freya
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Speed of Sound in Wood
I dislike hijacking threads so instead of butting in to the "Multiple piece soundboard?" thread, I've started this one. For several years I've been
interested in Lucchi's measuring equipment but knowing that I'll never have the $$ to get one of his meters I post this as an FYI.
Attachment: LUCCHI eng.pdf (516kB)
This file has been downloaded 467 times
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jdowning
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The use of ultrasonic techniques has had a wide application in the engineering field for many years to detect potentially dangerous flaws in highly
stressed components such as cracks or inclusions in metallic welds or more recently to determine important material properties of materials such as
grain size in metals.
A similar non destructive testing procedure is also used to determine the structural properties of wood - in the log or at the saw mill as part of the
lumber grading process so has significant commercial importance given the variable nature of wood as a material. This methodology and equipment used
enables important properties such as elasticity to be determined as well as flaws in the wood (cracks, knots, rotted areas etc), grain direction and
so on.
The equipment essentially consists a signal generator, a signal receiver, and a box of electronics to measure the time between the signal pulse and
its receipt. This equipment is expensive - costing several thousand dollars like the Lucci meter.
However with the improving electronic technology of recent years it has been possible to determine flaws and mechanical properties of timber more
simply by hitting the end of a log (or piece of wood) with a hammer to produce a sound pulse and to measure the time it takes for the pulse echo to
return. From this the speed of sound can be determined parallel to the grain which in turn can be used to determine modulus of elasticity in the
direction of the wood grain and then the important stiffness to weight ratio. The speed of sound across the grain can similarly be determined.
The stiffness to weight ratio is important for stressed wooden structures such as aircraft frames where lightness and strength is critical but it is
also used to evaluate the acoustic 'goodness' of instrument sound board material where maximum stiffness along and across the grain combined with
light weight (or rather density) is used as one criterion in sound board selection.
A couple of years ago violin maker Don Noon proposed a very simple, low cost method to determine the velocity of sound parallel to the grain in violin
top billets and hence their acoustic 'goodness' (or Radiation Ratio) - important to know before all of the exacting time consuming work in shaping a
violin top is undertaken. His method - discussed at length on the Maestronet forum for violin makers - is to place a microphone close to one end of
the billet, strike the other end with a very light hammer and record the audio signal received by the microphone on a PC. The audio clip is then
subject to spectrum analysis using the free open source software Audacity from which the speed of sound etc can be determined. The procedure at the
time was in the experimental stage but the results appeared to be promising.
I am currently undertaking trials to determine the practical limitations of this method in assessing the acoustic quality of thin sound board blanks
as well as to, hopefully, study the effect of sound board treatments. I shall report my findings later on this forum as they become available.
So - no need to spend thousands on fancy ultrasonic hardware - all that is required is a personal computer, a cheap microphone, and Audacity
software.
On the other hand, having found the 'perfect' sound board blank - by whatever process - is no guarantee that an acoustically superior instrument will
result. That is all in the hands of the individual luthier.
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jdowning
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As this topic is specifically about the Lucchi Meter this paper by J.E. McLennan about comparative testing of the meter will be of interest. "The
Lucchi Elasticity Tester" publication can be downloaded here :
http://www.phys.unsw.edu.au/music/publications/mclennan/lucchi.pdf
Interestingly my preliminary trial measurements on mild steel calibration bars using the Don Noon's method gives a velocity of sound of 5159
metres/sec and Modulus of Elasticity of 20.4 GPa (or N/m³) - so compares well with both the quoted commercial values (4700 -5200 m/s and 20 GPa) and
those measured by the Lucchi meter (5137 m/s and 20.7 GPa).
However, all of this will eventually be presented in greater detail as part of another related topic on this forum.
It should be noted that the Lucchi meter - operating as it does with transmitter and receiver devices - if I understand correctly can measure speed of
sound through curved pieces of wood (like completed violin top plates) whereas the Don Noon method measures reflected sound pulses travelling in
straight lines, so is limited to the assessment of straight, flat billets of wood.
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