Volume 12, Issue 4, April 1941
Index of content:
12(1941); http://dx.doi.org/10.1121/1.1916128View Description Hide Description
A new microphone is described which consists of a moving coilpressure element combined with an improved ribbon pressure gradient element to give a cardioid directional characteristic. The theory of operation is reviewed, and consideration is then given to variations in directivity caused by diffraction, separation of the elements, and disparities in their phase and response characteristics. It is then shown how these variations are largely eliminated by equalization in the electrical circuit so that the resulting directivity is practically independent of frequency throughout the range from 70 to 8000 cycles. The use of a moving coilpressure element makes high efficiency possible, while the design of an unusually rugged ribbon element provides a marked reduction in noise due to air currents. Several useful directional patterns in addition to the cardioid pattern are provided in the new microphone, and the theory and merits of these patterns are presented. Finally some of the results which were obtained in field trials of the new microphone are di‐cussed.
12(1941); http://dx.doi.org/10.1121/1.1916129View Description Hide Description
12(1941); http://dx.doi.org/10.1121/1.1916130View Description Hide Description
The coefficient of absorption (2αν−2) of ultrasonic waves was measured by a radiation pressure method at frequencies from 7 to 50 Mc. The average value is given for each frequency. These cluster closely around , and there is no indication of a drop in 2αν−2. Errors entering into absorption measurements are discussed.
12(1941); http://dx.doi.org/10.1121/1.1916131View Description Hide Description
The theory of the propagation of sound in cylindrical tubes is presented. Formulas are developed for the calculation of the attenuation factor for the first mode (“plane” waves), and these formulas take into account both the resistive and reactive components of the tube wall impedance. The nature of the higher modes of sound propagation in the tube is illustrated.
12(1941); http://dx.doi.org/10.1121/1.1916132View Description Hide Description
12(1941); http://dx.doi.org/10.1121/1.1916133View Description Hide Description
In spite of its interest for acoustics, the intensive difference limen for sound has not been determined over a comprehensive range of frequency and amplitude. In the present experiment we have determined the difference limens for discrete intensive discrimination at five frequencies ranging from 128 cycles to 1000 cycles with intensities from 0.02 bars r.m.s. pressure to 3.4 bars (−65 db to −15 db). Both the physical conditions and the psychological judgment have been carefully controlled. The data yield a value for Δ db of 2.4 db in the middle range of intensities (−35 db to −15 db or 35 SL to 70 SL.)
12(1941); http://dx.doi.org/10.1121/1.1916135View Description Hide Description
Theoretical explanations of the action of the vibrating clarinet reed have been based upon the assumption of the partial closing of the aperture under the reed. This assumption is not supported by experimental evidence. The motion of the reed was observed stroboscopically, with the tone produced by an artificial embouchure. The actual frequency of the reed has been determined and other details in the mode of vibration observed.