Flute acoustics: measurement, modelling and design - School of ...

CHAPTER 6. MATERIAL AND SURFACE EFFECTS 96
Figure 6.1: A micro-CT image of a small bore section.
course of the experiment by a maximum of nearly 40%, with oiling making little difference.
The pipes that were humidity treated for only 1 hour each day increased in mass by a maximum
of 10% (for pipe 3) and by 5% (pipe 5). The input diameter of all of the humidity treated
pipes decreased by approx. 3% as a result of the treatment, as the moisture raised the wood
grain on the bore surface.
Selected acoustic impedance spectra for the five pipes are shown in Figures 6.2 to 6.6. In
each figure the impedance measured before treatment is shown in black. The impedance measured
after five weeks of humidity and oiling treatments is shown in red, and the impedance
after one further week of drying is shown in green.
Exposing the bores to warm humid air (for prolonged or minimal duration) raises the grain
significantly. The acoustic effect of this is seen in the impedance spectra of the two un-oiled
but humidity-treated pipes. In Figure 6.4 (pipe 3) both impedances measured after treatment
show a greater attenuation and the maxima and minima are shifted to the left in frequency.
This is presumably due to the extra compliance of the rough surface. In Figure 6.3 (pipe 2) the
attenuation decreases during treatment (red trace) and increases after the pipe has dried out.
This shows that the roughness of the raised grain is ameliorated when the surface is wet since
the water fills the bores of the wood.
The acoustic impedance spectra of the oiled pipes show the same general trend as those
of the un-oiled pipes. Here however, the impedance spectra before and after treatment are
roughly the same. This is despite visible raising of the grain caused by the humidity treatment,
suggesting that the oil fills the pores of the wood and compensates for the extra attenuation
caused by the raised grain. In Figure 6.5, we again see that the attenuation is significantly reduced
when the wood is wet.
Samples were taken from stubs of each pipe at weekly intervals. These were imaged with
moderate success using confocal microscopy, however the images obtained are difficult to interpret.
I also investigated the use of micro-CT, a technique that uses X-rays to probe the sample
(Figure 6.1), and scanning electron microscopy (SEM). For standard SEM the samples need
to be dry—so it is impossible to investigate the surface characteristics of wet wood using this
technique. Such a task could be accomplished using environmental scanning electron microscopy
(ESEM), although this was not attempted for this thesis since the required equipment