[sdiy] Note volume compression in the Hammond organ

[Tom Wiltshire]

Hi All,

I’ve got a question about the Hammond organ (B3/C3, but others too) and I’m struggling to find an answer, so I thought I’d ask here to see if anyone knows.

I’ve heard it said that the way the harmonic volumes add up in the Hammond organ is not linear, and that there is effectively some compression of the sound as more notes are added. I’ve never been clear how exactly this works, and if this is true or simply a way of expressing the fact that doubling the power of a waveform doesn’t double its amplitude. So does the Hammond harmonic addition add power or amplitude?

This is important since all the tones in the organ come from the same 91 pitches of the tone wheel generator. This means that a given ‘C’ note will appear as the fundamental (8’ pitch) or one note, the second harmonic (4’ pitch) of the note an octave lower, and the sub harmonic (16’ pitch) of the note an octave higher, plus being re-used as the 3rd, 5th, etc of other notes. If you hold down a bunch of notes, you could easily get 4 or 5 copies of the same tone being fed to the output. How do they add up?

I’ve tried to find some reference that deals with this, but the only really complete document is Hammond’s original patent from 1934. I’ve tried digging through that too, but it’s 37 page long and written like a patent, not an explanation (obviously).

If anyone knows, or knows where I should look to find out without having to read 37 pages of Patentese, I’d be grateful!

Thanks,
Tom