Ear Domes - Low Frequency Loss

Summary: “Perhaps most significantly, the data shows that closed domes simply do not work as they are intended. Responses for open and closed domes are virtually identical, and the double-dome condition provides a modest (approximately 5 dB) increase in low frequency output.”

Procedure

A fully featured RITA hearing aid from a major hearing aid manufacturer was programmed for a flat 45 dBHL hearing loss to an NAL-NL1 prescriptive target with no reduction for acclimatization. The hearing aid was programmed to a first fit, and no attempt was made to fine-tune the response. The response of the hearing aid was verified in 9 ears (6 male and 3 female). The ear couplings shown in Figure 1 were evaluated by means of an AudioScan RM500SL real-ear analyzer using the Speechmap protocol with a calibrated speech signal delivered at 65 dBSPL:

• Open dome (8 or 10 mm)
• Closed dome (8 or 10 mm)
• Double dome (8 or 10 mm)
• Custom earmold with slim tube
• Custom earmold with regular #13 slim tubing

Results

Results of probe tube measurements were predictable for some of the conditions. For example, the open-dome condition showed considerably less low frequency output than did the custom earmold conditions. The responses for the closed dome and double dome, on the other hand, did not conform to prior expectations. These domes are meant to occlude the ear canal and provide more low frequency gain or, in some instances, reduce the likelihood of feedback. The feedback issue was not addressed in the current investigation. However, the degree of increase in output in the low frequencies for the closed and double domes was less than might have been expected.
In the open-dome condition (Figure 2), virtually no gain is present below 2000 Hz. With the exception of Subject A, high frequency responses approached or, in some cases, exceed target. Note that in this and subsequent data, there is a consistent “notch” in the response at 1500 Hz.
In the closed-dome condition (Figure 3), for all but one of the ears tested (Subject D, left ear), the responses are remarkably similar to those obtained in the open dome condition. It appears that, with the exception of Subject D LE, the ear canal is insufficiently occluded by the closed dome to allow for significant increase in low frequency gain.
In the double-dome condition (Figure 4), a modest increase in low frequency response is observed in most ears. Responses for Subject A continues to show little change from one condition to the next. High frequency responses are consistent with open- and closed-dome conditions.
In the custom earmold with slim-tube condition (Figure 5), low frequencies are fully engaged with little change in the high frequencies. The notch at 1500 Hz is more pronounced, perhaps because the increase in low frequency output makes it more apparent. The responses for Subject A are resistant to any change in ear acoustics.
FIGURE 4. Double dome, 9 ears.FIGURE 5. Custom earmold with slim tube, 9 ears.

The primary effect of using #13 tubing (Figure 6), as compared to a slim tube with a custom mold, is a lessening of the 1500 Hz notch that has been present in all other conditions. Otherwise, the effect of the change from slim tube to #13 is different for each ear. Low and high frequencies alike are affected by the change in tubing diameter. Note in particular how the response for Subject A in this condition finally begins to approach target, particularly for the right ear.
In each of the above examples, it is notable that there is at least one frequency, and usually more, where the difference between the lowest and the highest output observed is greater than 20 dB in the 1500 to 4000 Hz range.
When we look at averages of all of the data (Figure 7), a number of things become clear. High frequency responses are least affected by differences in ear coupling devices. A custom earmold with #13 tubing affords the response that most closely approximates the desired target. Both of the earmold conditions (slim tube and #13 tube) were the only instances where output in the low frequencies was allowed to approach, and in some instances to exceed, target for the low frequencies. We also see that a notch at 1500 Hz is most evident for the earmold with slim tube condition, and was much smaller for the earmold with #13 tubing condition. This suggests that there is something about the acoustics of the slim tube (perhaps a standing wave or other acoustic phenomenon) that is creating this artifact.
Perhaps most significantly, the data shows that closed domes simply do not work as they are intended. Responses for open and closed domes are virtually identical, and the double-dome condition provides a modest (approximately 5 dB) increase in low frequency output. Therefore, when attempting to fit RITA hearing aids on hearing losses with a significant degree of low frequency hearing loss, custom earmolds should be used.

Was that a non RIC with a slim tube?

Tells you little about the actual response of a current RIC and lots about the impedance of slim tubes in general.

What is the difference between a slim tube and a #13 slim tube. For my bad ear, I’m definitely going to try the custom mold. Audi said to try power receiver and custom mold on the slim tube that I’m currently wearing (and like).