This is a multi-part message in MIME format.
---------------------- multipart/alternative attachment
Dale,
=20
You wrote: =20
=20
"...interesting to note that when...booming the board with the fist to =
hear its innate resonance & if your face or hand is near any of the nose =
bolt holes just how powerful a blast of air comes thru there!"
=20
That is an interesting observation. I think a rush of air that comes =
from striking the board doesn't come from the periodic vibration of the =
board (which makes up the boom sound it makes when you strike it), but =
rather from the one-time relatively slow and high amplitude displacement =
caused by the initial strike itself. Sort of like the way a ride-cymbal =
or a gong swings when you hit it, but the swinging is only incidental to =
the sound you hear, which come from much faster vibrations within the =
material. I think the frequencies from the strings that we want to =
transmit would be more in the range of the board's self-vibration than =
in the relatively very slow movement of the initial displacement when =
you strike it. That isn't to say there aren't similar losses at =
relevant frequencies, I just don't know if you could tell a whole lot =
about them by feel. What you've witnessed would definitely be a loss as =
far as it goes. Part friction, part acoustic short circuit. =20
=20
An analogy to the distinction as to whether the air coming through says =
anything about the acoustic effects of the hole at relevant frequencies =
could be made by making an experiment with a ported loudspeakers. If =
you push the cone in with your hand, air will come out the port, and it =
comes out for all practical purposes perfectly in time to fill up the =
extra space in the room made by the cone going in, so there will be no =
pressure change, any any potential acoustic product that might occur by =
repeating this in and out movement is cancelled out. That's one kind of =
acoustic loss.
=20
The other kind is friction loss. In loudspeakers, a factor is =
considered "purely" lossy, or Q-reducing, when the output at any =
reasonable playing level is so disorganized or attenuated as to have =
negligible acoustical effect, destructive or constructive, compared to =
the energy dissipated as heat. Compressions and rarefactions going =
through tiny pin-holes in an enclosure are such an example. Their =
waveforms coming out the other side are very distorted in the amplitude =
domain and maybe in the time-domain also. Basically these pin-holes =
have a current limiting function as well as an energy dissipating =
function, so the higher the amplitude, the more distorted the exiting =
waveform. Somebody with a better scientific or engineering background =
can correct me if my terms are not correct.=20
=20
In the case of soundboard holes, I think conduction of coherent =
soundwaves at most frequencies and realistic amplitudes will probably =
almost always turn out to be substantial, or at least significant, =
compared to the friction losses. (But that doesn't necessarily mean it =
will be substantial or significant at important frequencies compared to =
overall output of the instrument.) =20
=20
But then there's also the other, third component, which is the reactive =
one. Reactive paths involve energy storage and time-delays. =20
=20
Going back to the loudspeaker, if we now whack it (or sharply tap it), =
instead of just pushing it, we'll get a similar puff of air, but if the =
speaker is relatively undamped (it might be hard to find one undamped =
enough, but it helps if one of the wires is disconnected) there will =
also somewhat of a drum sound that sometimes even has a recognizable =
pitch. Imagine hitting the big gong that's suspended -- the gong swings =
an inch or a few, but the sound comes from the faster vibrations -- it's =
the quivering of the gong, not its slow swing back into plumb, that =
makes the sound. The reason for referring to the ported loudspeaker is =
that at the quivering frequency, the air going in and out of the port =
will be delayed so that it is in almost perfect timing to add to the =
effect of the cone's movement. When the cone goes out, the air goes =
out, so the acoustic output is reinforced by the "hole" at this point, =
even though the first puff of air that came out of the port -- =
corresponding to the swinging of the gong -- represented acoustic =
canceling. This reversal, phase reversal, is because there is now a =
reactive path. =20
=20
Now if we excite the cone with a periodic source, say by coupling it to =
a string, there will be acoustic reinforcement from the hole/port until =
the frequency of the string used is so low that the port output simply =
follows the backwave of the cone in phase (non-reactively). Now we =
don't know yet if at the relevant frequencies the holes act more like =
the port does at or above its tuning, or more like the way it does below =
its tuning, as from from a slow push. We hope either more like the =
latter or like neither. The reactive reinforcing behavior of the port =
in a speaker happens when the spring of air inside the cabinet aligns =
through the cross-sectional area of the opening with the mass of the air =
inside the tube, and we can get a pretty good idea how all this works by =
just taking a weight and suspending it from a rubber band. =20
=20
Use a weight that's heavy enough or a rubber band that's thin enough so =
the natural periodic bouncing motion is slow, so that it's easy to =
observe the timing of it relative to motions of your hand. Hold the end =
of the rubber band and bob the weight up and down. If you do it very =
slowly, the weight will go up and down with your hand. As you increase =
the speed, you'll reach a frequency where the weight will be moving up =
and down opposite to the motion of your hand. At this frequency, the =
resonant frequency of the band/weight system, it will take only very =
small motions of your hand to keep the weight bobbing quite a bit. This =
corresponds to the tuning of a ported speaker, and represents the lower =
limit of substantial bass response (and just like your hand, the =
excursions of the cone are controlled by the opposing forces of =
reactance of the system, reducing distortion or preventing the speaker =
from mechanically bottoming out.) Connecting to some other current =
discussions about soundboard stiffness and mass etc., there is now also =
a very high impedance relationship of the weight/spring to your hand, =
and energy is reflected back to your hand and thus dissipated very =
slowly. If your hand was a string it would have long sustain at this =
frequency. Now continue to move your hand up and down faster, and the =
motion of the weight will diminish, and will continue to diminish as you =
increase the frequency. So at higher frequencies there is neither =
constructive nor destructive interference coming out of the port; the =
higher frequencies "see" the port as a sealed hole, and speaker behaves =
as it would in a sealed box in these frequencies.
=20
And if we re-did this whole experiment with the weight in thick oil to =
add substantial resistance to the system (or by adding big air-brakes =
made out of cardboard to the weight), we'd have some idea of the =
resistive or friction-loss component. You'd see that resonant motion is =
attenuated -- but also that losses from destructive interference below =
resonance would be substantially reduced. =20
=20
So now we have a couple of common-sense analogies to all three =
components of the soundboard hole's behavior -- 1) acoustic short =
circuit; 2) friction losses; and 3) reactive (mass and/or spring). =20
=20
What I don't know how to do is to quantify these effects relative to =
overall acoustic output, because I don't know what physical context to =
put it in. Even in the case of simple resistance, I think we still have =
to compare the amount of resistance to the pressure differential at the =
opening (analogous to voltage), or compare that path to the other =
available paths. In the case of certain kinds of reactance, we'd have =
to know even more.
=20
I said in my earlier post that since there is no enclosed body of air =
between the vibrating membrane and the hole, there isn't any mass/spring =
system involved in soundboard holes. Now I'm not so sure that the =
elasticity of air in propagating soundwaves might itself not make such =
an equation possible for analyzing the effect of the holes. =
Intuitively, it seems like there would be a frequency where the mass of =
the air in the hole would substantially inhibit conducting =
air-vibrations through the hole. (On the other hand, if, owing to the =
thinness of the board that frequency were so high that its wavelength =
was comparable to the hole size, you might as well forget about the =
reactive component and just look at the current capability, location and =
resistance of the hole -- both because the air won't act as "a" mass =
when the hole and wavelength sizes are similar, and also because we're =
talking about frequencies up around the second partial of C8, or higher =
depending on the actual hole.) That might be modelable as an inductor, =
but to get any quantitative results from that you have to put it in =
context of system capacitance and/or resistance. It's that system =
component -- in the soundwaves themselves or in the atmosphere (or in =
the pressure differentials, or the impedances of other available paths) =
or whatever, that I'm not grasping. I don't think I ever dealt with =
anything like that. (Well, of course I did, by living at all, but I'm =
saying consciously or explicitly, you know what I mean.) So (Terry) my =
confidence that I could figure this out in my earlier reply to your post =
may have been premature.
=20
If it turns out these holes aren't so good, it seems like an =
acoustically sound answer to the problem could come quite cheaply and =
conveniently in the form of the usual suspension systems used for =
loudspeakers -- either a sealed version of the little accordion-like =
"spiders" attached to the voice-coil behind the cone (outer-edge =
attaches to the SB, inner edge to the nose-bolt), or something like a =
foam or rubber half-roll surround. Maybe the stiffness could even be =
chosen to support desired impedance characteristics in the soundboard.
=20
I'll be interested to hear anything you notice from the experiment you =
proposed with your Steinway D.
=20
Trent Lesher
-----Original Message-----=20
From: Erwinspiano@aol.com [mailto:Erwinspiano@aol.com]=20
Sent: Sat 1/31/2004 10:28 PM=20
To: pianotech@ptg.org=20
Cc:=20
Subject: Re: Dipole Effects and Leaks in Soundboard; was Isolated =
Air-Movement
In a message dated 1/31/2004 4:35:49 PM Pacific Standard Time, =
tlesher@sachnoff.com writes:
As for the holes, although there's no substitute for somebody taking =
frequency spectrum measurements from a variety of positions and then =
plugging the holes up and doing it again, I think I can handle that =
question a little more satisfactorily than the other one. I wish I =
still had the equipment to take some measurements myself, but various =
upheavals took their toll some years ago. But give me some data if you =
want and I'll see what I come up with.=20
Best r egards,
Trent Lesher
(tuning student, amateur pianist)
Trent
Interesting post. This may be trivia but it is interesting to note =
than many times when I'm working on an unstrung piano & booming the =
board with the fist to hear its innate resonace & If your face or hand =
is near any of the nose bolt holes just how powerful a blast of air =
comes thru there! It would seem that a great deal of air is continually =
leaking thru these holes. Might this be a friction loss as you describe? =
WOuld the effeincy of the whole mechanism indeed increase if the hole =
were sealed or eliminated or are they benificial? I think I'll get =
someone to play vigorously on My Stwy D and then I'll see if I can feel =
any significant exchange from underneath the piano around the bolt =
holes.
Dale Erwin
=20
=20
=20
=20
EVEN MORE IMPORTANT NOTICE: Never mind the message below. It's intended =
for law-client-related communications, and the computer people at the =
law firm where I work seem to have made it impossible to keep it from =
being added to one's e-mails, otherwise I would turn it off and it would =
not appear below. For purposes of this message it may be considered as =
meaningless, random clutter. Piano tuners, technicians, rebuilders, =
makers, designers, scientists, craftsmen, artisans, musicians, =
engineers, enthusiasts and friends need not trouble themselves. Despite =
what the notice says, you are hereby notified of nothing (except that =
you are hereby notified of nothing).
****** IMPORTANT NOTICE ******
This e-mail, and any attachments hereto, is intended only for use by the
addressee(s) named herein and may contain legally privileged and/or
confidential information. If you are not the intended recipient of this
e-mail, you are hereby notified that any dissemination, distribution or
copying of this e-mail, and any attachments hereto, is strictly =
prohibited.
If you have received this e-mail in error, please immediately notify me
at (312) 207-1000 and permanently delete the original and any copy of =
any
e-mail and any printout thereof.
---------------------- multipart/alternative attachment
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/52/49/22/16/attachment.htm
---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC