"There's a lot of valuable sound that can be obtained through a simple modification of the earmold."

BTE hearing aids represent less than 20 percent of the total hearing aid market in the United States, although this figure is somewhat higher for SHHH members. In the years before in-the-ear (ITE) hearing aids were developed and became common, there was a great emphasis in the professional literature on the acoustics of earmolds used with BTE hearing aids. With the current focus on smaller and smaller hearing aids, it is easy to overlook the potential acoustic advantages of modifying earmolds for people wearing BTE hearing aids.

Let's consider how the sound generated by the hearing aid receiver (the "loudspeaker") reaches the eardrum of the hearing aid user. The amplified sounds are delivered through a tiny cylinder to a tube that emerges from the case of the hearing aid. This tube terminates in a nub to which a "tone-hook" is snapped or screwed in place (this is the bent plastic tubing that fits over the top of the ear). The earmold itself is connected to the tone-hook via a long, narrow tubing. Ordinarily, this tubing, which is the actual sound channel through which the amplified sounds are delivered to the ear, is a constant diameter from the tone-hook to the tip of the earmold. In this "normal" situation, a great deal of sound (as high as 130 dB) is being forced to travel through a long and narrow channel.

Acoustically, this eliminates many of the high frequencies as the sound travels from the receiver to the tip of the earmold. It is easy to conceptualize this effect if one thinks of a megaphone which is specifically designed to counteract it. Who ever heard of a megaphone that looked like a narrow cylinder? In a megaphone, in which sound is delivered through tubing that increases in diameter the acoustic effect is a preservation and an emphasis of the original high frequency content of the sound source.

This is exactly the effect that occurs when BTF earmolds are made to accommodate "horn-type" tubing. This can be the so-called "Libby Horn" or a more recent development, the CPA-type (contiuous flow adapter) mold. For high-frequency enhancement, the tubing system is designed in the "horn" system so that it gradually widens in diameter from the receiver in the hearing aid through to the tip of the earmold. The acoustical effect of this "megaphone" transmission is a substantial (10-15 dB) increase in the high frequency content of the sounds amplified by the hearing aid (as measured in the ear canal) as well as an extension of the high-frequency range of the hearing aid/earmold system (i.e., more high frequencies being transmitted).

There's a lot of valuable sound that can be obtained through a simple modification of the earmold. Unless there are some definite contraindications (like the inability to control acoustic squeal), I would suggest that a "horn" bore be routinely included in BTE earmold. In the Continuous Flow Adapter system, the earmold itself is modified rather than the tubing. Some options permit emphasis on the low rather than the high frequencies for people with just low-frequency residual hearing. (My thanks to Brad lngrao for his instructional comments on the CFA.)

It is also possible to modify the response characteristics of the hearing aid by controlling the site of the earmold vent (this vent extends from the surface of the ear mold right through to the tip of the earmold). The larger the vent, the fewer low frequencies which are amplified. Because of changes in the acoustical resistance, they get "shunted" out of the ear canal. Of course now with the electroacoustic flexibility incorprated in most modern hearing aids, it is relatively easy to achieve this same result electrically rather than acoustically.

But these vents can still play an important role in improving listening comfort (like improving the quality of one's own voice). The length of the earmold within the ear canal will also aFfect the acoustical response of the hearing aid. Generally speaking, the longer the portion of the earmold in the canal, the greater the increase in sound, particularly for the lower frequencies.

From Hearing Loss, July/August 1999