T-coils and Telephones

by Neil Bauman, Ph.D.

November 30, 2015

A man explained:

While talking on the phone the other day, I accidentally turned on the T-coil program in my hearing aids and got a boost in volume and since the microphone was muted in that particular program, cut out the noise in the room. I also tried this with my iPhone with similar success. I didn’t know about this.

I have tried to figure out why this works and can only think that since most speakers use some form of a magnetic medium to move the “cone”, so to speak, that that magnetic field wakes up the T-coil in my hearing aids. Or something like that. So my question is this: Why does this work? Once again no audiologist ever mentioned such a phenomenon to me.

Unfortunately, the benefits of T-coils are still a well-kept secret today. Although audiologists know about t-coils and telephones, few ever seem to mention them. Fortunately, that is now slowly changing.

T-coils have been around for a long time now. Would you be surprised to know that the first hearing aid to have a T-coil was 77 years ago—way back in 1938!

Did you know that T-coils were actually invented for the express purpose of helping hard of hearing people hear better when using the telephone? In fact, the “T” in T-coil stands for “telephone”. Originally it was a “telephone coil”. Now it is often shortened to “tele-coil” or more commonly just “T-coil”.

In the days before digital hearing aids, it was often referred to as a “T-switch” as there was a switch on hearing aids back then often labeled “O”, “M”, “T” for “Off”, “Microphone” and “T-coil” respectively. You flipped the switch to the “T” position to activate the T-coil when you wanted to make a phone call.

You are correct in understanding how T-coils work. Originally telephone earpieces were small loudspeakers. The voice coil moving past the stationary permanent magnet created a varying magnetic field that the T-coil picked up. (If you know how a transformer works, you can think of the t-coil and the voice coil in the telephone handset as the two parts of a transformer, separated by an air core.)

Essentially, when current flows though a wire it creates a magnetic field. An equal and opposite current is induced into any other wire within this magnetic field. That’s how the telephone transfers the sound signal to your T-coil wirelessly.

All was well for a number of years. Then the phone manufacturers wanted to make phones smaller and lighter and less power hungry. (This is especially true in cell phones.) Thus, they embraced a new technology—the piezoelectric crystal. When current flows through a piezoelectric crystal, it caused the crystal to vibrate in unison to the varying voltage. These vibrations push the air molecules and produce the sound waves you hear in the earpiece.

Unfortunately, by doing away with the voice coil in the phone’s earpiece, the magnetic field generated now was so small that T-coils couldn’t pick it up. Thus the T-coils in hearing aids were useless for use on the phone.

Things would have stayed this way if hard of hearing organizations such as the Hearing Loss Association of America (HLAA) hadn’t petitioned the FCC to require phones to be hearing-aid compatible—meaning that hard of hearing people could once again use their T-coils to hear on the phone.

Eventually the FCC made it a requirement that a certain percentage of the models each phone company carried needed to be t-coil compatible. This percentage is slowly being increased to 100% so eventually, we will be able to hear on any phone via our T-coils.

In order to make modern phones T-coil compatible, phone manufacturers didn’t go back to using miniature speakers containing voice coils. Instead,  they just added a corresponding coil in the phone’s ear piece to create the required magnetic field.

Thus, today you should be able to use your T-coils with most phones, whether they are landline, cordless or cell.

However, there is still a problem with T-coil use. You see, T-coil technology is very directional. It’s just one of the laws of physics. This means that in order to transfer a good signal, both the coil in the phone and the T-coil in your hearing aid need to be in the same plane. Since the speaker voice-coil in early phones had to be horizontal, T-coils in hearing aids were horizontal too.

This was (and still is) fine for phone use, but T-coils are useful in many other situations including counter and room loops. The problem is that room loops need to be oriented vertically, and horizontally-oriented T-coils do not pick up a useful signal from vertical room loops.

Some hearing aid manufacturers decided to mount the T-coils at a 45° angle. This only reduces the signal by 3 dB (not enough to worry about) so it appeared to be a good solution, but in retrospect, it just introduced a host of other problems related to directionality.

For example, depending on which way the T-coil was aimed at the 45° angle inside your hearing aid (i.e. front to back as opposed to side to side), when using a room loop, you might have to face the wall in order to hear the speaker at the front. (And it’s decidedly difficult to speechread someone when you are facing the wall!).

The solution to this is to mount all T-coils vertically, and to mount all corresponding coils in phones vertically. Then T-coils could be used effectively on phones and in looped venues without directionality problems.

Why don’t they do this? As I see it, the phone companies have always mounted their coils horizontally and don’t seem to want to change—even though there is no reason to mount these coils horizontally since they don’t use miniature loudspeakers anymore.

It is up to all of us hard of hearing people to advocate for vertically-mounted T-coils in hearing aids, and vertically-oriented corresponding coils in all phones so we can have barrier-free communication, which is only fair and right.