Vocalist.org archive


From:  Reg Boyle <bandb@n...>
Date:  Sun Apr 7, 2002  12:34 am
Subject:  Re: [vocalist] For Sally.... :)

Dear Lloyd and Sally,
On consideration I need to correct two things
I have written here and add a third. :) I always seem to get
drawn in further than is safe when it comes to this highly
complex operation.
The first was my error about the 90 phase shift
in the representation of the cords as a resonant device. In fact
each component contributes 90 degrees, making it 180 in
total. Inductance/capacitance in an electronic device and
mass/tension in the cords.
The second is my confusion of the visual
asymmetry. The various planes to be considered resulted in
my describing a "contact" asymmetry when I realise now
I meant an asymmetry of the ratio of "open : closed".
The cords will always contact along their whole length
but the time for which they do that, will depend upon the
wave shape. The most simple example is the square wave
of 50% duty cycle where the cords would be in
contact, (closed), for 50% of the period of the cycle, and
wide open for the other 50%. The time for the transition
would be very short. This is probably the only state of
complete symmetry of the open/closed ratio. A departure
from a 50% duty cycle would disrupt the symmetry resulting
in a totally different harmonic distribution as in the case of a
saw-tooth. A saw-tooth wave would have a very short
period of contact and a linear rate of motion towards its
fully open position which would also be very short.

I assume I do not need to mention here that the
fundamental frequency of both the square wave
and the saw-tooth are identical and that all that
we are concerned with is the harmonic content and
their relative amplitudes.

This leads into further consideration of the
ultimate wave shape of the air flow. As with any filter, its
performance is governed by its load. A "pi" filter, as I'm
suggesting the cords represent, needs a non-reactive load
in order to work effectively. While it is reasonable to
imagine that nature provides the means to attain such a
loading arrangement, it also appears to have provided the
flexibility to distort the value of this ideal match. Such a
match would also be essential to the efficient transfer of
the undistorted pressure wave to the resonant areas,
for without it, the all-important harmonics would be
suppressed and the mis-match would reflect on the
operation of the cords as an ideal filter.
Thanks for your patience in allowing me
to exercise my ravings once more, blame mike! :)
Now where does this leave falsetto?..(shrug)..
..wafting in the breeze. I'm off to a 5 hour rehearsal, bye.
Reg.








  Replies Name/Email Yahoo! ID Date  
18524 Re: For Sally.... :)Sally Collyer   Sun  4/7/2002  
18530 Re: For Sally.... :)Reg Boyle   Sun  4/7/2002  
18545 Re: For Sally.... :)Lloyd W. Hanson   Sun  4/7/2002  
18557 Re: For Sally.... :)Reg Boyle   Mon  4/8/2002  
18560 Re: For Sally.... :)Reg Boyle   Mon  4/8/2002  
18652 Belting ?bjjocelyn@p...   Wed  4/10/2002  

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