Just because you have a hearing aid does not mean that your hearing problems are solved. When you have normal hearing, your ears are very good at discriminating
between noise and the sounds you want to hear. Not so with a hearing aid, particularly if you are wearing only one.
The hearing aid is basically a microphone, amplifier and earpiece. Unfortunately the microphone picks up all sounds and noise then amplifies all by the same amount. The wearer
often has great difficulty discerning what is going on.
In many situations this problem can be largely overcome by a hearing loop, fed by an audio amplifier. The loop is placed around the room or hall and the radiated signal is then picked up by a hearing aid fitted with a T-coil (or Telecoil; see the sidebar, “The origin of the Telecoil”).
Alternatively, the signal can be picked up via a Cochlea implant or even a loop receiver, as described elsewhere in this issue, driving conventional headphones/earbuds.
Hearing loss increases with age so it is common for hearing loops to be used, for example, in halls and places of worship which older people frequent. In fact, many modern buildings are so equipped these days.
In the home, of course, the problem can be just as difficult, especially when shared with those without hearing impairment. But it is unusual for hearing loops to be installed in the home.
Until now, that is: in this article we describe how to set up a basic hearing loop for the home or for small to quite large meeting rooms, to Australian, New Zealand and IEC (International Electrotechnical Commission) standards – and how to drive it.
This could be done using a commercially made amplifier specifically intended for hearing loop applications but equally could be a standard commercial amplifier or even one of the many amplifier designs published by SILICON CHIP.
Professional hearing loop installations can cost many thousands of dollars, especially when retro-fitted (most new public buildings these days have them installed during construction in appropriate areas as a matter of course).
However, a do-it-yourself installation along the lines set out in this article can provide excellent results and save a heap of dollars. It is relatively easy to fit and can be made small or quite large, depending on the area needed to be covered.
Fig.1: the basic arrangement for a hearing loop. Signal from the room PA is amplified and coupled into the loop. The resulting magnetic field is detected by suitably equipped hearing aids or receivers.
Fig.2: a hearing aid equipped with both T-coil and microphone to cover both signal sources. Many hearing aids will have a switch to select both. Obviously, the loudspeaker is tiny enough to fit in the ear.
What’s a hearing loop?
In its simplest form, a hearing loop system comprises a signal source, an amplifier and a large loop of wire around the room or hall. As this loop forms a coil with an AC current flowing through it, it radiates an electro-magnetic wave which is in sympathy with the signal source.
This radiated signal can be detected by a hearing aid equipped with a T-coil or indeed, a loop receiver (with headphones) designed for the purpose. Fig.1 shows the arrangement but we will explain just how this works shortly.
Where a hearing loop is fitted, it doesn't usually cover the entire area. Hence a "map" is needed, such as this one in a church, to show deaf people with hearing aids where to sit.
If you want to set up a hearing loop in your home you should be able to get satisfactory results without any special equipment. For larger setups in halls, the magnetic field produced by the signal in the loop needs to be set to the required level, so that all hearing aids with T-coils will operate correctly.
In a later article in this series we will show how to build and calibrate a signal level meter to measure signal levels from the installed loop.
Our hearing loop is suitable for use in a home, office, hall, church or similar building. We include design graphs and tables to make it easy to select the wire size and its length, along with the amplifier power requirements for a particular installation.
For large loops, say in a community hall or church, you will need a signal pre-conditioner. In a later issue we will present a suitable design to allow a standard amplifier to be employed. The pre-conditioner provides stereo signal mixing, audio compression, treble boost to provide compensation for loop inductance and treble rolloff above 5kHz.
Other articles will provide circuit and construction details for an induction loop receiver (see p62 of this issue) and a microphone loop driver.
Now let’s describe the basics of a hearing aid.
How does a hearing aid work?
As we mentioned earlier, in its simplest form a hearing aid comprises a microphone, an amplifier and a miniature loudspeaker. In normal use the sound picked up by the microphone is amplified and processed, depending on the complexity of the hearing aid. The amplified signal is then reproduced via the loudspeaker which is closely coupled to the wearer’s eardrum at a level which compensates for the loss of hearing. Fig.2 shows the general internal arrangement.