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Custom instruments and some thoughts around open vs closed fittings

Reading Time: 4 Minutes
26/09/2018

When talking to students, I like to talk about fitting as an art. Of course, it also contains lots of science (audiology) but I try to create the image of a fitting process being sometimes unpredictable and focus on the fact that the perception of the individual customer is "The Truth" for them.

Fitting hearing instruments can be full of dilemmas that require the best decision taken in agreement between the HCP and the customer. Mastering the fitting process, as well as the optimal acoustical coupling, is the main reason to me why Audiologists and HCPs are needed today and will be continued to be needed tomorrow.

Fitting dilemmas

Regarding dilemmas, it is about finding the right balance between comfort and speech intelligibility, the right balance between size and acoustical performance and sound quality, and last but not least, the right decision about the openness of the fitting mainly defined by the venting parameter. Do I go open or more closed? Today there are good reasons for both options.

Open or closed

During my studies I was told the rule to fit hearing aids "as open as possible" to minimize the occlusion effect, to make use of the parts of hearing that do not necessarily need amplification (which in almost all cases are the lower frequencies), and to increase wearing comfort by more ventilation. These arguments in my opinion are still valid for a lot of fittings but probably not any longer for all fittings. Please keep in mind that I started my education as an HCP back in 1984. This was a time where only analog technology was available in hearing aids and gain was applied linearly in most cases. There were no such things as different types of noise reduction that can increase the listening comfort or adaptive directionality that can help to follow a conversation in acoustically challenging environments. For these features to be most effective, the vent-effect shouldn't be too big (Magnusson et al., 2013), hence the fitting should be potentially more closed and not "as open as possible" depending on the individual person. And of course, the feedback risk decreases due to a smaller vent when less sound can escape the ear canal (Blau et al., 2008).

Open fittings

If you favor an open fitting you most likely will go with a BTE or maybe, depending on the ear canal size, with a RITE instrument. In BTE instruments, the entire technology is placed behind the ear so there is only a little space needed in the ear canal for the tubing; so, the ear canal can be mostly left open.

Closed fittings

But in the case where you agreed with your customer to go for a more closed fitting, you may consider fitting a custom instrument. Please note that the term custom instrument in this context includes all the different types and sizes spanning from the full-concha instrument to the IIC / Invisible-In-the-Canal. Custom instruments to some extent block the ear canal which is no surprise since the technology and especially the largest parts, typically, the battery and the receiver, ask for some space. Hence, the fitting will potentially be more closed and less unprocessed sound can get into the ear canal. Therefore, a relatively higher amount of the sound that is processed by the hearing aid is audible. The signal processing and the feature functionality can potentially have a stronger effect (Keidser et al., 2007).

Getting the most out of custom instruments

If you want to get the most out of custom instruments you may try to fit small devices like CICs and IICs and try to position the instrument as deeply in the ear canal as possible. These instruments are only equipped with one microphone and consequently there is no directionality applied by the instrument. There are at least two good reasons to do so: reason number one is the fact that the instrument can be built smaller since directionality that is based on two microphones requires some millimeters of distance between those microphones and this space is simply not available. Secondly, the natural pinna effect can be fully used when the microphone is placed in the ear canal.

A deeply inserted microphone is also far less prone to wind noise since it is well covered. It is also possible in many cases to use the phone in the same way as before. No need to hold the phone somewhat differently. In general, higher sound pressure levels are measurable when the residual volume between tip of the instrument and eardrum is relatively small. This allows for smaller receivers and can reduce power consumption. When the instrument is placed deep enough in the ear canal, it also potentially reduces the occlusion effect. And of course, these kinds of instruments support the customer’s desire for invisibility as best as possible.

But please don't get me wrong: I'm still a true believer that the paradigm "as open as possible" is absolutely valid for many fittings. But it might not be the best solution for all customers and it is in the best intent of the HCP/Audiologist to identify the best solution together with each and every customer.

What are your thoughts on open vs. more closed fittings? We welcome your comments.

For more about the Bernafon Zerena family of hearing instruments that includes our widest range of instruments to choose from, go to the professional page on our Bernafon website.

References:

Blau, M., Sankowsky, T., Stirnemann, A., Oberdanner, H., & Schmitt, N. (2008). Acoustics of open fittings. Presented at Acoustics’08, Paris, France.

Keidser, G., Carter, L., Chalupper, J., & Dillon, H. (2007). Effect of low-frequency gain and venting effects on the benefit derived from directionality and noise reduction in hearing aids. International Journal of Audiology, 46, 554-568.

Magnusson, L., Claesson, A., Persson, M., & Tengstrand, T. (2013). Speech recognition in noise using bilateral open-fit hearing aids: The limited benefit of directional microphones and noise reduction. International Journal of Audiology, 52, 29-36.

 

About the author:

Reinhard Winter
Reinhard Winter, Hörakustikermeister
Reinhard works as International Product Manager Audiology at Bernafon. His main area of focus is on solutions for the very hard of hearing. Before joining Bernafon in 2005, he worked as a hearing care professional and audiological consultant for around 20 years – 10 years running his own shop in Germany. In 2009, Reinhard started to teach hearing aid and hearing aid fitting. In his private time, Reinhard enjoys spending time with his family, preferably in combination with playing sports and being out in nature.

 

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