There are many different ways a hard-of-hearing person can use a telephone with their hearing aids. The three most common are:
(1) Holding the telephone near the microphone on the hearing aid.
(2) Using a telecoil (T-coil), holding the hearing aid against the phone, and
(3) Connecting the hearing aid to the telephone by means of Blue Tooth technology (usually through the remote control).
The techniques and engineering used in these three methods differ markedly. In this blog I am going to discuss only the T-coil method.
There is a substantial difference between the signal (magnetic flux output) created by landline telephones and cell phones. The T-coil technique works well with most landline phones, but not with most cell phones.
The manufacturing of conventional landline phones is regulated by laws, such as the Americans with Disabilities Act (ADA), that are designed to give people with hearing loss access to telecommunications. For many years, most landline telephones have been required to include a strong unshielded receiver in the handset that connects efficiently to a hearing aid.
In contrast, cell phones have tiny receivers with low magnetic flux output, i.e., a weak signal used to connect the phone and the hearing aid. This limits their effectiveness for people wearing hearing aids, although Federal Communication Commission regulations do require cell phone manufacturers to offer some models that are compatible with hearing aids.
Image from Hearing Loss Association of America, Albuquerque chapter, https://www.hlaabq.com/Telecoils.html.
HOW T-COILS WORK
Traditional landline telephones and most hearing aids have magnetic coils (small coils of wire built into the unit). To use a T-coil on a landline phone, the wearer switches the hearing aid to the “T” position and holds the phone against the hearing aid. Touching the phone to the aid allows the magnetic signal from the telephone to pass directly into the hearing aid. It is important to note that this is a magnetic connection, not an air-borne sound connection. Using a magnetic connection eliminates feedback from the signal.
It is important to stress the need to have the distance between the speaker in the telephone and the T-coil in the hearing aid very small—less than half an inch. Increasing this distance quickly destroys the magnetic signal.
PROGRAMMING A HEARING AID FOR T-COIL FUNCTION
The usual way to create special listening programs is first to create the basic (Universal) program. Then you open the Program Selection menu and select the program you want to generate–in this case T-coil.
When you use this approach, the computer-generated T-coil program usually has several major faults. That is because the amplified sound is far too complex.
The signal coming out of the telephone has already gone through compression, frequency modification, and clarity enhancement. You do not want to add “compression” or other high-tech features to a signal that has just been compressed. All you want the hearing aid to do is strengthen the telephone signal.
You do not want or need feedback reduction; since the phone signal is magnetic, there is no feedback loop. You also do not need automatic noise reduction, compression, output reduction, frequency transposition, or most other high-tech features. Such features become a hindrance when you run the telephone signal into the T-coil of the hearing aid.
However, you do need to pay careful attention to the frequency response (6 dB/octave or flat response is usually best) and gain, but you should turn most other features off. Therefore, for the telecoil program, reduce the AGC to the minimum and turn off frequency transposition. You can also turn off digital feedback reduction as long as you are using a pure “T” input (i.e., the microphones are turned off).
Check to make sure your settings are appropriate by using a listening scope (attached to the hearing aid), setting the instrument to T, and listening to a landline phone call. If it sounds loud and clear to you, it will probably sound loud and clear to your patient.
