Fletcher-Munson curves



The Fletcher–Munson curves are one of many sets of equal-loudness contours for the human ear, determined experimentally by H Fletcher and W A Munson, and reported in a paper entitled "Loudness, its definition, measurement and calculation" in J.Acoust. Soc Am.5, 82-108 (1933).

Background
Until recently, it was common to see the term 'Fletcher–Munson' used to refer to equal-loudness contours generally, even though a re-determination was carried out by Robinson and Dadson in 1956, which became the basis for an ISO standard:ISO 226.

It is now better to use the term 'Equal-loudness contours' as the generic term, especially as a recent survey by ISO redefined the curves in a new ISO 226 :2003.

According to the ISO report, the Robinson–Dadson results were the odd one out, differing more from the current standard than did the Fletcher Munson curves! It comments that it is fortunate that the 40-phon Fletcher–Munson curve on which the A-weighting standard was based turns out to have been in good agreement with modern determinations.

Recording and Mixing
The human ear is less sensitive to bass and treble at low volumes than at high volumes. So it is important to consider - when you record that a very loud instrument and play it back at a lower level, it might lack bass and treble. In order to return these to original condition, you may need to boost the lows (around 100 Hz) and the highs (around 4kHz) when recording loud rock groups. The louder the group, the more boost you need. It also helps to use cardioid mics with proximity effect (for bass boost) and a presence peak (for treble boost). [Bartlett, Practical Recording Techniques, 2005]