It is most likely electromagnetic energy from the theft detectors which is
picked up by the hearing aid. This energy can vary in both magnitude and
frequency dependent on the particular system. In some theft detection
systems, only the receiver is turned off when you deactivate the system
hence the transmitter will always be active as long as the power is on.
Many hearing aids will pick-up the interference in both microphone and
telecoil settings. However, ultrasonic theft detection systems are known to
cause problems, too (only in microphone setting).
> Apparently more stores are using those little labels and the detectors
> at the doorway. My son wears a hearing aid with a bone vibrator.
> He can "hear" the detectors at the doorway when he walks by them.
> Some are so loud that they cause him pain. He has mystied more than
> one store manager when he told them that their antitheft detectors
> were not turned on.
> Jim Lellman