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The Hammond Organ and Additive Synthesis

by Jo Kennedy

On Wednesday 18 November 2015 I took part in YSWN’s workshop on the Hammond Organ and Additive Synthesis. A small group of us, having successfully battled public transport, the M62 and the elements, were warmly welcomed at Valley Wood Studio by Katherine. Here, residing in the basement, is a vintage Hammond M100 organ and its accompanying Leslie 145 speaker, both much loved by our tutor for the afternoon, Barkley.

My interest in attending this workshop came from having recently started a music degree and choosing a first-year module in computer composition. I’ve not got a music technology background so the opportunity to spend a couple of hours understanding additive synthesis from first principles, in a small group, with a friendly expert to hand sounded too good to miss.

It turns out you don’t need a computer and a space-age looking user interface to do additive synthesis – Mr Hammond did it in 1935 with cogs, magnets, wires, precision engineering and mains electricity, and took his inspiration from an even earlier, much more unwieldy organ-like machine invented in the 1890s called the Teleharmonium (think steampunk and you get the picture).

The Hammond Organ and Additive Synthesis

Unfortunately, there are no Teleharmoniums still in existence but the principle behind both instruments is the same – they make sound ‘synthetically’ by generating electrical currents that are then converted, via amplifiers and speakers, into sound in the form of simple sine waves. Depending on what type of sound you’d like, you can combine several sine waves together in whatever proportion you like (hence the term additive synthesis) to get a specific timbre.

So far so good? At this point in the afternoon I had to take a moment to think how this compares with what is going on in my computer when I’m using a Logic synth. The difference is Teleharmoniums and Hammond Organs generate sine waves using moving parts – they are electromechanical devices, whereas my computer synth makes a sine wave using an electrical oscillator – probably only a few millimetres in size. But beyond that the principle of additive synthesis is the same.

Back to the workshop… and going into a bit more detail, we learnt that a Hammond organ makes its sine waves using a tone generator. When you switch the organ on and it whirrs and crackles into life, inside, a long axle with lots of different sized cogs – called tone wheels – starts spinning round (and keeps on spinning until you turn the organ off). When the teeth of any of these cogs are allowed to make a connection with a metal plate an electromagnetic current is generated.

The frequency at which the cogs on any given wheel touch the plate corresponds to a particular frequency of sine wave. So, with a spinning axle crammed with enough wheels at enough sizes you can generate a sine wave corresponding to every fundamental tone for several octaves of the equal temperament scale, along with the first few upper harmonics for that tone.

To make the electrical connection happen and the sine wave sound on a Hammond – i.e. to get the cogs to touch the plate – you press down the correct key on the keyboard for the note you are after – e.g. Middle C, and pull out one or more of a set of drawbars located above the keyboard.
If you just want to hear the fundamental – i.e. the single sine wave vibrating at 261.60Hz (the frequency of middle C) you only pull out the first drawbar, and the tone wheel corresponding to that frequency connects. By pulling out more drawbars you allow other tone wheels corresponding to the upper harmonic of middle C to connect too – resulting in several sine waves being sounded together.

Barkley gave us short renditions of A Whiter Shade of Pale by Procol Harum and Green Onions, by Booker T. Jones – two tunes which famously use the Hammond. And there I was… back in my youth!

I’ve not mentioned the special relationship the Hammond Organ has with the Leslie speaker. Suffice to say, Mr Leslie designed and built the speaker after getting his newly purchased Hammond home and being dismayed at the sound quality compared with the showroom. Hammond wasn’t too pleased about Mr Leslie’s intervention but the speaker caught on with the buying public and has been used in combination with the early Hammond organs ever since.

Turn it on and a lovely black horn shaped speaker starts revolving in its top portion whilst in the lower half of the cabinet a bass speaker starts to billow, like it’s slowly breathing.

I know all this equipment is old and big and heavy and goes wrong, and needs frequent oiling, but there is something strangely satisfying about physically pressing down a key, or flicking a switch, or turning a real (as opposed to virtual) knob and knowing you have initiated an actual mechanical process that is going to result in something quite tangible happening.

Thanks to Barkley and Katherine for such an informative and friendly event!

The Hammond Organ and Additive Synthesis 1

More about drawbars and harmonics

The big consoles – A,B and C series organs – have nine drawbars.

The M here has nine for the upper register keys and eight for the lower. Drawbar eight on the lower register is actually two harmonics combined; it has a little red dot to indicate.

This is the regular system:

  • Drawbar One (brown) is the Sub Fundamental
  • Drawbar Two (brown) is the Sub 3rd Harmonic
  • Drawbar Three (white) is the Fundamental
  • Drawbar Four (white) is the 2nd Harmonic
  • Drawbar Five (black) is the 3rd Harmonic
  • Drawbar Six (white) is the 4th Harmonic
  • Drawbar Seven (black) is the 5th Harmonic
  • Drawbar Eight (black) is the 6th Harmonic
  • Drawbar Nine (white) is the 8th Harmonic

So brown drawbars are the subs or bass harmonics, white drawbars are the even harmonics and thus just octaves above the first white fundamental, and black the odd harmonics. The first white drawbar pulled out will equate to the actual tone in relation to key number.

Additional information kindly provided by Valley Wood Studio