Chapter 19: Hearing Rehabilitation: Surgical and Nonsurgical

Part 1: Hearing Amplification

Hearing Aid Basics

Components of a Hearing Aid

• Microphone picks up sound and converts it into electrical signals.

• Amplifier/signal processor is the heart of a digital hearing aid. A variety of digital signal processing (DSP) techniques can be used to process sound, identify and selectively amplify speech, recognize and suppress ambient noise, etc.

• Receiver converts the processes electrical signals back to acoustic energy and delivers sound to the ear, acting as a loudspeaker.

• Power source is usually a zinc-air disposable battery, or more recently a rechargeable nickel metal hydride or silver-zinc battery.

Hearing aid components are housed in a custom-made acrylic shell. Hearing aids can be analog, digital, or hybrid (analog-digital combinations). Analog and hybrid models are obsolete. All hearing aids dispensed today are digital.

Electroacoustic Properties of Hearing Aids

• Hearing aid is characterized by three parameters: frequency response, gain, and OSPL-90.

• Frequency response of a typical hearing aid extends up to 3 to 4 kHz; however, some hearing aids have an extended high-frequency response.

• Gain is the ratio of output power to input power. Gain depends on the input frequency and input intensity.

• OSPL-90 (SSPL-90)—output or saturated sound pressure level. Describes how much sound energy is generated by the hearing aid when input level is high at 90 dB.

• Linear processing—hearing aid amplifies by same factor regardless of input level. This technique is obsolete.

• Nonlinear processing (compression)—hearing aid amplifies softer sounds more and louder sounds less. This method is utilized in digital hearing aids and allows for more comfortable listening in ears with sensorineural hearing loss with much recruitment (narrowed dynamic range).

• Directional microphones enhance the signal-to-noise ratio and allow better understanding of speech in noisy environments. Many hearing aids allow the user to switch between directional and omnidirectional microphones.

• Some digital hearing aids use DSP techniques to recognize different listening environments and optimize their performance in real time. These devices can suppress feedback and amplify speech selectively while suppressing ambient noise.

• Two major parameters predict patients’ success with hearing aids. These are word recognition scores and dynamic range. Patients with word recognition scores of at least 50% and a wide dynamic range are usually successful hearing aid users.

• Dynamic range is the difference between the uncomfortable loudness level (UCL) and the speech reception threshold (SRT).

• Dynamic range = UCL − SRT.

• MCL (most comfortable level) approximately bisects the dynamic range.

• The narrower the dynamic range of an ear, the more difficult to fit a comfortable hearing aid.

Styles of Hearing Aids

• The four basic styles of hearing aids are as follows:

  1. BTE (behind the ear)

  2. ITE (in the ear)

  3. ITC (in the canal)

  4. CIC (completely in the canal)

• Several variations of these basic styles are offered by various manufacturers, such as mini-BTE, invisible in the canal ...