Above we have a graphical representation of it (blue) with the much more recent revisions (red). The revisions are based on the findings by Fletcher and Munson and are known as the “Equal Loudness Contour.”

Equal Loudness is a much more descriptive name and what these curves show is how the human ear perceives frequencies. Our ears aren’t perfect, and their shapes determine how we pick up sound. Much like how holding a tube to your mouth makes some frequencies more prominent and “honky” the tube in our ears favours certain frequencies.

These curves show the relative volumes for each frequency that we perceive as having an equal volume. You will notice there is a dip in each line between 1000Hz and 10kHz. That is the frequency that we are most sensitive to.

So even though it may be playing at a slightly lower volume than a tone at 500Hz, we hear it as the same volume.

You will notice that the graph also has many curves on it. This is because our sensitivity depends on the volume. At louder volumes, bass is more pronounced (seen by the flatter line).

To explain it better, let’s look at the bottom red line. This represents music played at the quietest volume. The lower the curve is at a frequency, the more sensitive we are to it.

So at that 1000Hz to 10kHz dip, it can be very quiet indeed to be audible, but low frequencies are very hard to detect. This means that to hear the bass, we have to boost the bass frequencies.

Compare this to the top red line, which represents loud music. The bass frequencies are a much gentler slope in comparison to the mid range, which means that the bass doesn’t have to be as loud relatively to the rest of the frequencies.

So what does this mean for mixing?