flexures of the source consequently care must be taken to isolate the senors from the acoustic radiation and evanescent waves vibrations of the noise source propagate along the source as flexural waves these waves have a frequency dependent propagation velocity which is generally lower than the acoustic velocity in the propagating medium because of this velocity difference the pressure disturbances due the noise source vibration cannot radiate into the medium thus evanescent waves are established pressure fields generated by such waves drop off rapidly with distance from the noise source disappearing within a few inches consequently positioning an acoustic sensor a short distance from the noise source effectively decouples it from the evanescent waves acoustic radiation generated when the noise source flexural waves encounter a stiffness discontinuity maintain relatively high levels at great distances from the noise source isolation of an acoustic sensor from these waves may be realized by positioning a low acoustic impedance layer between the acoustic sensor and the acoustic noise source it should be noted that all references herein to 34 low 34 and 34 high 34 acoustic impedance are relative to the acoustic impedance of the propagating medium refer now to fig5 a wherein a cross section of an array of acoustic particle acceleration sensors 51 mounted on a baffle 50 adjacent to a metallic plate noise source 55.