Re: Tuned metal bodies

[Michael Carley <mjcarley@xxxxxxxxxxx>]

> From: William Lippe <lippe@xxxxxxxxxxxxxxxx>
> 
> >> apart from the question of playing the harp in other positions, which
> >> Tim raised...doesn't the metal body have only one 
> >> natural/resonance frequency?
> >	It may have its fundamental at one frequency, but all wavelengths 
> >corresponding to the equation 
> >	lambda=1/i or  Lambda being the wavelength, and i being all 
> >integers. 1 is the original wavelength of the frequency the body is tuned 
> >to.  This means that all of those frequencies (fundamentals) will vibrate  
> >the entire harp (sympathetically) also. Unfortunately, I don't have to do 
> >the calculation to tell that the frequencies pointed to in Winslows post or
> >the Harp Handbook will not all be fundamentals of the tuned body, and hence I 
> >can't believe these harps won't have odd volume shifts.  Bonking the comb 
> >and hearing a pling instead of a thunk just tells me "We did what we said 
> >we would do, now we just have to see what we did does."
> 
> yup..i understand about those harmonic waves you mentioned.
> i was just wondering about the effect of the hand/palm dampenning any 
> waves that may have been resonated off the metal cover. a bell won't 
> sound if the hand is hugging tightly around it, right? perhaps just a 
> plonk sound?
> 
> perhaps the cover works by air acoustics?
> you know..like how some of those hi-fi speakers have hollow tubes
> connected to the front or back panels, to resonate the natural
> frequency (and also the harmonics) of the groove? makes the bass sound
> tighter and punchy. perhaps, the shaped metal cover does the same
> effect? i am just speculating anyway and i know little about acoustics.
> i am equally stunned and awestruck by Winslow amazing knowledge!

I'm not sure quite what's going on. It is possible to tune a lump of
metal to a number of different frequencies which are not harmonics of
each other (a bell for example). What shape is the new coverplate? If
it's set up properly, then it could be designed to have a set of resonant
frequencies which would boost certain tones. If you picked the right ones,
you might be able to boost the high/low end, say.

I very much doubt that it's an acoustic effect. The dimensions involved
are of the order of 10mm so the frequency is about 340/0.01 34 KHz which
is a long way up. Going across the harp, the length is about 100m so you're
down to about 3.4 KHz which is audible but not liable to resonate with
a reed. 3.4 kHz is about 4 octaves up from middle C, about an octave above
the top end of a diatonic (unless I'm wrong, quite likely I admit).

Most likely, I think, is that the more substantial coverplate resonates
at lower frequencies than the thinner metal types, without actually being
"tuned" to the harp, so that you get better response at the lower 
frequencies, giving you a fuller sound (which is all subjective anyway).

I hope some of that made sense.

-- 
Ahmet Ertegun introduced the former secretary of state to 'my friend' 
Wilson Pickett, then on probation for taking a loaded shotgun into a
bar
	'Henry Kissinger, my man,' said Pickett, hugging him.
	'Mr Pickett, a pleasure,' said Kissinger.
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