Shortly after the inception of A-weighing, a 1938 article noted that as a metric, it didn’t represent true low-frequency loudness. In 1969, a criterion was introduced to evaluate low-frequency by calculating the C–A equivalent sound level of the whole spectrum, and if the difference is greater than 10 dB, the spectrum contains low-frequency energy. The 1997 German standard DIN 45680 proposed a similar criterion but limited the bandwidth from 8 to 100 Hz. Since 1999, the WHO has recommended the use of the Botsford criterion warning that it is not valid on unbalanced spectrums. The purpose of this paper is to present a plausible way to evaluate the low-frequency using its own bandwidth, because the Botsford criterion is not useful on unbalanced sound spectrum, and it should be noted that the same criterion is included in an ANSI/ASA standard. In order to have an international noise descriptor, the ISO 1996-2:2017 low-frequency definition is used, which means to calculate the difference between the C-weighting and the A-weighting in a one-third octave band between 16 and 200 Hz, in order to have a prognosis of the real low-frequency sound level contained in any sound spectrum, demonstrating the robustness of this specific descriptor.

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