FITTING NON-LINEAR HEARING AIDS
A Coggle Diagram about NAL-NL1 (1999)
Overall we want to keep the overall loudness at normal levels (or even below normal).
At the same time, the aim is to maximise speech intelligibility for any & all of the input levels above the compression threshold.
• So: the aim itself wasn't loudness equalisation as such BUT: whilst trying to achieve alms ① & ② it appears that loudness equalisation occurred in the low frequencies & mid frequencies.
• AIM 1 : Restore normal loudness perception for speech .
• AIM 2: Maximise intelligibility.
How do we achieve this?
↳ By using the 1997 Moore & Glasberg loudness model (to achieve aim 1) & by using a modified SII formula (to maximise intelligibility as per aim 2)., NAL-NL1 & NL2
these are software driven prescription rules .
Targets worked out relative to the input PTA thresholds.
There is no loudness scaling involved & no supra threshold measures., Recap about Conventional Prescription Formulae :
→ Linear Amplification = As the input increases the output increases by the same amount (therefore: N0 COMPRESSION)
→ Non-Linear Amplification : (aka Compression)
The slope is less than 45 degrees & the Gain varies with input level., Prescription Rule 0ptions for Non-Linear, Using non-linear rules in the clinic
↳ Most rules are accessible through the NOAH REM Screen.
↳ the Computer performs the calculations.
↳ Targets will appear on the screen for the specified input.
↳ we match the targets for 2 or 3 input levels (hence non-linear).
↳ We fit to targets & use fine tuning as well based on patients' comments., For fitting a non-linear HA :
We need more than 1 target → need low, mid & high inputs, Fitting Linear Hearing Aids :
1 target will suffice. and Which Rationale/ Rule is preferred then?
• (Keidser & Grant 2001) Find increased preference for equalisation (NAL-NL1) over normalisation (IHAFF).
• (Ching 2003) *find (found?) overwhelming preference for NAL-NL1 over DSL i /o (largely normalisation) for adult HA wearers.
• (Johnson & Dillon, 2011) show that average prescribed loudness of NAL-NL2 & DSL m i/ o agree well with lab & field trials of HA loudness
• When it comes to speech-in-noise testing the NAL-NL2 & the DSL m [i/ o] gave the same predicted SI.
• There's an ongoing large study that compares NAL & DSL & the prescription rule is not a predictive factor.