Fletcher Munson Curve - Simplified
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Fletcher Munson Curve
The Fletcher-Munson curves are a set of experimentally established graphs that show how loud a sound at one frequency must be to be perceived as equally loud as a sound at another frequency.
You must understand the Fletcher Munson Curve If you want to get better at mixing.
It is a graph that illustrates an interesting aspect of human hearing as the hearing perceived by our ears is non-linear.
As the actual loudness varies (while listening to music on your studio monitors or headphones), the perceived loudness our brains hear will change at a different rate, depending on the frequency.
At lower listening levels (for example 50 dB SPL), Mid-range frequencies sound more prominent to the human brains, while the Low and High-frequency ranges seem to sound sunken. At higher listening levels (for example 85 dB SPL), the Lows and Highs sound prominent, almost the same as mid-range frequencies.
Our ears are most receptive to midrange frequencies (around 2.5 kHz – 4 kHz) at lower volume levels. Increasing volume strengthens lower and higher frequencies. This flattens out the listening curve, creating an illusion of power and clarity. It’s very difficult to hear the bass at low volume, it sounds a lot more powerful when you turn the music up. This effect is known as the Equal Loudness Contour.
History of Equal Loudness Contour
The first analysis on the topic of how the ear hears various frequencies at different levels was carried out by Fletcher and Munson in 1933. They were researchers at Bell Laboratories who illustrated, in 1933, that the human ear (and brain) perceive different frequencies in a shifting manner dependent on level.
Their measurements showed that your ear is most sensitive to frequencies in the range of 2.5 kHz – 4 kHz and that frequencies above and below those points must be louder, in absolute terms, to be perceived as being of equal loudness. They also showed that the amount of increase of loudness in those other frequencies to achieve that perceived equality varies depending on what the overall SPL (Sound Pressure Level), or sound intensity, is in the first place. This analysis was done by measuring people’s subjective experiences.
The original Fletcher-Munson curves have since been superseded by a series of generic Equal Loudness Contours like the modern ISO 226:2003 which better represent human hearing as a whole.
This is one of the reasons why traditional, voiceband or narrowband telephone calls limit audio frequencies to the range of 300 Hz to 3.4 kHz. Our ears are more sensitive to the Audio Frequency range between 2.5 kHz – 4 kHz.
Picture Attributes: By Lindosland at en. Wikipedia – this image was created by Lindosland Dec 2005 using OpenOffice Draw, Public Domain, https://commons.wikimedia.org/w/index.php?curid=16477782
Definition of Terms
SPL
On the left-hand side of the graph, you can observe the Sound Pressure Level increasing in Decibels(dB) from -10 dB to 130 dB vertically. The Sound Pressure Level (SPL) is the intensity of the sound waves hitting your ears.
Frequency
On the bottom, we have the Frequency varying from 10 Hz to 100 kHz horizontally. (However, we only look into the Audible Frequency Range, which is from 20 Hz to 20 kHz)
Phon
It is a term used to describe the loudness of a sound as felt by us humans. It is different from SPL. SPL measures the actual intensity of the sound while the Phon tells us how loud we feel that the sound is.
Fletcher Munson Curve illustration
You make a person who has a ‘good ear’ sit down in a very quiet place.
Play a sound of 1000 Hz at a 20 dB level. You call this level of loudness 20 Phons. Ask him/her if he/she can hear it.
Play a sound of 200 Hz at 20 dB and ask him/her whether the second sound feels as loud as the first sound.
If he/she says no, then you slowly increase the volume (dB) of the second sound until he/she says that the 200 Hz sound feels as loud as the first sound. Now you note down the dB, say, 35 dB at which he/she felt this ‘Equal Loudness’.
Play a sound of 100 Hz at 20 dB and ask him/her whether the third sound feels as loud as the first sound.
If he/she says no, then you slowly increase the volume (dB) of the third sound until he/she says that the 100 Hz sound feels as loud as the first sound. Now you note down the dB, say, 46 dB at which he/she felt this ‘Equal Loudness’.
So to conclude, to feel a loudness of 20 Phons, 1000 Hz should be at 20 dB, 200 Hz should be at 35 dB & 100 Hz should be at 46 dB
You make the same person sit down in a very quiet place
Play a sound of 1000 Hz at 60 dB level. You call this level of loudness 60 Phons. Ask him/her to take note of the loudness he/she felt.
Play a sound of 200 Hz at 60 dB and ask him/her whether the second sound feels as loud as the first sound.
If he/she says no, then you slowly increase the volume (dB) of the second sound until he/she says that the 200 Hz sound feels as loud as the first sound. Now you note down the dB, say, 65 dB at which he/she felt this ‘Equal Loudness’.
Play a sound of 100 Hz at 60 dB and ask him/her whether the third sound feels as loud as the first sound.
If he/she says no, then you slowly increase the volume (dB) of the third sound until he/she says that the 100 Hz sound feels as loud as the first sound. Now you note down the dB, say, 70 dB at which he/she felt this ‘Equal Loudness’.
So to conclude, to feel a loudness of 60 Phons, 1000 Hz should be at 60 dB, 200 Hz should be at 65dB & 100 Hz should be at 70 dB.
This trend of low and high frequencies having to need a lot of boosting in SPL to achieve equal loudness is more noticeable in lower loudness levels. In the higher loudness levels, 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
Why do we need the Fletcher Munson Curve?
One of the most significant aspects of a Good Mix is obtaining the ideal balance of frequencies most pleasing to the listener. But how are you supposed to do that when the perceived balance of frequencies changes as the volume changes?
Suppose you were mixing your new song at a very low SPL. You finished the mix and exported the file for consumer listening. One of your friends is now playing your music in his HiFi system. Your soft-melodious song sounds so good when he plays it at a lower volume.
But when he turned up the volume, it started to sound harsh. WHY?
This is because, at the time of mixing, you were hearing the song at a low SPL (I’m talking about the Sound Pressure Level produced by your monitor speakers/headphones, not about the Volume of your DAW fader or the Audio file itself) Due to the Fletcher-Munson Effect, the low and high frequencies had to be boosted to have an equal-loudness effect at lower SPL. So during mixing, you had erroneously boosted the levels of low-frequency sounds (Kick drum, Bass guitar etc) and also increased the levels of high-frequency sounds (Shakers, Hi-hats, Cymbals etc)
Now when you hear the music at a low volume, you’ll hear the song soft and pleasant – just almost the same as you heard during mixing.
According to the Fletcher Munson graph, the low and high frequencies do not need much boosting at high volume levels. So when you turn up the volume, the low and high frequencies which have already been boosted will be louder than required. Therefore you’ll find the song sounding harsh.
The solution to this problem is to mix the song at a higher SPL so that it sounds soft even at high volumes. And when you want to hear it at lower volumes, the ‘Loudness’ controls of a HiFi system can be used to manually create the ‘Equal Loudness Effect’.
Rule of the thumb is “For your mixes to sound their best at high volumes, you need to EQ it at a high SPL”
Note: This does not mean that you have to adjust the volume fader of your DAW. DO NOT DO IT. It will change the overall loudness of your audio file. Instead, adjust the volume knob of your monitor speakers/headset.
What monitoring level do you need to set?
You need to get the most accurate response from your monitors in your studio every time you go to mix or master a track.
An SPL Meter is required to measure the acoustic sound pressure level (SPL) generated by your monitors. You’ll need an SPL meter with a C-weighted filter option, which is flatter than the A-weighted response which is commonly used for general measurements. The SPL meter will also need a ‘slow’ or ‘averaging’ mode. SPL meter is not very expensive. Interestingly, some smartphones also have an app to measure the SPL level.
Now you need to work out at what volume you want to listen to your audio in your studio. An ideal SPL amount would be 85 dB as this level is close to the more flat portion of the Fletcher Munson Curve (refer to the picture above).
But this figure is very loud and was intended for larger spaces such as a cinema. Most home studios are much smaller and an 85 dB SPL will be extremely loud and it may damage your ears. You need to protect your ears as well. So for small home studio rooms, 70-76 dB SPL C is good enough. A good rule of thumb is that your volume level should be low enough to allow for conversation without raising your voice. If you need to shout to be heard, your monitors are too loud.
You can also utilise this new monitor level to find a comparable level in your headphones. It’s very hard to correctly estimate loudness on headphones as you can not aim your standard SPL meters at headphones for SPL measurements. Hopefully, as technology advances, there will be in-ear SPL monitoring systems in the future. Who knows?
This also means that no matter how good your Track is Mixed and Mastered, different people will perceive your audio differently. Recordings sound best at the same volume at which they were mixed. Play a CD too soft and you’ll lose the lows and highs, too loud and it’ll sound hyped and unnatural. Sadly for us, there is no ‘perfect’ volume to mix at in order to be able to monitor all frequencies equally. That is why we need to take note of the Fletcher Munson Curve as the graph represents an average value taken from the listening experiments conducted on hundreds of people. Audio Engineers
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