Audio feedback affects Theremin tone?

Hello,
I, being a piano Geek am wondering if the wood species that the 'Theremin with speaker' cabinet
is built with could impart a specific overtone harmonic sequence feed-back to the pitch-rod.
Certain spruce trees growing in higher mountainous altitudes,(near the tree line), have been praised for their acoustic and resonant properties. Also, straight grained mahoganies have been extolled as acoustically superior.
From what wood was the RCA Theremin cabinets made?
Mahogany? And, also what were the dimensions and weight? And how is the cabinet constructed--- more horse-hide glue, less screws--- tongue and groove?
All these specifics lend to a cabinet's --- or a cavity's resonance.

The spruce wood species generally has characteristics that make it better for planar dispersion of acoustical energy, and a high frequency response.
Mahogany is better suited to dimensional dispersion of acoustical energy. And, mahogany has a lesser frequency amplitude, but certain harmonics are enhanced. This wood is better for acoustical resonant cavities: such as RCA phonographs!
The RCA Theremin cabinet might have had a thoughtful boost towards it's specific sound just to save some money on the cabinet costs.
I could be wrong, but think a minute on it! I did.
Good Luck!
teslatheremin





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Hi Teslatheremin,

I agree with everything you say here.. The properties of the wood used, and the cabinet dimensions etc could (would?) play a significant part in the modulations being fed back -

Parameters I can think of (in possible order of importance) are:

Coupling: The acoustic coupling of the antenna to the chassis is likely to have a significant effect - for example, coupling via a angular component (a right angle at the side of the Theremin, as in the Moog EW models) would probably generate more movement on the horizontal axis than an antenna mounted on the top of the Theremin.

Antenna: material, dimensions (thickness, length, construction[solid/tube etc]) and any other factors influencing its response / resonance/s to audio frequencies. I think these factors will even influence the tone quality in response to an external loudspeaker, but obviously to a far lesser extent than if coupled physically to the speaker via a chassis. Coupling via chassis to antenna could be significant if the Theremin has a large cabinet in close proximity to an external loudspeaker.. but with modern (small) Theremins I do not think there will be much coupling.

Cabinet: Its dimensions,construction, materials, resonances, and generally everything which relates in any way to the spectrum of audio frequencies that get to the antenna coupling point.

Speaker: The response charactaristics and its coupling to the chassis.

Probably, the best way to 'emulate' a RCA would be to copy the physical layout and materials used on the RCA as closely as possible (dimensions of cabinet,materials, type of wood, antenna/s etc) and to get a speaker and amplifier matching the charactaristics of the RCA as closely as possible ( The mass of the weighty electronics would also need to be taken into account.)

- then, into this, build a modern Theremin capable of replicating the waveform from the RCA. My original ideas on emulating the RCA were based purely on evaluating the waveforms from an RCA system (Theremin+original amplifier and speaker) and building a circuit capable of producing these waveforms (+ many others)..

I am now inclined to thinking that this would turn out a bit like a good electronic piano.. No matter how good the emulation was, you will never capture the full richness of an acoustic piano, 'simply' because one needs the large carefully tailored mass of the piano, Tonal qualities of the wood, and the sympathetic interactions of the tone board etc, and these cannot be crafted into electronics yet (I doubt they ever will be). I thought that, because the Theremin was a 'purely' electronic instrument, there was no comparison - there were no acoustic factors involved in the output waveform, so any Theremin could be 'easily' replicated if one had a sophisticated enough reconfigurable analogue computer..

I now think I was probably wrong.

A year ago I posted this: (http://www.thereminworld.com/forum.asp?cmd=p&T=3966&F=3&p=2)
>> [i]I believe the difference between a Clavinova's "emulation" of a Boesendorfer grand piano sound, versus a Theremin "emulating" another Theremin is huge.. I do not believe it is possible to COMPLETELY emulate an accoustic instrument electronically.. The mere physical aspects - resonances in the cabinet / toneboard etc, the locale of the output sounds and harmonics, the effect of the players non-obvious interaction with the mechanical components... Emulation is good - but it is not (and I think probably never can be) perfect.

However - I do believe it would be within the realms of possibility and even achievability to make a Theremin which created tones IDENTICAL to OTHER THEREMINS - simply because both are electronic instruments producing waveforms which can be analysed and duplicated, and the number of factors which define the tone (which are far simpler than what one gets from even the simplest acoustic instrument) can be broken down and rebuil

The part of this discussion that confuses me is that, regarding the RCA theremins, there were only a couple of theremins made that had an intergral loudspeaker in a floor-length cabinet; most notably the one once owned by Dr. Samuel Hoffman now owned by Peter Pringle.

The RCA (style) theremin that is most often mentioned and revered is Clara Rockmore's custom theremin. Like the vast majority of RCA theremins, this one used a satellite speaker on an electrical tether. There would be no acoustical-mechanical interactions of any consequence.

They do all use the warmer-sounding, antiquated tube technology.

However, Fred's recent observations may provide a way to get closer to "that sound" (minus Clara's talent and skill, of course) with modern electronics. It will be interesting to see what becomes of it.

[i]"Like the vast majority of RCA theremins, this one used a satellite speaker on an electrical tether. There would be no acoustical-mechanical interactions of any consequence." - Jeff S[/i]

Jeff - You may be right about this.. and I may be "barking up the wrong tree" (or just plain barking - LOL! ;)

I started looking at this seriously because I could hear a difference (which I percieve as an improvement - as in, closer to the RCA sound) when comparing my H1 Theremins with internal speaker to them played via much higher quality studio amplifier and speaker.. My hypothesis seemed like a simple explanation -

- But there may be an even simpler explanation.. this being that the lower quality amplifier / speaker, and the resonances etc of the box, is what produced the sound I prefer.. It may have nothing to do with antenna feedback.

Further examination of the hypothesis shows a possible problem or flaw in it - one which is now inclining me to think that it may be wrong..

The effect of any audio vibration on the antenna will be a function of how much capacitance is changed by said vibration.. This, in turn, will be a function of the distance of the antenna from the other 'plates' (grounds).. With a vertical antenna, I would not expect (I could be wrong) the background capacitance 'seen' by the antenna to vary much as a result of antenna vibration, as this background is likely to have quite a wide dispersal.. I think it will be the players body and hand which will form the major 'directional' capacitive component against which any vibration would be referenced..

If the above is true, then the effect of vibration would be most significant when the player is closest to the antenna (highest frequency audio) and almost completely insignificant as the player moves away from the antenna (lower audio frequencies)..

The above contradicts what I believe I heard, and also makes harmonic contribution from antenna vibration extremely unlikely.. The only time there will be significant change to the waveform as a result of antenna vibration would probably be with the hand about 5cm or less from the antenna - a zone where any additional harmonics are likely to have an insignificant effect on the tone (added harmonics quickly dissapearing into ultrasonics).

At this time, I do not have any answers.. Testing should provide the answers, or turn up more surprises / questions..

I must also be wary - it is easy to delude oneself and hear thing which are not there.. I was extremely exhausted when I believed I heard my H1 Theremin sound like a Claramin.. It might have been a bit delusional!

Ok, playing Devil's Advocate ...

Suppose for an instant that the differences in acoustic properties alter the radiation resistance of the air surrounding the pitch antenna. Such a mechanism would alter reflected radio waves.

Just another crazy avenue to explore ... :)

If you are looking for external mechanical mechanisms, it would seem unlikely that you are putting out enough sound pressure to move the antenna much.

Don said(playing devils advocate.. not sure if that makes me the devil..)-: [i]"Suppose for an instant that the differences in acoustic properties alter the radiation resistance of the air surrounding the pitch antenna. Such a mechanism would alter reflected radio waves."[/i]

I dont think "reflected radio waves" is of likely relevance (my perspective has always been to see the operation of the Theremin 'antenna' as simply a capacitance sensor, having little relationship to 'antenna' in the classical RF sense).. But it is possible I suppose that audio waves could cause 'layering' of the density of the dialectric (air) between the 'antenna' and the player. (however, if this was the mechanism, it should apply to all Theremins, regardless of their construction)

Interestingly, there was a lot of displacement of air from the ports in the speakers I constructed the H1's into.. The wire connecting from the circuit board to the EQ inductor+Antenna (the antenna was constructed with the EQ coil mounted vertically under the antenna - there was no wire connecting between the inductor and antenna, as one end of the inductor was bolted directly to the antenna) had lower capacitive sensitivity than the antenna, but was still sensitive - and this wire was positioned over the top port hole.. At higher volume levels I did see this wire vibrating (but had no time to correct this possible 'problem')

I am sure that the (unintentional/accidental) construction of the H1 contributed to it producing a 'sound' I did not expect - I will be exploring the cause/s ASAP..

What I now doubt, however, is that these cause/s will relate to a common mechanism between the H1 and some of the great Theremins of the past - Obviously, I hope I will find that I have stumbled upon the key to "that sound".. But I doubt it! - And, if I have stumbled on it, well - it would almost be embarrasing! Years of careful evaluation and designing would be overtaken by an accident caused because I was rushing to build a batch of Theremins which I regarded almost as high-end toys..

If it was the vibrating wire between oscillator and lin-coil/antenna, you should be able to find a slight frequency modulation which increases with the volume. That could lead to an interesting timbre since the first and most important sidebands of the modulated signal would be f_osc +/- f_audio (like with AM). This would lead to an increased part of 2nd harmonic (2*f_audio) after the mixer, giving a slightly more "stringy" and "earthy" timbre.

"Devil's advocate" was meant to play support of non-capacitive sensor aspects of the theremin.

Sounds like you found your culprit, though ... air-induced FM. Gives me yet-another-crazy-idea ... purposely installing an electrically-tuned circuit in the port of a bass reflex cabinet. :)

Hi Thierry,
You say: [i]"That could lead to an interesting timbre since the first and most important sidebands of the modulated signal would be f_osc +/- f_audio (like with AM). This would lead to an increased part of 2nd harmonic (2*f_audio) after the mixer, giving a slightly more "stringy" and "earthy" timbre."[/i]

I agree - And I think this is the most likely cause of the change in timbre I heard when comparing between the sound played via external amplifier and the sound played through the internal speaker.

This FM effect could be easily implemented on almost any Theremin - one could take the audio signal back to some form of capacitance modulator (varicap diodes for example) which modulates the frequency of one of the oscillators - and as the level of modulation would be a function of the audio signal level, this would give distortion proportional to volume.. One could easily filter the audio prior to feeding it to the 'capacitance modulator' to introduce frequency dependent distortion as well..

My gut feeling, however, is that the 'randomness' of an acoustically coupled signal is likely to sound better.. it may be that a small, low signal level audio coupling ( perhaps a reasonable quality flat PC speaker bolted to a block of wood, with a couple of wires bolted to this wood to form a capacitor that vibrates with the audio ) will be a simple way to get this timbre.

Some experiments should make the situation clearer - I will do these ASAP.. I would love to hear from anyone who looks into this - either theoretically or practically. I will post anything I discover - but it will be a while before I have time to explore this fully.

Does anyone know if Fred Mundell (rip) had explored this further or someone else?