Re: [Harp-L] Re: The Comb Debate

Vern writes:
> Rick Epping wrote: 
>>  And if a reed plate
>>  can't transfer a reed's vibrations, why is it that when plucking the reeds
>>  of an unmounted 270 Super Chromonica reed plate, the reed tuned closest to
>>  the resonant frequency of the plate itself always sounds dull, only to ring
>>  normally once mounted onto the comb?  It seems as if the loose reed plate is
>>  vibrating sympathetically with the reed tuned closest to it, thereby
>>  absorbing some of its energy.

The free plate is acting as an "undamped vibration absorber"
See:http://paws.kettering.edu/~drussell/Demos/absorber/DynamicAbsorber.html

Nice illustration of "undamped vibration absorber," Vern. I think we agree that a reed plate can transfer energy to the comb.

>>  Once mounted directly onto a comb, the reed
>>  plate has no effective resonant frequency itself but rather contributes to
>>  the resonant frequency of the plate/comb assembly, which is lower than the
>>  pitch of any of the reeds and unlikely to vibrate as much.
It is not so much that the plate-comb assembly has a different resonant frequency. It is so highly damped that it has hardly any resonance at all. You can confirm this by tapping the free plate and hearing it ring. Then tap the Plate-and-comb assembly and hear only a dull "click".

Yes, but ~play~ the harmonica and the the entire instrument vibrates, does it not? And it vibrates at that reed's characteristic frequency.

> > ...So a comb vibrating along with the reeds is thus perhaps not a good thing in 
>>  that, like the reed plate, it would thereby absorb energy from the reeds and
>>  detract from their ability to swing freely, affecting
>>  their response and loudness, if not tone.
Because it is so highly damped, the plate-comb assembly does very little vibrating. Because its mass is very much higher than that of the reed, it absorbs very little of the reed's energy. Look at it this way. If the reed can move the plate, then the force exerted by the reed times the distance moved by the plate is energy absorbed by the plate or plate-comb assembly. However if the assembly is so massive it can't be moved very far by the reed, then little energy is absorbed. The force exerted by the reed is the same but the displacement is smaller so the energy product is also smaller.

Let us not forget that the reed is not the only thing making the entire body of the harmonica vibrate when it is ~played~. There is that air moving at that same frequency right? That's the sound we all hear.

I'm sure you have played harmonicas with metal combs, Vern. Have you not noticed that they vibrate in your hands and mouth to a greater degree than wood or plastic ones? In my experience, those metal-combed harps vibrate much more than wood or plastic ones. I'm not the only one who has observed this. Regardless of the amount of vibration, I contend that all harmonicas vibrate and that such vibrational energy cannot be ignored when analyzing the flow of energy that creates the final emitted sound.

Experiments studying the forces exerted by the vibrating reed on the rest of the harmonica assembly when it is hanging inert, not being played (and dampened by the player's hands) are fine, but they don't tell the whole story of the physical phenomena afoot when the insstrument is played while held tightly in the hand and cupped to the mouth.

I remain steadfast in my claim that if there is enough vibrational energy transferred to the body of the harmonica that the player can feel it, it must be significant in terms of the formation of the sound emitted from the instrument. My hands are not super-sensitive sophisticated instruments like strain guages or inertial sensors, so this vibrational energy is ~not~ small and insignificant in spite of the argument you build in this post. We're not just talking about the forces exerted by the single free reed on the rest of the instrument, but the entire system that vibrates at the characteristic frequency of that particular reed, the air moving through the instrument, the player's airways, embouchure, and so on.

>>  A hard, high-mass comb might
>>  resist vibrating along with the reeds more than a flexible, low-mass comb,
>>  leaving the reeds unimpeded and unaffected.  A soft gasket between the reed
>>  plate and comb would partially isolate the plate from the comb, preserving
>>  some of its ability to vibrate sympathetically and absorb energy from the
>>  reeds.
You are correct. However, after the mass of the plate-comb assembly is large enough so that the amount of absorbed energy is minuscule, further increases in mass will produce no perceptible changes in harp performance. Besides that, the comparatively powerful stream of breath can easily make up for tiny losses of energy to the plate-comb.

It occurs to me that we should be specific about which type of harmonica we are referring to here. My experience and thinking are almost solely based on playing diatonics, where is suspect yours is primarily based on playing chromatics. Perhaps the vastly heavier body of the chromatic does have a different impact on sound production in this discussion. Certainly the fractional mass of a single reed is far less with a chromatic than a diatonic. And the significantly higher mass of the chromatic may mask the vibration of the instrument when played.

The plate-comb assembly is absorbing as much energy from the reed at the instant that it is plucked as it is when blown at the same amplitude. Therefore a plucked reed will give you an idea of the loudness of the sound absorbed from the reed and emanated from the comb and passing to your ear.

Vern old pal, with all due respect, I'm afraid you are over-analyzing here. Nobody blows their instrument so softly that it the sound is the same amplitude as when just plucking the reed. I don't know if it's even possible. You are thinking "trees;" I am thinking "forest."

I attached a reed so that it protruded out the back of the harp and could be easily plucked while the other reed of the same pitch is being blown. The plucked reed can be plainly heard when the harp isn't being blown. However, when the harp is being blown, neither I nor listeners could hear it. The sound of the blown reed completely masked the sound of the plucked reed. This is an easy experiment to duplicate and I encourage you to do so.

Easy to duplicate but I don't see its relevance. It's clear that the contribution of the mass of a plucked reed is infinitesimal compared to the overall sound of a blown instrument.

Because of this masking, any affect of the material on the sound passing through the comb is imperceptible. If a change of the entire amount of energy absorbed directly from the reed is imperceptible, then the DIFFERENCES among materials will be even farther below the threshold of perception!

Now hold on a minute. I think you've walked yourself out onto a theoretical limb here. Clearly the mass of a single vibrating reed contributes little to the vibration of the played instrument, especially in a chromatic. I don't think anyone would argue against that. You have shown that the mass of the reed itself isn't significant enough to contribute to the vibration of the entire harmonica. But you haven't addressed the existence or the impact of the very real vibration of the harp as felt by the player.

I say that that "comparatively powerful stream of breath" is what makes the harp vibrate at the characteristic frequency of the reed and that the acoustical properties of the combs are what cause the different amounts of vibration to be transferred to the body of the harmonica.

This is less a matter of kind and more a matter of degree. Although many of the mechanisms discussed in this thread may indeed operate, the amount of energy involved is too small to be heard in the presence of the main source of sound.

I completely agree if you are talking about the contribution of the mass of the reed. I strongly disagree if we are referring to the entire system, including the energy of the vibrating column of air formed by the player's airways. There is no doubt that the whole system is vibrating at the sympathetic frequency of the reed -- at levels of energy far exceeding the energy of the mass of the vibrating reed itself.

This has been confirmed by the SPAH and Buckeye tests.

So far as I am aware, nobody ever mentioned the impact of the mass of a single reed, or the vibration of the harmonicas themselves in the experiments (and the most recent SPAH experiment even employed a jig ~designed~ to mask the mass of the different combs). Let us avoid the temptation to extend the results of the experiments farther than the boundaries of their hypotheses.

Vern, as always I am bowled over by the expanse of your knowledge and experience in engineering and acoustics in general and as they apply to harmonicas specifically. Your ability to analyze the most minute details of how harmonicas work is astonishing. But, in this case I think that you may have let your analysis of the details obscure the larger picture. I think both Rick's and your observations about transferring the energy of single vibrating reed to the body of the harmonica, while very interesting, are difficult to extrapolate to the real-world situation that involves many other larger sources and effects of energy in the dynamics of a played instrument.

The bottom line is that, if we can't agree that the whole harmonica vibrates, we are at a stand-still in this discussion. :-)

Respectfully,

Michelle



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