ABSTRACT
In 1933, two researchers, Harvey Fletcher and W. A. Munson, at Bell Labs conducted one of the most significant experiments in psychoacoustics. Their experiment was based on a series of tests taken by a group of listeners. One important thing that the equal-loudness contours teach people is that they are more sensitive to mid-frequencies—an outcome of the lows and highs roll-off that can be seen on the various curves. The fact that people's frequency perception alters in relation to levels is a fundamental mixing issue. When listening at soft levels, people hear more of the direct sound coming from the speakers and less of the sound reflected from the walls. Sound energy is absorbed, mostly as it encounters a surface. The small amount of energy speakers emit at quiet levels is absorbed by walls to a degree that only a fraction of it reflects back to people's ears.
