Assistive Listening System
A system that helps hard of hearing people to hear in meetings, movies, plays, classrooms, etc. Generally there are four types of Assistive Listening Systems: Radio, Infrared, Hard Wired, and Induction Loop. Induction Loop is the subject of these pages.
A wire that carries electrical current. Usually if we speak of a single wire all by itself, we just call it a wire. If we are speaking about a group of wires bundled together in a cable, we usually call them conductors. Cables can have 2, 3, 4, 6, .... or many conductors, depending on what the cable was intended for.
Measured in Henrys, milihenrys or microhenrys, inductance is a term that indicates how much opposition to change in current a conductor or coil gives. By analogy, think of a flywheel on a motor shaft. When the motor is turned on, the flywheel does not immediately start turning at the maximum speed of the motor. It takes a few seconds for the motor and flywheel to reach maximum speed. Likewise, when the motor is turned off, it takes a few seconds for the flywheel and motor to stop turning. This is because of inertia, and inductance can be thought of as electrical inertia. When current is turned on to a coil of wire, it takes a certain amount of time for current to reach it's maximum value. Factors that affect the amount of inductance include the number of turns a coil has and whether or not the coil is formed around a ferrous core. This is because a conductor carrying an electrical current always has a magnetic field around it, and it takes a certain amount of time for that field to get established.
Inductive Reactance (XL)
Measured in Ohms, Inductive reactance is an electrical term that relates to alternating current (AC) like resistance relates to direct current (DC) Inductive reactance is given by the formula:
XL = 2 X PI X Frequency X Inductance
This formula indicates XL will increase when frequency increases.
An electrical term that indicates how much load a device will present to the preceding stage. For example, speakers usually are rated at 8 Ohms Impedance. A lower number means a greater load will be placed on the driving amplifier and a higher number means a lessor load will be placed.
Z = square root of (R squared + XL squared)
Because XL is a function of frequency, impedance is also. But for audio considerations, frequency is usually taken to be 1000 Hz.
PA amplifiers usually have an input jack that is called "Line-In" or "Aux". Some other audio equipment have output jacks called "Line-Out". A patch cord can be connected between Line-out to Line-In, since the "Line" signal level is standardized at about 1 V. The line-out and Line-In jacks are usually of the "RCA" type. By using a patch cord, equipment such as CD players, DVD sound, VRC sound, can be broadcast over an audioloop.
This term is not applicable to audioloops, but since the words "Line" and "Power" are both used in a discussion about audioloops, it may be helpful to define it. Line Powered means a telephone that gets its power from the telephone line. It used to be that all telephones were line powered, but now some are appearing that have an AC adapter that plugs into a wallplate.
A microphone is a transducer that converts acoustical sound energy to electrical energy. Most microphones are either the dynamic type or the
electret type. They are available as omni-directional which means they are equally sensitive in all directions, or uni-directional, which means they are more sensitive in one direction.
Electret microphones have a tiny preamplifier included in the microphone package. This preamplifier needs a small amount of power to operate, and this power is called phantom power. Amplifiers that support electret microphones supply the phantom power on the microphone jack. Some electret microphones contain their own battery.
A dynamic microphone is a speaker in reverse. Both have a coil of wire, called the "voice coil" that is mechanically attached to a diaphragm and is suspended between the pole pieces of a permanent magnet. In the case of the dynamic microphone, sound waves cause the diaphragm to move in accordance to the sound waves, and movement of the voice coil generates an electrical current.
Microphones generate a very small signal level, usually from 10 MV to 40 MV. Therefore, the amplifier for a microphone has to have a large DB gain to make a microphone useful. PA amplifiers usually have several microphone input jacks and gain controls for each input.
An Induction Loop that is small and worn around the neck like a necklace. It plugs into a receiver just like a headphone.
In these pages, a power amplifier for a DIY audioloop means an audio amplifier that is capable of driving loads in the 4 to 16 ohms range, at relatively high power - 30 to 100 watts. Power amplifiers designed for single turn loops can drive a much lower impedance, and are called "constant current" amplifiers.
Radio and Infrared Assistive Listening Systems employ a transmitter that broadcasts program material to the audience. Users in the audience must use receivers matched to the transmitter, to receive the broadcast. Induction Loop systems do not require receivers for people who have telecoil equipped hearing aids; however, Induction Loop receivers are available for people who do not have the telecoil feature.
An electrical term that indicates how well a conductor will carry current. Resistance is measured in Ohms and is given by the formula E = IR, or R = E/I where R = Ohms, E is voltage, and I is current. A conductor that has a low
resistance will carry more current than a conductor with a high R. By analogy, a small garden hose pipe will not carry as much water as a 4 inch main pipe.