By Jennifer Groth
This year marks the 10th anniversary of the modern age of open fittings. Ten years ago this spring, the ReSound AIR thin tube mini BTE was introduced, creating a new product category that started a massive shift in the market from custom hearing aids to models placed behind-the-ear. In the decade since, a body of knowledge has built about open fittings, and what is and isn’t different compared to conventional hearing aid fittings. Gradually, some of the misunderstandings are being weeded out and guidelines are emerging. For example, one common misconception that was often heard 10 years ago was that an open fit would add 15 or so dB of gain at the ear canal resonance, and that it therefore was a good idea to decrease programmed gain by this amount. Once everyone stopped and thought about how sound pressure does not add together in the decibel domain (recall that a doubling of sound pressure = 6 dB), it was easier to understand that the enhanced gain provided by the open fit was a bit more modest than first assumed.
Verification has also come into question as audiologists wondered whether they should be doing something different in this process. Do we need different prescriptive targets for open fits? Can we even match targets with open fit hearing aids? Are there any measurement pitfalls to be aware of?
First, prescriptions: while some hearing instrument manufacturers do offer proprietary prescriptions for open fittings, this mostly reflects the way they deal with some technical issues rather than an audiological rationale. These issues are the lack of low frequency gain, the slight enhancement of gain at the ear canal resonance, and the greater tendency for feedback. After all, why would the SPL required at the eardrum to achieve your fitting goals depend on how the sound gets there? It’s pretty safe to say that your favorite prescription is as valid for open as well as conventional fittings. In fact, your favorite prescription can help you decide if you should even be doing an open fit. For example, anyone requiring more than 15 dB of low frequency gain is probably not a great open fit candidate.
Next, target matching: it has been widely reported that manufacturers’ out-of-the-box fittings do a poor job of hitting real ear targets, although ReSound has been found to exceed others in this area (Aazh & Moore, 2007). More recently, these same authors examined the match to target achievable with open fit hearing instruments. Similar to their 2007 study, they found that the initial fit with no individual adjustment was not a great match to prescriptive targets, but that more than 80% of their open fittings were acceptable after fine-tuning. The limitations they ran up against were not surprisingly lack of low frequency gain and feedback in the high frequencies. This goes back to the point above regarding prescriptive targets…if the client needs significant low frequency gain or has severe high frequency hearing threshold levels, they may not be well-served with an open fit. Maximum stable gain for an open coupling to the ear canal and a BTE mic placement is generally in the 15 to 20 dB range, and a good feedback cancellation algorithm can allow 10 to 15 dB more stable gain in everyday situations (that is, not just sitting still in your office). This means that individuals requiring much more than 30 dB of high frequency gain are very likely to have feedback issues with an open fit.
Finally, measurement pitfalls: it was brought up early on that the combination of open ear canal, effective feedback cancellation, and proximity to the reference mic on the REM equipment could “trick” the AGC loop on the equipment to lower the signal level and give misleading results for insertion gain or aided gain measurements.
This is because these gain measurements are derived by subtracting the signal input level and the real ear unaided gain (for insertion gain) from the real ear aided output. If the equipment thinks the signal level is higher than it actually was, this will mess up the calculation and underestimate the insertion gain or aided gain. Lantz et al (2007) found evidence of this for open fittings at high gain levels, while other investigators have not found it to be a concern provided gains are about 25 dB or lower in the high frequencies (Mueller & Ricketts, 2010). REM equipment manufacturers have addressed this potential issue by providing a stored equalization function, which allows you to apply the correct signal level but run the actual measurement with the reference mic shut off. Shaw (2010) found that using the stored equalization method provided equally reliable results as measuring with the reference microphone on, so we would recommend using this feature if it is available.
Another potential measurement pitfall is related to the different type of coupling used for many open fittings. Silicone domes are often used to achieve an open fit instead of custom earmolds. Because these domes have more freedom of movement in the wearer’s ear canal, it is conceivable that this might be the cause of less reliable real ear measurements than with a custom earmold or custom hearing instrument. My colleagues Jespersen & Moeller (2013) examined how the different coupling methods to the ear canal affected test-retest and inter-examiner reliability, and found both to be comparable for hearing aids fit with domes and hearing aids fit with earmolds. As long as a best practice protocol is followed for probe tube placement, test environment, and test setup, they recommend business as usual for real ear verification of open fittings with domes.
REFERENCES
Aazh H, Moore B. The Value of Routine Real Ear Measurement of the Gain of Digital Hearing Aids. Journal of American Academy of Audiology (18) 2007; 653-664.
Aazh H, Moore B, Prasher D. The Accuracy of Matching Target Insertion Gains with Open Fit Hearing Aids. American Journal of Audiology (21) 2012; 175-180.
Jespersen CT, Moeller KN. Reliability of Real Ear Insertion Gain in Behind-the-Ear Hearing Aids with Different Coupling Systems to the Ear Canal. International Journal of Audiology (52) 2013; 169-176.
Lantz J, Jensen OD, Haastrup A, Olsen S. Real-ear measurement verification for open, non-occluding hearing instruments. International Journal of Audiology 2007; 46(1): 11-16.
Mueller G, Ricketts T. Open canal fittings: Ten take-home tips. The Hearing Journal 2006; 59 (11): 24-39.
Shaw P. Are real-ear measurements (REM) accurate when using the modified pressure with stored equalization (MPSE) method? International Journal of Audiology 2010; 49(6): 463-466.