The Rhodes Piano by Bob Moog

[The Real MC]

From my library of Keyboard magazines I found this intriguing article.  Enjoy.

The 'tone bar' referred to in this article is the tine and mounting post.

A tone-producing mechanism of the Rhodes is shown in the drawing.  When a key is depressed, a hammer hits the wire-shaped tone bar, which then vibrates like a reed.  The motion of the free end induces a signal in the pickup coil.  Three processes of interest to us occur as the tone decays.  First, at the beginning of the tone, the tone bar vibrates simultaneously in many modes, or patterns.  The faster modes die out more rapidly than the slower ones.  The tone bar is shaped and mounted in such a way that these higher-frequency modes result in a pleaseing bell-like attack that decays much more rapidly than the 'sustain' portion of the tine.  Second, the amplitude of the tone bar's motion at the pickup coil is much greater at the beginning of the note than at the end, and is greater for loud notes than for soft notes.  When the tone bar motion stays within the range of the pickup coil's pole piece, the coil output waveform is a reasonably accurate replica of the tone bar's motion.  However, on loud tones when the tone bar swings outside the pole piece's range, the coil's output waveform is 'distorted' in much the same way an overloaded vacuum tube or transistor amplifier distorts signals, and its harmonic content is thereby greatly increased.  Thus the beginnings of the notes have more harmonic content than the ends, and loud notes have more harmonics than soft ones.  These relationships appear to be desirable in any polyphonic instrument that produces percussive (piano-like) timbres.

The third process in Rhodes tone production has to do with the way the tone bar is mounted.  The mounting post is not entirely rigid.  Vibrations can pass from the tone bar up through the post to the resonator bar.  The resonator bar is also a reed-like structure, free to vibrate at the end opposite that which is fastened to the post.  Nothing strikes the resonator bar, and no pickup is located near it.  It serves only to store vibrational energy.  The resonant frequency of each resonator bar is tuned to be very near that of the tone bar to which it is attached.  As soon as the tone bar is struck, it begins to vibrate the resonator.  The resonator vibration builds up, literally sucking vibrational energy out of the tone bar.  At one point, nearly all the fundamental frequency energy is in the resonator, from which it then begins to flow back to the tone bar.  If you have access to a Rhodes you can actually see this happen, especially on the lower notes; remove the top of the instrument, strike a low note, and watch that note's tone bar and resonator.  First the tone bar will be vibrating and the resonator above it will be still.  After a second or so the resonator will be vibrating and the tone bar will be almost motionless.  This exchange of energy will keep on going for several seconds.  Of course, the pickup coil signal doesn't null out (disappear) because the tone bar is never completely motionless, as long as the key is held down.

From a musical acoustics point of view, this energy flow out of and into the tone bar has an effect which is similar to the flow of energy from one harmonic to another in an acoustic piano string.  The sound is always moving.  The energy interchange imparts complex envelopes to the sound's overtones and introduces phase shifts that slightly detune the fundamental with respect to the overtones.

The 'Rhodes sound' thus has three features that contribute greatly to its aural appeal: (1) a bright, bell-like attack, (2) velocity-dependent brightness that decreases as the sound gets softer, and (3) a warm moving quality.  Each of these is closely related to specific elements of the instrument's mechanical design.  The ultimate success of any instrument depends not only on the existence of features such as these, but also on the care and taste with which the instrument designer proportions and adjusts the instrument's design parameters.

Bob Moog, April 1980, Contemporary Keyboard

[O.Lahoz]

:shock: Fascinating and detailed observation. Understanding like works the thing, can help to improve the sound.

Viva Bob Moog!!!

[gunnar]

Nice article!

[BJT3]

Very interesting, I never thought about it that way before.

[Rob A]

Moderation note: I made this post sticky since I refer to it fairly frequently, and it's one of the "hidden gems" here in the forum.

[dnarkosis]

FYI in this context:

http://home.swipnet.se/~w-42826/rhodes/rhodesaction4.html

[bolero]

that was great!! and written by Bob Moog, no less

[Fendersocks]

Interesting analysis - well, Bob Moog knew a lot about oscillators and resonating filters.

What he does not elude on is the question what influence the torsion in the resonator has. He speaks about the migration of the energy's "center of gravity" -  passing the torsion in the resonator, the oscillations will probably be refracted and reflected in a very complex pattern.
What is known about the balance of physical theory and empirical curiosity in Mr. Rhodes' own development work? It occurs as a bit strange, if you compare the geometry of the resonators to some special antennas / aerials, where the position of the feed point must be accurately met, that the fixation mounts and the distances between fixation points and oscillator feedpoint are equidistant over the whole tone range.

Moreover - Bob did not look for the influence the resonator has on the pickup's magnetic field. In the beginning, the tone bar's motion certainly generates the major contribution to the signal - but later on, that role goes over to the resonator which partly closes the field lines' distribution around the coil.

[longcat]

Thankyou Bob, lovely and detailed analysis.

I'm going to go take the top off my mark II and watch the vibration pass back and forth between the tone bar and the resonator on a low note x

[mantrak]

Nice find. Both Harold Rhodes and Bob Moog were geniuses. Both made revolutionary, epic instruments. There is only one more person in their league, Laurel Hammond.   

[David Aubke]

I recently learned that one of the namesakes of my day job - Bill MacDonald  of Stewart-MacDonald Mfg. - used to live in some sort of multi-family dwelling that was also home to Bob Moog. This was while Bob was developing his Theremin kit and apparently testing the strength of his marriage with stacks of equipment occupying every inch of their apartment.

[orangefizz]

A slightly different diagram (maybe from a newer/older manual?):

[orangefizz]

This one is a profile view (from the Mark V):

[orangefizz]

The third process in Rhodes tone production has to do with the way the tone bar is mounted.  The mounting post is not entirely rigid.  Vibrations can pass from the tone bar up through the post to the resonator bar.  The resonator bar is also a reed-like structure, free to vibrate at the end opposite that which is fastened to the post.

Where is the "resonator bar"? I don't see this on any diagram I've come across. The hammer strikes the tine/spring, which is attached to a post that connects to the tone bar. Is the resonator bar part of the tone bar?

[The Real MC]

Where is the "resonator bar"? I don't see this on any diagram I've come across. The hammer strikes the tine/spring, which is attached to a post that connects to the tone bar. Is the resonator bar part of the tone bar?

The tone bar is the resonator bar.  It is designed to resonate at a particular frequency.  Every solid mass has a resonant frequency, all the resonator bars in the piano are different lengths set to the pitches of the keys.  When the tine is set in motion, the resonator bar resonates by way of sympathetic vibration.  Same principle of sympathetic resonation in a string instrument.

The exchange of energy between the tine and the resonator bar is what gives the tone a longer decay.  Otherwise the decay would be very short.

[orangefizz]

Ah OK, so he was referring to the tone bar. I hadn't seen the words "resonator bar" in this context.