Fletcher Munson Curves (Equal Loudness Contours) Made Easy

fletcher munson curves

What are Fletcer Munson Curves (Equal Loudness Contours)?

The Fletcher Munson Curves are a bunch of curves which show how sounds of different frequencies and at different sound pressure levels have to be boosted/subdued in order for the human ear to perceive an equal loudness level across all frequencies. Therefore they are also called Equal Loudness Contours.

While trying to do some sound attenuation, I remember an audio expert telling me that unless I understand what this particular graph says, I shouldn’t be working on any sound-related projects.

Whether you are producing electronic music, doing stage mixing or even using a sound level meter, having a good grip on the Fletcher Munson curves will give a niche in have sound loudness levels under your control.

In 1933, Two scientists named Dr. Harvey Fletcher and Dr. Weldon A. Munson conducted a series of experiments trying to find out how our ears are particularly sensitive to sounds from different frequencies.

The Fletcher Munson Curves generally called the Equal Loudness Contours is actually a graph which shows the results of the experiments conducted by them.

Their research showed that the sensitivity of the human ear to pick up sound across different frequencies varied drastically.

Here’s the graph

fletcher munson curves
The Fletcher Munson Curves graph

If you are unfamiliar with the terms used in this graph, don’t worry, I’ll explain each term as we go further.

Definition of Terms

On the left-hand side of the graph, you can see the Sound Pressure Level increasing in Decibels(dB) from 0dB to 120dB vertically.

The Sound Pressure Level (SPL) is basically the intensity of the sound waves hitting your ears.

On the bottom, we have the Frequency varying from 20Hz to 10000Hz horizontally.

For starters, the frequency of a sound wave determines how bass’y or tweet’y it feels like to hear.

For example, sounds coming from a rumbling car engine or a thunder or a kick drum are usually low-frequency sounds while, the chirping of birds, hi-hats or scraping a chalkboard are usually high-frequency ones.

And then you encounter this term called the ‘Phon’. When I first heard this term, I had no idea what it meant.

It is actually a term coined by the International Standards Organisation (ISO) after they went through the findings of this research.

Phon is a term used to describe the loudness of a sound as felt by us humans. It is different from SPL in that, the SPL is objectively telling us what the intensity of the sound actually is while the Phon tells us how loud WE feel that the sound is.

Also, the Phon is calculated relative to how loud we feel a sound of 1KHz or 1000Hz of frequency is.

You can see that in the graph itself. At 1000 Hz frequency, see how the Loudness Level given in Phons is equal to the SPL given in dB.

‘Oh boy, why am I doing this?’ is probably what you are feeling now. So let’s have an illustration.

First, you make a guy who has a ‘sound’ ear sit down in a very quiet place and play a sound of 1000Hz at 10 dB level. This is your standard loudness, so you call this level of loudness as 10Phons.

After that, you play a sound at, say, 100Hz at 10dB and ask him whether the second sound feels as loud as the first sound.

If he says no, then you slowly increase the volume (dB) of the second sound till he says that the 100Hz sound feels as loud as the first sound. Now you note down the dB, say, 45dB at which he felt this ‘Equal Loudness’.

So to conclude, to feel a loudness of 10Phons,

1000Hz should be at 10dB.

100Hz should be a 45dB.

As simple as that.

Now that you have learned what a ‘Phon’ is, you a now more intelligent than 87% of Sound Producers out there!

Nah, just kidding,  every pro sound producer knows this term well.

But this I can say, now that you know what the terms shown in the graph mean you’ll find the remaining part of this article very very easy.

The Fletcher-Munson Effect

Without further ado, we’ll now look at the curves themselves!

Here on, I would suggest that you don’t see these curves as some continuous curvy lines (which may make you feel dizzy) but as individual points plotted at different stages and then connected together.

Human Auditory Curve

So, from the bottom, the first one is a dotted curve which is called the ‘Threshold of Audibility’ or the ‘Human Auditory Curve’

This curve gives us an idea of how much the minimum volume should be for a particular frequency before we start to hear it. So for example, a sound with a frequency of 500 Hz needs to be at least 5dB to be barely audible to a human ear.

And as you follow each individual curve above it, you will notice that the lower frequencies (20Hz to 300 Hz) and higher frequencies (8KHz to 10KHz) need to have a higher SPL to make the listener feel the same level of loudness when compared to mid frequencies (around 1KHz to 5KHz).

Read that again slowly if you want.

Let’s take for example the 40 Phons curve. 1000hz needs to have an SPL of 40dB to attain 40 Phons, while 100Hz needs to be at 62dB to have the same loudness of 40 Phons and also, 10000Hz needs to be at around 51dB SPL to have a loudness of 40 Phons.

This trend of low and high frequencies having to need a lot of boosting in SPL to achieve equal loudness is more pronounced in lower loudness levels than in the higher loudness levels- where the curves start to flatten out and the low and high frequencies do not need as much boosting as before to be equally loud as the mid frequencies.

This phenomenon is called the Fletcher Munson Effect and it has important applications in the field of acoustics.

The Fletcher-Munson effect in action

One such area in which the Fletcher Munson curve has gained much popularity is Sound Mixing where basically the volume levels of different frequencies are adjusted to make the music/audio sound good.

To illustrate, have you ever heard a soft-melodious song that sounds good only when you play it at a low volume and when you raise the volume high it starts to sound harsh?

This is because the person who did the mixing did it when he himself was hearing the song at a low volume level.

And how do I know that?

Well, we have learned from our graph that at low volume levels, the low and high frequencies have to be boosted to have an equal loudness effect.

So our mixing guy boosted the levels of low frequency sounds like that of a bass drum or bass guitar and also increased the volume of sounds with high frequencies like shakers, hi-hats, and cymbals.

Now when you hear the music at a low volume, you’ll hear the song soft and supple- just as the mixing guy intended.

But again, according to the graph, at high volume levels, the low and high frequencies do not need much boosting. So when you increase the volume higher, the low and high frequencies which have already been boosted will be louder than necessary.

And hence you’ll find the song sounding harsh.

The solution to this problem is to mix the song at maximum volume so that it sounds soft even at high volumes.

And when you want to hear it at lower volumes, the ‘Bassboost’ / ‘Loudness’ controls of a music system can be used to manually create the ‘equal loudness effect’.

So to conclude, having the concept of the Fletcher Munson Curves in the back of your head will help you solve many challenges related to sound processing.

Now that you have taken the pain to learn what the Fletcher-Munson curves are all about, you can do whatever you did (acoustically speaking) way better!

(Note: The sounds used to generate the Fletcher-Munson curves are pure-tones )