Measurement of individual loudness functions by trisection of loudness ranges

Loudness-balance measurements with monaurally impaired subjects have shown that the shape of the loudness versus sound-pressure curve among hearing-impaired persons varies significantly. But the effectiveness of adjusting the compression characteristics of wide-dynamic-range compression hearing aids-the compression ratios, the variation of compression ratio with level, and the threshold of compression-to restore normal loudness growth for the individual patient has never been properly tested; individual loudness measurements have been too uncertain to permit meaningful individual adjustments. Recent investigators have reported standard deviations of such measurements in normal-hearing subjects of 6.4 dB and 7.8 dB. This investigation describes a method of measuring loudness function with a standard deviation in normal-hearing subjects of the order of 1 dB, both significantly lower than that of previous methods and sufficiently accurate for individual-subject adjustments.

Each of nine normal-hearing subjects-seven of them inexperienced and one a 9-year-old was asked to make three successive loudness trisections within an amplitude range of 40 to 80 dB SPL, providing six points from which to plot a loudness-function curve between these limits. The individual and average curves were validated as accurate loudness functions by comparing them to the curve defined by the equation of loudness versus amplitude in current Standards. In a second validation experiment, the loudness functions of masked ears measured by trisection were compared to the loudness function of those ears measured by loudness balance between masked and unmasked ears.

The difference between a loudness function based on the average of subject trisections and the loudness function defined by the ANSI Standard loudness equation was -1.92 dB at the lowest trisection level and +0.05 dB at the highest level. The standard deviations of subject responses were 1.63 dB for the lowest trisection level and 0.68 dB for the highest level, with an average of 1.1 dB. The across-subject standard deviation of the test-retest differences for three subjects was less than 1.7 dB for the first three lower level responses and less than 0.8 dB for the remaining three responses.

A trisection procedure for measuring loudness function showed validity and significantly less variation than previous loudness-measurement procedures. Such a procedure, once it has been validated for hearing-impaired subjects, makes it possible to test hearing aid design and fitting strategies that are based on individual-patient loudness functions.