Tech Logic VoiceLink II User Manual page 20

Feedback (Mic Squeals)
To reiterate: Batteries in the transmitter is our greatest grievance.
Life expectancy of "Duracell" batteries given conscientious care is
about two years. We have experienced much shorter service life
from other brands and batteries that are not recycled on occasion.
Batteries should be stored in a fully charged condition and stored
in a cool place.
Evidence of weakening batteries: (1) shortened service per charge,
(2) batteries won't take a charge, (3) lack of output, causing noisy
operation.
Feedback is the result of a resonant sound
coming from the speaker into the microphone
creating a closed loop audio signal.
This can only be avoided by not walking under or
in front of the speaker. Or reduce the gain
(volume).
Relocation of speaker is another consideration to
prevent the close proximity of microphone to
speaker.
Lapel microphones are the most prone to
feedback because of distance from mouth to
microphone. Therefore, if the problem persists,
you may want to consider using another
microphone such as a collar, headband with a
boom, or a unidirectional microphone plugged
into the pendant or body-pack with a lanyard
worn around the neck.