In the 1930's two scientists at Bell Labs named Harvey Fletcher and Wilden Munson came to the conclusion that the way humans hear specific frequencies is in fact much more complicated than anyone had previously thought. They developed the Fletcher Munson Curve, which helps us to see how frequencies are perceived. Humans do not hear the entire frequency range at the same loudness level. Furthermore; as amplitude changes, so does our ears response to the frequency spectrum. In other words certain parts of the frequency spectrum (lows, mids, highs) may seem louder to us than others, and when amplitude (volume) increases or decreases our ears hear these frequencies at different loudness levels. For example; as you turn your system louder, bass and treble sounds will increase while the mid-range frequencies will begin to get drown out. This will make more sense after we look at an Equal-Loudness Contour graph. While Fletcher & Munson were the first to create the Equal-Loudness Contour, today we use a different contour compiled from a much broader test group of subjects. The idea is still the same though, and most Equal-Loudness Contours are still refereed to as a Fletcher Munson Curves at present time.
Equal-Loudness Contour Graph
The Y axis represents Sound Pressure Level (dB SPL) or volume, in simple terms. The X Axis represents the frequency range. Phons are simply a term used to rate equal loudness. As you can see, our ears are most responsive to the middle range of the frequency spectrum. Most likely because, this is where human speech occurs in the frequency range. (score one for evolution) At lower volumes you can't hear all of the frequency range at the same level. Mids are exaggerated while bass and high frequencies are not heard as well. At higher volumes the curve begins to flatten out and we begin to hear at a flatter response than we did at lower volumes. If you're still having trouble understanding this whole idea, the following real world example may help you out. Have you ever seen the "Bass Boost" button on an amplifier or car stereo? The purpose of this button isn't to blast bass. In fact, it's actually meant to be used when listening at low volumes, and its existence can be directly contributed to the Fletcher Munson Curve. The bass boost setting is something like an EQ that amplifies low and high frequencies so we can hear them more in line with the middle range frequencies at low volumes.
If it's not obvious by now, the level your monitoring your productions or mixes at is extremely important. If you're monitoring at to low of a level (usually never a problem for most beat freaks) the mid range of the frequency spectrum is going to be louder so you're going to end up compensating for this by cranking the low and high end of your project. If you're monitoring at too loud of a level the bass is going to begin drowning out the mid range and the high end of the spectrum will be ripping through your ears. So... What's the best level to monitor at? It's widely accepted that right around 80-85 dB SPL is the best spot to work at. At this level our ear's frequency response is just about as flat as is humanly possible. The best way to ensure that you're monitoring at this level is to get a dB SPL meter (buy & return or rent one, if you're broke like me) Once you have one of these you can mark on your interface or mixer where 83 dB SPL roughly lies. That way you're always sure you're hearing everything that exists in your project.
Hopefully this whole concept makes a little more sense now. Now, go take this idea back to the lab with you. Thanks again for reading and as always, if you have any questions, don't hesitate to email me at: firstname.lastname@example.org