Acoustical characterization of a portable pneumatic infrasound source.

A portable infrasound source based on a pneumatic siren design is described. The source is capable of producing narrowband tone bursts over a frequency range of approximately 0.25-10 Hz. The acoustical characteristics of the infrasound source are presented, including measured sound pressure levels and horizontal directivity. The methodologies for processing the received acoustic signals are reported, and comparisons of measurements are made with a simple multipole model of the source. Measurements indicate that the pneumatic infrasound siren can be useful for a variety of applications for which a controlled, known infrasound source is required.

Acoustic radiation from an infrasonic ball-valve siren.

The concept of a ball-valve siren is developed through experimentation and theoretical modeling. The ball-valve siren is a source transducer developed for the purpose of establishing the concept of infrasound generation through the modulation of compressed air flowing through a rotating ball valve and released into the atmosphere, in the context of a siren. Directivity, frequency response, and propagation experiments were performed for the fundamental frequency component, and the results compare favorably to an empirical model based on monopole and dipole radiation. The results show that a small ball-valve siren can generate useful infrasound radiation with nominal directivity at frequencies in the range 1 to 8 Hz.

Elastic softening of sandstone due to a wideband acoustic pulse.

Elastic wave propagation experiments were performed on a thin bar sample composed of Texas "moss" sandstone in order to study nonlinear elastic effects in the time domain. The present experiments utilized a pendulous hammer to produce axially propagating transient signals with strain amplitude between 15 and 130 microstrain in the mid-audio band. Particle velocity along the bar axis was measured with a laser Doppler vibrometer, focused at various locations along the bar. Nonequilibrium dynamics and nonlinear elasticity effects were observed on the propagating pulse as it reflected between the ends of the bar. The same effects were also observed at a single location along the mid-length of the bar using a continuous-wave 1 MHz probe signal, propagating transverse to the bar axis. The results demonstrate larger strain amplitudes and greater reduction in the Young's modulus than in previously reported measurements that employed narrowband excitation. Nonlinear attenuation of the axial pulse is also observed, which increases with excitation amplitude. The present results indicate significant conditioning of the sandstone, particularly "softening" of the Young's modulus of up to 20%, primarily during the tensile phase of propagation, with a "slow dynamic" memory that is similar to that reported in previous investigations.