A question about "prescriptive targets"

Hearing Aids
#1

Hello!

I’m currently researching the topic of how hearing aids are being tuned to compensate a hearing loss, and in a variety of sources I’m constantly meeting the term “prescriptive targets”. If I understood correctly, PTs are basically gain values for specific frequencies on specific loudness levels (loud, moderate and soft).

My question is, how are these values being derived? If they are based on conventional audiogram, they should be very inaccurate, because audiogram is typically measured in +/- 5 dB steps, and while that’s fine for determining the level of hearing loss, for tuning the hearing instruments it looks too imprecise; for example, for me 5 dB is a difference between hearing the tone clearly (during audiometry) and not hearing it at all.

Then there is a topic of acoustic difference between headphones used for audiometry and various types of domes/earmolds used with hearing aids which alternate the acoustic environment in the ear and therefore hearing thresholds will be slightly different. You will probably say that In-Situ audiometry is the answer, but in Genie 2 for example In-Situ has the same 5 dB steps, so while it should be more accurate, it still has the same problem of having too big increments.

Can specialists please explain, how do you work around that (if you do), and how do you make prescriptive targets fine-tuned and accurate?

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#2

I have had exactly the same question and have not seen an adequate answer. Think maybe the prescriptive target is based on interpolation from data from many other users. Would love to hear other explanations.

#3

Search for “fitting formula” or “fitting rationale”.

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#4

There is an audiology website where anyone can sign up for continueing education. It might be a way to learn this type of information. I think it is audiology.com

#5

nvm, ChatGPT provided somewhat good answer to my question, here it is if anyone is interested:

Question:
What about the fact that the standard audiogram has too large a measurement step (5 dB)? Does this mean that the amplification calculation will be inaccurate?

Answer:
You’re right that the standard audiogram has a measurement step of 5 dB, and this might seem like a rather coarse step, especially when it comes to fine-tuning a hearing aid. However, there are several factors that help avoid significant inaccuracies in calculating amplification, even when using an audiogram with such a step.

1. Smooth Amplification and Compression

Modern hearing aids use not only linear amplification but also nonlinear compression. This means they can adjust the level of amplification depending on the input signal level (for example, soft sounds are amplified more, loud sounds less). As a result, even small differences in hearing thresholds within 5 dB are smoothed out by flexible compression, helping to avoid significant inaccuracies.

2. Individual Hearing Characteristics

Formulas like NAL-NL2 take into account statistical data and hearing characteristics for different types of users. Even if the audiogram has a coarse step, this data helps to adjust the calculation so that the amplification is as accurate as possible for an average user with that type of hearing loss.

3. Interpolation

Hearing aid fitting software can use interpolation between audiogram points. This allows them to more accurately estimate hearing thresholds at intermediate frequencies and apply smoother amplification.

4. Clinical Adjustment

After the initial calculation based on the audiogram, testing and adjustment of the hearing aid settings are typically done directly with the patient. A clinician or audiologist can manually adjust the amplification based on the user’s subjective experience and additional tests (e.g., speech intelligibility tests).

5. Frequency Interdependence

Hearing loss often affects not just individual frequencies but a range of frequencies. Formulas like NAL-NL2 take into account that changes in one frequency can affect neighboring frequencies, which helps smooth out potential inaccuracies caused by a coarse measurement step.

6. Real-World Usage

In real-world conditions, users rarely encounter pure tones like those in an audiogram. Hearing aids amplify entire spectra of sounds, including speech, and operate in various acoustic environments. This also helps compensate for potential inaccuracies arising from a 5 dB measurement step.

Conclusion

While a 5 dB step may indeed seem large, modern methods of amplification calculation and hearing aid technologies help minimize the impact of this margin of error on the user’s actual sound perception. This is achieved through nonlinear amplification, data interpolation, clinical adjustment, and consideration of frequency interdependence. As a result, the user receives a fitting that provides comfortable and effective sound perception, despite the initial limitations of the audiogram.

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