If you’ve conducted any independent study into audio theory and sought to understand why it is that we hear the way that we do, it’s likely that you’ve come across the Fletcher Munson Curve. Among one of the most imperative principles to grasp in the field of audio, this particular curve can be transparently defined as how our central nervous system perceives loudness. More specifically, as the actual loudness increases, the perceived loudness adjusts at a frequency-dependent rate.
When listening to a passage of audio at low listening volumes, the high and low frequencies seem to fall behind the mid-range frequencies, causing the mid-range to appear at the forefront.
When listening to a passage of audio at high listening volumes, the mid-range seems to fall behind the high and low frequencies, causing them to appear more prominent.
This phenomenon is critical to take into account when conducting or approving a final mix. Regardless of listening level, the truth of the matter is that the overall balance of the passage of audio remains the same despite our ears adjusting to perceived loudness. Below you’ll find a Fletcher Munson Curve displaying the frequencies (Hz) across the horizontal, x-axis. The intensity (dB) of the volume measured in SPL is displayed vertically on the y-axis:
Beyond just understanding this curve, it’s necessary to pay homage to the brilliant minds behind this phenomenon; Harvey Fletcher and Milden A. Munson. In the 1930’s, the two researchers accurately measured, produced, and published a set of curves that showcased differences in the human ear’s sensitivity to loudness in relation to frequency. The conclusive result of these curves also demonstrated that the human ear is most sensitive to frequencies between 3kHz and 4kHz. If you’ve ever wondered why sibilance controllers (De-Essers) are oftentimes defaulted to affect 3kHz – 4 kHz and typically a bit higher, it’s due to the ear’s perceived harshness of this frequency range.
The Fletcher Munson Curve can also commonly be referred to as an “equal loudness contour” due to its ability to display how we can compensate for our ears heightened perceptions of certain frequencies. These contours typically range from the extremely soft level of 0 dB (SPL) all the way up to 130 dB (SPL) which is commonly known as a destructively loud level that can permanently inflict damage upon the ears when exposed to this level for only one minute and fifty-three seconds (represented by the Occupational Safety and Health Administration). In more recent years, Fletcher-Munson Curves have been updated to reflect more reliable information as advancements in technology have corrected slight deviations in contour.
So what does this all mean to you as an audiophile? From the perspective of an engineer, this principle is essential to take into account whether you’re recording or performing post-production services such as mixing or mastering. Understanding the Fletcher Munson Curve will allow you to take into account these discrepancies when monitoring back at various levels, when deciding how on or off-axis to place a guitar cabinet microphone, or when conducting the final quality control checks when mixing and mastering. Always trust your ears, but first, understand how they operate!
If you’re interested in learning about more audio techniques, consider F.I.R.S.T. Institute’s Recording Arts & Show Production program, which teaches students audio fundamentals in just 11 months.