摘要:

The acoustic band structures have been computed for cubic arrays of spherical hydrogen balloons in air. Stop bands are obtained for the face-centered-cubic (fcc) and body-centered-cubic (bcc) lattices, however, there is no band gap for the simple cubic lattice. These gaps are largest for a volume fraction of ∼38% and, at low balloon pressure (1.10 atm) and small latex wall thickness (10−3times the inner radius), the corresponding gap/midgap ratios are about 0.2 for the fcc and 0.1 for the bcc structure. As the pressure or wall thickness increases, the stop bands diminish and ultimately vanish.

ISSN:0001-4966    

DOI:10.1121/1.417964

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The implementation of structural intensity measurement using an accelerometer array allows active attenuation of broadband frequency vibration using a modified filtered-x algorithm. In this work, a multi-input channel accelerometer error filter outputs a single channel error signal proportional to the propagating power. The filter is specified in the frequency domain as a transfer function from a model of bending waves in a finite beam. Integrity of the beam model and active control simulations is verified by monitoring an acceleration in the far field. The best overall performance is achieved when the error sensor array is in the far field of the primary and adaptive control force actuators. Broadband application of the intensity error technique in physical model-based simulations showed 15- to 25-dB attenuation except at a few isolated frequencies. These preliminary simulation results support new approaches to active stuctural control through error sensor array processing.

ISSN:0001-4966    

DOI:10.1121/1.418268

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This letter reports a simple new technique for the ultrasonic velocity measurement on thin elastic layers of fractional wavelength thickness using broadband transducers. An external trigger to the oscilloscope with continuously variable frequency and an intensity enhancing strobe signal allow direct measurement of the wavefront time shift down to 13 ns. The technique makes it possible to measure the sound velocity of thin layers with the thickness down to 10−2λ. Using a pair of 2.25-MHz transducers, we have accurately measured the sound velocity of a 26-μm-thick aluminum foil.

ISSN:0001-4966    

DOI:10.1121/1.417965

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP