In Search of the Digimold

We live in the digital era. While a great deal of attention has focused on technology and the algorithms of digital products, relatively little attention has been given to earmolds in the recent press.

Don't you think the time has come to develop a high tech, nearly perfect, digital earmold? Let's set off in the search of the world's first 'DigiMold'.

What are the factors, issues and options that would need to be considered to create the DigiMold?

Are the newest motorized gun injectors superior for impression taking?

An injector gun is an attractive tool for impression taking. It can be assembled within a minute or two and does not require any advanced maintenance. The impression material is stored in a double-barrel cartridge, which can be easily attached to the gun. When injected into the ear, the silicone cures within 2 to 3 minutes. The injector gun offers a 'mess-free' work environment and there is little material wasted. Only the necessary amount of silicone is mixed in the mixing canula and injected directly into the ear. It is also believed that the use of a gun injector, particularly the motorized one, sends the right message to patients that we are using the latest equipment and techniques to give them better service.¹

Unfortunately, gun injectors can dispense only light silicones. Most of the guns are just not powerful enough to push a standard viscosity silicone through the mixing canula. This means that if a gun is used, the light impression material will not stretch the ear tissue, and the resulting mold may be loose, have an insecure fit and may be susceptible to acoustic feedback. A case of inadequate ear tissue stretching is provided in Figure 1.




This problem would not occur with standard viscosity silicone. When standard silicone is injected into the ear, the material flows up against the oto-block, spreads and stretches the tissue in the canal out toward the ear aperture. The canal is opened up and its increased diameter is captured in the impression. Impressions, which reflect the ear in the more expanded state, are more desirable for any type of earmold, including molds for mild and moderate gain hearing instruments.

Since gun injectors, except the newest DS 50 injector, are incapable of dispensing standard viscosity silicones, manually operated back-loaded syringes are best for DigiMold impressions.

Should the patient chew to make a 'dynamic' impression?

In many instances, the patient's jaw movements can modify the size and shape of the impression while the material is setting in the ear.² Impressions A, B and C in Figure 2 were taken with various jaw positions from the same subject's ear. Impression A was taken with the jaw wide open, impression B with the jaw closed, and impression C while the subject chewed. There is a gap at the anterior wall between the control ear investment and the closed, as well as the chewing impression. The open-jaw impression has the canal about one-millimeter wider than that obtained in the chewing impression. Such an ear canal widening is typical with mandibular movements

Earmolds made while chewing may have significant feedback. Earmolds made from closed-jaw impressions may lack proper acoustic seal and retention. In addition, a loose-fit earmold may require frequent pushing back into the ear by the patient in an attempt to achieve a more secure fit and to stop feedback. This may lead to the ear tissue irritation and discomfort, which can be incorrectly interpreted as resulting from the mold's tight fit. All this is usually preventable if an open jaw impression is taken. A DigiMold built from an open jaw impression would have better anatomic definition of the ear, a more secure fit, less feedback and better comfort.³

Would a soft body DigiMold better prevent acoustic feedback?

Research has found that making an earmold from a soft material would not guarantee a proper seal.⁴ Proper earmold seal results from the mold fitting exactly, not from its softness. The exactness of the fitting is primarily defined through the impression taking process and through the mold manufacturing process. The seal of the DigiMold would depend little, if at all, on the softness of the mold material.

Results in custom hearing aid fittings prove this clearly. The majority of shells for custom instruments are manufactured from either a hard acrylic or hard ultraviolet resin. A recent study found that with a proper impression taking technique and competent manufacturing not more than 1% of custom in-the-ear hearing aids required a remake due to acoustic feedback, including high gain instruments.⁵ If soft earmold materials were as critical for acoustic seal as it is commonly claimed, most custom powered aids would never be successfully fitted.

Would a soft earmold at least prevent acoustic feedback while the patient speaks?

Sorry, they will not! The likelihood of breaking acoustic seal by jaw movements is similar for both soft and hard earmolds.⁶

So maybe a soft body DigiMold would be more comfortable?

Theoretically, yes. Practically, no. Human ears appear to be softer than most soft earmolds. This means that it is the ear tissue that has to conform to the earmold, not the earmold to the ear. Our DigiMold could still be made from the traditional hard acrylic and be very comfortable.⁷ A soft material would just be an option.

What is the best earmold style in terms of acoustic seal?

Research does not show that any particular earmold style offers a more efficient acoustic seal. Two separate studies found that canal style earmolds sealed at least as efficiently as full concha earmolds, sometimes even better.^6,8 The secret is that, in most earmolds, the acoustic seal occurs between the ear canal aperture and the canal second bend. Jaw movements break the seal at the front ear wall, as shown in Figure 2. In such a situation the path of feedback is as shown in Figure 3; along the front ear wall, above the tragus, and then directly to the microphone of the hearing aid hanging over the ear. The path of feedback for loosely fitting earmolds is similar. The escaping sound always chooses the shortest way from the ear canal toward the microphone. In this regard, the size of the DigiMold at concha is not critical for enhancing the mold seal.



Should the DigiMold have the Libby horn tube?

The Libby horn tube is a smooth one-piece earmold tube with varying diameter. The internal diameter of the tube is 2 mm at the earhook, and 3 or 4 mm at the end of the earmold canal, depending on the tube. Beside other things, the Libby horn tube determines the smoothness of the aided response and the effective real ear high frequency gain.⁹ How does it work?

The modern hearing aid receiver is a high impedance source. It generates high sound pressure, which moves only a small volume of air. The eardrum is a moderate impedance load. A low impedance load responds to low pressures, but requires large air volume movements. An impedance 'transformer' is required to efficiently transfer the energy from the receiver to the ear. A Libby horn tube is such a 'transformer'. It works as follows: The high pressure from the receiver moves a small volume of air in the small diameter of the tube. This in turn, moves a larger volume of air in the next larger bore of the tube, and the pressure drops progressively until the desired conditions are met.¹⁰

In a conventional earmold tube, an increase in gain potentially causes the spectral peaks to amplify sound beyond the comfort level, causing the wearer to turn the aid down or suffer. The Libby horn boosts the signal in the 2.7 kHz area and above. It should not be feared that this boost would increase the likelihood of feedback. Acoustic feedback in hearing aids develops typically below 2700 Hz, often between 1700 and 2500 Hz.¹¹ Figure 4 shows the effect of the 3-mm and 4-mm Libby horns with a 1500 ohm damper placed at the end of earhook, as compared to a standard 2-mm tube without damping.¹¹ The Libby horn is a better option than a drilled horn because to be acoustically effective, the horn bore must be drilled about 17 mm in length, rendering a nearly impossible task in many earmolds.¹²



Would a vent be necessary in the DigiMold?

A vent is a channel drilled in the body of the mold that connects the lateral surface of the mold with the medial end of the canal stalk. The presence of a vent in the DigiMold would not only reduce the low frequency components in the hearing aid response but it also would reduce moisture condensation in the earmold tube that adversely affects hearing aid performance. In addition, vents equalize air pressure in the ear canal, reduce the occlusion effect and contribute to greater earmold comfort for the wearer. The lack of a vent can lead to fungal infection.

Would it make a difference which earmold lab makes the DigiMold?

In many cases it would. There is a great variety of earmold manufacturing processes and not all of them produce earmolds with the same fitting exactitude. Proper earmold lab technician training is crucial. The technician's work is one of the critical factors, which determines the mold's easy insertion, seal, comfort and sound direction.

If you take care to obtain the best possible ear impressions, choose an earmold lab that will make the best use of them. Get your DigiMold built right!

Summary

As discussed, the success of designing and fitting the DigiMold would depend on a variety of factors. The factors that count most are the impression taking technique and material. Taking open jaw impressions with a standard viscosity silicone is recommended. The DigiMold could be made practically in any style, and from either a hard or soft material. A vent is recommended. The 3 or 4 mm Libby horn tube is a desirable option. The appropriate coupling between the receiver and ear canal in many instruments determines the smoothness of the aided response, the effective real ear high frequency gain, and in the final analysis, the patient's acceptance of amplification.


References

1. Chester Pirzanski C. Selecting material for impression taking: The case for standard viscosity silicones. The Hearing Journal, 2000, 53 (10); 45,48,49,50
2. Pirzanski, C. Critical Factors in taking an anatomically accurate impression. The Hearing Journal, 1997, 50(10): 41,44,46-48
3. Pirzanski, C. An alternative impression-taking technique: the open-jaw impression. The Hearing Journal, 1996, 49(11):30,32,34,35
4. Macrae, J. Static pressure seal of earmolds, Journal of Rehabilitation Research and Development. 1990, 27(4), 397-410.
5. Maye V. Field return analysis by account. Starkey Labs Canada, Internal study, 1997
6. Pirzanski, C. Chasin, M. Klenk, M. & Purdy, J. Attenuation variables in earmolds for hearing protection devices. The Hearing Journal, 2000, 53(6):44-45,48-50.
7. Berge, B. & Pirzanski, C. Earmold acoustics and technology. On-line AuD course on Earmolds for Pennsylvania College of Optometry, School of Audiology, 2000
8. Kuk, F. Maximum usable insertion gain with various earmold configurations. Journal of the American Academy of Audiology, 1994, 5:44-51.
9. Libby, E. Smooth wideband hearing aid responses - The new frontier. Hearing Instruments, 1980, 30(10):12-13,15,18,43.
10. The earmould, current practice and technology, British Society of Audiology, 1994
11. Vonlanthen A., Hearing instrument technology, 1995, pp 278-279
12. Killion, M. Problems in the application of broadband hearing aid earphones. In: Studebaker G.A., Hochberg J., eds. Acoustical Factors Affecting Hearing Aid Performance. 1st ed. Baltimore: University Park Press, 1980, pp 219-264
13. Katz J., Handbook of Clinical Audiology, William & Wilkins, 1985.


Chester Pirzanski, B.Sc. is a process engineer, trainer and lecturer at Starkey Canada, Mississauga, ON. Chester is available to teach on ear impressions, email: chester_pirzanski@starkey.com

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