There are two reasons to make sure a hearing instrument fits well – one acoustic, the other anatomical.
The acoustic reason – A loosely fitting hearing instrument can cause feedback (whistling). This happens when the instrument does not fit snugly and the amplified signal finds its way back to the microphone. For comfortable, interference-free hearing, a snug fit in the ear canal is essential.
The anatomical reason – A hearing instrument that does not fit snugly can work its way out of the correct position when its wearer is chewing, yawning or simply talking. This is because the shape of the ear canal is influenced by movements of the jaw. You can feel this by putting the tip of your finger into the ear, then slowly opening and closing your mouth.