Glossary of Terms
As the area of Assistive listening has grown so too has the language used to describe the different technologies, and how we use them. This Glossary aims to provide basic definitions of terms that may be unfamiliar. Please let us know what we have missed- and if you have a definition send that to us too
- Assistive Listening
- This is a collective term given to any aid ( except hearing aids) that help the hard of hearing or profoundly deaf to understand a situation. It includes FM, Infrared, induction loops, Vibrating /flashing alarms,visual displays and for some, even sign language.
- Field Strength - Magnetic Field strength
- Terms used to describe the magnitude or level of the magnetic field. It is equivalent to the volume or loudness of the audio signal.
- Background noise
- In the context of induction loops, this term refers to the background magnetic noise, not the acoustic noise. It is audible signal picked up by a loop receiver or T-coil when the Induction loop system is turned off
- Neck Loop
- A personal induction loop. Instead of the loop being installed around the room, it is worn around the neck over clothing. To use the neck loop, it must be connected to the audio source. The output of the audio source must be compatible with the neck loop. The hearing aid must be switched to the T' position
- Speech intelligibility
The quality or condition of being understandable or intelligible; Able to be understood; Capable of being understood.
The speech Intelligibility Index, SII, is an international index for quantifying the intelligibility of speech Unfortunately, Assistive listening standards do not yet require any minimum SII. IEC60118-4:2006 does not test the intelligibility of the system.
- Induction loop - Hearing Loop
Is a term used to describe an electromagnetic communication or detection system such as a vehicle detection system or an Assistive Listening system for the hard of hearing. As induction loops are used in a range of applications , when used for assistive listening, they are often described as Audio Frequency Induction Loop Systems, AFILS.
Common name, particularly in the USA, for an Audio Frequency Induction Loop system
- Standard Superloop
- A propriatry system that is suitable for compensating for metal loss, is always Ultra Low spill, and provides almost 3D d area coverage.
- Multi- Loop
- 2 or more loops connected in series or parallel such as a Figure 8 Loop, Super 8 Loop or a SuperLoop
- Figure 8
- Describes a loop pattern that scribes a figure 8 or series of figure 8's
- Custom SuperLoop
- This describes a bespoke (custom) design SuperLoop with non standard performance characteristics tailored to technical or budgetary constraints
- Signal to noise ratio
- A comparison of the desired signal to the background noise
- The sum of the d.c resistance and the frequency dependent inductance of an electrical circuit
- AFILS is an acronym for Audio Frequency Induction Loop System
- Loop Amplifier
- This is like an audio amplifier but it has been designed to drive a current around a cable. It is often referred to as a Loop Driver
- Loop Driver
- See Loop Amplifier
- Perimeter loop?
- This is the simplest induction loop cable configuration. It is a single or multiturn cable that is run around the perimeter of the room
- FM System
This is a radio system using Frequency Modulation (FM)
An Assistive listening FM system must be specially designed to meet the needs of the hard of hearing. The system will need automatic gain control and the recivers will need higher output gain than typical and ideally complete with a neck loop option.
Wireless microphones are also radio based FM systems and can be used to complement Induction loop systems
- Auto SCART
- An amplifier with this function will automatically detect which pin on the SCART connector has audio and connect it. Simplifying the set up with a TV.VCR, DVD etc.
- SuperLoop (standard)
Comprises separate 2 figure 8 loops interleaved over the area ,driven by 2 independent loop amplifiers, a master and slave with a 90 0 phase shift applied between their input signals.
A standard SuperLoop, SLS system, will:
- Provide very uniform coverage over the space in both the vertical and horizontal planes*
- Provide ultra-low spill control
- Almost eliminating interference problems with microphones, video signals etc
- Dual Action Automatic Gain Control
- This is proprietary 'Always On' technology which controls the level of the input signal to within defined limits. Unlike any other automatic Gain control system it does not shut the output down after any sudden short duration high level sound bursts so every word of every sentence is heard.
- Cross counter loop
- Cochlear Implant?
'Bionic ear', a surgical implanted electronic device that gives a sense of sound to the profoundly deaf or severely hard of hearing.
They are fitted with a T-coil.
- Metal loss
- Metal in close proximity to an induction loop may 'absorb' the signal. (Metal Loss) If this is not compensated for in the induction loop system design, the system will not perform to the IEC60118-4:2006 standard.
The unwanted magnetic signal the exists outside of the area that the loop is intended to cover.
It is the name given to the signal generated by the system that exists outside of the loop. Magnetic fields fall off very rapidly with distance, however, 2 - 3 times the width of the loop away, a signal level greater than -32dB may still be present. A room 5m wide may still be radiating above the acceptable background noise level 10m - 15m away.
(-32dB is the maximum level of background magnetic noise that the standard IEC60118-4 deems acceptable
- Almost 3D area coverage
(See Standard Superloop System)
The field strength in the horizontal plane changes with direction.
When facing the longitudinal cables of the array, the field strength created by the system will be equal in the horizontal and vertical planes. This is not true when facing the short sides of the loops.
- FSM (Field Strength Meter)
- This is an instrument designed to measure the strength of the magnetic field. Such as the univox FSM2
- Speech Intelligibility Index, SII
- This is an internationally recognised index to quantify the intelligibility of speech. The index range is from 0 to 1 where 0 means the speech has no intelligibility and 1 means it is 100% intelligible
Super 8 Loop
Super 8 In Phase
Super 8 Out of Phase
Describes 2 or more loops connected in parallel and typically driven by 1 amplifier. There are 2 types.
- This has no null fields and is ideal for covering large areas on a budget.
- Has similar performance to a figure 8 loop. Null fields will exist where the loop cables of adjacent loops run together
- Speech Transmission Index, STI
Is a measure of speech transmission quality. It measures the physical characteristics of the transmission channel and its ability to maintain the characteristics of a speech. The channel can be anything from a room, a station concourse to a telephone line. Factors affecting the STI are: Speech level, frequency response, non linear distortions, background noise level, echoes, reverberation time, sound masking.
The index range is 0 to 1 where 0 is bad and 1 is excellent. For most situations 0.5 is OK for the hard of hearing >0.7 should be the aim
- Room Acoustic Speech Transmission Index, RASTI
- This is an earlier standard to STI. It was declared obsolete by the IEC in June 2011
- Video Synch
This function enables manual adjustment of the time delay on the audio signal to re -synchronise the audio with the video. ie. to have the lips move in time with speech.
Video /audio synchronisation has always been an issue depending where in the TV set up, the audio is fed from Now, with the introduction of digital TV's, this issue is becoming more common as digital video processing takes a lot longer than the audio signal processing causing a time delay.
SOUND as we perceive it is the vibration or local pressure deviation from ambient of air or some other medium acting on our ear. The pressure deviation can be measured in air using a microphone. The sound pressure level for the threshold of human hearing is 20?Pa (micro Pascal) and is generally used as the reference level for all other sounds when expressed in decibels. ie 20?Pa is equivalent to 0dB
Sound pressure is inversely proportional to distance from a point source. So when measuring sound pressure, it is important to state the distance from the source.
- SuperLoop (Custom)
This is a superLoop system that has been tailored to meet certain requirements or constraints of a particular application. Parameters that can be controlled by custom design are:
- The degree of spill control ( less pill control can mean a cheaper solution)
- Field strength versus height
- Degree of compensation for metal loss
- Deaf awareness
- Is a way of interacting with the hard of hearing and profoundly deaf that helps them to understand without being patronising or drawing attention.
Induction loop system designed for installation at a ticket counter, reception desk etc. Often supplied as a kit with a microphone and a loop coil