How Far Off Is That German Gun? How 63 German guns were located by sound waves alone in a single day, Popular Science monthly, December 1918, page 39, Scanned by Google Books: http://books.google.com/books?id=EikDAAAAMBAJ&pg=PA39
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John L. Spiesberger (June 2001). «Hyperbolic location errors due to insufficient numbers of receivers». The Journal of the Acoustical Society of America109 (6): 3076-3079. Bibcode:2001ASAJ..109.3076S.doi:10.1121/1.1373442. PMID 11425152.