Full Document: ANSI S3.22: Specification of Hearing Aid Characteristics

6 Recommended measurements, specifications and tolerances
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6.1 Curves
It is recommended that all published curves of gain, response, or output versus frequency be plotted
on a grid having a linear decibel ordinate scale and a logarithmic abscissa scale, with the length of one
decade on the abscissa scale equal to the length of 50 + 2 dB on the ordinate scale.

Section 6.1, titled Curves, outlines the recommended aspect ratio for graphing. This used to be a requirement. Basically, it says 50dB vertical for a decade of Frequency, which is 15dB vertical / octave. The idea was that by controlling the aspect ratio it would be easier to compare one product to another (back in the days when paper datasheets where how you sold products). Most likely today, the datasheet is done mainly for compliance.