Apart from the programming options available in the manufacturer's software, hearing instrument performance can be significantly changed by employing numerous acoustic options in the earmold. These options fall into three groups, each of which affects a different portion of the hearing aid response curve.
Venting, which modifies the low frequencies, can be applied to occluded earmold styles either through drilling a constant-diameter channel parallel to the sound bore or by the use of inserts featuring different diameter vent holes. Non-occluded styles, of course, offer the ultimate in venting.
Dampers, working in the mid-frequencies, primarily remove unwanted peaks in the response curve, and take several forms.
Horn effects, or enlargement of the sound bore as it passes from the earhook through the earmold, enhance the high frequencies.
Other factors which will change the acoustic properties of a hearing aid fitting include tubing length and diameter, canal length, and bore diameter, although the fitter may have little control of these in any given fitting.
When considering acoustic options, remember that hearing aid wearers most frequently complain about the apparent loudness and clarity of their instruments. This is a vital reason for the success of Open-Fit instruments, because they do not occlude the ear and thereby keep in unwanted low frequencies. Likewise, though you have extensively counseled your patients about reasonable expectations, some will always be looking for the hearing aids to perform miracles.
In general, and within the limits of each case, less low frequency amplification is perceived as "less loud," fewer peaks are perceived as "less harsh," and additional high frequency amplification is "adding sound quality." While there will always be exceptions, your selection of circuit response and acoustic options will usually follow these guidelines in the quest for the most natural, most acceptable sound.
The study of earmold acoustics began with the vent, which allows acoustic energy to leak out. Venting is employed to achieve one or more of the following results:
- To reduce a feeling of fullness caused by wearing an earmold
- To reduce resonating low frequency sound produced by the aid and mold, and
- To improve speech clarity
Parallel venting is an effective method of modifying the low frequency component of an amplification system without having any effect on the high frequency area. Actual effects are determined by the location, diameter and length of the vent. Diagonal venting is to be avoided because of possible interference with the high frequency response and the introduction of feedback problems.
Changeable venting systems, such as the Select-a-Vent, eliminate drilling in the dispenser's office while allowing trial-and-error modifications that are reversible if they are unsuccessful.
Dampers cause acoustic resistance to the sound passing from a hearing aid. They smooth the frequency response in the range between 1 and 3 kilohertz, although damper placement makes a considerable difference in exactly which frequencies are affected. Dampers can be as elementary as lamb's wool or as refined as the Knowles Acoustic Dampers, which come in seven different color-coded acoustic resistances housed in a metal sleeve which fits inside the earmold tubing.
Horns were first applied to earmolds in the late 1950s in the "Acoustic Modifier" -- a short-canal, wide-bore mold. Enlarging a sound bore will enhance high frequencies passing through it; reducing it will reduce the high frequency components. The key to horn theory involved the change in bore diameter, not its shape. With the extended high frequency range of many of today's hearing aids, a horn of some type should be employed to preserve the performance and achieve a successful fitting. Probably the most popular style is the Libby Horn -- a tapered, one-piece sound tube which is replaceable in the dispenser's office.
Additionally, various earhooks contain elements which affect regions of the frequency response. Whatever your choices, make certain that your amplification goal is realistically attainable. Don't chase one or two decibels, because your patient is not likely to notice the difference.