Posted by Leland on January 15, 1999 at 13:38:16:
In Reply to: Re: Re: Re: valve posted by mike on January 14, 1999 at 23:06:33:
In this case, it's the idea of removing that "pop" that happens when you pull a valve slide, _then_ press the valve. Say for example, on a certain BBb tuba the C in the staff plays flat and the Ab just below plays sharp. If you're into slide-pulling (I'm not that much, but I'll do it if I have to), have a slurred passage that goes C-Bb-Ab or something similar, and you want to move the 1st valve slide for better tuning, there is a real possibility of a nonmusical or frack-inducing "pop" or "thoomp" caused by moving the slide during the Bb and engaging the valve for the Ab.
Sorry for the non-graceful explanation, but that's the idea behind vented valves. If tolerances are close enough, there shouldn't be any whistling, although I could see it happening if tolerances are a bit loose or the valve slide is really thrown around. The vent itself is in a location that never contacts an "active" pipe, meaning that it won't be sitting there buzzing when you play a note.
Piston valves can be vented, albeit with much more risk involved -- a hole is drilled into the side of the piston where a valve tube appears, so that the vent pasage goes from that hole to the vents on the top & bottom of the piston. The extra risk is there because it's already a much closer fit than most rotors.
How's that?
Leland