DIGITAL HEARING AID CIRCUITRY

 

Until about 1996, almost all hearing aids amplified incoming sound as an analog signal. Since that time, the hearing aid industry has replaced analog with digital technology as the standard way of processing sound. Digital processors change the incoming signal into small binary units before amplifying. This conversion allows for processing at much faster speeds. Greater speed means more things can be done with the signal before it leaves the hearing aid.

 

Most of the hearing aids I currently fit at Dobbins Hearing Service are digitally based processors. I still recommend and fit analog hearing aids when the occasion warrants, but the number of manufacturers currently producing analog models is rapidly dwindling. This trend will certainly continue as the price difference between the two technologies continues to be reduced.

 

There are several features that are available in today's digital hearing aids which did not exist in analog aids. Other features which were available in analog units have been greatly improved upon in the current digital versions. Below are discussions of a number of terms which relate to digital hearing aids.

 

Channel/Band - Hearing aids amplify sounds across a wide pitch range, from low tone to high tone. A single channel aid amplifies all of the pitches as a unit whereas multiple channel aids divide the pitches into discrete groups and then amplify. Each group of pitches is termed a channel or a band. Hearing aids currently have from 2 to 20 channels. More channels result in more precise amplification.

 

Automatic Volume Control - Within each channel, we usually instruct the hearing aid to amplify soft sounds more and loud sounds less. This allows the hearing aid to keep all sounds within the user's range of comfort. The ability to change itself as the input changes is the automatic volume control feature advertised on some aids. Because each channel changes separately, the number of automatic volume controls in a hearing aid will be the same as the number of channels. Most hearing aids with this feature do not require a user operated manual volume control on the outside of the aid.

 

Digitally Programmable - Any hearing aid that can be changed by the audiologist in the office via attaching it to the computer is termed digitally programmable. This ability is extremely useful to get the best settings for your exact hearing loss and to have the flexibility to change the settings as your listening requirements or hearing changes. Hearing aids can be programmed again and again to achieve the best results for each person's lifestyle.

 

Programs/Memories - The setting of a hearing aid is sometimes termed its program as it has been set or programmed for the wearer to function well in most situations. Some listening situations are more difficult than others and require a different setting in the hearing aids. Hearing aids which have memories contain from two to four different programs and allow the wearer to change the setting to suit the environment. Examples are aids with separate settings for noisy places or for using the telephone.

 

Noise Reduction Circuit - Hearing aids are designed to amplify and clarify speech. Not all sound in the world, however, is human speech and sometimes non-speech sounds interfere with the understanding of speech. Noise reduction circuits can tell the difference between human speech and some other non-speech sounds and then act to reduce the amplification of the interfering sounds while maintaining the amplification of speech. These circuits are most successful when the background noises are very different from speech such as in situations with fan or motor noises.

 

Directional Microphones - All hearing aids have one omni directional microphone which picks up sound from all directions. By contrast, hearing aids with directional microphones only amplify sounds in front of the wearer. This is very useful in situations where there are noises behind the wearer, especially noises composed of speech such as in a restaurant. To achieve this effect, the hearing aids must be built with two microphones each, one facing forward and the other backward. The directional effect may be manually controlled by pushing a button or may be automatic as part of the programming in the hearing aids. The most effective hearing aids for understanding speech in difficult situations combine noise reduction circuitry with directional microphones.