Experimental demonstration of the effect of wall vibrations on the radiated sound of the horn and a search for possible explanations.

The question of whether the in∞uence of wall vibrations of various musical instruments involving resonant air columns like brass wind instruments, wood wind instruments, organ pipes and so on, is strong enough to be objectively observable in their radiated sound is still a controversial issue. Instrument makers and musicians are making a strong claim that wall thickness, material and conditioning are crucial factors for sound quality and response of wind instruments while acousticians often remain sceptical. In this work a further attempt is made to identify the claimed in∞uence of wall vibrations on the sound of horns and to flnd or exclude possible explanations. The experiments have been made with an artiflcial mouth sounding the instrument continuously while gushing sand was slowly damping the walls of the instrument more and more. In order to eliminate the efiect of difierent surrounding acoustics the horn was mounted in a box damped with styrofoam which was fllled with sand during the experiment. Damping of the bell as well as decoupling of the mouthpiece, which was connected to the instrument using a piece of ∞exible rubber tube, has been quantitatively verifled using miniature accelerometers. Big difierences in harmonic levels have been observed but mechanical feedback to the mouthpiece as a possible explanation can now be excluded. A change in radiation impedance caused by vibrating walls seems to be a reasonable explanation and has to be further investigated.