Dr Jonathan Kemp Tutor and Research Associate Acoustics and Fluid Dynamics Group The Department of Physics and Astronomy The University of Edinburgh Room 4201 James Clerk Maxwell Building Kings` Buildings Mayfield Road Edinburgh EH9 3JZ Tel: 0131 650 5257 E-mail: jonathan (at) ph.ed.ac.uk Business Home Page: http://www.kempacoustics.com Personal Home Page: http://www.ph.ed.ac.uk/~jonathan




I completed my PhD in 2002 here at the Acoustics group and teach at St. Andrews University, the Open University and now also as a part time lecturer in musical acoustics at Edinburgh University. The technique of acoustic pulse reflectometry is used in musical acoustic research here. Develpments to the technique are set out in my thesis, following on work from David Sharp's thesis. An acoustic pulse (or audible click) is produced by a loudspeaker and is directed down the object to be measured. The reflections from the object are measured by microphone and sampled by computer. Accurate determination of the internal profile and input impedance is possible from this, and also leaks may be detected. Figures 1a and 1b show the portable pulse reflectometer system developed here, which has been used in the workshops of musical instrument manufacturers. Currently the bore reconstruction algorithm used in acoustic pulse reflectometry assumes that acoustic propagation in the object under test is in the form of plane waves. In reality the propagation satisfies the boundary condition that the wavefronts must be perpendicular to the wall. The plane wave assumption is therefore perfect for cylindrical objects and works well for conical musical instruments since the apex angles tend to be very small. For the bell section of brass instruments however the rate of flare in the tube becomes very large and the plane wave assumption is no longer valid. In order to provide a theoretical basis for the study of propagation in instruments of large flare, the theory of higher order modes has been studied. Higher order modes are sound waves with a non-uniform pressure distribution perpendicular to the direction of propagation. My work in this area is in collaboration with Noam Amir at the Tel Aviv University, in Israel. A calculation of acoustic input impedance and pressure fields including higher order modes for the bell section of a trumpet are shown in figures 2a and 2b. Note how the wavefronts in figure 2b are perpendicular to the wall. The agreement with experiment is better when higher modes are included in the calculation. The study of the theory of higher order modes can be expected to lead to their inclusion in the bore reconstruction calculation, leading to better reconstructions of instruments with large rates of flare.





Figure 1a. The portable pulse relectometer

Figure 1b. The portable pulse reflectometer measuring a renaissance cornet

Figure 2a. Multi-modal calculation verses experiment for the input impedance of the last 50.4cm of a trumpet. Click here to view full size

Figure 2b. Multi-modal calculation of the pressure field in the last 50.4cm of a trumpet. Click here to view full size

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Last Updated November the 20th, 2007
Contact: jonathan (at) ph.ed.ac.uk