THE MILLER VALVE

A PATENTED NEW ROTARY VALVE DESIGN FOR BRASS MUSICAL INSTRUMENTS

U.S. No. 5,798,471 U.K. No. 2,322,224
Ger. patent pending

by Robert Miller

A musical instrument valve needs to function well in two aspects. It needs to seal and move reliably and quickly to engage or disengage a tubing loop and, ideally, it should not alter the sound wave as it passes through the valve. The traditional rotary valve moves well. Because the rotor spins on a small central bearing shaft friction is low, the mechanical advantage of an actuating lever will speed rotation and the small gap between the mating surfaces is reliably maintained for a good seal. However, it alters the sound wave substantially. As the sound wave passes through the valve it travels across two to four relatively sharp edges. This causes turbulence in the airstream and removes some of the energy from the sound wave. The sound wave also encounters a change in bore size and shape. This again removes some energy from the sound wave and causes a small portion of the wave to reverse direction. These defects become more pronounced at high sound volume levels and as the number of valves used on the instrument increases. The path of the sound wave changes direction substantially as it passes through the valve and this causes the wave to reflect off the walls of the valve passage, further altering the sound.

A traditional piston valve provides somewhat less sound alteration. The sound wave does not encounter any sharp edges as it passes through the valve and the bore size is usually constant.

However, the change in direction of the path of the sound wave as it passes through the valve i s still substantial. The movement characteristics are less reliable. The sealing surface, which is the out side of the piston, is also the bearing surface of the piston and thus , there i s more bearing surface to generate friction. The movement of the piston is entirely linear. However, it is difficult to make fingers accommodate linear motion throughout the valve stroke. The result is that some side pressure is applied to the piston during movement with a corresponding increase in friction. On large instruments such as the tuba and euphonium the large bore requires large piston valves. As the mass of the piston increases so does the energy needed to overcome the inertia of the valve.

The Miller valve is a rotary valve which incorporates the movement advantages of the traditional rotary valve and betters the sound alteration characteristics of the piston valve. It consists of a hollow outside cylinder (casing) and a hollow inside cylinder (rotor) which spins on a central bearing shaft. Inside the rotor three hollow tubes are soldered in place to provide constant size bore passages which direct the sound wave into and out of the loop or straight through the horn. Since the cylinder rotates, the tubes which engage the loop can be placed on either side of the main tube and the deviations necessary to avoid interference as the tubes cross the rotor can be shared by all three passages.

 
Each passage is continuously circular in cross-section, both within the passage and at the inlets and outlets. Thus, there is no compression or expansion of the sound wave as it passes through the valve. All of the passages have a minimal lateral deviation as they cross the rotor. As a result, there is less reflection of the sound wave off the walls of the passage as it passes through the valve and less alteration of the sound. The outside loop engaging passages each deviate about one-half of the bore diameter as they cross the rotor and the center passage deviates about one bore diameter. Thus the total lateral deviation between the engaged and unengaged positions is about equal and there is little change in tone color when engaging and releasing the valve.

This valve is made of brass and has the resonance associated with a traditional brass instrument. It is made with traditional rotary valve construction and each valve is hand lapped for a tight seal. The valve operates quickly because the rotor is hollow and the actuating lever uses increased mechanical advantage to spin the rotor. The Miller valve is the most open, free-blowing valve available. It has excellent response in all registers. The sound of the valve is warm and relaxed at soft to medium volume levels and remains focused and clear through the loudest fortissimo.

 

Since the tubing loop ports are located 45° above and below the main tube ports this valve can be easily ganged together. For instruments which require a shorter loop than is possible on this valve, one of the valve passages which engages the loop can be altered so that both loop ports are closer together. In these configurations this valve can be used on most brass instruments.



© 2000 International Trombone Association. Reprinted by permission.