What Is Hz?
Hz = cycles per second. Doubling the frequency raises the pitch by one octave. Your ear hears ratios, not differences, which is why synths use logarithmic scaling.
Cycles Per Second
When a speaker cone pushes air forward and pulls back again, that's one cycle. If it completes 440 of those cycles every second, we hear the note A above middle C. The unit is named after Heinrich Hertz: 440 cycles per second = 440 Hz.
A low bass rumble might be 40 Hz, the cone lumbering back and forth 40 times a second. A piercing whistle might be 4000 Hz: four thousand tiny vibrations every second. Human hearing spans roughly 20 Hz to 20,000 Hz, though most musical content lives between 100 Hz and 4000 Hz.
Octaves and Ratios
Here's the key insight: going from 100 Hz to 200 Hz sounds like the same interval as going from 1000 Hz to 2000 Hz. Both are a doubling, one octave. But on a linear scale, the first jump is 100 Hz wide while the second is 1000 Hz wide.
Your ear perceives pitch as ratios, not differences. That's why every synthesizer, every piano, every musical scale is built on multiplicative relationships. The twelve notes of an octave divide a 2:1 frequency ratio into twelve equal multiplicative steps, each a factor of 21/12 ≈ 1.059.
The demo below shows the same set of frequencies on two scales. On the linear scale, octaves bunch together at the bottom and spread apart at the top. On the log scale, octaves are evenly spaced, matching how you actually hear them.
Notice how the linear scale compresses low octaves and stretches high ones. The log scale shows even spacing, matching your perception.