[Harp-L] Re: The Comb Debate

[Rick Epping <rickepping@xxxxxxxxx>]

> ------------------------------
>
> Message: 3
> Date: Sat, 4 Sep 2010 14:06:33 -0700
> From: Vern <jevern@xxxxxxx>
> Subject: Re: [Harp-L] Re: The Comb Debate
> To: Harp L <harp-l@xxxxxxxxxx>
> Message-ID: <52AB8E0E-CEFC-4860-B1E0-BEA4AFDDDD6B@xxxxxxx>
> Content-Type: text/plain;       charset=us-ascii
>
> >
> > 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


Agreed.

>
>
> >> 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".
>

I accept that this may be.  Whatever the resonance frequency of the
plate-comb assembly, its vibrations will contribute little or nothing to the
sound heard by the audience.  It may, however, be enough to aid the player
in controlling the instrument.  In his paper, "A Right to Sing", <
http://www.josephshore.com/a_right_to_sing1.htm> Joseph Shore quotes the
vocal pedagogist William Vennard in relation to the notion of the singer
directing the voice to different parts of the body:

"It is now recognized that the sound vibrations cannot be directed at all.
They enter every nook and cranny possible, and also set all the bones into
vibration. Where they reach a mass of the right natural frequency, be it air
or be it bone, it will be agitated by sympathetic resonance, and may produce
a sensation of which the singer will be conscious. But these vibrations add
nothing to the tone that reaches the ears of the audience."

Shore adds:

"Bodily vibrations are the RESULT of resonation happening in the vocal
tract. They are NOT causes of resonation, or evidence of any addition of
resonance to the vocal tract resonation." He goes on to say, however, that
these vibrations are very useful to the singer as biofeedback information.
The lack of additive effect of upper head resonance on vocal tract
resonation may point to a similar lack of any additive effect of plate-comb
resonance on external sound production in harmonica playing.

>
> > ...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.
>

We seem to be in agreement here.  My point is that the harder and
higher-massed that the comb is, the less it will act as a vibration
absorber.  A loose reed plate will act as a relatively strong undamped
vibration absorber, a comb with a soft gasket somewhat less so, a pearwood
comb even less, and a stainless steel comb will act least of all as
a vibration absorber.

> >
> >> 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.
>

Here's the crux of the matter.  The amount of energy absorbed by, say, an
ABS plastic or pearwood comb is small compared to the overall energy
produced in the airstream.  But most of the energy in the airstream resides
in the note's fundamental frequency and very little in its upper partials.
 A reduction of energy due to absorption by the plate-comb assembly may
result in an imperceptible reduction in the energy of the fundamental
frequency but cause a significant reduction in the strength of the upper
partials, which are responsible for a tone's brightness and definition.

Numerous players have observed that a high-mass comb material such as
stainless steel or DymondWood produces a brighter tone than does pearwood or
ABS.  This may be due to the fact that such hard and heavy materials absorb
very little of the reed's energy, most significantly the energy in the
reed's upper partials.

>
> 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.
>

I think we are in agreement that little energy is emanated from the comb and
passes to the ear through the air, but some of this energy may pass to the
player's ear by way of the teeth and bone, serving as feedback.


> 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.



Any additive effect of the sound passing through the comb may be
imperceptible to anyone but perhaps the player, but the main effect of sound
passing through the comb, I would propose, is not additive, but subtractive
in terms of the amount of energy produced by the reeds, especially with
regard to the upper partials which have comparatively little energy to begin
with.  Even a small reduction of energy to the upper partials can result in
a noticeable reduction in brightness of tone.


>
> This has been confirmed by the SPAH and Buckeye tests.
>

I attended the first SPAH test and agree that from where I sat there was
little difference in the tone of any of the comb samples.  However, any
difference in loudness, or overall strength of sound due to vibration
absorption by the comb material would have been difficult to note, as the
strength of sound is due more to the amount of breath pressure applied.  And
any difference in the strength of the upper partials would have been
difficult to hear from the audience, since upper partials decay rapidly over
distance.

In my experience, the bright tone I perceive when playing a harmonica with a
hard, high-mass comb is similar to the bright tone I perceive when playing a
finely adjusted harmonica with embossed reed plates.  And as is shown in the
case I mentioned in a previous post of the engineer who preferred the
recorded sound of the off-the-shelf Special 20 to the customized harmonica,
such perceptions may not necessarily reside in the player alone.

In summary, the effect of any plate-comb assembly vibration may be twofold:
its own vibrations may provide a direct feedback mechanism to the player,
and the strength of these vibrations may have an inverse effect on the
strength of the reed's vibrations, especially in the reed's upper partials.

Best regards,
Rick