Introduction

Our auditory system is a wondrous and complex array of physical, neurological and psychological constructs, and hardly anything about it is simple. But one of the most difficult and confounding aspects of our auditory system, when we try to make recordings, is what we call the Equal Loudness Contours (also referred to, sometimes, as Fletcher-Munson Curves, also Phon Lines).

These contours are all the more bedeviling because their behavior is so smoothly masked by our consciousness. Mostly, we aren‘t really aware of their existence. But in the recording biz we ignore them at our peril. They have a profound effect on the frequency response of our recordings and their existence calls into question the very possibility of ever having truly accurate recordings, especially when working with multitrack techniques.

Equal Loudness Contours

What are the Equal Loudness Contours? Basically, they are curves that are the inverse of frequency response curves, so that they are literally curves of, well, equal loudness. When Fletcher and Munson did the first ones (in the ‘30s), they started with 1 kHz. at some known sound pressure level as a reference, and measured all other audible frequencies in comparison to 1 kHz., determining at what sound pressure level each frequency sounded “equally loud” as 1 kHz. The resulting curve is known as (duh!) an equal loudness curve, and it is, as I said above, the inverse of a frequency response curve. Whew! If the low frequency response of a system falls off, then it needs greater level at low frequency to provide “equal loudness.” Got it? Not a biggie.

What’s all the fuss about? Why are we discussing this?