Some pitches sound louder than others.
Displayed are equal-loudness curves. Depending how familiar you are with isoclines, they might mean the opposite of what they look like to you. Where the curve dips down, that means that instruments playing that pitch sound louder for their deciBel level.
It’s crazy how non-monotonic these curves are. It’s like there is a general pattern that higher-pitched instruments sound louder per deciBel, but there are many pitch ranges where the pattern reverses. Strange, right?
For reference, here are some of the pitches in that 12k-20k range where some of the curviness happens.
Reminds me of the Lab colour scale — a nonlinear scale which is attuned to how we humans perceive colour, which differs from the linear scales (RGB, CMYK) along which colour “actually exists”.
Acoustic science, you just blew my mind.