A friend of mine wondered what kind of sound is the sound of a loco having compound engine,i.e. a two cylinder loco having the other side of the loco having cylinder for high pressure steam and the steam is directed not to chimney but to (larger) low pressure cylinder on the other side of the loco.
We have had such locos in Finland and they had a special control valve (called the "wip" ["piiska"])that allowed both sides of the loco work as high pressure cylinders to enable the loco to start in case it has halted so that the normally high pressure cylinder is in "dead" position.
When this loco starts, how soon the cylinders have been connected from parallel to series with this special valve, and how does the sound of the loco change -- is it like switching to second gear as every second exhaust blast probably dissapeared...
In compound settings the exhaust was much softer than for a simple locomotive. There would also be only half the number of exhaust beats. (2 per revolution)
Under "simple" valve settings the locomotive would sound just like a normal 2 cylinder simple, (4 beats per revolution) but I imagine alternate beats would be unequal as the amount of steam passing would be unequal (1:2.25 approx.)
The maximum speed in the simple setting is probably about 20% of the locos maximum speed, but the setting uses a lot of the boiler's capacity so the change-over speed is going to be as early as possible, dependent on tthe load being started, gradient and track curvature.
The "special valves" were a wonderful playground for inventors so there were many different types, from fully automatic to entirely manual. The French for example liked everything under control and adjustment of the driver while other railways had much lower opinions of drivers capabilities and made the system automatic. Some systems simply added live steam into the receiver between HP and LP cylinders so that the bigger LP cylinders did all the work while the HP cylinder had HP steam at both inlet and outlet. As this HP receiver inlet was very restricted the effect died away as speed increased from stopped. However that system would mainly be on 4 cylinder Loks.
3 cylinder compounds would sound different again :-)
My guess, for a two cylinder compound, with equal cut-offs would sound:
- starting: chuff/CHUFF/chuff/CHUFF/chuff/CHUFF
- just rolling: chuff /CHUFF /chuff /CHUFF (cutoff being wound back a little)
- changeover chuff./...../chuff./.....
- accelerating to running speed: fluff.../...../fluff.../.....
- at running speed fluff....../....../fluff....../....... etc.
The action of the cutoff is interesting. Some railways used levers on quadrants and others the wheel with nut on a threaded rod. Depending on ease of operation, drivers often set the cutoff and then drove on the throttle (inefficient) while others manipulated power by using both or mostly the cutoff.
In fact, all I've actually heard are recordings of French and ex Austrian compounds. The French locos were quite boring as the drivers aim in life seemed to be to use as little steam as possible, while the Austrian (Jugoslavia, I think) were much more interesting, particularly the change-over while losing momentum uphill!