| Abstrakt: |
An experimental investigation of the focusing of intense, airborne pressure pulses by an ellipsoidal reflector is reported. Short-duration N waves were generated by weak sparks at the near focus F1 of four different prolate ellipsoidal surfaces. Reflection then concentrated the rays at the far focus F2. Measurements were made with a wide-band microphone, primarily along the axis of the reflector. A few transverse measurements in the plane of F2 were also made. In the axial measurements the reflected (upright N wave) and edge-diffracted (inverted N wave) components of the signal are distinct at points distant from F2, approach each other as the focus is neared, and merge at the focus. At the focus the waveform is U-shaped, or cusped. Beyond F2 a 'droopy' N wave emerges and, where the edge wave can be resolved, it now arrives first and is phase inverted relative to its prefocal version. The more intense reflected wave does not experience phase inversion. Some features of the observed waveforms are consistent with recent calculations by Hamilton, which are based on lossless, linear theory. Other wave characteristics, including pulse amplitude and location of the head and tail shocks, are strongly influenced by nonlinear effects. © 1997 Acoustical Society of America. [ABSTRACT FROM AUTHOR] |
