Think of the tone as a static load. It doesn't change. It's not that cut and dry, but a generalization. That 39Hz note in a song could be measured but it's amplitude won't be the same, all other factors being equal because the dynamic load present on the amplifier before and after that note from the dynamics of the music.
Think of it this way; your faucet on your sink breaks and the water is flowing at a constant rate (i.e. the tone), and your other faucet breaks but this one has the water pulsating while sometimes reaching the same constant rate as the other faucet , and sometimes not.
A rollercoaster is another good example. Say we have 12 hills all the same height, length, and profile. The forces exerted will be the same throughout the ride. We build a different coaster with one hill the same size as the first coaster, but all others vary in difference to this one and the forces will differ for the duration of the ride. This is rudimentary, but it gives a fundamental understanding, or a picture if you will.