The equal-loudness contour
There is a particular quirk/flaw in the human hearing that is very good to know about in mastering.
Maybe you have stumbled upon this when mastering (or mixing) to loud or to silent and when adjusting the volume some frequencies has been to exaggerated or subdued. This is because the human hearing isn't flat. Our perception of sound pressure is different over the frequency spectrum and it differs depending on sound pressure.
For example when listening to a 80dB sine wave at 100Hz it is perceived as 10dB lower than it actually is. When listening to the same frequency at 40dB it is perceived as 20dB lower than it actually is. That’s a 10dB decrease of perceived sound pressure and at 50Hz that number almost doubles. I made an infographic (at pinterest) of the difference of the hearing threshold and 100dB.
The equal-loudness contour and Fletcher Munson curves
The equal-loudness contour is a measurement of human perception of sound pressure over the frequency spectrum. The research is based on the similar Fletcher-Munson curves from 1933 and is often referred to the Fletcher-Munson curves.
By human perceived equal loudness of sine waves at different frequencies and sound pressure levels (dB).
To get around this you have to master on a consistent and comfortable level. If not you will perceive the mix in different ways every time you change that volume. This will make you approach the master in many different ways and you will even counteract your own changes.
If you work in a minimalistic way this is easier to handle though if you use many instances (plug-in/outboards) the counteractions will be all over the place. This can invite phase-, loudness-, clutter- and ringing-issues. A good minimalistic first approach to every master is Eq, compression, limiter. Keep it simple.