The operation of the pressure gradient microphone is compared with that of the pressure microphone It is shown that the pressure gradient microphone may be used to measure particle velocity in a sound wave. The advantages of the pressure gradient microphone in making loudspeaker measurements, particularly outdoors, are pointed out and experimental data are given for some arrangements which were tried out. The characteristics of the distribution of particle velocity in a complex sound field are studied theoretically and experimentally with a ribbon microphone. A method is described for measuring the energy density in a sound field and some measurements which were taken in complex sound fields in rooms are discussed. It is shown that a combination of three pressure gradient or velocity microphones with a pressure microphone, placed adjacent to each other, may be equivalent in eliminating interference patterns to four pressure microphones placed at distances large compared to the wavelength and with random distribution. A microphone for measuring energy flow in a sound field is described.
Directivity has been found to be desirable in sound collecting systems to improve the ratio of direct to generally reflected sounds and to otherwise discriminate against undesirable sounds. The bi-directional ribbon microphone is a pressure gradient instrument in which the response corresponds to the velocity component in a sound wave. By using a mass controlled element the velocity of the ribbon is in phase with the velocity in a sound wave. By a suitable combination of this microphone with a pressure-operated nondirectional ribbon microphone, a uni-directional ribbon microphone is obtained. The pressure ribbon microphone is resistive controlled and the response is a measure of the pressure component in a sound wave. The velocity of the ribbon in this type of pressure microphone is in phase with the pressure. Combination of the outputs of the pressure and velocity ribbon microphones produces a uni-directional characteristic. The response as a function of the angle θ between the normal to the ribbon and the direction of propagation of the incident sound is expressed by R = R0(1 + cos θ), where R0 is the sensitivity of either the pressure or velocity microphone for θ = 0. Results with an experimental microphone utilizing these principles verify the theory with respect to the phase relations and the directional characteristics. This microphone has been found to be useful in sound motion picture recording, radio broadcasting and sound reinforcing systems where the direct sounds originate in front and undesired sounds to the rear of the microphone.
A new type of loudspeaker is described in which a single mechanism is coupled to two horns: a straight axis high frequency horn and a folded low frequency horn. A theoretical analysis of the combined system is given and experimental data are shown which indicate smooth uniform response from 50 to 9000 cycles, and an efficiency of the order of 50 percent over a large portion of this range.
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