A standing-wave tube as an absolutely known source of sound power

An open ended standing-wave tube has been examined as an absolute sound power (ASP) source, having an absolutely known radiated sound power. The ASP source can serve as an acoustical instrument, principally in reverberation chambers, for three purposes. The first is for calibration of the presently used arrays of microphones for measurement of total radiated sound power by steady sources. The second is for measurement of the influence of acoustical diffusion devices, such as moving vanes, on the radiated power of a source. A third application is to the steady-state measurement of the total cross section for sound absorption. The power measurement scheme makes use of concurrent measurements of sound pressure, both amplitude and phase, at two separate points inside the tube and on its axis. From these, the time-averaged product (Umov vector) of the particle velocity and pressure can be obtained. The product gives accurately both the sound intensity within the tube and the radiated power from the open end. Optimum accuracy for measurement of intensity is achieved by large microphone separations (one-quarter wavelength). Comparison of indicated sound power radiated from the tube with the usual sound power measurements done in both an anechoic and a reverberant environment are made.