LETTERS TOTHE EDITOR function for the case treated and the directivity function of the elementY This means that if the pattern function of the element is less than A, at any value of 0 less than 90 ø the sector coverage of that array is necessarily limited to that angle. The results here are limited to the case of uniform spacing. Since it is conceivable that a nonuniformly spaced array could be steered, the method might be extended to include this case. However, the analyses on the nonuniform array of the many authors who have treated the problem of arrays of arbitrary spacings thus far have not led to results that seem amenable by an extension of the method presented here. For such arrays, the procedure remains "cut and try." z S. Several investigators have derived expressions for the pressure spectrum of the sonic-boom N wave. Where the rise time is zero, Howes has shown that the pressure spectrum is a published spherical Bessel function. The present analysis shows that the spectrum with a finite rise time, not exceeding onetenth of the duration, can be closely approximated by multiplying two spherical Bessel functions, or, equivalently, by adding their logarithmic curves. WITI-I RECENT INTEREST IN TI-IE SUPERSONIC TRANSPORT AND TI-IE associated sonic-boom problem, several investigators--notably Zepler and Hard/Young, TM Howes, 4 and AustinS--have derived expressions for the spectrum distribution of N waves. Young •' has published curves for the case of zero rise time and has also shown a Volume 43 Number 4 1968Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 160.36.178.25 On: Sat

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