Hearing aids do not always function well with wireless handsets. Hearing aids operate by using a microphone to pick up sound waves, converting the sound waves into electrical signals to be amplified. Distortion or amplification of unwanted sound (noise) often occurs.
The FCC's hearing aid compatibility requirements address hearing aids that operate in either of two modes - acoustic coupling ("M" rating) or inductive coupling ("T" rating). Hearing aids operating in acoustic coupling mode receive through a microphone and then amplify all sounds surrounding the user, including both desired sounds, such as a telephone's audio signal, and unwanted ambient noise. Hearing aids operating in inductive coupling mode turn off the microphone to avoid amplifying unwanted ambient noise, instead using a tele coil to receive only audio signal-based magnetic fields generated by inductive coupling-capable telephones. The FCC's "M" and "T" ratings indicate whether a handset can be expected to function well with a hearing aid and are generally marked clearly on the handset packaging. M4 and T4 are better than M3 and T3. The "M" or "T" rating does not guarantee that the and set will function without distortion or noise, so NDTC recommends that you test the handset before purchasing.