Wilson and McArdle (2014) report that for more than 50 years, audiologists and dispensers have been taught if the octave frequencies (250, 500, 1000, 2000, 4000, and 8000 Hz) provide thresholds that are less than 20 dB apart, there is no need to test the inter-octave-frequencies (750, 1500, 3000, and 6000 Hz). This clinical protocol is called the “20 dB Rule.” Despite Wilson and McArdle’s (exhaustive) search, they found little supporting scientific data as to the origin of the 20 dB Rule. Therefore, the authors engaged in a retrospective, descriptive, correlational, and cross-sectional study. They report over a million audiograms from veterans served as the data source and their analysis included thresholds from three inter-octaves (1500, 3000, and 6000 Hz) and the octave thresholds on either side.
With regard to the octave thresholds for 1000 and 2000 Hz, 62 percent of the time, the differences between these two octave thresholds were greater than 20 dB. With regard to 2000 and 4000 Hz, the differences between the two octave thresholds were split equally (half the time the differences were greater than 20 dB, half the time the difference was less than 20 dB). With regard to the octave thresholds obtained at 4000 and 8000 Hz, some 74 percent of the time, the differences between the two octave thresholds were less than 20 dB. Further, as might be expected, for octave pairs across the audiogram, the higher frequency generally had poorer thresholds than the lower frequency.
The authors conclude that “unless there are other medical indicators or hearing aid fitting requirements that necessitate greater precision, inter-octave thresholds may be estimated using the mean threshold of the bounding octave frequencies….” They report that the 20 dB Rule is a good criterion with regard to the necessity of establishing the threshold for the inter-octave frequency.
For More Information, References, and Recommendations
Wilson RH, McArdle R. (2014) A Treatise on the Thresholds of Interoctave Frequencies: 1500, 3000, and 6000 Hz. Journal of the American Academy of Audiology (JAAA) 25:171-186.