Posted by John Swensen on April 07, 1999 at 17:50:39:
In Reply to: Compensating/non-compensating?! posted by Paul on April 07, 1999 at 17:17:05:
With a regular tuba, each valve adds an additional length of tubing to the total length of the tuba. Because only one length of tubing is added for each valve, if the valve slides are the correct length for an individual valve like 1 or 2, combinations (like 12 or 14)
will be too sharp. The 12 combination is not too bad, but combinations like 124 are very sharp and are usually played 234 on 4-valve instruments.
A popular way to get around the sharp combinations problem is to add one or two extra valves; on a 5-valve tuba the fifth valve is often a flat whole step that works well with the fourth valve, plus the fifth valve can be used alone or in combinations with other valves for fixing other out-of-tune notes.
With a compensating tuba, actuating the fourth valve routes the tubing back through additional paths through the valves, so that a combination like 14 adds the length of 1, plus the length of 4, plus an additional length in valve 1 to compensate for the combination of 1 and 4. Similar additional lengths are added in valves 2 and 3 when they
are depressed along with valve 4.
Some tubists don't like compensating instruments because of the additional resistance of the extra tubing paths back through the valves.
Another alternative, found most commonly in French horns, is a double instrument, where the air goes through valves 1, 2, and 3 exactly once, but through one of two independent sets of ports. If the fourth valve is not activated, the path with normal length slides is used, but if the fourth valve is activated, the alternative path through valves 1, 2, and 3 is selected, with a full set of slightly longer slides.
The extra weight of the second set of slided, plus the added mass of valves big enough for two sets or ports are the primary disadvantages of double tubas.