Study of the optimal active control of a trombone

Abstract

In the brass instrument family, the sound can be modified or attenuated using a mute, which is usually inserted in the bell of the instrument. The objective of this paper is to study the principle and the technological feasibility of an active mute using loudspeakers placed in front or around the instrument bell. This mute must reduce the acoustic power emitted by the instrument while avoiding any impact on its playability. At this stage, an optimal control is considered and no real-time controller is implemented. Results show that an active control placed outside the trombone is theoretically feasible and can be efficient to reduce the acoustic power up to 2000 Hz by placing a ring of sources around the bell and a source in front of the trombone. The instrument input impedance is very slightly affected by the control. In accordance with numerical simulations, the experiment showed that placing a control speaker very close in front of the bell of the instrument modifies the pressure field of the instrument in such a way that it allows to obtain a power attenuation greater than the predicted one. The control is technologically achievable but requires a high power for the closest source.