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Glossary | Microphones
| Induction Loop Systems |
FM Radio systems | Infra-red systems
Below are some of the questions we get asked most frequently, and
a few more beside. If you can't find the answer you are looking
for - please contact us
Glossary
Glossary of commonly used terms
Microphones
Microphone introduction
Types of microphone
Dynamic and condenser microphones
explained
Omni-directional
and uni-directional microphones explained
Balanced and unbalanced
microphones
Boundary microphone
Leaded (wired) or radio
microphones explained
Radio microphone non-diversity
Radio microphones true
or space diversity
Radio microphone receivers
Radio microphone compatibility
Hand held
or lapel (tie clip) radio microphones
Use of Microphones
Microphones proximity effect
Care of Microphones
Induction Loop Systems or Audio Frequency Induction Loop Systems
(AFILS)
What are induction loop systems?
How does an induction loop system work?
How do I compare different amplifiers
specifications all stated to be IEC 60118-4 complain?
What is the latest revision to the IEC 601
18-4?
Does a loop system affect sound for non-hearing aid users?
Can other circuits interfere with the hearing aid set on the ‘T’ position?
Can I install a loop system if background
interference is above the critical level?
How do I test for
electromagnetic background noise?
How do I use a Field Strength Meter (FSM)?
How do I install an induction loop
cable?
How do I set up the loop system?
How do I connect a loop
system to an existing Public Address System?
How do I install an Induction
Loop System without an existing Public Address System?
Do I need to switch the loop amplifier
off after use?
How do I use a microphone?
When is it not possible to install
a loop system?
Does metal in a building
interfere with a loop system?
What is overspill?
Can the loop cable
cause interference to other circuits?
Do I have to check the loop system
on a regular basis?
What are the advantages of a
Portable Induction Loop System?
Do I have to train my staff how the
loop system operates?
How do I keep the system running and
ensure good customer care?
Do I need a license for an induction
loop system?
Can I use radio microphones on
the same frequency together?
Glossary of commonly used terms
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Assistive technology, assistive devices,
assistive listening devices
These are all terms used to describe any item used to assist
hearing - e.g. hearing aids, visual alerts, vibrating alarm clocks,
vibrating and flashing smoke alarms, pager systems and audio frequency
induction loop systems (AFILS).
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Hearing loss, hearing impaired, hearing
impairment
Generic expressions covering various medical conditions.
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Deaf community, deaf culture, deaf clubs
Terms referring to the greater body people who are hearing
impaired or deaf.
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Induction loops, audio frequency induction
loops (AFILS), hearing loops
These are all terms commonly used by the deaf community to
refer to audio frequency induction loops (AFILS) - one of the most
commonly used assistive devices to improve hearing and communication
with the hearing impaired.
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What are induction (inductive) loop systems?
People who are hearing impaired, find it difficult to hear in large venues because of poor room acoustics. The problem is made worse by the distance that sound has to travel before it reaches the listener, background noise and competing sounds. Induction loop systems help to reduce unwanted background noise so that sound can be heard more clearly with the hearing aid set to the ‘T’ position.
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How does an induction loop system work?
We have dedicated a whole page to the topic of how an induction
loop works, for more information, please click on the link below...
How induction loops (or
inductive loops) work
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How do I compare different amplifiers?
A majority of loop amplifier manufacturers specify the coverage
area based on 100mA/m instead of the 400mA/m. Therefore it is truly
impossibly to compare different amplifiers with the manufactory
claimed coverage area. An amplifier configured for 400mA/m need
be 16 times as powerful!! And since power output is costly this
16 times difference are likely to come up in the price tag. This
is therefore not a popular subject amongst most manufactories. In
addition the coverage is highly dependent on loop figuration and
metallic environment and is always less than the theoretical free
field value.
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What is the latest revision to the IEC
601 18-4?
The new revised IEC 60118-4 with 400mA/m as the new reference overrides
the old vastly misunderstood 100mA/m standard. A level of 400mA/m
must be achieved in a loop system according to the new standard.
It is NOT sufficient with only 100mA/m which is a widely spread
misunderstanding of the old standard. This has caused and is causing
many weak and noisy installations based on cheap and under-powered
amplifiers. The 100mA/m has therefore been removed in the revised
standard and should not be referred to any longer.
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Does a loop system affect sound for non-hearing
aid users?
Using a loop system with a television, hi-fi, microphone or other sound source means a hearing person can hear sound at a level that is comfortable for them. The hearing impaired person using the loop system can get louder sound by adjusting the volume on the hearing aid. Sound from a TV speaker is unaffected by a loop system.
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Can other circuits interfere with the
hearing aid set on the ‘T’ position?
Hearing aid users (with hearing aid on the ‘T’ position) may hear magnetic interference from electrical equipment and wiring, such as fluorescent lights, light dimming circuits or 3 phase power cables. In extreme cases this manifests itself as a continuous loud hum or buzz. Clearly this is unacceptable for the hearing aid user. It is important that any site allocated for an induction loop system is checked for electro-magnetic background interference (in accordance with BS6083 Part 4). It is advisable that test is carried out before the equipment is fitted. A Field Strength Meter (FSM) is used for the purpose of testing for electro-magnetic background interference. An induction loop system must not be installed or be in use if the electro-magnetic background interference exceeds the critical level of -20dB across the majority of the loop area.
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Can I install a loop system if background
interference is above the critical level of -20dB?
If just a small proportion of the looped area has electro-magnetic background interference above the critical level of -20dB then a loop system can be installed but signs must be displayed clearly marking the area that has interference.
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How do I test for electromagnetic background
noise?
Using a Field Strength Meter (FSM) will determine the level of electro-magnetic
background noise. A Field Strength
Meter can be hired from us to carry out this test prior to fitting
one of our kits. Full instructions on how to test for electro-magnetic
background interference are supplied with the meter.
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How do I use the FSM?
Field Strength Meter (FSM) operation.
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How do I install the induction loop cable?
You can install the induction loop cable yourself - or you may want to ask someone who is good at DIY to do it for you. Full instructions for installation are included with every kit that we supply. The loop amplifier is normally placed close to the source of sound (e.g the stage area) and connected directly to it via a plug-in lead or microphone.
Installing the loop cable itself is the trickiest part of the installation as it is usually placed round the edge of the room at skirting board level (or you can hide it under the carpet if there is one).
Take care to install the cable securely - especially round door
frames, window frames or openings (staple pitch < 15cm). You
can route the loop cable up and down over door frames, window frames
and openings as this will have little effect on the overall system
performance.
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How do I set up the loop system?
Once the induction loop system has been installed it must be calibrated
to the correct output level of +12dB on peak signals using a Field
Strength Meter (FSM) in the middle of the room at 1.5 metres above
floor level. Instructions for set up are supplied with all our loop
induction kits.
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How do I connect an induction loop system
to an existing public address system?
You will need to check if the existing public address (PA) system
has an auxiliary or tape output socket. This would normally be found
on the rear of the PA amplifier. An audio lead (supplied with the
loop kit) is connected from the auxiliary or tape output socket
on the PA amplifier to the loop audio input socket. If the PA amplifier
does not have an auxiliary or tape output socket, a separate microphone
will be required for use with the loop.
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How do I install an induction loop system
without existing public address system?
If the area for the induction loop system does not have public address
system, a microphone will be required. The microphone is probably
the most important part of the induction loop system - get this
wrong and any good induction loop system can sound poor. With this
in mind each of our induction loop kits has the correct type of
microphone to be used for each particular venue.
TOP
Do I need to switch the loop amplifier
off after use?
The induction loop amplifiers in each of our kits have been designed
to be left on continuously including the home induction loop system
that has a DC power supply. We would advise that a dedicated fused
spur be used for the mains supply to all of the induction loop system
kits (except home loop system). The installation of a dedicated
mains fused spur removes the chance of the induction loop system
being switched off deliberately or inadvertently. You will also
not have to remember to turn your system on before each use or performance.
If customers cannot use the system because it has not been switched
on, then you could be in breach of the Disability
Discrimination Act (DDA).
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How do I use a microphone?
Microphone techniques.
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When is it not possible to install a
loop system?
An induction loop system must not be installed if the electro-magnetic background interference exceeds the critical level of -20dB across the majority of the loop area. If this is the case then we can supply an alternative method of meeting your requirement, for example by using our Infra-Red (IR) systems.
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Does metal in a building interfere with
a loop system?
Metalwork, particularly aluminium, can have an unpredictable effect
on loop system performance. Other external factors can affect the
performance of an induction loop system. Reinforced concrete floors,
steel lintels, corrugated steel cladding and other types of metalwork
all have an effect on the performance of the induction loop system.
Any aluminium based building product extensively used throughout
the building may have a dramatic effect up on the performance of
the induction loop system. In extreme cases it may prohibit the
installation of a loop system because its effect has degraded the
sound quality and strength to an unacceptable level.
If you are in any doubt of the construction of the building it is advisable to run a temporary loop around the venue and test it first. This saves having to deal with problems after the loop has been installed. You may want to use our test system hire service.
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What is overspill?
Overspill is the magnetic field outside the area of the loop system. Induction loop systems are designed for hearing aid users to be present within the area of the loop. As the magnetic field will travel through walls, ceilings and floors there will always be some overspill. This means that hearing aid users outside the looped area may be able to overhear conversations if their hearing aids are switched to 'T' position. The magnetic field of a simple one loop system will overspill into adjoining rooms and rooms directly above and below. If you are fitting more than one loop please use our consultation service.
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Can the loop cable cause interference
to other circuits?
In some circumstances, an induction loop may cause interference in other circuits within a sound system (e.g. a public address system). This is likely to happen if the induction loop cable is installed parallel to, and in close proximity to, audio input cables. Microphone cables are especially vulnerable to this kind of interference. Carefully routing of all cables so as not to run in parallel or cross/touch normally prevents this problem.
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Do I have to check the loop system on
a regular basis?
Ideally, you should use a Field Strength Meter (FSM) with level indicator LED’s and speaker output to check the system (available as an optional extra). This lets you check magnetic field strength (+12B on peaks of sound) and quality of the sound. Non hearing aid users are unable to hear the loop system unless they use a hearing aid set to the ‘T’ position or use a FSM. We would advise that any venue that has an induction loop system should use an FSM on a regular basis.
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What are the advantages of a portable
induction loop system?
Portable induction loop systems can be used when required in any venue and packed away after use. They are useful if you do not need a permanent system or if you need to use the loop in different rooms or venues. The loop cable on an easy to use drum makes the temporary loop set up quick and easy.
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Do I have to train my staff to operate
the loop system?
You must let your staff know you have had an induction loop system
installed for people who are hearing impaired using a hearing set
to the ‘T’ position and make sure they know how to use
it. If customers cannot use the system because your staffs have
not been trained how to use it, then you could be in breach of the
Disability Discrimination Act (DDA).
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How do I keep the system running and
ensure good customer care?
Make sure that at least one named person is responsible for keeping the system working to its optimum performance in your building. With an induction loop system, designated staff should be responsible for looking after the loop system and auxiliary components such as radio microphones. In major venues, it is good practice for users to be able to contact at least one member of staff if there is a problem during performances. All staff who have contact with customers should also know about the system.
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Do I need a licence for an induction
loop system?
No, you do not need a licence to operate an induction loop system.
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Microphone introduction
A microphone ("mic" for short) is a device for
converting audible sound into an electrical signal. To accomplish
this task with optimum efficiency and accuracy requires a type of
mic that is appropriate to the particular situation. There are many
different types of mics some are designed for very specific applications
and others have a more general purpose.
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Types of microphone
Microphones can be categorised in several different ways.
The most important of these categories are described below.
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Dynamic and condenser explained
All types of microphone incorporate some form of diaphragm.
This is a small surface which vibrates in sympathy with the sound
pressure waves reaching the microphone. However, dynamic and condenser
mics vary in how these vibrations are used to produce an electrical
signal.
In a dynamic microphone, sound is converted to an electrical signal
by the vibrations of the diaphragm causing the vibration of a coil
in a magnetic field, effectively an electrical generator on a very
small scale. As this produces sufficient signal level for direct
connection to a PA system, no amplification of the signal is required
within the mic. Dynamic mics are most useful for close-proximity
applications (i.e. 0 to 15 cm) such as lead vocals, guitar amplifiers,
etc.
In a condenser mic (also called a capacitor mic), sound is converted
to an electrical signal by the vibrations of the diaphragm causing
changes in the capacitance of a charged capacitor. This is achieved
by the diaphragm itself being one of the plates of the capacitor.
As this produces a very small signal level, some initial amplification
of the signal is required within the mic itself. This internal amplifier
may be powered either by an internal battery or by power supplied
from the mixer. Condenser mics are most useful for larger distances
between the sound source and the mic (i.e. 15 cm upwards), such
are encountered with lecterns and with overhead miking of drum kits,
choirs, theatre stages etc. They can be more prone than dynamic
mics to making a "popping" sound when used close-up with
a "breathy" sound source such as a voice or a wind instrument,
though this problem can be reduced with the windshield fitted. They
are capable of a higher quality sound than dynamic mics, and the
best versions are therefore extensively used in studio recording
work. They are the best type of microphone for use with an Audio
Frequency Induction Loop System (AFILS).
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Omni-directional and uni-directional
microphones explained
Omni-directional mics pick up sound with equal sensitivity
from all directions. This is not normally useful for PA work, because
in PA work each mic is targeted at a single sound source (so that
the amplification given to that sound can be controlled separately
from others, and so that pick-up of unwanted sounds can be minimised).
Their application is generally limited to recording work (particularly
of ambient sounds) and to sound-level measurement.
Uni-directional mics pick up sound with greater sensitivity from
the front than from other directions.
There are several variations on this theme:
Cardioid mics have a gradually reducing sensitivity from the front
to the back, with very little sensitivity at the back.
Super-cardioid mics reduce their sensitivity from the front to the
sides at a faster rate than cardioid types, reaching a minimum sensitivity
at an angle of around 120-140°, measured from the front. The
sensitivity then increases again towards the back, but the sensitivity
at the back is still very much less than at the front.
Hyper-cardioid mics provide even less sensitivity at the sides than
do super-cardioid types, at the expense of a little more sensitivity
at the back. Therefore, a monitor speaker should never be placed
directly behind this type of mic. Their minimum sensitivity is at
an angle of around 100-120°, measured from the front.
'Rifle' or 'shotgun' mics are the most directional type, so-called
because of their long rifle-like barrels. They are generally used
only for long-distance miking (more than 2 metres from the source),
e.g. for theatrical work, and should be located such that the back
of the mic is not exposed to unwanted sounds.
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Balanced and unbalanced microphones
Unbalanced mics, where one of the signal-carrying conductors
of the connection between the mic and the system is provided by
the signal earth or 'screen' of the connection. With this arrangement,
the inter-connecting cable is prone to pick-up of interference from
stray magnetic fields and radio signals, and so these types of mic
are suitable only for use with moderate lengths of cable (up to
around 10 metres).
Balanced, where the two signal-carrying conductors of the connection
are separate conductors from the signal earth or 'screen' of the
connection. This arrangement is highly immune to pick-up of interference,
and so may be used with very long lengths of cable (up to 200 metres),
provided it is of good quality.
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Boundary microphones
A boundary mic is a special type which when placed on a surface
utilises the sound energy collected at that surface. Such mics are
generally equally sensitive to sounds in all directions above the
surface. Typically used for speech, where a convenient surface such
as a desk or lectern is available.
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Leaded (wired) or radio microphoness
Leaded or wired mics connect to the PA system by means of
a cable. The cable usually attaches to the mic by means of a 3-pin
XLR or 1/4" jack plug.
Radio (or "wireless") mics contain a battery-powered radio
transmitter. The radio signal from this transmitter is picked up
by a receiver which is connected to the PA system. The mic and the
receiver are purchased as a pair and are referred to as a "radio
mic system".
Radio mic systems use a frequency-modulated (FM) radio signal at
VHF or UHF frequencies. (Strictly, each system does not operate
at a single frequency, but rather uses a narrow range of frequencies
called a 'channel'. The frequency that is quoted is the carrier
frequency, the frequency at the centre of the channel.) The frequencies
used are either "licensed" or "de-regulated".
Use of a licensed frequency requires payment of an annual license
fee. Use of the de-regulated frequencies is free.
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Radio microphones - non-diversity
Non diversity means that the radio signal (usually picked
up on a single aerial) is processed by a single set of receiving
electronics. These types are prone to "drop-outs" - temporary
interruptions of the audio signal caused by temporary reductions
in the received radio signal strength (due to reflections of the
signal and physical obstacles in its path). The receiver aerial
is generally best set vertically. Some single-channel receivers
are equipped with two aerials, even though they have only one set
of receiving electronics - these may perform a little better than
single-channel receivers with only one aerial, but fall far short
of the performance obtained from true diversity receivers.
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Radio microphones - true or space diversity
True or space diversity means that the radio signal is picked
up on two aerials, each connected to a separate set of receiving
electronics - the output of the receiver is provided from the set
which is giving the best quality signal at any moment in time, or
is a combination of the two. True or space diversity receivers are
much less prone to drop-outs than single-channel types. The aerials
are generally best set at between + and - 30 to 45 degrees from
the vertical, i.e. spreading apart at between 60 and 90 degrees
to each other.
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Radio microphone receivers
Some receivers provide an audio output intended for connection
to a line input of the PA system, whilst others have outputs intended
for connection to a mic input. Some types may provide both kinds
of output, or a single output of adjustable level.
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Radio microphone compatability
When several radio mics need to be operated simultaneously, each
system must be set to a different frequency. Furthermore, in order
to avoid intermodulation interference between the systems, the frequencies
selected must be chosen from a compatible set for the particular
make and type of system being used. The maximum number of frequencies
in a compatible set will depend upon the quality of the system.
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Hand held or lapel (tie clip) radio microphones
Hand-held mics are generally about 6 to 7 inches (15 to 18
cm) long and 1.25 inches (3 cm) in diameter. They may be held in
the hand or placed in a clip on a mic stand. Note that many radio
mics have a slightly larger diameter than leaded (wired) types,
and therefore will not fit into 'standard' sized mic clips.
Mic clips fix to the stand by means of a screw thread, of which
there are three common types:
In case your mic clip doesn't fit your stand, thread adaptors are
available.
Lapel (tie clip) mics are generally much smaller and are either
body-worn (e.g. clipped to a lapel or tie, or attached to a head-set).
A body-worn mic is also known as a lavalier mic.
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Use of microphones
To get the best results, it is important to choose an appropriate
type of mic for the job, and to use it correctly.
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Microphone proximity effects
Most PA mics are uni-directional types, and all uni-directional
mics exhibit what is known as the "proximity effect".
The result of this effect is that sounds which are made very close
to the mic are picked up with a greater bass response than sounds
which are made further away. This is most important for presenters
and vocalists to understand, because the difference that a change
in working distance makes to the sound of their voice can be quite
dramatic. It is especially significant for deep-voiced vocalists
(usually male), because a greater proportion of their voice is in
the frequency range which is subject to the proximity effect. At
a working distance of greater than about 4 to 6 inches (10 to 15
cm), the proximity effect can be ignored. As the distance decreases
from this down to zero, the amount of bass emphasis increases.
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Care of microphones
Mics contain delicate precision-engineered components, and if you
want your mics to continue to perform as well as when they were
new, you must look after them very carefully. Even ruggedised stage
mics will benefit from careful treatment. Following these simple
do's and don'ts will help considerably:
• DO keep them in padded protective boxes or pouches - preferably
individually - when not in use (especially during transport).
• DO clean the integral windshield from time to time, when
this is accessible. Follow the maker's instructions (especially
for expensive mics!), but in the absence of any instructions the
wire-meshed end of most types can be unscrewed and then gently washed
in warm soapy water - allow to dry thoroughly before re-attaching
to the main part of the mic.
• DON'T check them by tapping them or by blowing into them
- speak (or sing) into them instead, and educate users to do the
same.
• DON'T drop them or allow them to be subjected to other sudden
shocks.
• DON'T store them in damp conditions or expose them to extremes
of temperature.
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