The self-sustained oscillation of a brass wind musical instrument involves a complex aerodynamic coupling between a multimode mechanical vibratory system (the lips of the player) and a multimode acoustical vibratory system (the air column of the instrument). In this thesis the behaviour of the coupled system near the threshold of oscillation is investigated using a simplified model in which a single mechanical lip mode is coupled to a single mode of the acoustical resonator by air flow through the lips. The theoretical threshold behaviour is compared with the measured threshold behaviour of a trombone sounded by an artificial lip reed mechanism. Comparability between theory and experiment is ensured by using model parameter values
derived from mechanical response measurements on the artificial lips and input impedance measurements on the trombone.
The mechanical response measurements can be used to classify mechanical modes of the artificial lips unambiguously as either "inward striking" or "outward striking". Each of the
embouchures considered is found to have at least one mechanical mode of each category. The experimentally observed threshold frequencies of the coupled system suggest a behaviour which passes smoothly from "inward striking" to "outward striking" character as the trombone slide is extended or the embouchure parameters changed. It seems unlikely that this type of behaviour can be explained using a lip model with only a single degree of freedom.
After a discussion of the theory of laser Doppler anemome!ry (LDA), the technique is applied to the problem of measuring the instantaneous acoustic particle velocity within a standing wave pipe driven by a loudspeaker. The resulting Doppler signals display quasi-periodic amplitude
modulation with a fundamental frequency equal to the frequency of the acoustic field. The phenomenon of amplitude modulation is investigated in some detail.
Two different methods of analysing Doppler signals are compared: the digit~l Hilbert transform and the Disa analogue frequency tracker; the analogue tracker is found to offer the greater signal-to-noise ratio and dynamic range. Experiments are carried out to establish how phase
errors introduced by the analogue tracker can be minimised:,
Velocity measurements extracted from Doppler signals using the analogue tracker are compared with the velocity deduced by applying basic theory to probe microphone pressure measurements. It is found that the acoustic particle velocity amplitude can be measured accurately
over the entire frequency range considered, and the phase of the acoustic particle velocity also agrees well with theory, but not at low frequencies. LDA is successfully applied to the measurement of multi-harmonic sound fields. The technique of ensemble averaging velocity signals is shown to be particularly useful.
LDA is used to measure the velocity in the backbore of a specially designed transparent mouthpiece, driven by the artificial lip reed. Although significant levels of turbulence are encountered, it is shown that acoustic components can still be clearly distinguished in frequency domain representations of the measured velocity. However LDA measurements in the mouthpiece are restricted to conditions where the acoustic particle velocity amplitude and the turbulent intensity are sufficiently low to ensure that the bandwidth of the Doppler signal is less than the bandwidth of the apparatus used to capture or process the Doppler signal.
LDA measurements in brass instrument mouthpieces should provide a better understanding of the air flow into the mouthpiece and may lead to an improved model for self-sustained oscillation of the coupled system which more accurately describes the air flow.