Magnetic MM or MC phono cartridge, is “velocity sensitive” device. It means that cartridge electrical output is proportional only to it’s stylus tip velocity, with which it tracks LP groove, and not the frequency.

That’s why manufacturers declare cartridge output voltage at 5 cm/s RMS velocity and don’t include any frequency. It will have this rated output at all frequencies within it’s usable bandwidth, provided velocity will stay constant. Let’s find out how this velocity relates to amplitude of a record groove. From a simple equation for motion that follows a sine wave:

Follows a derivative of velocity at time t  being:

Which at 0⁰, when peak velocity is reached, becomes:

Now, let’s find out what amplitude has a standard 1kHz groove at 5 cm/s RMS:

So, for a 300Hz tone at the same 5 cm/s RMS, this groove has to be:

This means, that cartridge tracking 1kHz groove with 11.2 μm wide amplitude, will output the same voltage, as 300Hz groove with 37.52 μm amplitude. Sounds simple enough ? But there is a catch. We are feeding this cartridge voltage to our phono preamp, which has RIAA equalization. There is no problem for 1kHz, as this is unity gain EQ frequency, but for 300Hz – there will be 5.48 dB boost applied. To retain the same 0dB level, we must compensate by lowering velocity:

And the resulting groove amplitude becomes:

This is our 0dB reference for 300Hz after RIAA equalization. So, what about HFN record +12dB 300Hz track with 45.5 μm groove?

That’s far out man… (pun intended)