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

[Vern <jevern@xxxxxxx>]

> 
> 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  

>> 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".  
> 
> ...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.  
> 
>> 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.

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.

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.

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!

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.

This has been confirmed by the SPAH and Buckeye tests.

Vern