What is a prescriptive fitting formula and which one should I use with my patients?
Prescriptive fitting formulas are extremely helpful in the process of fitting hearing systems. They help hearing aid professionals to develop a pre-adjustment for the hearing devices. For most fitting formulas this involves at least an analysis of the air conduction hearing threshold and the assessment of the frequency-dependent amplification requirement. Various fitting formulas also take other patient parameters into considerations, e.g. the bone conduction threshold, level of discomfort, sex, age or type of ear mould.
Any setting of a prescriptive fitting formula should be seen as a default setting, which calls for additional fine-tuning with the customer.
Comparison of current fitting formulas (DSLv5.0 adult vs. NAL-NL2)
DSLv5.0 stands for “Desired Sensation Level” and is a fitting formula which aims at calculating the best possible loudness balancing. Especially with high-frequency hearing losses, this leads to a significant increase of the not clearly audible frequencies. This may not be a good choice for people who have lived with a hearing loss for a while and use a hearing device for the first time, they can be overwhelmed with the sensory overload and this again may lead to discomfort. However, for patients who have been wearing a hearing device for a while and who are used to higher gain values, DSLv5 can be the best choice. Moreover, this formula also takes the measured levels of discomfort into account and the MPO of the hearing device is adjusted accordingly.
NAL-NL2 is a fitting formula developed by the National Acoustic Laboratories (Australia) and which is generally well suited for first time users of hearing aids. It is also ideal for people who can tolerate only little gain in the high frequency range. The formula considers the hearing threshold, the degree of the hearing loss, whether a patient is used to hearing aids, age, sex and other parameters. The discomfort threshold, however, is not taken into account, which means that the MPO needs to be set manually.
Manufacturer-based fitting formulas
Various manufacturers of hearing devices develop their own fitting formula together with their hearing aids. This makes sense for the manufacturers as fitting formulas nowadays do not only set gain parameters, but also affect features. These features are supposed to be most effective with the manufacturer-based fitting formulas and enable the customer to make the most of the manufacturer’s technology.
Fitting formulas for children – DSLv5.0 pediatric
Especially for children it is important to balance the loudness completely, as the language acquisition progresses fast in childhood. In order to do so the brain must receive enough stimulation from the birth onwards. This is only possible with an adequate gain and a hearing device with is set relatively linear.
Resources for More Information
For more information, visit https://www.inventis.it/en-na or check out these resources:
- Measuring and Assessing Aided Audibility Using the SII
- Let's Get Real About Outcome Measures: Practice Solutions for Evaluating Success
- Roll up your sleeves: (Hands-on) REM basics with CARL