A useful feature available on many hearing aids today is called the "telecoil" or commonly referred to as T coils. Some folks call it t switch or T setting. T coils or Telecoils have existed within hearing aids since the late 1940s.

The image above shows what a telecoil at several times actual size looks like. A Telecoil is nothing more than a tiny coil of wire around a core that will induce an electric current in the coil when it's in the presence of a changing magnetic field.
The most common use of the telecoil is with the telephone. Although most modern hearing aids are designed to minimize feedback, hearing aids are susceptible to interference from other electrical energy sources and will create feedback like an annoying "buzz" or "hum". For example - digital cell phones, cell phone transmitting towers, some cordless telephones, close proximity to AM-FM radio stations, burglar alarms and motion detectors, electrical shavers, fluorescent lights, computer monitors and other electronic equipment, large TVs, light dimmer switches, fax machines, garage door openers, and other devices can create this annoying "buzz" or "hum" in your hearing aid.

The Telecoil works by detecting and converting magnetic energy into electrical energy in much the same way that a microphone converts acoustic energy into electrical energy. The Telecoil receiver serves essentially the same purpose as the microphone.
When the Telecoil is turned on it allows for much clearer hearing without feedback. Telecoil are beneficial and they may represent the only consistent and reliable option for successful telephone use -- and the Telecoil shuts out other background noises.

What is another new use for a Telecoil? Meetings or Churches or concerts...or at home. Some places like church or meeting halls or concerts will have a "loop" installed. Meaning, a wire (telephone like) will run around the room, either at the floor or the ceiling, exposed or concealed. The wire creates a circle or loop that joins itself, and plugs into an output plug on the PA system box.

When anything is said into the microphone -- from the speaker, it sends the sound to the loudspeakers but also through the "loop," creating a magnetic induction field within the loop. This means that anyone in the room within the loop with their Telecoils on will be able to hear clearly all that is said into the mic...sometimes even better than people with normal hearing.
If you are in a church or meeting hall ask if there is a "loop" installed or look for this sticker....

Q & A.
Do all digital hearing aids have a t coil or t switch or t setting in them - answer: No. T coils take of real estate in the hearing aid. Some of the smaller mini behind ears do not have space. Definitely no space in an In the Ear.
Today...If you wear hearing aids and you are in a worship place, auditorium, business, and or even at home with a loop you can now have the sounds broadcasted from the stage or speaker directly to your personal in-the-ear loudspeakers.