The high-pitched whistling sound sometimes heard from a hearing aid is a common acoustic event known as feedback. This noise occurs when the amplified sound intended for the wearer’s ear escapes the ear canal and is picked up by the device’s own microphone. The resulting sound loop is continuously re-amplified, creating the distinct, unwanted squeal. Understanding this mechanism helps in identifying and resolving the specific causes of the noise.
The Science of Hearing Aid Feedback
The whistling is the audible result of an instability in the electronic-acoustic system of the hearing aid. A hearing aid operates by converting incoming sound into electrical signals via a microphone, boosting those signals in an amplifier, and then converting them back into louder sound waves via a receiver (speaker). This amplified sound is meant to travel down the ear canal toward the eardrum.
Acoustic feedback occurs when a portion of the sound waves produced by the receiver leaks out of the ear canal and reaches the external microphone. The microphone then picks up this escaped sound, treats it as a new input, and sends it through the amplifier again. This cycle of leakage and re-amplification repeats rapidly, creating an oscillation that manifests as the high-frequency whistle.
Whistling Caused by Physical Seal Failure
The most frequent reason for feedback is a failure to maintain a proper acoustic seal between the hearing aid and the ear canal. When the seal is compromised, sound energy escapes through the gap, allowing it to easily travel back to the microphone. This leakage can be due to improper insertion, where the device or earmold is not pushed deeply or securely enough into the ear.
Over time, the shape of the ear canal can change due to weight gain or loss, causing a previously snug custom earmold to become loose. Similarly, the small plastic tubing connecting a behind-the-ear device to the earmold can harden, shrink, or crack, which pulls the earmold out of position or creates a tiny exit point for sound to escape.
The wrong size dome or ear mold will inherently create gaps along the ear canal walls, providing a path for the sound to escape. Any damage to the device casing, such as a hairline crack in the shell, can also allow sound to leak out and trigger the feedback loop. Even simple movements like chewing, yawning, or talking can momentarily shift a slightly loose-fitting device, briefly breaking the seal and causing a burst of whistling. A correct and secure fit is the primary defense against this type of feedback.
Whistling Caused by Internal Reflection or Excessive Gain
Feedback can also happen even when the physical fit of the device is secure, often due to a blockage inside the ear canal that reflects sound. Excessive buildup of cerumen, or earwax, acts as a physical barrier in the canal, preventing the amplified sound from reaching the eardrum. Instead, the sound waves bounce off the wax mass and are redirected back toward the hearing aid’s receiver and microphone, initiating the feedback cycle.
Another common cause is setting the amplification level, or gain, too high for the ear’s current acoustic environment. When the volume is excessive, the sheer power of the sound energy delivered into the ear canal may be too great for even a good seal to contain. This forces some sound to leak out around the device and be captured by the microphone.
Sound can also be reflected back to the microphone by external objects held close to the ear, such as a telephone receiver, a hand when adjusting the device, or a hat or scarf covering the ear. In rare cases, the issue is internal to the hearing aid itself, where a dislodged internal microphone or a component failure can create a mechanical path for the receiver’s sound to reach the microphone.