Summary:

The Fletcher-Munson graph says that at stronger sound pressures, the ear's ability to perceive is leveled off in the way that we perceive all frequencies as equally strong and strong sound the experience becomes richer, i.e smaller red areas in the diagram. The treble is roughly the same regardless of loudness.

IMPORTANT! Each curve shows the threshold for how weak sound an ear can perceive at a loudness*). So frequencies below the curve (red areas) are sounds (frequency) that we cannot perceive at a given loudness*).
*) The reference loudness of the curves is defined at 1 kHz (yellow arrows).

At 80 dB loudness curve we can see all frequencies as equally strongears (green area).The treble is roughly the same regardless of loudness.

What level shall I mixing & master at to get it right?

One rule is to start level around 80 dB, the green area in the graph above. At that level 80 dB loudness the ear respond it’s not quiet, but not loud either. It’s the best level for hearing the frequency spectrum as balanced as possible.

But we listen to music at different volumes has to be taken, that make it difficult and many compromises. - We often listen to a lower volume at home in the sofa.

Test your ears with CubaseINDEX online Tonegenrators! ==>HERE

For those who want to know more:

How to read Fletcher-Munson complex diagram? It looks too far complex for my eyes. - Don't worry, I shall explain the diagram easily.

First, the curves in the diagram show the threshold what the ear can perceive at different loudness, and also the frequency ranges with different loudness.There are 12 curves in the diagram represent different sound pressures measured at 1 kHz, the weakest at 0dB and the strongest at 110dB.
This is very important to know and to understand for sound designers - that our ears are frequency perception is different depending on loudness.

To read the graph, remember you should look only at one curve at the time!
For example, you want to know the ear frequency perception at loudness*) at 50dB at 100Hz and 10kHz. Find the curve that hit 1 kHz at 50 db and then follow the curve to the left 100 Hz gives 18 dB (red area) i.e. the bass requires stronger sound pressure +18db compared to a freqency of 1kHz. To right 10 kHz treble +10dB compared to a freqency of 1kHz (red area).

IMPORTANT! Each curve shows the threshold for how weak sound an ear can perceive at a loudness*). So frequencies below the curve (red areas) are sounds (frequency) that we cannot perceive at a given loudness*).
*) The reference loudness of the curves is defined at 1 kHz (yellow arrows). 

At 80 dB loudness curve we can see all frequencies as equally strongears (green area).The treble is roughly the same regardless of loudness.

1.Now, we start with to find out what the weakest sounds we can hear. Find the curve in the bottom, i.e. hit 0 dB at 1k (lowest blue arrow). Below this curve there are red areas, these red areas below 0dB curve is sound which is too weak for ours ears to hear.

To read out the weakest sounds a human can perceive is to find the area below the curve who are not red. We see a tiny white area nearly out of the diagram who is not red at about 2500 Hz to 3200 Hz.

We have now found out, what the weakest sounds we can hear at sound pressure level at 0 dB, and also which frequency ranges the ears are most sensitive for 2.5 to 3.2 kHz

2.Next we shall read out how much we need to increase loudness to hear a wider frequency band than 2,5 to 3.2 kHz for example 100 Hz to 10k.

From above example we find the curve in the bottom i.e. hit 0 dB at 1k (lowest blue arrow).

If we're looking to the left to 100Hz and see the volume must be increased to 40dB the read area goes up to 100Hz (left side Y-axis at 0dB). To hear 10k we need to increase volume about 18 dB.

We have found out at a volume of 0dB, and to perceive 100Hz the loudness must be increased to 40 dB (left side Y-axis at 0 dB) and to hear 10k we need to increase loudness about 18 dB.

3. Next we shall read out how the ear's frequency ranges with different loudness. When we increase the loudness to 50 db or louder at 1k we will notice a  "secrets" about ears perceive frequency at different loudness.

At in above example, we read out that we need to increase loudness 40 dB to hear 100 Hz at 0dB loudness, now we shall see what we get at 50 dB loudness. 
Find the curve, hit 50 dB at 1k (middle blue arrow). Below this curve there are red areas, these red areas below 50dB curve is sound which is to weak for ours ears to hear.

If we're looking to the left and at 100Hz the loudness must be increased to 18 dB the read area goes up to 100Hz (left side Y-axis at 50dB) and to hear 10k we need to increase loudness about 10 dB.

When we increase loudness from 50 to 80 dB, we see that 100 Hz needs only to increase by 4dB, compare to 18dB for 50dB loudness, and 40dB at 0 dB loudness for ours ears to hear. The treble is roughly the same regardless of loudness, therefor treble is a minor issue.

4. If I simplify the meaning of the diagram for a studio technicians and others who work with sound design, can we say in summary that a stronger sound pressures (loudness) the ear's ability to perceive is leveled off in the way that we perceive all frequencies as equally strong. Precisely that we perceive a wide frequency band is used in mixing, mastering and PA in the arts, i.e. that with strong sound the experience becomes richer.

The disadvantage and the danger is that the ear can only withstand a short time with strong sound which, in its extension, can cause permanent hearing damage. (See the article "Take care of your ears" from Sounonsound below!

In addition, we often listen to a lower volume at home in the armchair. The fact that we listen to music at different volumes has to be taken into account by mixing and mastering, which makes mixing and mastering very difficult. For example, we often listen to a lower volume at home in the armchair.

The question is for whom to listen and at what volume. Many use a sound pressure at 80-85 dB the green field in the diagram. Unfortunately, there is no exact answer, each music genre has its optimal mastering level, often it becomes a compromise.

I hope I explain the Fletcher-Munson diagram so that can give some help with your sound design work.

Jan Karqvist

Test your ears with CubaseINDEX online Tonegenrators!  ==>HERE