Hagmann valve
The Hagmann Valve is a rotary valve for trombone that was developed by René Hagmann in the early 1990s. His intention was to address some of the problems with the popular Thayer Valve, in particular the maintenance issues caused by its relatively complex design.
A standard rotary valve turns the air flow as much as 90 degrees when it is engaged. This does not substantially affect the sound of instruments like the French Horn which already have lots of turns in the air flow, but part of the characteristic sound of the trombone comes from its long, straight air flow. The angles on a Hagmann valve are dramatically less (60 to 66 degrees) which makes it much easier for the player to maintain proper tone with the valve engaged. While the Hagmann Valve does not achieve as low an angle as the Thayer Valve (which can be as low as 20 degrees), it is much easier to maintain and the tone differences are nearly inaudible.
Many leading trombone manufacturers now offer Hagmann valves as an option, especially on their professional ranges.
Sources
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