| Pages:
1
2 |
jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
|
|
The test sample was flooded with water first on one side then on the other and left overnight to dry out. before acoustic testing.
What became immediately apparent is that on applying water to one side the test sample became bowed - curving away from the moistened face. This means
the the oil offered little or no resistance to the water being absorbed by the wood - the wood cells swelling as they readily absorbed the water on
one side so causing the test piece to bend longitudinally. The test piece became straight again on drying.
Acoustic testing of the dried out test piece showed no significant change from when saturated with oil on both sides.
The test piece was originally prepared by cutting a larger section of spruce down the middle to create two matching quarter sawn pieces - one of
thickness 2.5 mm being used for this current sequence of tests.
The remaining piece has now been prepared to a thickness of 1.5 mm and will be acoustically tested both free of oil and oiled on one side. It has been
previously demonstrated that the depth of penetration of the oil - when wiped on to one surface - is estimated to be less than about 0.3 mm. This
further test is to determine if this level of oil penetration, being proportionally greater than in the thicker specimen, has any significant effect
on speed of sound and internal damping. In this case the oil layer would occupy about 20% of the cross section area of the 1.5 mm thick specimen
compared to 12% in the case of the 2.5 mm thick specimen. A thickness of 1.5 mm is about the thinnest practical limit for a lute or oud sound board
and 2.5 mm is about the maximum upper limit.
It is not known if such a thin test piece will yield a clear frequency spectrum so this next trial will be of interest from that perspective alone.
|
|
|
jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
|
|
The thinner 1.5 mm thick test piece produced good results with five clear harmonic peaks on the frequency spectrum analysis. Speed of sound un-oiled
was 5388 m/s - no different from that of the 2.5 mm thick test piece.
For the oiled case, speed of sound was 5366 m/s - so no significant change due to oiling giving a sound radiation coefficient R of 10.5.
From the waveforms, the estimated loss factor due to internal friction un-oiled was 0.010 whereas that for the oiled case was 0.012. So for the thin
1.5 mm sample the applied oil does cause a relative significant increase of about 20% in the damping of the sound vibrations (i.e. reduced sound board
resonance) whereas for the thicker 2.5 mm test piece there was no significant difference in the loss factor between the oiled and un-oiled cases.
So - what might be the consequences of applying a non drying vegetable oil to a sound board based upon these trials?
The oil is non drying - i.e. it will never solidify, ever.
If wiped on in the manner of a drying oil finish, it will only be absorbed into the first few wood cell layers of a sound board up to an estimated
depth of about 0.3 mm. This oil layer will cause increased internal damping of sound waves (reduced resonance) longitudinally and radially (the latter
has not been tested here) - the thinner the sound board the greater the effect. There may be some slight reduction in the Sound Radiation Coefficient
due to the oil layer increasing the sound board density - more so in thinner sound boards (but not measured in these tests).
Both a reduction in the Sound Radiation Coefficient and increase in the loss factor are considered to be detrimental to the 'acoustic goodness' of a
sound board material - particularly in the case of plucked instruments. However it is difficult to estimate how significant or otherwise (good or bad)
any such small changes to the sound board acoustic properties as a result of oiling may affect the acoustic performance of a completed instrument -
there being many critical factors in the design and fabrication of an instrument than just the acoustic goodness of the sound board material.
Also, over time the oil will attract dirt and grime. Furthermore contamination by oil of the wood cells on the sound board upper surface might
complicate future bridge and pick guard replacement or sound board inlay repairs.
So - all in all - I would agree with fernandraynaud that coating an oud (or lute) sound board with a non drying vegetable oil is not a good idea.
Now to remove the bottle of Almond oil from my workshop as it is very easy to contaminate wooden bench tops, wood stock and wooden components when
handling the stuff.
|
|
|
oudistcamp
Oud Junkie
Posts: 144
Registered: 5-27-2009
Member Is Offline
Mood: No Mood
|
|
Thank you for the tests,results and conclusion.
I will let you know how the face looks, and what the oud sounds like, when I see it next month.
|
|
|
jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
|
|
I forgot to add that non drying vegetable oil does not provide a barrier to the ingress of moisture to sound board wood - but then neither does a
drying oil, water vapour entering not through the oil layer but via the internal surfaces of the sound board that are normally left untreated.
Both moisture and high temperature in wood reduces speed of sound as well as increases the loss factor to the (temporary) detriment of the acoustic
performance of an instrument - a condition familiar to many who play oud or lute during the hot humid days of summer.
|
|
|
| Pages:
1
2 |
![[*]](images/xmbprobrown/default_icon.gif){kind=icon}
