Charles Edward Ebbing

Reverberation‐room qualification for determination of sound power of sources of discrete‐frequency sound

In practice, many sources of sound such as motors, pumps, fans, blowers, and transformers radiate sound having discrete frequency components. The reverberation room is a particularly convenient environment in which to determine the sound power produced by such sources provided that the measurement uncertainty can be reduced to an acceptable value. If the measurement uncertainty lies within standardized limits, the room is said to “qualify” for measurements on sources of discrete‐frequency sound. The causes of the various uncertainties in the determination of sound power have been considered. It is shown that, in effect, the discrete‐frequency room qualification procedure allows the measurement uncertainty to be estimated for a monopole‐type sound source. The problems involved with discrete‐frequency and swept‐frequency qualification procedures are discussed, and experimental qualification data for several rooms with and without vanes and other accessories are presented. Methods of generating the frequenci...

Experimental Evaluation of Moving Sound Diffusers for Reverberation Rooms

The acoustic power flowing from a monopole source is proportional to the radiation resistance presented to the source. This value is determined by the modal structure and the acoustic impedance of the walls of the chamber. The rotating sound diffuser modulates the modal structure and with it, the radiation resistance. Comparison of the time average of the radiation resistance as the monopole source location is changed with and without the diffuser rotating indicates the effectiveness of a given diffuser design in removing the dependence of acoustic power output on the source position. The rotating sound diffuser evaluated was a symmetrical biconical surface of revolution rotating at speeds up to 30 rpm. Fifty percent of the surface area of the diffuser was covered with ribbed fiberglass, the remainder was left uncovered. In order to measure the radiation resistance an acoustic impedance head was fabricated from a modified loud speaker. Displacement of the diaphragm was sensed using a noncontacting fiber‐o...

Use of Mean Squared Pressure along a Reverberant Room Diagonal for Sound Power Measurement

The mean of the squared pressure along part of a room diagonal is frequently used to determine the sound power using the reverberant room technique. The spatial pressure along a room diagonal has been evaluated experimentally and theoretically for a source located in the corner of a 300‐m3 and a 150‐m3 reverberant room. For frequencies between 100 and 2000 Hz, the average number of maxima in the mean squared pressure 〈p2〉 is given by N=4L/πλ, where L is the length along the diagonal and is independent of absorption, room size, and stationary diffusers. The variation of 〈p2〉 at a maxima is given by 〈p2〉=An cos(πkx/4), where the An possesses a gamma probability distribution that depends on absorption. It is shown that the value of the mean of 〈p2〉 for white noise with a 6‐Hz bandwidth depends on the traverse length selected and is independent of damping while the standard deviation is improved by adding damping. Experimental results substantiate this conclusion.