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. The study of earmold acoustics began with the vent, which allows acoustic energy to leak out. 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.
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. 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. Additionally, various earhooks contain elements which affect regions of the frequency response.