Programmability does not negate the utility of earmold/plumbing modifications.
|Brian Taylor, AuD, is the professional development manager at Unitron in Plymouth, Minn. Darrel Teter, PhD, owns a private practice in Highlands Ranch, Colo, was a member of the original IHAFF protocol committee, and has served as a consultant for Phonak for more than 35 years.|
It is a popular belief among some hearing care professionals that digital electronics has rendered knowledge of basic earmold modifications unnecessary. This statement is supported by the following observations:
1) Most clinicians rely on software to electronically adjust the gain and frequency response of the hearing aid. Although this is certainly an effective way to modify the acoustic parameters of a modern hearing aid, there may be times when adjustments to the acoustical plumbing of the device are useful.
2) Many graduate-level audiology programs offer little hands-on or academic training on earmold modifications and how they affect the acoustical characteristics of the fitting. For example, the Handbook of Clinical Audiology, long a staple reading for audiology graduate students, eliminated its chapter devoted to earmolds after the third edition, which was published in 1985.
3) Published reports in the pertinent journals are scarce, as a recent search uncovered less than a dozen articles related to earmold acoustics and earmold modifications.
The purpose of this article is to revisit some of the more practical considerations surrounding the selection and modification of earmolds and how those factors contribute to a more successful hearing aid fitting.
Death of Mechanical Alterations?
The belief that digital electronics supplants the need for modifications to earmolds is not without merit. It is indeed true that many changes to the acoustic parameters of a modern hearing aid can be readily adjusted with a few clicks of a mouse or computer keyboard, using the manufacturer’s fitting software. Many traditional earmold modification techniques that lost favor in the ’80s and ’90s due to the surge in custom product popularity need to be re-examined with the surge in behind-the-ear (BTE) product use since 2005.
One of the likely factors contributing to the disregard of earmold acoustics and how they contribute to a successful fit can be attributed to the popularity of open-canal (OC) technology. Mini-BTE devices, which are usually coupled to the ear with a noncustomized thin tube and dome ear tip, comprise a significant part of total OC sales. As others have demonstrated,1-3 the fitting range of mini-BTE OC products can be extended with the use of a custom earmold. Although no longer considered an open-canal fitting, mini-BTEs—when coupled to a custom-made earmold using either a thin tube or a thin wire—maintain their cosmetic appeal.
When sales of OC products are combined with conventional BTE hearing aids, well over half of all hearing aids sold in the United States have the potential to be coupled to the ear with a custom earmold. Thus, it is prudent to revisit some of the basic science surrounding earmolds and earmold acoustics, especially as it relates to thin tube/thin wire mini-BTE technology.
Earmold Materials and Styles
Before going into some of the details surrounding the use of custom earmolds for thin tube/wire mini-BTEs, it is important for the dispensing professional to have knowledge of the basis of earmold selection and modifications. The first consideration is when to use various earmold materials and styles.
|FIGURE 1. An example of a properly inserted otoblock.|
Before this discussion, however, the assumption must be made that the ear impression is taken correctly. The ear impression, using the correct size oto block, must make contact with the entire canal wall, as shown in Figure 1. In addition, the otoblock should extend beyond the second bend of the ear canal, and regardless of the earmold style being ordered, the ear impression material should completely fill the entire concha. Before sending the finished ear impression to the manufacturer, the practitioner should carefully inspect the impression and ensure all the major landmarks of the ear have been accurately cast.
As conventional wisdom would dictate, the choice of earmold material and style depends a great deal on the extent of the hearing loss and the professional’s need to meet specific gain and output requirements. Kuk4 determined that the canal portion of the earmold contributes the most to maximum gain requirements of the BTE. This would suggest that skeleton and canal style earmolds are appropriate for hearing losses up to about 75 dB HL.
Another important consideration is the type of material used to make the earmold. In general, softer materials, such as silicon and vinyl, are used for hearing losses greater than 75 dB HL. Table 1 outlines the three major earmold materials, along with their performance advantages.
|TABLE 1. Three common earmold materials, along with advantages of each material.|
Earmold Material Characteristics
Regardless of the earmold style or material, Lybarger5 noted three requirements of a properly fitted earmold:
1) Acoustic seal. The earmold needs to direct sound toward the eardrum without acoustic feedback.
2) Comfort. The earmold should fit as comfortably as possible without causing irritation to the external ear.
3) Aesthetic appearance. The earmold needs to be as inconspicuous as possible within the limits set by the acoustic requirements of the patient.
More than 44 years since Lybarger made these observations—and even though electronic feedback cancellation algorithms exist in many modern hearing aids—these three basic requirements still hold true. In fact, given the proliferation of mini-BTE products, these three considerations take on a renewed importance addressed in the final section of this article. First, however, the basic acoustic factors affecting earmold performance will be reviewed.
There are three major types of vents found in a hearing aid shell or earmold: parallel, diagonal, and trench. All three vents are designed to accomplish the same thing, which is to provide some reduction of amplified low frequency sound—that is, allow low frequency sounds to leak out of the ear. Venting also allows for some pressure relief, which results from bone-conducted sound getting trapped in the closed-off ear canal when a tight-fitting hearing aid is inserted into the ear canal (the sound is generated from the condyle area of the mandible, which is located close to the ear canal). This problem is especially apparent when patients talk or chew.
The most commonly used vent is the parallel type; however, there are instances when a professional may have to substitute a different style vent. It is easy to forget that the vent type can have a significant effect on the reduction in low frequency energy, as diagonal vents reduce low frequency energy more than parallel vents with the same internal diameter. This is illustrated in Figure 2, which compares the same hearing aid with identical acoustical settings. The only change is the style of vent. Notice in this particular example how gain is reduced through 3000 Hz with the use of a diagonal or side branch vent.
|FIGURE 2. The effects of vent style on gain and frequency response. The hearing aid, earmold style, and vent diameter were held constant and the vent type was changed. The green curves show aided gain for a parallel vent and the purple curves show the aided gain for the same product and earmold style using a diagonal vent. The orange curve is the REUR.|
Tubing length and internal diameter can have a pronounced effect on the frequency response of the hearing aid. Tubing diameters are standardized according to the internal diameter. The most common tubing is size 13. Here is a summary of common tubing options with the corresponding inside/outside tube diameters (in mm):
|FIGURE 3. The effect on hearing aid response for various tubing diameters measured on an ear simulator. The internal diameters for size 13 and 14 tubing are listed above. The .031-inch diameter is similar to today’s thin tube internal diameter.5|
Figure 3 provides a good example of how the gain above 1000 Hz is affected by the internal diameter of the tubing. In this example, five different internal diameters are compared in a Zwislocki coupler. The .031-inch (0.80 mm) internal diameter curve in Figure 3 closely approximates the internal diameter of a modern thin tube. Even though the reduction in gain resulting from tubing resonances can be compensated through software adjustments, if the proper correct factor is not properly verified, thin-tube fittings can result in inadequate mid and high frequency gain.
As sound travels through the tubing, standing-wave resonances occur. These create peaks in the frequency response of the hearing aid, resulting in acoustic feedback or poor sound quality. To some extent, these peaks and valleys can be smoothed out electronically using the fitting software.
However, many manufacturers still rely on plastic or metal dampers placed in the earhook to smooth out the response. For example, in many high-gain BTEs, a 680 or 1000 ohm damper is used to smooth out the frequency response of the hearing aid. The location of the damper also has a significant effect on the frequency response. For the clinician, knowing the type and location of a damper is essential when replacing it, as the damper impacts sound quality.
Sound Bores and Horns
Changes in sound bore diameter affect the high frequency response of the hearing aid. Increases in bore diameter alone, however, yield little or no change in the high frequency response. Figure 4 shows the aided and insertion gain differences for a standard, bell, and open sound bore, respectively. Notice how low frequency gain is affected due to changes in the venting characteristics relative to the sound bore, but high frequency gain is unaffected. This is due to the fact that changes in the high frequency response can only be accomplished mechanically with a horn, and not with a larger sound bore within the earmold. Simply increasing the size of a sound bore has no appreciable effect on the hearing aid’s response in the high frequencies.
|FIGURE 4. The response curves for the same hearing aid using earmolds with three different sound bores. Note there is not a significant difference in the high frequencies. The differences seen in low frequency gain are due to changes in vent type for each of the earmolds.|
For an acoustic horn to work effectively, a gradual increase in internal diameter over a specific length is required. If the length is too long or too short, the increase in high frequency gain will not be attained. With a properly constructed horn, an increase in high frequency gain can be readily achieved without increasing the hearing aid’s output. The 3 mm or 4 mm Libby horn, along with well-designed Continuous Flow Adaptors (CFA), are proven methods for obtaining more high frequency gain mechanically rather than electronically. When using a Libby horn tube, it is important not to modify the length of the tubing, as shortening the length can render the horn ineffective. The mechanical action of a horn is governed by the length and taper of the horn. In simple terms, for any horn to be effective, the length of the horn must be at least 17 mm. With the proper length maintained, up to 12 dB of gain between 2 kHz and 5 kHz can be obtained (Figure 5).
|FIGURE 5. The difference in high frequency gain between an earmold with standard tubing and sound bore (purple curve) compared to 4 mm Libby horn (blue curve). The REUR is the orange curve.|
Custom Molds and Thin tube/wire Mini-BTEs
Rose1 outlined several benefits associated with coupling a thin tube/wire mini-BTE product to a small custom earmold. These include greater patient comfort, improved retention, and an extended fitting range. Regardless of the advantages of coupling a custom earmold to a thin tube/wire product, there are several acoustical changes that should be considered when closing off the ear canal.
It has been documented that open-canal products do not provide adequate gain in the low frequencies for losses greater than about 40 dBHL. Over the past several years, manufacturers have offered “closed” and “double” domes designed to close off the ear canal, increase low frequency gain, and thus extend the fitting range of these products.
Figure 6 from Teie6 shows mean data for nine patients using five different coupling options for a thin-tube mini-BTE. The hearing aid was programmed for a flat 50 dBHL hearing loss and the prescribed target gain is shown in red. For the non-custom coupling systems, gain through 1500 Hz is significantly compromised. There are two important clinical considerations that can be taken from this study, which both underscore the importance of being knowledgeable about the mechanical properties of earmolds and tubing:
|FIGURE 6. The mean aided output for five different coupling options using the same mini-BTE in relation to a prescriptive fitting target for a flat 50 dB hearing loss. From Teie.6|
1) On average, gain at 500 to 1500 Hz is inadequate when a noncustom coupling system—such as a closed dome or double dome—is used. In practical terms, this suggests that, for losses greater than 30 to 35 dBHL in the low and mid frequencies, audibility is compromised when a noncustom dome or eartip is used with a thin tube/wire fitting.
2) There is a decrease in gain at 1500 Hz for the thin tube fitting, as described by Teie.6 This reduction in gain at 1500 Hz with the thin tube fitting is especially prominent when the earmold with thin tube is compared to the earmold with standard #13 tubing. This finding is consistent with tubing resonances associated with the small internal diameter of the thin tube, which needs to be accounted for during the fitting process by possibly increasing gain in that frequency region through adjustments using the fitting software.
Extending the fitting range. Custom earmolds can extend the fitting range of mini-BTE devices. Figure 7 shows the real ear aided response (REAR) of a receiver-in-the-canal (RIC) product for a patient with a flat 50 dB hearing loss. As expected, the earmold with the smallest vent—or the 1.5 mm Select-a-Vent (SAV) shown as the purple REAR curve—provides the least amount of low frequency gain reduction.
|FIGURE 7. Custom made earmold coupled to thin wire mini-BTE device for a 50 dBHL loss (Purple: 1.5 SAV; Green: large cavity vent). The bottom purple and green curves are the insertion gain curves. The top purple and green curves are the REAG responses in relation to the REUR (in orange).|
An added advantage of using custom earmolds with thin tube/wire products is that the low frequency response can be tailored by modifying the size of the vent. This is especially important when trying to reduce occlusion-related issues commonly associated with closing off the ear canal to obtain more gain in the low frequencies. As shown in Figure 7, a significant amount of low frequency gain can be modified by changing the vent size. As expected, the green curve in Figure 7, which represents the larger vent diameter and shorter vent length, has significantly less low frequency gain compared to the earmold using a 1.5 mm parallel vent (purple curve). Custom earmolds coupled to thin tube/wire devices have been shown to extend their fitting range to flat moderate to severe hearing losses.
The basic tenets of a well-fitted hearing aid are a smooth broadband response with sufficient audibility, comfort, and sound quality. As Figure 8 illustrates, the perception of amplified sound can be modified through mechanical changes to the coupling system. In order to accomplish these things, while minimizing common problems like feedback and occlusion, considerable attention must be given to how venting, tubing, and other mechanical aspects of the coupling system are made. All can make a significant impact on the quality of the final fitting.
|FIGURE 8. The general effects of venting, damping, and acoustic horns on the frequency of a hearing aid. Note on the bottom schematic a typical frequency response is superimposed on the figure. Adapted from Berger, 1970.7|
- Rose D. The return of the earmold. Hearing Review. 2006;13(9):14-19. Accessed August 5, 2009.
- Rose D, Ghent R, Wright N. Extending candidacy for thin-tube fittings by modifying the earmold. Hearing Review. 2008;15(10):12-15,48. Accessed August 5, 2009.
- Taylor B. Optimizing open canal fittings with custom earmolds. Advance for Audiologists. 2009;11(3):30-34.
- Kuk F. Maximum user real-ear insertion gain with ten earmold designs. J Am Acad Audiol. 1994;24(4):299-312.
- Lybarger S. Earmolds. In: Katz J, ed. Handbook of Clinical Audiology. 3rd ed. Baltimore: Williams & Wilkins; 1985.
- Teie P. Ear-coupler acoustics in receiver-in-the-aid fittings. In press, 2009.
- Berger K. The Hearing Aid: Its Operation and Development. Livonia, Mich: National Hearing Aid Society; 1970.
Correspondence can be addressed to HR at [email protected] or Brian Taylor, AuD, at .
Citation for this article:
Taylor B, Teter D. Earmolds: Practical considerations to improve performance in hearing aids. Hearing Review. 2009;16(10):10-14.