Fletcher Munson Curve : What is it and how it affects your mixes : Magroove Blog
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Fletcher Munson Curve : What is it and how it affects your mixes

July 20, 2019 • 6 min read

There is a lot to be aware of when mixing and mastering audio. Music is closely related to a lot of science, and some of it can get quite complex. One of the key pieces of knowledge to improve the quality of your work is an understanding of the Fletcher Munson Curve. When we’re young, we’re all taught that music travels through vibrations into the ear, and while this is true, there is much more to it than that. If you are hoping to create professional-quality mixes, one of these audio phenomena that you need to understand. It is to do with how our ears and brain perceive sound. Here, we simplified all you need to know.

Summary of the Fletcher Munson curve (and How Mixes May Sound Depending on Volume)

An overall mix will be impacted judging by the volume listen to it. It is important when you are mixing to listen both at moderate and loud volumes (don’t listen too loud for too long, though). Most people who are listening to your music will (hopefully) want to listen to the song loudly. The mix should sound good at volume, too.

In summary:

  • Low volumes mean that the mid-range sounds stand out more, as our ears can pick them out with ease.
  • High volumes bring the lows and highs into more prominence within the mix.

Magroove tip: We recommend the 90dBSPL curve for mixing and mastering. It allows you to mix and produce for long enough, and don’t have to take a break every few minutes. The curve is also flat enough so that the perception isn’t drastically out. You can assure you are around 90dBSPL using a decibel meter instrument or a phone app (less precise, depending on the phone may be just too inaccurate).

What is the Fletcher Munson Curve?

The Fletcher Munson Curve is our perception of certain audio frequencies displayed on a graph. Our perception is impacted by the Fletcher Munson phenomenon. So what does this mean? The phenomenon is a quirk of the brain. We perceive different frequencies in a song differently based upon the volume (amplitude) of the sound we’re hearing. Our ears have sensitivities in certain areas of the frequency graph.

The curve simply shows the sensitivities our ears have to certain frequencies when played at the same volume as other frequencies. To put this simply; one sound played at the same decibel volume as another may be perceived to be louder or more prominent due to the sensitivity of human hearing.

Fletcher Munson Curve Graph

How to Read the Graph

When you see the graph, it can be confusing. First, the blue and red curves are two different references displayed on the same graph. Blue is the actual Fletcher Munson Curve, red is another famous perceived loudness curve, taken from the ISO 266:2003. So take on of them and ignore the other, unless your intent is to compare existing loudness curves.

Now, to the curves! It plots the SPL (sound pressure level) against the frequency. Be sure not to read the graph upside down. The lower frequencies, displayed in Hz, should be at the left of the graph.

There are multiple curves on the same graph. These are separations of the actual volume, with 10 dB between them. This is important to show how sensitivity changes based on the SPL. So the machine or amplifier is reproducing every frequency with the same dBSPL (volume) level, but our ears experience things very differently depending on the frequency being played and whether they are being played loudly or quietly.

The trick

Pick one single blue curve from the graph. The number written on the middle of the curve is the dBSPL level all frequencies are being reproduced at. The higher points mean that we are less sensitive in that particular region of our human hearing. Meaning the volume needs to be boosted to reach the same perceived volume as the other frequencies. The curve shows how loud the frequency needs to be in order for us to perceive the same dBSPL written value.

If you look at the graph, you will also notice a dip around the 3k-4k region of frequencies. This is the resonant frequency of the ear canal. This means that our ears are most sensitive to sounds in this range. Another quirk of the graph you will notice is that when we lower the volume we hear less bass and low-end sounds. This is a further argument for the fact you should reference at different volumes in order to check your mix.

Mixing and Mastering Using the Fletcher Munson Curve

There are many relationships between the volume in dB and our human hearing. It is vital to understand these when you are mixing and mastering songs. Our ears should only be exposed to certain volumes for short bursts of time. Any longer exposure will lead to damage. For instance, a sound which is as loud as a tractor can start to cause damage to human hearing within 30 minutes. For a full graph explaining time exposure permissible before damage starts to occur, we recommend this guide. But what does this have to do with the Fletcher Munson Curve?

Mix smartly, using the curve in your favor

Well, when mixing, it is important to find a balance between loud enough, with a relatively flat hearing curve, but is also not too loud that we start to experience hearing damage or fatigue. Some people don’t realize that human ears can get tired. You are less likely to create an effective mix if your ears are tired. You can miss the detail and become sloppy. Regular breaks in mixing are always required for both the quality of your work and to protect your hearing.

As a general piece of advice, it is always best to check your mix and reference on as many speakers, monitors and headphones as you possibly can. This gives a wide variety of different perspectives on the audio, both in terms of volume and frequency response. Don’t be afraid to listen on terrible speakers once you think you have a mix you are happy with. Many people who aren’t audiophiles listen mainly through phones and car speakers.

Why We Perceive Sounds Differently Based on Frequency

It’s a curious phenomenon if you don’t understand the science behind it. There are multiple theories on our perception of sound, and even tricks you can play on yourself such as auditory illusions.

Within the 1k to 5k range of hearing, humans have an “intelligibility region”. The human voice definition is within these frequencies and this means that we have evolved in order to hear these frequencies clearly. Being able to communicate clearly and hear one another is key to human life. You can see this 1k to 5k range represented within the Fletcher Munson Curve.

This has a functional use, as many telephones are designed to only reproduce these frequencies, as it is all we need to be able to hear the other human voice. Also, the intelligibility range of frequencies serves a purpose when it comes to speakers and sound systems, too. Stadium speakers and emergency systems to provide audible warnings have to follow standards and regulations regarding intelligibility. The frequencies within which the human voice lies must be clear and audible. Standards are also in place regarding echo, to make sure the sound doesn’t become hard to understand.

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