of
phase") will cancel each other out. The cables used in such systems
are designed to carry two versions of the signal and manipulate the polarities
of these signals to eliminate noise.
This will make more sense when we look at how balanced cables work, but
first we need to take a step backwards and look at unbalanced audio cables.
Traditional unbalanced cables use two lines to transmit the audio signal - a
hot line which carries the signal and an earth line. This is all
that is required to transmit audio and is common in short cables (where noise
is less of a problem) and less professional applications.
Note: Internal componentry (in sound mixers
etc) is also unbalanced.
Unbalanced audio cables are commonly associated with the 1/4" phono jack connector and the RCA connector. However any
single-pin connector used for audio is unbalanced. 3-pin XLRs
can also be used for unbalanced cables. For more information about these
connectors, including how to wire them, see Audio Connections.
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Microphones are a type of transducer - a device which converts energy
from one form to another. Microphones convert acoustical energy (sound waves)
into electrical energy (the audio signal).
Different types of microphone have different ways of converting energy but
they all share one thing in common: The diaphragm. This is a thin piece
of material (such as paper, plastic or aluminium)
which vibrates when it is struck by sound waves. In a typical hand-held mic like the one below, the diaphragm is located in the
head of the microphone.
Location of Microphone Diaphragm
When the diaphragm vibrates, it causes other components in the microphone to
vibrate. These vibrations are converted into an electrical current which
becomes the audio signal.
Note: At the other end of the audio chain, the loudspeaker is also a
transducer - it converts the electrical energy back into acoustical energy.
There are a number of different types of microphone in common use. The
differences can be divided into two areas:
(1) The type of conversion technology they use
This refers to the technical method the mic uses
to convert sound into electricity. The most common technologies are dynamic,
condenser, ribbon and crystal. Each has advantages and
disadvantages, and each is generally more suited to certain types of
application. The following pages will provide details.
(2) The type of application they are designed for
Some mics are designed for general use and can be
used effectively in many different situations. Others are very
specialised and are only really useful for their
intended purpose. Characteristics to look for include directional properties,
frequency response and impedance (more on these later).
The electrical current generated by a microphone is very small. Referred to
as mic level, this signal is typically
measured in millivolts. Before it can be used for
anything serious the signal needs to be amplified, usually to line level
(typically 0.5 -2V). Being a stronger and more robust signal, line level is the
standard signal strength used by audio processing equipment and common domestic
equipment such as CD players, tape machines, VCRs, etc.
This amplification is achieved in one or more of the following ways:
Dynamic microphones are versatile and ideal for general-purpose use. They
use a simple design with few moving parts. They are relatively sturdy and
resilient to rough handling. They are also better suited to handling high
volume levels, such as from certain musical instruments or amplifiers. They
have no internal amplifier and do not require batteries or external power.
As you may recall from your school science, when a magnet is moved near a
coil of wire an electrical current is generated in the wire. Using this electromagnet
principle, the dynamic microphone uses a wire coil and magnet to create the
audio signal.
The diaphragm is attached to the coil. When the diaphragm vibrates in
response to incoming sound waves, the coil moves backwards and forwards past
the magnet. This creates a current in the coil which is channeled from the
microphone along wires. A common configuration is shown below.
Earlier we mentioned that loudspeakers perform the opposite function of
microphones by converting electrical energy into sound waves. This is
demonstrated perfectly in the dynamic microphone which is basically a
loudspeaker in reverse. When you see a cross-section of a speaker you'll see
the similarity with the diagram above. If fact, some intercom systems use the
speaker as a microphone. You can also demonstrate this effect by plugging a
microphone into the headphone output of your stereo, although we don't
recommend it!
Technical Notes:
Dynamics do not usually have the same flat frequency response as condensers.
Instead they tend to have tailored frequency responses for particular
applications.
Neodymium magnets are more powerful than conventional magnets, meaning that
neodymium microphones can be made smaller, with more linear frequency response
and higher output level.
Condenser means capacitor, an electronic component which
stores energy in the form of an electrostatic field. The term condenser
is actually obsolete but has stuck as the name for this type of microphone,
which uses a capacitor to convert acoustical energy into electrical energy.
Condenser microphones require power from a battery or external source. The
resulting audio signal is stronger signal than that from a dynamic. Condensers
also tend to be more sensitive and responsive than dynamics, making them
well-suited to capturing subtle nuances in a sound. They are not ideal for
high-volume work, as their sensitivity makes them prone to distort.
A capacitor has two plates with a voltage between them. In the condenser mic, one of these plates is made of very light material and
acts as the diaphragm. The diaphragm vibrates when struck by sound waves,
changing the distance between the two plates and therefore changing the
capacitance. Specifically, when the plates are closer together, capacitance
increases and a charge current occurs. When the plates are further apart,
capacitance decreases and a discharge current occurs.
A voltage is required across the capacitor for this to work. This voltage is
supplied either by a battery in the mic or by
external phantom
power.
The electret condenser mic
uses a special type of capacitor which has a permanent voltage built in during
manufacture. This is somewhat like a permanent magnet, in that it doesn't
require any external power for operation. However good electret condenders mics usually include a pre-amplifier which does still
require power.
Other than this difference, you can think of an electret
condenser microphone as being the same as a normal condenser.
Technical Notes:
Every microphone has a property known as directionality. This
describes the microphone's sensitivity to sound from various directions. Some
microphones pick up sound equally from all directions,
others pick up sound only from one direction or a particular combination of
directions. The types of directionality are divided into three main categories:
To help understand a the directional properties of
a particular microphone, user manuals and promotional material often include a
graphical representation of the microphone's directionality. This graph is
called a polar pattern. Some typical examples are shown below.
Captures sound equally
from all directions.
Uses: Capturing ambient noise; Situations where sound is coming from
many directions; Situations where the mic position
must remain fixed while the sound source is moving.
Notes:
Cardioid
means "heart-shaped", which is the type of pick-up pattern these mics use. Sound is picked up mostly from the front, but to
a lesser extent the sides as well.
Uses: Emphasising sound from the direction
the mic is pointed whilst leaving some latitude for mic movement and ambient noise.
Notes:
This is exaggerated version
of the cardioid pattern. It is very directional and
eliminates most sound from the sides and rear. Due to the long thin design of hypercardioids, they are often referred to as shotgun
microphones.
Uses: Isolating the sound from a subject or direction when there is a
lot of ambient noise; Picking up sound from a subject at a distance.
Notes:
Uses a figure-of-eight
pattern and picks up sound equally from two opposite directions.
Uses: As you can imagine, there aren't a lot of situations which
require this polar pattern. One possibility would be an interview with two
people facing each other (with the mic between them).
Some microphones allow you to vary the directional characteristics by
selecting omni, cardioid or
shotgun patterns.
This feature is sometimes found on video camera microphones, with the idea
that you can adjust the directionality to suit the angle of zoom, e.g. have a
shotgun mic for long zooms. Some models can even
automatically follow the lens zoom angle so the directionality changes from cardioid to shotgun as you zoom in.
Although this seems like a good idea (and can sometimes be handy), variable
zoom microphones don't perform particularly well and they often make a noise
while zooming. Using different mics will usually
produce better results.
When dealing with microphones, one consideration which is often
misunderstood or overlooked is the microphone's impedance rating. Perhaps this
is because impedance isn't a "critical" factor; that is, microphones will still continue to operate whether or
not the best impedance rating is used. However, in order to ensure the best
quality and most reliable audio, attention should be paid to getting this
factor right.
If you want the short answer, here it is: Low impedance is better than
high impedance.
If you're interested in understanding more, read on....
Impedance is an electronics term which measures the amount of
opposition a device has to an AC current (such as an audio signal). Technically
speaking, it is the combined effect of capacitance, inductance, and resistance
on a signal. The letter Z is often used as shorthand for the word
impedance, e.g. Hi-Z or Low-Z.
Impedance is measured in ohms, shown with the Greek Omega symbol Ω.
A microphone with the specification 600Ω has an impedance of 600 ohms.
All microphones have a specification referring to their impedance. This spec
may be written on the mic itself (perhaps alongside
the directional pattern), or you may need to consult the manual or manufacturer's
website.
You will often find that mics with a hard-wired
cable and 1/4" jack are high impedance, and mics
with separate balanced audio cable and XLR connector are low impedance.
There are three general classifications for microphone impedance. Different
manufacturers use slightly different guidelines but the classifications are
roughly:
Note that some microphones have the ability to select from different
impedance ratings.
High impedance microphones are usually quite cheap. Their main disadvantage
is that they do not perform well over long distance cables - after about 5 or
10 metres they begin producing poor quality audio (in
particular a loss of high frequencies). In any case these mics
are not a good choice for serious work. In fact, although not completely
reliable, one of the clues to a microphone's overall quality is the impedance
rating.
Low impedance microphones are usually the preferred choice.
Microphones aren't the only things with impedance. Other equipment, such as
the input of a sound mixer,
also has an ohms rating. Again, you may need to consult the appropriate manual
or website to find these values. Be aware that what one system calls "low
impedance" may not be the same as your low impedance microphone - you
really need to see the ohms value to know exactly what you're dealing with.
A low impedance microphone should generally be connected to an input with
the same or higher impedance. If a microphone is connected to an input with
lower impedance, there will be a loss of signal strength.
In some cases you can use a line matching transformer, which will
convert a signal to a different impedance for matching
to other components.
Frequency response refers to the way a microphone responds to
different frequencies. It is a characteristic of all microphones that some
frequencies are exaggerated and others are attenuated (reduced). For example, a
frequency response which favours high frequencies
means that the resulting audio output will sound more trebly than the original
sound.
A microphone's frequency response pattern is shown using a chart like the
one below and referred to as a frequency response curve. The x axis
shows frequency in Hertz, the y axis shows response in decibels. A
higher value means that frequency will be exaggerated,
a lower value means the frequency is attenuated. In this example, frequencies
around 5 - kHz are boosted while frequencies above 10kHz
and below 100Hz are attenuated. This is a typical response curve for a vocal
microphone.
An ideal "flat" frequency response means that the microphone is
equally sensitive to all frequencies. In this case, no frequencies would be
exaggerated or reduced (the chart above would show a flat line), resulting in a
more accurate representation of the original sound. We therefore say that a
flat frequency response produces the purest audio.
In the real world a perfectly flat response is not possible and even the
best "flat response" microphones have some deviation.
More importantly, it should be noted that a flat frequency response is not
always the most desirable option. In many cases a tailored frequency response
is more useful. For example, a response pattern designed to emphasise
the frequencies in a human voice would be well suited to picking up speech in
an environment with lots of low-frequency background noise.
The main thing is to avoid response patterns which emphasise
the wrong frequencies. For example, a vocal mic is a
poor choice for picking up the low frequencies of a bass drum.
You will often see frequency response quoted as a range between two figures.
This is a simple (or perhaps "simplistic") way to see which
frequencies a microphone is capable of capturing effectively. For example, a
microphone which is said to have a frequency response of 20 Hz to 20 kHz can
reproduce all frequencies within this range. Frequencies outside this range
will be reproduced to a much lesser extent or not at all.
This specification makes no mention of the response curve, or how
successfully the various frequencies will be reproduced. Like many
specifications, it should be taken as a guide only.
Condenser microphones generally have flatter frequency responses than dynamic.
All other things being equal, this would usually mean that a condenser is more
desirable if accurate sound is a prime consideration.
