 |
Venting, which affects frequencies from 1000Hz and below. |
Venting:
External vent - This is a channel that is routed along the outside of the earmold.
Diagonal vent - A diagonal vent joins the sound bore in the earmold. This vent allows for a higher chance of feedback, and a reduction of high frequency gain. It is used when size doesn’t allow for a parallel vent.
Parallel vent - The vent hole runs parallel with the sound bore in a parallel vent. Low frequencies are reduced, yet high frequencies are not affected. This is the preferred vent style.
S.A.V. (Select-A-Vent) - The Select-A-Vent system allows the dispenser to choose what size vent to use. In this system, a tree of five vent sizes and one solid plug gives the dispenser the option of easily changing vent size. If you find that one vent size is wrong, it can be replaced easily without modification to the earmold.
|
 |
Damping, which affects frequencies between 1000-3000Hz. |
Dampers
Lambs Wool - The first damping material used with hearing aids. The density of
the lamb’s wool determines the damper effect. It is effective, but not easily controlled.
Sintered Steel Pellets - A series of steel pellets with different degrees of porosity which give different levels of acoustic resistance.
Star Damper - Star dampers are made of silicone and have no moisture build-up problems. They must be cut to different lengths and measured to know the exact effects.
Knowles Acoustic Dampers - These dampers are a refinement of acoustic control. Each damper is in a metal housing with a color coded plastic screen at one end. There are six dampers available: 680, 1000, 1500, 2200, 3300, and 4700 in Ohm values. Knowles Acoustic
Dampers are the most convenient to use, but moisture build-up can be a problem
|
 |
Horn effects, which affect the 3000Hz and above range. |
Horn Effects:
In acoustics, the belling of a tube will enhance high signals passing through the tube. If a tube narrows toward the end, the high frequencies will then be reduced. Many of today’s hearing instruments produce high frequency signals out to 7000-9000Hz. Therefore it is important that the tube and earmold not restrict the resonance in the sound channel and cut off the high frequencies. Thus, in many fittings, a horn of some kind may be essential. |
