Earmold Modification

The hearing aid dispenser has developed a great utilization of the "earmold" as an integral part of the hearing aid fitting system. The modification of the earmold or the hearing aid shell allows for enhancement of many acoustical qualities and can lead to greater client satisfaction.

An earmold may influence the frequency response of the hearing aid and can either emphasize or reduce the energy in the frequency response through a change in acoustic resonance. It is important to know the acoustic performace you desire when you combine the earmold with the hearing aid.

The number of earmolds that are "modified" in some way is approximately 90%. Experience has demonstrated to the hearing aid dispenser that an acoustic change produced by an earmold option will result in a more natural sound to a client than one produced in the hearing aid alone.
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General Acoustic Considerations

The following graph indicates how earmold modifications can influence the frequency response of a hearing aid.

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Venting

PARALLEL vents (where the sound channel and the vent are parallel) are generally preferred as feedback is minimized.

DIAGONAL vents intercept the sound bore at an angle and can result in a greater tendency to feedback.

EXTERNAL or SURFACE vents are grooved on the outside of the earmold.

(326 X 107)

Sizes

Vents can be bored to specific sizes within the limitations of the space available. Conventional sizes recommended by Mid-States are:

small .031 medium .062 large .125

Effects

There is a predictable acoustic change occurring when an earmold is modified with a vent. Obviously, as mentioned earlier, the size of the ear can alter the magnitude of the change. Individual hearing impairments will also react differently to various acoustic modifications. When the hearing aid dispenser combines the appropriate hearing aid with the correct earmold modification, a client will receive a satisfactory fitting.

The following are several general comments on the effects of venting on the low frequencies.

.031 vent is primarily a pressure vent and it will have very little effect on the frequency response above 400Hz.
.062 to .093 vents will tend to increase the acoustic energy in the frequency range between 500 Hz and 1000 Hz. The acoustic energy increases as the vent diameter increases.
.125 to .150 vents will also increase the acoustic energy in the frequency range between 500 Hz and 1000 Hz. However, at some point, the vent may become large enough, in a small ear, to shunt the energy below 1000 Hz. Large vents may result in feedback more frequently with ear level hearing aids with an SSPL90 in excess of 115db.

Select-A-Vent/Positive Venting Valve

These insert vents provide the hearing aid dispenser with the flexibility of changing sizes by merely changing the plug-in vent or screw-in vent. No special tools are needed to remove or insert the S.A.V. plugs, however, a small removal tool is necessary with the P.V.V. With the friction fit S.A.V., the insert may be cemented in place once the final choice has been made.

When ordering S.A.V., indicate the size of vent plug desired. The entire set of plugs will be included with each order at no extra charge. Orders for P.V.V. will include only the size plug requested.

Insert #

S.A.V. Vent Size

P.V.V. Vent Size

.156

.125

.094

.093

.062

.031

.020

PLUG

Damping

Attenuators used in conjunction with earmold modification influence the center frequencies. The amount of effect is determined by the density of the attenuator or damping plug, the number of attenuators used, and the location in the system.

Density is the same as damper resistance or acoustic ohms and it is determined by the material. Sintered balls, fiber plugs, even lambs wool, all create some acoustic resistance and influence the mid-frequencies, some more than others. The greater the density(resistance), the greater the reduction in the peaks of the frequency response. This smoothing effect created by any form of attenuator or damping plug therefore reduces the output and gain of the hearing aid.

There is an additive effect when using dampers. It is important to remember that the damping plug will reduce both gain and output. The placement of the damping plug will influence the frequency response. Generally, the closer the damping plug is to the end of the tubing nearest the earmold, the greater the effect. All manufactured dampers will produce predictable effects on the frequency response of a hearing aid. The effects will vary, however, between hearing aid models, and when coupled to different ear cavities. Lambs wool and other similar material will produce variable acoustic results and should be used accordingly. The dampers available from Mid-States are the 680 ohm, 1500 ohm, and 2300 ohm dampers. Others may be secured and added to the earmold system after it is fabricated.

Horn Coupling

Horning or trumpeting is a technique used to emphasize the high frequencies in a hearing aid to create a "Horn" in the end of the earmold. The construction consists of an increase in the diameter of the sound channel to a size allowable within the constraints of the canal dimensions.

High frequencies can be enhanced by enlarging the bore of the sound channel. Another earmold modification technique which increases the experience of high frequencies is to shorten the canal. Therefore, if we have a shortened canal with a large bore, we have created a high frequency emphasis earmold.

The reverse is also true. The most effective way to "choke" or reduce high frequencies through earmold modification is to reduce the diameter of the bore or lengthen the canal or both.

When requesting the "Horn" option for an earmold, the diameter will conform as closely to the standards reported in the literature as possible within the limits of the size of the ear canal.

High frequencies can also be reduced by the use of diagonal or angular vents as mentioned earlier. While this type of vent may reduce the high frequency response, it is important to realize that feedback may also occur more often than with a parallel vent.

Earmold Modification Information

A pressure vent is recommended for all molds where possible. This improves the comfort factor greatly.
If feedback does occur with a vent, the vent can be easily plugged. However, to drill a vent where none exists is quite difficult.
In general there is less chance of feedback with a parallel vent than with a diagonal vent.
For high frequency emphasis a short canal and a large vent is a common choice. Also consider using a bell bore.
For frequency response smoothing and alteration, consider Killion of Acoustic Horn construction.
For difficult to seal, more severe losses, the soft textured materials will be most satisfactory.
The vent may be used for pressure equalization, increase of low frequency response, or reduction of low frequencies.
Standard vent sizes are: small - .031, medium - .062, large - .125. Other sizes can be requested and will be supplied if canal diameter permits.
Suggested bore sizes are: small - .055, standard - .118, large - .125. Larger bores and belled bores may be requested.
Tubing length and diameter will cause changes in location and height of the frequency response peaks in a predictable fashion.
A dental lathe with appropriate accessory tools is highly recommended for in-office modifications.
For more details consult The Hearing Aid by Berger or the NIHIS text by Sandlin.
Dampers exist in several densities.
Moisture build-up in dampers is a common problem. Reduction of the moisture is possible by using an air blower, or the dampers can be replaced.
Accurate measurement of tubing and placement of dampers is essential to maintain published frequency responses.
Ear canal sizes will vary and could therefore affect final frequency response curves.
The higher the ohm rating for dampers, the greater the attenuation.
More attenuation occurs as the damper is moved closer to the earmold, however, this also increases the moisture build-up. It is therefore suggested that the damper be placed in the earhook if possible.

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