Understanding Fletcher–Munson Curves to Improve Mixes and Masters

There's a saying, 'Music sounds better when it's loud'. Well it's true, but not in the way you may think.  Fletcher-Munson Curves, more commonly known as the Equal Loudness Curves, were discovered by Harvey Fletcher and Widen A. Munson. The discovery was from an investigation with the aim of finding the cheapest way to broadcast a telephone call. What Fletcher and Munson Discovered What Fletcher and Munson discovered was how humans perceived the volume of different frequencies.Play the two audio examples below focusing on the volume of each. 1kHz sine wave 3.5kHz sine waveWhat you should notice is the 3.5kHz sine wave sounds louder than the 1kHz sine wave. However, they're both the same reference level. This goes back to the point of humans not perceiving frequencies at the same volume level.  The frequency we're most sensitive to is the 3.5kHz tone. This is due to the ear resonating at that frequency. For example, if one was to blow into an ear like one would blow across a glass bottle top, it would give the frequency 3.5kHz. The range of frequencies we're most sensitive to is 1kHz to 5kHz. It was this discovery that allowed telephones companies to save energy, broadcasting the most useful frequency range and, thereby, save money. It's this frequency range that gives us the telephone effect. You can re-create the telephone effect by using a band-pass filter. If you don't have the option of selecting a band-pass filter, you can create one yourself, such as the one below.Band-pass filterTo create the band-pass filter above: set the low-cut filter to 1000Hz, with a 48dB slope and Q value of 0.75high-cut filter to 5000Hz, with same slope and Q valueWith Band-Pass Filter: The Telephone Effect Without Band-Pass FilterWhat Fletcher and Munson also discovered is how poorly we hear low and high frequencies at a low volume.    Why This Matters The point we hear all frequencies at their most equal in volume is around 90dB. This is why professional mix and mastering engineers work at, or close to, a sound pressure level of 90dB. It's this equal balance of frequencies that results in a better listening experience. This is why music often sounds better loud, as 90dB is considered loud. You may wish to calibrate your studio monitors to 90dB to give you a more accurate listening experience.You may also wish to read my tutorial How to Calibrate Studio Monitors.   Mix and Mastering at Lower Levels  Mixing and mastering at the flattest frequency range point will help give you a production that translates better. However, working at 90dB can cause fatigue as it's considered to be quite loud, especially when 120dB is considered the threshold of pain. When working at lower volumes I recommend using a EQ present that adds the appropriate amount of bass and treble that appears lost at lower volumes. Some EQ plug-ins, like the Gullfoss, have a function to composite for this producing a flatter frequency range at lower listening levels.ConclusionThe Fletcher-Munson curve is a discovery that all producers should bear in mind when producing music. You need to bear in mind, however, that all music is not produced to be played at 90dB. For example, a soft piano solo. After all, there's no point getting the perfect master that was created whilst working at 90dB, if it's most likely going played at a lower volume level. Common sense is as important as theory.