Seminar 1: Prescription Formula

Question 1

What is a prescription fitting rule?

Answer 1

A prescription fitting rule is a formula used to calculate the ideal target output for a hearing aid based on audiometric thresholds or loudness measurements (MCL - Most Comfortable Level).

Question 2

What are the most commonly used prescription fitting rules in the UK?

Answer 2

The most commonly used prescription fitting rules in the UK are NAL (National Acoustic Laboratories) and DSL (Desired Sensation Level).

Question 2

How do prescription fitting rules work?

Answer 2

Prescription fitting rules use audiometric thresholds or loudness measurements to calculate the ideal target output for the hearing aid. This ensures the hearing aid provides appropriate amplification tailored to the user’s hearing loss and listening needs.

Question 2

Why do we use prescription fitting rules?

Answer 2

Prescription fitting rules are used to ensure that hearing aids provide the correct amount of amplification tailored to an individual’s specific hearing loss and listening needs. They help achieve optimal speech intelligibility and listening comfort.

Question 3

Which is the best rule to use?

Answer 3

*The choice of rule depends on the individual needs and goals of the user:

• NAL (National Acoustic Laboratories): Focuses on maximizing speech intelligibility. It uses target insertion gain for different input levels, providing less gain in lower frequencies and more gain in higher frequencies. This is particularly beneficial for adults who need clear speech understanding.
• DSL (Desired Sensation Level): Focuses on comfortable listening levels and making speech sufficiently audible without causing discomfort. This rule is often preferred for children as it encourages them to wear their hearing aids by ensuring comfort.
• Manufacturer Prescriptions: These are designed to make the best use of the specific features and technology of a manufacturer’s hearing aids. However, they lack objective validation and are often based on customer satisfaction questionnaires, making them less reliable.

Question 4

What does the line in the middle of the LTASS represent?

Answer 4

The line in the middle of the LTASS represents the average intensity of each frequency in speech.

Question 4

How important are prescription fitting rules in the hearing aid fitting process?

Answer 4

• Prescription fitting rules are crucial in the hearing aid fitting process as they ensure that the amplification provided by the hearing aid matches the specific needs of the individual’s hearing loss.
• Proper fitting can significantly improve speech understanding, comfort, and overall satisfaction with the hearing aid.

Question 4

What are the main characteristics of the NAL fitting rule?

Answer 4

• NAL focuses on maximizing speech intelligibility by providing target insertion gain for different input levels.
• It provides less gain in the lower frequencies and more gain in the higher frequencies.

Question 4

What are the main characteristics of the DSL fitting rule?

Answer 4

• DSL focuses on comfort and aims to make speech sufficiently audible to allow speech perception without causing discomfort.
• It calculates target output for input levels, which is particularly useful for children to encourage consistent hearing aid use.

Question 4

What is an SPL-ogram and what information does it show?

Answer 4

• An SPL-ogram is a graph that displays all relevant audiological information on the same graph, including the soft, moderate, and loud sounds the hearing aid needs to amplify.
• It helps audiologists visualize and verify that the hearing aid is providing the correct amplification across different frequencies according to the prescription targets.

Question 4

What are the limitations of prescription fitting formulae?

Answer 4

• They can be used as a starting point
• They should be modified according to patient feedback
• They may not fully reflect hearing aid performance in the real world

Question 4

How do we know whether the hearing aid output meets the prescription targets?

Answer 4

• To determine if the hearing aid output meets the prescription targets, real-ear measurement (REM) is often used.
• This involves placing a small microphone in the ear canal to measure the sound delivered by the hearing aid.
• The results are compared to the prescribed targets to ensure accuracy.

Question 4

What does the LTASS (Long-Term Average Speech Spectrum) show?

Answer 4

• The LTASS shows how speech varies in intensity across all frequencies that are included in speech.
• It provides an average intensity level for each frequency and indicates the range of variation (typically represented as a shaded area showing ±1 standard deviation).

Question 4

How are manufacturer prescriptions different from NAL and DSL?

Answer 4

• Manufacturer prescriptions are designed to optimize the features and technology of their specific hearing aids.
• They are often based on internal customer satisfaction surveys and lack the objective validation that NAL and DSL have, although the latter have not been updated with the latest technology and features.

Question 4

How should the LTASS ideally map to your Real-Ear Measurements (REM)?

Answer 4

• Ideally, the REM should show that the hearing aid’s output matches the LTASS, meaning that the amplification provided by the hearing aid allows the user to hear speech sounds within the normal intensity range of speech at each frequency.
• This ensures that speech sounds are audible and clear across all relevant frequencies.

Question 4

Why is the shaded area in the LTASS important?

Answer 4

• The shaded area in the LTASS indicates the range of intensity variation for each frequency, showing the standard deviation.
• This helps in understanding the natural variability of speech sounds across different frequencies.