unwanted modes and to excite only those modes that have low damping and 

 which are far away in frequency from the next possible mode, and afford 

 therefore accurate and unambiguous measurement of phase velocity. 



It appears that one of the difficulties encountered with 

 the standing wave apparatus is the definition of the boundary conditions, 

 i.e. of the terminating walls. Even small amounts of dirt (Reference 34) 

 have been reported to ruin the performance. While this poses no insur- 

 mountable problem in the laboratory, it makes the instrument useless for 

 oceanographic runs of long duration. In an expendable tactical instrument, 

 however, the walls would probably stay clean during the single descent 

 of the instrument and these objections to the use of resonant cavities 

 would probably not hold. 



Most other methods of indicating standing waves and hence of 

 measuring wavelength or phase velocity are restricted to the laboratory. 

 Among others there is Debye-Sears' method of forming a diffraction grating 

 from a standing wave of high frequency, and Toepler's method of producing 

 a Schlieren image of a standing wave to determine the wave length directly. 

 These optical methods, however, appear to be unsuited for marine use. 



An important refinement in standing wave techniques is the scan- 

 ning interferometer, where instead of scanning the frequency in a fixed 

 resonator one scans the dimensions of a resonator at constant frequency 

 and uses the reaction of the wave on the exciting transducer to indicate 

 resonance. The output of a suitable transducer is recorded while the di- 

 mensions of the resonator are changed by, for example, a precise lead screw. 

 From a record of sufficiently many "fringes", to borrow a term from optical 

 inter ferometry, as a function of the position of the moveable boundary one 



38 



Arthur JH.l.tttlc.I(nr. 



