The Measurement of the Acoustic Impedance of Brass Instruments

An apparatus is described which is largely free of the limitations of earlier methods of measuring the input impedance of brass instruments. The concept of acoustic impedance is discussed and those earlier methods are briefly reviewed and are shown to suffer from the following disadvantages: (i) absolute calibration is difficult to achieve, (ii) the plane of measurement is not readily movable, (iii) the effect of steady air-flows cannot be studied, (iv) phase information is not easily extracted, (v) only sine wave test signals may be used, and (vi) measurements cannot be made when the instrument is being played. The new apparatus measures both the acoustic pressure and the particle velocity simultaneously using a horn coupled probe microphone and a hot wire anemometer and is controlled by an on-line mini-computer. The calibration procedures of both pressure and velocity transducers are described in detail in view of the fact that hot wire anemometers, in particular, are not commonly used in musical acoustics. The method used to make a swept sine wave measurement of the input impedance is described and the corrections necessary to achieve an accurate result discussed. The results obtained from measurements on a straight cylindrical tube are compared with theoretical predictions developed by the authors from many sources and show good agreement. Results of measurements on trombones are illustrated and an indication of the way that computer processing may aid analysis and intepretation is given.