Abstract
In Germany assessment of hearing loss for numbers is used to calculate the percentage hearing loss from speech audiometry and for plausibility checking with pure tone thresholds. It is common practice to take a graphical reading from the speech audiogram to determine the hearing loss for numbers. This study searches for a mathematical formula for the exact calculation of the hearing loss for numbers from the intelligibility values measured. With analytical methods on the basis of the standard curve following DIN 45626-1, two simple formulas for the calculation of the hearing loss for numbers were developed. The hearing loss for numbers a(1) is calculated as a(1)=p(1)+(50-v(1)) (p(2)-p(1))/(v(2)-v(1))-18,4 for two available measured values and as a(1)=p(1)-0,13 v(1)-11,9 if only one measured value is used, with v(i )being number intelligibility in percent at the level p(i) in decibels (dB) of measurement number i. Number intelligibility of all inserted pairs of values must be between 30% and 70% because the standard curve of DIN 45626-1 runs approximately linearly only in this range. The calculated value for the hearing loss for numbers is subsequently mathematically rounded up to 5 dB as well as for the conventional graphically determined value. With the presented formulas the hearing loss for numbers can be calculated exactly from the measured values of the Freiburg number test especially in matters of expertise if the conventional graphical determination of this value does not seem to be unambiguous.
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