ABSTRACT
Back in 1933, two researchers at Bell Labs, Harvey Fletcher and W.A. Munson, conducted one of the most significant experiments in psychoacoustics. Their experiment was based on a series of tests taken by a group of listeners. Each test involved playing a test frequency followed by a reference tone of 1 kHz. The listener simply had to choose which of the two was louder. Successive tests involved either a different test frequency, or different levels. Essentially, what Fletcher and Munson tried to conclude is how louder or softer different frequencies had to be in order to be perceived as loud as 1 kHz. They compiled their results and charted a graph known as the Fletcher-Munson Curves. A chart based on the original Fletcher-Munson study is shown in Figure 2.1. I am presenting it upside-down, as it bears a resemblance to the familiar frequency-response graphs that we see on some equalizers. A similar experiment was conducted two decades later by Robinson and Dadson (resulting in the Robinson-Dadson Contours), and today we use the ISO 226 standard (which was last revised just a few years ago). The formal name for the outcome of these studies is termed equal-loudness contours.
