theodoropoulos
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sound velocity in bridges
the mathematical formula of the sound velocity through wood is
c = square root of (coefficient of stiffness /density)
my question is :why do we use dense woods in the bridge,althought as mentioned they must have low velocity of sound according to the formula???
to be correct dense woods must have great coefficient of stiffness to compensate the great density..
please give me your lights....
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theodoropoulos
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i would appreciate mr jdowning's opinion....if possible
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jdowning
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I have always imagined that the bridge has a mechanical function - that is it transmits string vibration to the sound board by means of fluctuating
torsional forces? If this is the case then the mass of the bridge and the height of the strings above the sound board (as well as the area the bridge
occupies on the soundboard and its location relative to everything else) would be primary factors, not the velocity of sound in the wood that the
bridge is made from?
So a lower density wood might be set in motion more readily than a heavier wood and produce a faster response from the sound board. However, I am not
aware of any comparative, comprehensive scientific tests that have verified this concept - at least as far as the oud or lute are concerned.
Many surviving lutes, apparently, have bridges made from lower density woods (such as pear wood), stained black rather than being made from higher
density woods like ebony. The ancients presumably knew what they were doing - but perhaps did not use ebony because it was not available locally or
too costly? Or was there some other reason?
As far as the oud is concerned, 14th C writer Ibn al-Tahan al-Musiqi, in "Hawi al-funun wa salwat al- mahzun" (according to Dr H.G. Farmer's
translation) states that "As for the bridge, it should not be weighted by anything, and should not be made of ivory, ebony, gold, or any precious
thing, because it makes the sound of the oud dull". The author does not say why.
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theodoropoulos
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thank you for you fast reply.
i used ebony in my last oud and something went wrong with the sound...i dont konw what......i am gonnna use wild pear in the next one.An old man here
told me that he uses it for a better sound.
i imagine that the lenght o f the string 'needs' a certain density/weight/shape f the bridge....
my opinion is:wood of very low density absorbs the palm.wood of very high density does not allow the palm to move anything.so....a spectrum of
frequencies which send the string must be similar to selffrequency of the system bridge-top of the oud...in order to succed co-ordination which means
maximum volume
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jdowning
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I also fitted an ebony bridge to a lute that I refurbished a few years ago when I replaced the entire sound board. I am quite happy with the sound
balance and volume of the instrument (although I am more concerned with quality of the sound and response rather than loudness). Lute bridges have a
narrower "footprint" than oud bridges so this may have had some influence. I also seem to remember that I fitted a brace between the bridge and the
bottom of the bowl - as usually found on an oud - but cannot now find my notes recording what I did exactly. The original lute that I copied had no
bracing in the area below the bridge.
The old sound board had a sycamore bridge but had split along its centre joint due to shrinkage and the lute never sounded well. So I figure that
there were likely a number of more significant factors at play than the bridge material. It would be interesting to fit a pear wood bridge for
comparison purposes but I do not plan to do that - 'if it aint broke don't fix it'.
I do have a quantity of pear wood in stock, cut for peg blanks about 25 years ago, so will experiment with some of that for making a bridge on my
current oud project. My current lute project has an ebony bridge already fitted to the sound board so that will stay for now.
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theodoropoulos
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another thing is that in ebony we don't see the fibers..and i do not know if is important to a bridge.i believe that we must place the fibers accross
the wave's direction and not vertical.and what is the wave's direction??from the end of the string to the top of the oud..
as far as ebony bridges concerns i believe that if we decide to use ebony we must minimize the weight...
we have an institute of accoustic here in Greece http://www.iema.gr/home/?lang=en
and probably could help us ...take a look
and let me ask something else....
is there more torque in iraq ouds with float bridge than in normal ouds with fixed bridge??does this affect in loudness??
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jdowning
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I am only interested in traditional oud design so do not have an opinion about the modern floating bridge ouds. Violin acoustics might be of some
relevance?
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Marina
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I am proud to be at the same forum with you smart guys!!!!
I am very impresed when I do not understand something.
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theodoropoulos
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marina...i also like to learn from guys like mr jdwoning...and many other here.
as far as accoustics concerns all instruments have the same properties ,but violin differs because its highest frequencies are more difficult to be
balanced..
if you have any other usefull info about the bridges and generally accoustic ,they are are well accepted.
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Marina
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hi theodoropoulos!
I highly appreciate those informations, of course.
I was just joking & admiring.
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spyrosc
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Sound Velocity and choice of Bridge wood
I'm not an expert in acoustics or wood by any stretch, but I do have a Physics and Engineering background and I was interested in this topic. Here's
what I think:
1) You were wondering "why do we use dense woods in the bridge, although as mentioned they must have low velocity of sound".
The equation for the speed of sound you presented is correct and indeed it would appear that the highest the density, the lower the speed of sound
would be. However, that would be true only if the modulus E (which you call the coefficient of stiffness) stayed the same. I believe that that is not
the case.
As you pointed out earlier it is really the RATIO of the coefficient to density that matters, not just the density.
In fact, the speed of sound in hardwoods is roughly 4000 meters/sec whereas in softer woods it is about 3200 m/s a ratio of about 5 to 4.
The density of ebony is about 1200 kg/m3 (on the high side) whereas the density of pear wood is about half of that, 600 kg/m3 a ratio of about 2 to
1.
2) Even if the speed of sound was dramatically different in different woods, I believe the magnitude of the effect in the distances involved may not
be significant. Assuming that the distance on the bridge from the string tie-downs to the surface of the oud is about 1-1.5 cm (I measured a few of
mine) the amount of time it would take sound to travel that distance is of the order of about 2.5 microseconds (according to my calculations which
might be wrong). I don't think, therefore that the choice of different woods would make any noticeable difference to the human ear as far as sound
speed is concerned.
3) So why would you pick pear over ebony? I think there are two issues here:
a) Mechanical strength (those strings are really putting some stress on the bridge).
I don't have any reliable numbers on shear strength of different woods. Maybe somebody can find them. We can then see what order of magnitude
difference we are talking about. It might not be that much.
b) Energy dissipation (loss).
Unless I don't remember correctly I believe that Energy dissipation of the sound wave is proportional to the same modulus you used in the speed
equation.
So it appears that you might be losing more energy with a stiffer wood (such as ebony) than with pear and that is something important. You want as
much of the string energy to be transmitted to the soundboard.
I'm looking forward to some Structural Engineers out there, to give us some enlightenment.
Spyros C.
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theodoropoulos
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spyrosc your opinion is very worthy.my opinion is this...
as far as the loss of energy concerns we can transport our oud to an electric circuit.the roll of resistance plays the bridge.more Ohms correspond to
higher desity.So the denser woods the more thermal energy is lost..
the more loss of energy the lower loudness of the strings..\
BUT.....
why dont we put a very soft wood to gain less loss???what is the roll of Young modulus E???
it plays the roll of Hook's law in springs...but i am not sure i have understood it...i will read and wrie about it in a short time..
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theodoropoulos
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http://www.northstarguitars.com/tonewoods.html
take a look here...
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