The voice frequency
300 Hz sits in the heart of vocal territory: a typical adult female speaking fundamental is around 165-255 Hz with strong energy just above, and male voices place their first harmonics here. It is also (as D4, 293.66 Hz, give or take) squarely in singing range. The classic telephone band ran 300-3400 Hz precisely because keeping this window preserves intelligible, recognizable speech while discarding everything else.
Because so much musical energy piles up between 200 and 400 Hz - vocals, guitars, piano left hand, snare body - the region is the first suspect when a mix sounds thick or boxy. Mix engineers call excess energy here mud; a modest cut around 250-350 Hz is one of the most common EQ moves in existence.
Using a 300 Hz reference
As a test tone, 300 Hz is friendly: every speaker from a phone upward reproduces it, so it isolates level and room questions from bandwidth ones. Play it while walking around the room and you will hear it swell and dip - those are room modes, and at 300 Hz they are dense enough to blur together in most rooms, unlike the widely spaced bass modes below 200 Hz.
Sweep the surrounding octave with the sweep generator to hear where your room is uneven, or check the voice range against real speech with the mic test and spectrum analyzer.