SOME INFO ABOUT ACOUSTIC DAMPERS
At the following site, Mid States Laboratories |Modification Links
, there's an interesting discussion about the effects of dampers and tubes, some of which I thought I'd share here. For purposes of organization, I'll number these comments as follows:
1. The number of earmolds that are "modified" in some way is approximately 90%. Experience has demonstrated 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.
2. There is a predictable acoustic change occurring when an earmold is modified with a vent. 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.
3. 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.
4. 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.
5. There is an additive effect when using dampers. 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.
6. All manufactured dampers will produce predictable effects on the frequency response. 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.
7. 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.
8. High frequencies can be enhanced by enlarging the bore of the sound channel.
9. Another earmold modification technique which increases the experience of high frequencies is to shorten the canal.
10. 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.
11. A pressure vent is recommended for all molds where possible. This improves the comfort factor greatly.
12. For high frequency emphasis on a short canal and a large vent is a common choice. Also consider using a bell bore.
13. For frequency response smoothing and alteration, consider Killion of Acoustic Horn construction. The vent may be used for pressure equalization, increase of low frequency response, or reduction of low frequencies.
14. Standard vent sizes are: small - .031, medium - .062, large - .125. Suggested bore sizes are: small - .055, standard - .118, large - .125.
15. Tubing length and diameter will cause changes in location and height of the frequency response peaks in a predictable fashion.
16. 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.
17. Accurate measurement of tubing and placement of dampers is essential to maintain published frequency responses.
18. Ear canal sizes will vary and could therefore affect final frequency response curves.
19. The higher the ohm rating for dampers, the greater the attenuation.
20. More attenuation occurs as the damper is moved closer to the earmold, however, this also increases the moisture build-up.
Phonak also has an interesting article about Acoustic Tuning, which is accessible here: http://www.phonak.com/com_028-0580-xx_focus_2.pdf
Acoustic Options http://www.perfect-seal.com/acoustic.html
also has a straight-forward introduction to the three methods of "acoustic control." Among other things, it says:
1. 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.
2. Sintered Steel Pellets - A series of steel pellets with different degrees of porosity which give different levels of acoustic resistance.
3. 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.
4. 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.
5. 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.