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

CHAPTER 6. MATERIAL AND SURFACE EFFECTS 95
Table 6.1: The treatment schedules for the five test pipes.
Pipe 1 Pipe 2 Pipe 3 Pipe 4 Pipe 5
Bore oiling none none none once per
week
Warm humid none 23 hours 1 hour per 23 hours
air treatment
per day day
per day
once per
week
1 hour per
day
before and after oiling as a qualitative test of the acoustics of such a non-optimal material.
The extent of the humid air exposure varied among the pipes, and some were oiled while
others were not. The treatment schedules for the five pipes is summarised in Table 6.1. This
schedule is designed to simulate an extreme range of playing conditions. Pipe 1 is the control.
We would not expect to see much change in the impedance and surface characteristics of this
pipe over the course of the experiment. Pipe 2 simulates the flute of a most negligent owner.
The bore is not oiled and it is ‘played’ (i.e. exposed to warm humid air) constantly. Pipe 3 is
‘played’ for only 1 hour per day but never oiled. Pipe 4 is oiled diligently but ‘played’ constantly.
Pipe 5 is oiled diligently and ‘played’ for only 1 hour per day.
In conformity to Terry McGee’s standard practice, the oiled pipes were oiled initially with
raw linseed oil and thenceforth with a commercial bore oil. The oiled bores were allowed to dry
overnight before humidity treatment was started or resumed. Once a day the humidity treated
pipes were dried with a cloth swab. The air was humidified by bubbling it via a pump through
water held at 32°C. Acoustic impedance was measured using the methods outlined in Chapter
3, using two microphones with a separation of 40 mm.
Once each week, the input diameter to each pipe was measured, the pipe was weighed, and
the acoustic impedance spectrum of the pipe was measured.
6.3 RESULTS AND DISCUSSION
Figure 6.1 shows a single slice of a 3-dimensional image of a pipe, obtained using X-ray microtomography
(micro-CT). The surface shown is perpendicular to the axis of the pipe and the
slightly curved surface at the top of the image is part of the bore. Clearly visible are the wood
cells and some deformation extending up to a dozen cells deep caused by the reamer that was
used to cut the bore.
Terry McGee played the radiata pine keyless flute before and after oiling. Before oiling Terry
could not play any notes below A4 (the flute was designed to play D4 with all holes closed) and
the notes he could play were ‘weak and noisy’. In a test of the porosity of the timber, Terry
closed all the finger holes and the end hole and was able to suck air through the walls of the
instrument. Clearly, the attenuation of the acoustic waves with distance along the flute was
much larger than that for a pipe with smooth walls. After oiling the flute all of the notes were
playable, but they were weaker and more difficult to play than on a comparable instrument
made from a more typical flute wood. Terry’s account of the experiment may be found on his
web site at .
The pipes that were humidity treated for 23 hours each day increased in mass over the