Thankfully, more public places in Australia are being fitted with some type of hearing assistance system. There are four Assistive Listening Device systems available if you count AM & FM as Separate entities. These are:

1. AM Transmission Systems
2. FM Transmission Systems
3. Infrared Systems
4. Induction Loop Systems

Each has advantages and disadvantages which I will briefly describe.

AM and FM Transmission
Advantages: The system is not interrupted by physical obstacles in the reception field. Can be used on different channels in adjacent rooms, or in multi-lingual application.
Disadvantages: Susceptible to RF interference. Requires a special receiver for each person using the system. This can be a sensitive issue for the hearing impaired as the receiver is obvious when worn. Confidentiality cannot be maintained as the system can transmit through walls.

Infrared
Transmits signal via infrared light pulse. Again can be obvious as there is a need to use a special receiver.
Advantages: Does not transmit through walls giving low spillover and confidentiality
Disadvantages: Susceptible to interference from direct sunlight or lighting. Requires specialized receiver. Shadow zones can be formed by physical obstacles in the listening zone. System can change with age due to LED dropouts.

Induction Loop
Advantages: Does not require a specialized receiver. Listening area ca be controlled by loop layout design. Not interrupted by physical obstacles in the pick-up area.
Disadvantages: Can be susceptible to interference of mains wiring and flouro lighting/dimmers. Current required is dependant on metallic content of surrounding areas. Some spillover (this can be minimised by good loop design) Magnetic field strength can vary in complex layouts.

It should be note that in excess of 90% of all Assistive Listening Device (ALD) systems installed are of the Induction Loop type. This is due in the main to relatively low cost of the system,comparatively flexible application for the hearing impaired, and overall listening comfort for the end user.

An induction loop amplifier is controlled by current requirement of the loop - not voltage driven. It can therefore run down to 0(zero) Ohms as compared to a voltage (or speaker) type amplifier which is limited to a minimum 2 Ohms. It should be noted that use of anything other than a pure trans conductance loop amplifier is not the way to go.

They are in principle a Current Feedback design (opposite to most amplifiers that are Voltage Feedback designs). They use a current mirror of some sort in the feedback path and fix the voltage normally to a gain of 1). This makes them a voltage-controlled current source (VCCS), which is in contrast to the audio trades conventional op-amp, which is a voltage-controlled voltage source (VCVS).

Although, relatively new in the audio world, amplifiers of this type have a large bandwidth, are very fast and exhibit low noise.

In the past there was a tendency to use pure voltage or a voltage/current conversion (hybrid) amplifier, the end result being discomfort for the hearing impaired as the amplifier would overdrive the loop. It also had a tendency to be dangerous due to an inability to limit voltage to the system - thus overheating.

A good induction loop amplifier is one designed to AS1088.4 and is inherently safe, that is, capable of being driven into a short circuit ad infinitum. Do not consider using anything other than a compliant product.

Existing installations
There are numerous installations throughout Australia. Unfortunately many do not comply as they are:

1. Poorly specified by consultants with little or no knowledge of the consequences of their actions.
2. Incorporate incorrect amplifier product, that is voltage, or hybrid type, not pure trans conductance.
3. Installed by contractor companies or electricians with little or no knowledge of what is required to ensure a best possible audio output for the end user, the hearing impaired.

Many of the above are the result of early installations, fortunately today there is an improved awareness within the industry..... but there is still a long way to go, there are still "cowboys" out there - both manufacturers and installers who will take the least line of resistance (excuse the pun) where Assistive Listening systems are concerned.

However, I am pleased to advise that the following have provided compliant product, LM Audio, within their facilities.

University of Technology Sydney: 66 Systems
BHP Melbourne: 15 Systems
Adelaide Jockey Club: 4 Systems
IBM Australia: (all states) 31 Systems
Spencer Street Rail Terminal Melbourne: (in construction) 37 Systems
Qantas Terminal Sydney: 12 Systems
Victorian Parliament: 3 Systems
Victorian State Library: 6 Systems
Federal Parliament Canberra - numerous
Hornsby RSL Sydney: 7 Systems

There are many more, but the above will give you some idea of the move toward ensuring that the hearing impaired are properly catered for where audio access is concerned.