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EXPERIMENTAL INVESTIGATION OF DOPPLER SHIFT AND INFRASOUND GENERATION DURING WAVE PROPAGATION WITHIN THE BORE OF THE TROMBONE DURING SLIDE MOVEMENT

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Original languageEnglish
Title of host publicationProceedings of the Third Vienna Talk on Music Acoustics
Place of PublicationVienna
Pages9-14
Number of pages6
StatePublished - Sep 2015

Abstract

During glissando playing in the trombone the length of the approximately
cylindrical slide section within the bore is altered
while waves are propagating. Slide movements of 2 metres per
second are not unusual. The simplest way to visualise the effect
is in terms of the slide being represented by a moving reflector,
resulting in a (small but measurable) Doppler shift in the
wave coming from the mouthpiece before it arrives at the bell
for instance. An additional effect is to be observed in terms of
the volume of air within the instrument changing telescopically,
leading to a localised change in DC pressure (and a resulting
flow) which generates infrasound components within the bore
and also impacts on the sound velocities for forward and backward
going waves. Lastly there will be sections of bore with
moving walls which could introduce additional mean flow effects
and excitation of (mostly evanescent) transverse modes of
vibration.
In this study experimental data is presented showing the
pressures measured by microphones mounted in the mouthpiece,
in the water key (in the slide section) and at the bell of a trombone
while slide movements are performed. Some measurements
were performed using a fixed excitation frequency provided
by a loudspeaker mounted onto the mouthpiece. Moving
the slide results in changes in both the amplitude and frequency
of the signal being measured by microphones (in spite
of the input signal being produced by the loudspeaker being
fixed in frequency). Infrasound components were also detected
inside the instrument bore. Frequency tracking of audio was
combined with optical tracking of slide movement to provide
evidence concerning the nature of the physics of wave propagation
within the dynamically changing trombone bore and conclusions
drawn concerning any implications for perception and
synthesis.

ID: 22096702