A New Device for Measuring Local Acoustic Power Output of Subsonic Jets

Many devices have been developed for measuring noise intensity output from each volume element of a subsonic jet. However, none of these can measure, in a single operation, the total power output from the volume element. A new device will be proposed to fulfill this need with a good spatial resolution and a desirable frequency characteristic. We point out that in order to measure the power output in a single operation with a device of a manageable size, the device should operate in the spherical farfields (i.e., not in the plane wave fields) of the noise sources. Hence, our device will be made of many microphones mounted on a semispherical tubular skeleton. It will operate in the spherical farfield and will utilize the technique of interferometric phase‐cancellation to achieve a good linear resolution (close to half‐wavelength) and to measure the acoustic power output in a single operation with a good frequency characteristic (from 500 to 15 000 Hz), as confirmed by computer simulation and partially by experiments. Many microphones are needed to cancel out all the large sidelobes. Recently, a good and inexpensive electret microphone has become available with a weatherproof casing, to make the semispherical array economically feasible.