How To Use The Fletcher Munson Curve (A Simple Explanation)

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  • What is the Fletcher Munson curve?
  • Understand equal-loudness contours
  • Apply these concepts to your mixing toolkit

What Is The Fletcher Munson Curve?

Often referred to as equal-loudness contours, the Fletcher Munson Curve is related to physics and the way that the human ear responds to different frequencies. The Fletcher Munson Curve denotes the relationship between sound pressure level and frequency.

Although not as fun as learning a new plug-in or a new DAW, it is still an essential part of audio theory that you will need to be familiar with in order to get the best from your mixes.

What Is Perceived Loudness?

In order to understand how the FMC relates to our ears when mixing then we need to understand the perceived loudness of what we are listening to.

Our ears respond to different frequencies in different ways and at different volumes.

The interesting thing about the Fletcher Munson Curve is that although our ears hear tonal changes to sounds at different volumes, the sound actually remains the same, it’s the behaviour of our ears that provide a different audio quality, and different ‘loudness’ of certain frequencies.

Have you ever heard a producer suggest mixing at low level or checking your mix on laptop speakers?

This can be beneficial because our ears will pick up on different frequencies depending on the volume of the track and tone of the speakers.

For example, low or sub frequencies will be heard most clearly at higher SPL levels whereas at low volumes our ears will be more in tune with mid-range frequencies.

So this is where the Fletcher Munson curve comes in.

The Fletcher Munson Curve denotes the relationship between sound pressure level and frequency.

The Problem With Fletcher Munson Curves

Much like an untreated room or badly calibrated monitors, the Fletcher Munson Curve can understandably cause problems when it comes to your mixing process.

It’s all very well simply stating that sub and lower frequencies are more noticeable at high volumes, but let’s say you aren’t able to listen to your mix at a loud volume.

This could easily cause you to overcorrect your EQ and add more low end which, when played at a more suitable volume will be too overpowering.

By looking at the FMC graph we can roughly see how our ears are going to react to certain frequencies at certain levels and mixing is a case of trying to find tonal balance across the full spectrum regardless of the frequency content.

In reality this proves tricky, after all it’s difficult to mix a track intended to be listened to loudly if we can only monitor at low levels.

Think of modern dance music as an example.

Typically these contain a large amount of low end and sub frequencies so it’s no surprise that dance music is often played loud – you don’t get the same sense of power when listening on your phone speaker or quietly in the background.

Mixing At An Ideal Volume

The general consensus is that somewhere between 75-85dB is a good SPL to mix at as our ears are most responsive to the widest range of frequencies at that point.

Unfortunately for us, there is no ‘perfect’ volume to mix at in order to be able to monitor all frequencies equally. As we’ve mentioned it can be useful to listen on varying speakers at different levels to obtain consistency (in fact some producers prefer NOT to mix on top of the range monitors as most music consumers won’t have top of the range speakers).

In an ideal world, we would be able to create mixes that sound consistent regardless of what speakers or headphones the mix is played on. But as this proves very difficult we can use the Fletcher Munson Curve to our advantage and try and find the most suitable volume to monitor at.

The general consensus is that somewhere between 75-85dB is a good SPL to mix at as our ears are most responsive to the widest range of frequencies at that point.

Of course, another added complication is that we are still talking about PERCEIVED volume. Whilst we measure sound in decibels, what one person hears can be different in volume to what another person hears. We aren’t talking vast differences but this is one of the main reasons there isn’t a quick fix, ideal volume to monitor at.

It’s also important to keep in mind the issue of ear fatigue when monitoring at high levels.

While you may find mixing at 80dB is great for creating a consistent mix, spend too long at these levels and your ears will become fatigued and you’ll start to over or under correct your mix.

For this reason it is always worth taking time out from a mix, or even breaking it down into different sessions to avoid making mistakes.

The ‘Smiley Face EQ’

The Smiley Face EQ is a tried and tested technique to improve your mixes on certain instruments (often ‘low end’ instruments such as a kick drum).

The idea here is to remove some of the mid frequencies in your instrument and boost the lows and highs.

By doing this you can see where the technique gets its name from – the parameters on a graphic EQ actually look a little like a smiling face!

The same principle can be applied using a digital EQ plugin:

Generally this is a quick and easy technique to add hifi ‘sheen’ and remove ‘boxiness’ to a kick drum.

Although if used incorrectly, it’s also a quick way to remove low-midrange clarity, creating a hollow and ‘gutless’ kick drum.

Hence, it is worth spending time fine-tuning your parameters and finding where the power of the kick drum lies (usually around 50-75Hz).

So how does the Fletcher Munson Curve relate to this technique?

By removing the mid-frequencies we are taking out the frequencies that we are most responsive to at lower levels.

Due to the increase in low-end the sound will actually appear more powerful and bassy to our ears meaning there is no need to turn up the kick drum and therefore we are able to make tonal changes, rather than volume changes, to enhance the sound.

This technique is also a great example of using subtractive EQ, ie removing frequencies, to improve a sound. Sometimes less is more.

Of course it is imperative to ensure that the kick drum is in phase with the other microphones on the kit (particularly if mic’ing with a ‘kick in/kick out’ setup) otherwise you’ll be working with a reduced low end to start with.

Try the Smiley Face technique and add a touch of compression with a medium-long attack time to increase the power of your kick.

You’ll also find this can work wonders for snare drums, by accentuating the lower frequencies (where the power of the snare lies) with the high end (where the ‘crack’ of the snare lies) the result is a much more powerful sound that you often hear in rock or metal.

Approaching Your Mix

Hopefully, now you have a better understanding of how our ears react to sound when mixing. The Fletcher Munson Curve is useful to keep in mind when mixing, but remember you don’t need to memorise the graph to get good results, just be aware of how it works.

Next time you start a mix, keep in mind how your ears are reacting to what you’re hearing.

  • Are you mixing quietly but noticing a lack of low end?
  • Is there too much mid-range at a higher SPL?

Approach your mix with all this in mind and you’ll start to see big improvements in the finished product!