What Is Acoustic Feedback (The Whistle) and Why?

An amplifying system includes a microphone, a speaker and other components. If the sound leaves the speaker and enters the microphone, it becomes re-amplified again and creates an oscillation. When this acoustic energy, audible or not, is reintroduced through the system, it develops its own noise or whistle. In hearing aid systems this oscillation is difficult to control because the microphone and speaker are so close together. When the whistle is audible to the hearing aid user, others can also hear it.

High frequencies have the shortest wavelength, it is, therefore, easier for high frequencies to escape from the ear and reenter the hearing aid microphone. As a result of the high frequency hearing loss of most hearing impaired persons, they may not hear the acoustic feedback, but most other people can hear it. When others indicate that they can hear the acoustic feedback, the hearing impaired person becomes embarrassed, frustrated, and angry.

How to control acoustic feedback:

• Hearing Aid Fit -- The proper fit of the hearing instrument to the ear is most important. As time passes ear canal changes slightly in shape. As this happens, the hearing aid no longer seals properly the ear. The same loudness that was required from the hearing aid for the hearing impaired person to hear and understand, no longer can be maintained because this acoustic feedback occurs. The earmold must be remade or modified, so that the hearing aid seals properly in the ear. This change in the canal occurs every one to three years for most individuals. In order to experience increased power, the seal in the ear must be good. Severe or profound hearing losses may require remakes or modifications as often as every 3 to 6 months.
  
• Vent Size -- The vent is a passage through the hearing instrument or earmold, which allows the ear to breathe, equalizes the atmospheric pressure to the ear drum and allows excess amplification at various frequencies to escape from the ear canal. The vent needs to be reduced in size as acoustic feedback becomes more of a problem. In some cases, closing the vent becomes a necessity, and users may hear themselves walk, hear their own voices with an echo, and may experience moist ears. These problems should be discussed with the hearing health care professional, or hearing instrument specialist, at Preferred Hearing Aid Center.
  
• Reposition the Microphone and Speaker -- This is done, by using longer canals and larger hearing instruments, which increases the distance between the microphone and the speaker. Doing this can also increase the surface contact in the ear, thereby providing a more efficient seal. As the hearing loss becomes more severe, the microphone and speaker may need to be separated further by using behind-the-ear, power CROS or body type hearing aids. The correct type of hearing instrument for your loss should be discussed with your hearing health care professional, or hearing instrument specialist, at Preferred Hearing Aid Center.
  
• Electronically Modify the Sound -- Reduce the level of high frequency amplification. This frequently helps, but can also decrease the ability to clearly understand speech, difficult high frequency voices, accents, or understanding in background noise. This is the last choice as it detrimentally affects the user's hearing. A better way is phase-inversion. Many digital hearing instruments incorporate this invaluable feature. This feature, called feedback cancellation, will eliminate or reduce acoustic feedback without sacrificing the ability to hear speech and other sounds. This is the most desired method to control "whistling" or acoustic feedback.
  
• Remove Excess Earwax -- Earwax causes sound to reflect back to the hearing instrument instead of going through to the eardrum. Wax on the tip of the hearing aid is a tale tell sign that there is excess wax in the ear. When this occurs, the instrument should be cleaned every night or morning. People experiencing excess earwax should visit their doctor to have the earwax removed on a regular basis, at least once a year but as frequently as every month in some cases. Case history and experience will determine what degree of ear care is required for you.

Acoustic feedback problems can be a source of immense irritation and can occur when you least expect it: when you smile, when you eat, when you bend over. With patience and a good sense of humor, you can learn to live with slight acoustic feedback if it occurs only occasionally.