The truth behind the fitting range

Fitting ranges are touted important in the fitting process, but what role do they really play?

As the audiologist for Phonak Target fitting software, I sometimes receive questions about fitting ranges. They vary from, “How is the fitting range calculated in Phonak Target fitting software?…Do they differ to technical specification?” to how they translate over to the fitting process such as, “What parameters influence the fitting range?” and “How can I see this in the fitting software?” In order to address these questions, we need to first define how the fitting range is calculated and at which point/s in the fitting process it is being used.

As a hearing aid manufacturer, we are required to publish technical specifications about the products we develop. These specifications, such as gain, MPO, frequency range, etc., fall under American National Standards Institute (ANSI) or International Electrotechnical Commission (IEC) guidelines and are shown in datasheets. However, these guidelines do not apply to fitting ranges. This is because there are no industry standards for how they should be calculated. Therefore, manufacturers define, calculate and represent the capabilities of hearing instruments differently, thus creating the potential for confusion.

One approach to calculate a ‘fitting range’ is to show the maximum capability or strength of the receiver selected as a range of audiograms that could potentially be fit with that product. This approach provides quick, ‘standardized‘ information about what gain and output the hearing instrument can deliver and are often used in the hearing instrument selection process.

Another approach would be to take the calculated ‘fitting range‘ and automatically include the impact of the fitting rationale used, audiometric data, feedback test, acoustic coupling, receiver strength, etc. This approach considers individual ear information and better represents what is happening at the level of the ear. However, it does not represent a ‘range‘ but rather specific information needed for the fitting of the hearing instrument.

So…How are fitting ranges calculated on datasheets and in Phonak Target fitting software?”

Phonak uses both approaches in the fitting process from the initial hearing instrument selection to fitting. However, the philosophy is that each approach serves a different purpose and therefore is kept separate for the hearing care professional. For the calculation of the fitting range in Phonak Target, they are based upon measurements according to ANSI / IEC guidelines that reflect the strength or available output of the receiver. This ‘fitting range‘ is kept static in the fitting software. Its goal is to serve as a reference point, providing quick information about whether the hearing instrument is capable of the gain and output needed to address the end-users listening needs. This can be seen in the fitting software by comparing the entered audiogram to the calculated fitting range (Figure 1).

Figure 1 – fitting ranges for different receiver strengths with an audiogram overlaid

“What parameters influence the fitting range?” & “How can I see this in the fitting software?”

In contrast to the static ‘fitting range‘, when parameters such fitting rationale, acoustic coupling, receiver strength, experience level, and feedback test are entered, they are automatically applied by the fitting software. These parameters all influence the actual available gain and output of the hearing instrument. They are then reflected in the curve display and not the ‘fitting range‘ (Figure 2). This approach allows the fitting software to more accurately reflect what happens at the ear. By incorporating the individual hearing aid, hearing aid configuration, and ear characteristics and visually showing it in the curve display, the hearing care professional can see the available gain and MPO in relation to the individual applied gain and MPO.

Figure 2 demonstrates how the available output changes when the same audiogram has different receiver strengths applied. The overall gain applied for the hearing loss doesn’t change, however, the available output limit changes (see range from curves to grayed out area) giving a wider range of fitting flexibility.

Figure 2 – Effect on the “fitting range” using different receiver strengths xS for R ear and xP for L ear for the same audiogram and acoustic coupling (power dome) in the curve display

As more individualized information is used, the available output limits changes even further in the curve display. In Figure 3, the audiogram and acoustic coupling are exactly the same, but the ears have different RECDs. This results in the differences noted between the ears.

Figure 3 – Effect on the “fitting range”/available output on the curve display using xS receivers and power domes, but applies different individual ear characteristics (RECD).

While there are different approaches to calculating and visualizing ‘fitting ranges‘, Phonak’s philosophy is each approach serves a specific purpose. For published fitting ranges in fitting software, Phonak provides information about capabilities of the receiver specifically for the purpose of hearing instrument selection. This information is clearly displayed in the hearing instrument selection screen and labelled as the fitting range. When additional information such as fitting rationale, acoustic coupling, receiver strength, experience level, and feedback test are entered, then these parameters that influence the fitting are clearly seen in the curve display. Therefore, by keeping the fitting range separate from the curve display and clearly visible at all times in fitting software, the hearing care professional has the complete picture throughout the entire fitting process.

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