The Independent Recording Network not only created an Interactive EQ chart for reference, but also an interactive equal-loudness curve fit with intelligent nuances that are displayed as as you dive into the interface.
Now, this curve isn’t the updated equal-loudness contour released in 2003, but it does represent the previous standard accepted by the ISO which was the Robinson Dadson Curve established in 1956.
The Equal Loudness Contour is a curve in which different frequencies are considered equally loud at specified sound pressure levels. The overview of this curve is to essentially show you at which points throughout the human hearing spectrum do we perceive different frequencies with specified amplitudes to be equally loud. (For more on the history read: More Around The Bend)
I still love to refer to this chart over others because it’s essentially a teacher in a box. I can pull additional information as I play around with the hovers on the page, or I can really narrow down to the phon where ears sense an equal amplitude among different frequencies. As opposed to other mock drawings of the curve in black and white that sometimes skip makers for SPL and phons, this chart separates the frequency spectrum into five ranges, every SPL and phon is marked for clarity, and its simply easy to follow with its multi-colored layout.
You may not refer to this chart as much as the EQ chart IRN also built, but it doesn’t hurt to walk through this chart every so often.