< Whistles 1 >
As William is rather small model, I have neither enough space for a big whistle nor enough steam to drive it. However I wish to have deep tone and at least three-note chime, and also pitch-bending whistle like a full size one. So I will try as hard as I can.
Steam whistle pitch is determined due to following formula.
tube length + 'open-end correction' = 1/4 * wavelength
The open-end correction depends on whistle construction and steam pressure. As the pressure rises, the open-end correction decreases and whistle pitch increases. Therefore if you wish pitch-bending, you have to prepare a whistle with wide range of pressures.
In proportion to the steam pressure, opening length or gap width should be larger. 'U' shape or oval shape opening makes the length ambiguous. As a result, they can resonate with wide range of pressure. Triangle shape opening is identical but it makes poor sound.
Opening length and gap width can be adjusted as follows. If they are too small the whistle resonate with shrill voice (the third harmonic), and if too large the whistle sounds poor and husky. You can start with a small gap and increase it until the third harmonic sound disappears. It should be done with maximum steam from your boiler, instead of your breath. Incidentally, opening area should be smaller than the section area of the tube. If it is too small, you have to increase tube diameter itself.
Chime whistle doesn't sound like a chime without good volume balance. Due to the above-mentioned reason, the opening or the gap only control the sound quality, not the sound volume. Volume control should be done by steam volume itself for each whistle, adjusting tube diameter or inserting orifices.
To return to my William, there is no space for the whistle between the main frames, so I arranged to mount two whistles under the both running boards, simulating compressed air containers. Maximum size is 25mm dia. and 100mm length. Each whistle tube is divided into two parts by partition plate. One of the whistles generates higher two notes and the other has a turn back at the bottom to generate one deep note. I referred several chords of full size locomotive chime whistles and chose a chord 'C-G-Bb'. The middle note is determined by the tube length. The top and bottom notes are adjustable with positioning of half-round plates.
The higher two-note whistle has two openings in opposite side, upper and lower. I shifted the upper opening toward the edge of partition plate and also slightly tilted the plate, so as to allow condensed water to drain out of the tube.
A tube-cutter which was employed to cut the whistle tube from a long brass tube.
The oval-shaped openings was filed out, starting with a round file moving across the tube till the tube skin breaks and following with half round or flat file to get desired shape.
Two bushes and one stud were silver soldered onto the whistle. The former is for fixing the whistles under the running boards and the latter is for steam connecting from whistle valve.
The front plug is integrated to the gap-control disc. The groove between them is as a steam chest.
The gap is milled in a jig on the rotary table. The jig has round recess close-fitting to the plug. I started with 0.2mm gap for each note.
The partition plate is made from 1.5mm brass sheet. Each plug have 1.6mm slit to hold both end of the plate.
Before steaming up for adjustment, it is worth testing with a bicycle tire pump. Press the pump handle as fast as you can, then you get a boiler pressure. I prepared branched tube from whistle valve to whistles in advance, then connected two whistles together and let them go. The lower one-note whistle was louder than the others. Therefore I introduced a micro brass tube into the steam stud as an orifice. Then the volumes became well balanced.