Audio feedback affects Theremin tone?

The biggest surprise to me about the 16 Theremins I built for Ether-2010 was how much better they sounded when fully assembled and operating "stand-alone" than they did during the design / testing phase, where the audio from the Theremin was taken to my studio monitor amplifier and Tannoy Gold Loudspeakers.

There was a 'special' quality to the sound from the assembled units (which had a 38W RMS average quality IC power amp) and moderate (low compared to the Tannoy Gold's) quality speakers.. I was even asked by several people at the show whether there were 'tubes' in the Theremins.

I have a hypothesis -

I think that, when the Theremin is constructed into a good, solid housing in which the loudspeaker is also mounted, the audio from the speaker will be coupled to the antenna/s, and cause the antennas to vibrate with the audio..

Vibrating antennas will produce frequency modulation, which will be synchronous with the audio, and therefore result in 'distortion' of the audio waveform.. but as the system is a 'closed loop' Audio distortion will be fed back to the antennas , giving an extremely complex relationship.. Also, the acoustic charactaristics of the housing (resonances of the wooden box etc) will directly affect the tone quality of the signal from the Theremin due to the amplification / attenuation of specific frequencies.

If I am right, it might just be that all our attempts to emulate one of the classic Theremins which had its own, built-in loudspeaker, is doomed to failure - as the tone qualities would depend on factors such as the volume the Theremin was being played at, the mechanical construction, loudspeaker etc,
.. We could not hope to get close, unless we put loudspeakers back into Theremins!

I'd have to agreee with your hypothesis.

We certainly know that amp/speaker choice can have a profound effect on theremin sound. It's not much of a stretch to conclude that mechanical coupling between a theremin and it's loudspeaker and cabinet could introduce other potentially interesting modulations and resonances.

The difficulty would lie in getting a controlled, consistant result and one that most people would enjoy.

Pardon my ignorance but what does, "all our attempts to emulate one of the classic Theremins which had its own, built-in loudspeaker" refer to? Some of us may not have been privy to those conversations.

Hi Jeff,
[i]"Pardon my ignorance but what does, "all our attempts to emulate one of the classic Theremins which had its own, built-in loudspeaker" refer to? Some of us may not have been privy to those conversations."[/i]

You are right, my apologies.. A while ago I posted on the matter of recording (for posterity as well as other reasons) the sound(s) from old EMI Theremins played through their original amplifiers / speakers.. This led to some email exchanges with a (tube) Theremin designer and the owner of an EMI system - There are commercial interests, so none of this has, or can be, placed in the public domain at present. Suffice to say - I have been trying to achieve the EMI sound through additive analogue synthesis, based on a few samples.. For me, it is part of my larger goal of making my Theremin capable of producing any waveform which can be composed from the first 16 harmonics.

Getting an EMI waveform, complete with dynamic shape variation, is not too difficult - However, it sounded 'empty' compared to the EMI - even though the waveforms were quite comparable.

At the show, I heard a remarkable individual play "Hebrew Melody" along with a film of Clara playing this piece - and the sound from my cheap pitch-only Theremin was the closest to the "Claramin" that I had ever heard.. The player had spent a few hours with the 16 Theremins, and got to know their individually preset tones and characteristics, and had gone to the Theremin which he felt best suited what was being played.

It was this which caused me to think that I was missing something obvious.. I realized that the improved tonal difference between what I had heard through my studio monitors and the lower quality amplifier / speaker in the integrated Theremin (the latter sounding better) may relate to audio feedback via the antenna – and that many of the best sounding Theremins (Some EMI’s, Moog 201, 91 series, and Vanguard for example) had speakers built into the same cabinet which supported the antenna.

Below I paste what I wrote on LevNet about what was, for me, the highlight of Ether-2010:

---------------------------------

One event in particular happened which I must share.. I believe I might have met someone who could easily become the worlds greatest Thereminist..

The chaos of the event was awful - we never knew what was going to happen one minute to the next .. Films were shown throughout the day (from the Ohm+ collection) and when a film started, I would run to each Theremin and switch it to headphones.. I did not want to be distracted by obscene noises while some virtuoso performance was occurring (alas, some other exhibitors did not share my reverence, so often the film sound was drowned out by noise anyway)

I was getting myself a coffee, when a film clip of Clara playing "Hebrew Melody" started.. I went to silence the Theremins (the piece was mid-way by the time I got to them) when I heard someone playing along with Clara absolutely note perfect, on a Theremin which sounded just like a Claramin.. There was only one person playing one of my Theremins, and his technique was far from standard.. he was using both hands, one as a 'bias' and the other for fine articulation and vibrato.. but, apart from dynamics (my half Theremins had no volume control antenna) his performance was breathtaking.

When the piece ended, I congratulated him and asked how long he had been playing.. "Oh, I came here a couple of days ago, and played with each of these Theremins - I really like this one.." He had absolutely no idea that what he had done was astounding - no notion that the Theremin was a difficult instrument to play "I know that tune well" he said.. "If I know a tune, I can play it on anything"..

I gave him my card, I gave him Lydia's contact details, I bought him a drink and we chatted for an hour - I hope he shows up again - But he struck me as a free-spirit.. more interested in traveling

Er, EMI = RCA?

(Another) good question. In nearly ten years, I've never heard of an EMI theremin. A Google search brings up no mention of such a model; only electro-magnetic interference and the record label (EMI).

I have been thinking about ways to test Fred's hypothesis.

My first thought was to rush to the cupboard and dig out my soundbug (http://en.wikipedia.org/wiki/SoundBug) - but halfway there I remembered that both my theremins are away for modification at the moment, so I can't test it myself. Perhaps someone else has one?

However, a little online research showed that the magnetostrictive actuator has a very short excursion, so perhaps stripping down a conventional speaker and strapping the voice coil (is that the right term?) to the theremin enclosure would provide a more effective test.

It also occurs to me that causing the playing hand to vibrate at the appropriate frequency would be equivalent to making the pitch rod tremble - and this reminded me of something that featured on iPod websites a few years ago that might also work.

(It's rather risqué - but tastefully done - there's no nudity but still, wait until the boss/vicar/kids have left the room before clicking. You have been warned.)

http://www.ohmibod.com/

Well to add some further "evidence" to your theories...

Some guitar players prefer open-back, all in one tube amps to anything else (Fender twin reverb, for example).

In such a case, the speaker vibrates the tubes, the reverb springs, and has destructive interference at lower frequencies (since no attempt is made to deal with sound cancellation from the back of the speaker to the front).

oops.. sorry! I am still recovering from the SBC I think! ;-)
Yes, I meant RCA! LOL!

Gordon said: [i]"It also occurs to me that causing the playing hand to vibrate at the appropriate frequency would be equivalent to making the pitch rod tremble - and this reminded me of something that featured on iPod websites a few years ago that might also work."[/i]

I think (but this is all hypothesis) that it may be the closed-loop nature of the speaker feeding back to the antenna which is responsible for the 'extra' quality imparted to the audio output.. it would not be possible to vibrate ones hand at (say) 440Hz, and applying any vibratory signal directly to the antenna would cause different effects depending on the phase relationships etc.. Also, the antenna itself will have audio resonances.

I think that the whole acoustic system will be responsible for the quality of the output tone - but the issues of primary importance are likely to be the resonances of the cabinet and the resonances of the antenna.. These will cause changes to the waveform as a function of both pitch and volume, and they will not be a uniform function - The resonances will depend almost entirely on the mechanics, and changing any acoustic component will alter the tone.

I will be testing these hypotheses in the next few weeks.. by taking the Theremin output to a spectrum analyser and comparing the waveforms with the internal speaker turned on and turned off.

At the moment I am under a pile of paperwork as I must submit my company accounts in the next few weeks - Alas, I have not been keeping my books in order, and am in total chaos, and face a heavy fine if the accounts are not submitted on time - so it will probably be a few weeks before I can add anything more to this thread.

If it turns out that the hypothesis is true, or even partly true, then there are all sorts of possible ways to 'emulate' the effect.. One could build a filter array into which the Theremin output is fed, and use the output from this to directly modulate the variable oscillator - Or one could build a sealed dummy speaker unit containing acoustic resonance elements, speaker and microphone, and feed this back to the oscillator.. Or one could use a DSP to create a speaker / box emulation.

But I think it likely that the best results will come from having a real speaker in a good wooden enclosure as an integrated instrument.. its just a hunch though.

[i]"(It's rather risqué - but tastefully done - there's no nudity but still, wait until the boss/vicar/kids have left the room before clicking. You have been warned.)"[/i]http://www.ohmibod.com/ (http://www.ohmibod.com/) - GordonC

LOL! Gordon - One of the things I found facinating about the SBC event, was how people who had never encountered a Theremin before first 'engaged' with it.. I had always believed that the phalic inferences regarding the 'rod' and 'loop' were hyped.. but at least 50% of people made actions which were, well, surprising to me.. Cupping their hands 'round the antenna and attempting to play using vertical motions.. (due to the fact that the electricians disconnected my ground 'antennas' the ground coupling from box did cause minor pitch change as a result of vertical movements..)

The idea of having the pitch antenna mounted atop a vibrator.. Hmmm.. I think that might be asking for trouble! ;-)