By Jennifer Groth, MA, and EricA Koehler, AuD
What actually happens to the fitting when you select the specific coupling style?
Open-fit hearing instruments have become synonymous with small, behind-the-ear (BTE) styles fit either with a thin acoustic tube delivering sound to the ear canal or with a thin wire connecting the device with a receiver placed in the ear canal. However, an open-fit hearing instrument can actually be any style, provided it creates minimal occlusion of the ear canal. Likewise, the small BTE instruments can occlude to a degree that they no longer can be considered an “open fit. ”
While the decision to fit one of these appealing small BTE styles is an easy one, the decision of how to fit it is less so, as there are many options for coupling to the ear canal. For both thin-tube and thin-wire styles, there are a multitude of standard-sized silicone domes that can be used, and custom earmolds with a host of different venting (and other) options are also available.
To complicate matters further, the information presented in fitting software may inaccurately represent the impact of different coupling options on the real-ear gain, and may give the fitter a false sense of security regarding the frequency-gain characteristics of the sound being delivered. We have encountered hearing aid users with an open fit who would have experienced much greater benefit with an occluded fitting, and vice versa. One who comes to mind was a lady we met at an open house event. She had a bilateral, relatively flat sensorineural hearing loss with hearing threshold levels hovering in the 60 dB HL range. She had used full-shell custom hearing instruments for two decades, and came into the event still wearing her old custom devices despite the fact that she was “trialing” her third pair of receiver-in-the-ear (RITE) hearing instruments from another practice. Each of these pairs had been fit using open domes.
Although she loved them for the appearance and the comfortable way they fit her ears, her complaint continued to be that she couldn’t hear as well with them as with her own custom hearing aids.
While it seemed rather obvious to us that fitting her with open domes would preclude the gain levels she preferred, it is likely that her fittings with these different products did not look out of line as shown in the respective fitting software modules. But how can this be?
It is well-established that individuals with hearing threshold levels in the low frequencies exceeding 40 dB HL are much less likely to be bothered by the “boomy” sensation of their own voice and other self-generated sounds caused by the occlusion effect.1,2 Likewise, people with low frequency thresholds better than 40 dB HL are likely to prefer minimal or no amplification in the low frequencies. Therefore, it made sense that the first open-fit hearing instruments to gain wide popularity recommended a low frequency fitting range extending only to around 40 dB HL.
However, as more options for coupling to the ear became available, fitting ranges grew, and manufacturers diverged in how these different coupling options were handled in the fitting software. Anyone who fits hearing aids from more than one manufacturer has a lot to keep track of!
In this article, we will revisit a few guiding principles on selecting appropriate coupling options for individuals. In addition, we provide an overview of how several manufacturers adjust (or don’t adjust) the hearing instrument gains depending on the information you enter about the acoustic properties of the fitting. It should be emphasized that these adjustments (or lack thereof) are not necessarily “right” or “wrong” fitting/programming approaches; each manufacturer has a fitting rationale that it believes helps dispensers provide the best fittings possible.
Armed with these insights, we hope that dispensing professionals will more easily be able to decide on appropriate adjustments when fitting hearing instruments from one manufacturer to the next.
How do you choose a specific coupling option?
Many dispensing professionals have a “go to” solution resulting from positive experiences in fitting. While some prefer standard-issue domes, others would rather order custom earmolds for the majority of their patients. Because there are a variety of options within each of these categories, there is no right or wrong here. However, there are a few general points that are helpful to keep in mind:
- Open domes and “tulip” domes are the least occluding, and ideal for fitting those with normal or near-normal hearing thresholds below 1 kHz. However, feedback may be problematic. This leads us to our next point.
- Domes known as “power domes” or “double domes” do not provide an open fit as they significantly occlude the ear canal. It may be tempting to use them to solve feedback issues with an open fit, but they are more likely to generate occlusion-related complaints. Consider ordering a custom hollow earmold in these cases. A hollow earmold reduces occlusion with a smaller diameter vent than a solid earmold.3 This can help strike the balance between adequate high frequency gain and acceptable occlusion.
- The ear canal must be occluded to some degree in order to provide low frequency gain. In other words, individuals who have low frequency thresholds exceeding 40 dB HL and who are likely to need 10 dB or more of low frequency gain cannot be fit adequately with open domes or tulip domes.
- Individuals requiring a lot of low frequency gain won’t get it with power domes or double domes. A custom earmold is required for severe low frequency hearing losses. Our measurements indicate that, relative to an occluding earmold with no vent, gain is reduced by 3 dB at 1 kHz, 6 dB at 500 Hz, and 11 dB at 250 Hz with a power dome.
Does the coupling option affect the gain prescription?
This question can be answered in two ways: 1) theoretically, and 2) in terms of what actually happens in practice. From a theoretical perspective, there is no reason for a prescription that is specified in terms of gain or output in the ear to differ depending on how sound is delivered to the ear. Real-ear prescriptive targets attempt to provide particular sound levels at the tympanic membrane of the hearing instrument user. As long as these levels are achieved, it should not matter how they get there or whether they come through: a) an amplified pathway, b) direct unamplified sound, or c) a combination of the two.
In practice, however, some manufacturers do change the real-ear targets based on the assumed coupling to the ear canal. Figure 1 shows an example of how the real-ear insertion gain targets for one manufacturer were changed when the coupling to the ear was changed. The audiogram was for a sloping mild-to-moderate hearing loss, and the hearing instrument was a receiver-in-the-ear (RITE) style. When the coupling option was open, no low frequency insertion gain was prescribed, and less overall gain was prescribed relative to a more occluding coupling option.
Figure 1. The coupling option selected in the fitting software can have significant effects on the default prescribed real ear gain. Prescribed insertion gain when an open dome was selected was 10 dB or more less than that prescribed when a micro mold was selected for the same audiogram and user experience level. In this particular case, the manufacturer is probably adding corrections for the acoustic conditions on top of changes meant to accommodate gain preferences for an open fit. Other manufacturers do not show applied gain corrections for acoustic conditions in their simulated real ear gain and output displays, but only in coupler displays.
It is important that dispensing professionals are aware that this may happen with most manufacturers, and that they consider whether it is appropriate for the individual being fit. Each manufacturer also should be expected to provide a rationale for any prescription changes they make.
So, what really happens in the hearing instrument when coupling options are selected?
It is imperative to follow the manufacturer’s guidelines regarding whether a BTE hearing aid is being fit with a thin tube versus a standard tube, and to specify the receiver for an RITE instrument. These options have profound effects on the gain applied by the device and, if specified incorrectly, can result in vastly overfitting or underfitting patients.
In addition to this important information, the clinician has the opportunity in fitting software to select the coupling option used in the fitting. This would seem to be an important step that ultimately impacts the settings in the hearing instrument—or why else is it there?
One of the most frequent questions we get regarding ReSound products is what effect changing the “Physical Properties” of the fitting has on the gain provided by the device. It is apparent that different manufacturers not only call their coupling options by a different name in their respective software modules, but that changing these options can have different effects on the hearing instrument.
We decided to investigate how several manufacturers handle this information in fitting a RITE hearing instrument.
Hearing instruments from five manufacturers were tested. These included ReSound Alera 61DW, Phonak Audeo Smart IX, Oticon Agil Pro mini RITE, Starkey Wi RIE, and Siemens Pure 701. The lowest power receiver available from each manufacturer was attached to the device, and the device was mounted with putty on the HA1 coupler of an Audioscan Verifit, and a 65 dB SPL pure-tone sweep was used as the stimulus.
Measured this way, any differences in gains are attributable to what is provided by the hearing aid and are not affected by venting. The audiogram used for programming all instruments was the standard N3 audiogram4 shown in Table 1. Special processing, such as noise reduction, directionality, and frequency lowering, was disabled for the measurements.
Two measurements were performed on each device: one with the most open coupling option selected, and one with the most occluding coupling option selected. In the event that the manufacturer does make changes in device gains depending on coupling option, these extremes would best illustrate the strategy for doing so.
In general, we observed three different approaches to what happens when an open coupling option is selected relative to an occluding option in programming the hearing instrument gains. Each of the three approaches is illustrated with a graph showing the difference in measured coupler gain for the most open coupling option versus the most occluding coupling option available. The three approaches are:
- Do nothing. Hearing instrument gains are not affected by the coupling selected. This was the case for ReSound “Physical Properties” and Starkey “Acoustic Options” (Figure 2). While it may be useful for record-keeping purposes to save the coupling information with the fitting, it has no impact on the fitting and is really not a fitting tool in the sense that it does not change the settings of the hearing instrument. The real-ear gain and output displays in the fitting software do reflect the acoustic effects of the coupling selected in order to better simulate real-ear responses.
- Increase low frequency gain, and decrease high frequency gain. This was the case for Siemens “Acoustical Parameters” (Figure 3). The rationale for decreasing high frequency gain in the case of an open coupling to the ear canal is that the ear canal resonance will contribute to the sound pressure level (SPL) of the sound at the tympanic membrane. This effect has been estimated to be 5 dB or more in the 3 kHz region,5 so the correction we observed is fairly modest. Although we are uncertain about the exact rationale for an increase in low frequency gain with the open coupling, it may be related to the roll-off in the low frequency response due to the large venting condition (although technical limitations of some receivers may make this compensation difficult to achieve).
- Decrease low frequency gain, and decrease high frequency gain. This was the case for ReSound “Reconfigure,” Phonak “Acoustic Parameters,” and Oticon “Acoustics” (Figure 4). As discussed, some high frequency gain reduction for the open coupling makes sense to account for the enhancing effects of the ear canal resonance. The correction applied by the ReSound “Reconfigure” feature is based on an internal study of the effects of ear-canal coupling options on real-ear gain6 and is limited to the 3 kHz region. The high frequency correction for the other manufacturers is a bit broader, and reflects changes to the real-ear gain prescriptions, as well as corrections to account for the acoustic coupling to the ear canal. The rationale for decreasing the low frequencies is based on the knowledge that it will not be possible for a hearing instrument to compensate for the low frequency roll-off resulting from the open ear canal. Therefore, it does not make sense to expend energy in attempting to do so.
Clinicians can expect one of two things to happen to hearing instrument gain when they select one coupling option versus another. Either this will have no effect, or the gains will be changed. Some manufacturers may effect these changes as part of their prescriptive targets, while others may do it “behind-the-scenes.” And some may do both. The combined effects of these changes were illustrated for RITE hearing instruments from five manufacturers. These results can give clinicians an idea of how each of these manufacturers handles information that is entered regarding the acoustic coupling to the ear canal, and thus make informed decisions regarding the importance of entering the information, as well as decide what gain adjustments might be best for a particular individual.
Jennifer Groth, MA, is director of audiology communications, and Erica Koehler, AuD, is a research audiologist at the GN ReSound Technical Center in Glenview, Ill.