Mother's Under-$30 Hearing Aid

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

The other two types of hearing disorders are mixed hearing loss, caused by a combination of ailments, and central hearing loss, caused by auditory-nerve damage. The last is the most severe, and, as yet, there seems to be little hope for an absolute cure.

Of the four types of hearing loss, the first two can be partially or totally corrected by sound amplification. Persons with conductive hearing loss, particularly that which is brought on by aging, are the most likely to benefit from the use of hearing aids.

HEARING AID DESIGN

Because of the different types and severities of hearing loss, there's more to constructing an effective hearing aid than simply building an electronic amplifier. Tonal quality and volume range are just two of the important factors that must be considered.

To complicate matters, the ear isn't a linear receiving device to begin with; that is, it doesn't perceive all frequencies equally well. Instead, it's more sensitive to some than it is to others, and that sensitivity shifts as the intensity of the sound increases.

Fig. 1 tells the story. The lowest, and most contoured, of the lines represents the threshold .of hearing—the level at which sounds are barely audible. Notice how the line dips as the frequency nears 3,000 cycles per second (3,000 Hz, in technical terms). This means that we can hear sounds at 3,000 Hz that are about one-tenth as loud as the faintest 1,000Hz sound we're able to pick out (if this seems confusing, remember that hertz are a measure of frequency, not volume). At 10,000 Hz, though, the sound level must be ten times louder than it is at 1,000 Hz in order for us to hear it-or 100 times greater than at a sensitivity level of 3,000 Hz!

As a sound becomes louder, however, our ears begin to interpret all frequencies equally. That's why you notice a pronounced increase in the dynamic range of music when you turn your stereo up. What you're doing, in effect, is equalizing the loudness level to match your ear's frequency response curve.

And spectral sensitivity varies with the individual. Women, for instance, can generally hear higher frequencies than men can and are thus able to hear sounds men can't. Age also brings about changes in hearing.

Consequently, to do an acceptable job of amplifying sound for the hearing impaired, a hearing aid must be able to cope with the wide range of frequencies, intensities, and sensitivities encountered in everyday life.

The amplifier I'm going to tell you how to build has been designed with these criteria in mind. For example, it incorporates a volume control that spans a 10-to-1 range. More important, however, is its tonal quality. The device is heavily filtered to attenuate-but not completely eliminate-those annoying sounds so many hearing aids amplify. Far too many commercial units amplify all sounds equally, even those that you'd rather not hear. The background din at a cocktail party, for instance, may come through loud and clear enough to actually drown out the immediate conversation. By adding a tone control to the hearing aid and adjusting its frequency contour with the volume control setting, much of this unwanted babble can be filtered out. (Appropriately enough, this feature is called a clarity control.)

RSS | Comments | E-mail | Print