Vahid Naderyan

Wind-induced ground motion

Wind noise is a problem in seismic surveys and can mask the seismic signals at low frequency. This research investigates ground motions caused by wind pressure and shear stress perturbations on the ground surface. A prediction of the ground displacement spectra using the measured ground properties and predicted pressure and shear stress at the ground surface is developed. Field measurements are conducted at a site having a flat terrain and low ambient seismic noise. Triaxial geophones are deployed at different depths to study the wind-induced ground vibrations as a function of depth and wind velocity. Comparison of the predicted to the measured wind-induced ground displacement spectra shows good agreement for the vertical component but significant underprediction for the horizontal components. To validate the theoretical model, a test experiment is designed to exert controlled normal pressure and shear stress on the ground using a vertical and a horizontal mass-spring apparatus. This experiment verifies the linear elastic rheology and the quasi-static displacements assumptions of the model. The results indicate that the existing surface shear stress models significantly underestimate the wind shear stress at the ground surface and the amplitude of the fluctuation shear stress must be of the same order of magnitude as the normal pressure. Measurement results show that mounting the geophones flush with the ground provides a significant reduction in wind noise on all three components of the geophone. Further reduction in wind noise with depth of burial is small for depths up to 40 cm.

Mechanisms for wind noise reduction by a spherical wind screen

Spherical wind screens provide wind noise reduction at frequencies which correspond to turbulence scales much larger than the wind screen. A popular theory is that reduction corresponds to averaging the steady flow pressure distribution over the surface. Since the steady flow pressure distribution is positive on the front of the sphere and negative on the back of the sphere, the averaging results in a reduction in measured wind noise in comparison to an unscreened microphone. A specially constructed 180 mm diameter foam sphere allows the placement of an array of probe microphone tubes just under the surface of the foam sphere. The longitudinal and transverse correlation lengths as a function of frequency and the rms pressure fluctuation distribution over the sphere surface can be determined from these measurements. The measurements show that the wind noise correlation lengths are much shorter than the correlations measured in the free stream. The correlation length weighted pressure squared average over the surface is a good predictor of the wind noise measured at the center of the wind screen. [This work was supported by the Army Research Laboratory under Cooperative Agreement W911NF-13-2-0021.]

Design, construction, and evaluation of an omni-directional loudspeaker (Dodecahedron)

Dodecahedron loudspeaker (Dodec) is an omni-directional sound source in the shape of a 12-sided loudspeaker with each side being a pentagon. The omni-directionality of this sound source makes it mainly applicable in room acoustical and sound insulation measurements and research as it can excite and saturate the room as much as possible. Dodec is a good approximation of a point sound source. Commercially available omni-directional loudspeakers are not economically efficient for some purposes. This paper outlines a simple and inexpensive Dodec loudspeaker designed, constructed, and evaluated using ISO 140 and ISO 3382 standards as reference as part of an independent coursework. The Dodec was evaluated in an anechoic chamber. The measured directivity of this Dodec meets the standards for omni-directionality at all frequencies.

Design, construction, and evaluation of a binaural dummy head

Binaural-dummy-heads are often used as standard measurement devices where modeling of the human binaural hearing system is desired. The binaural-dummy-head imitates a human head (and torso) which is used in binaural recording as well as research areas such as hearing aids, sound localization, noise measurements, etc. Commercially available binaural heads are not economically efficient for some purposes. This paper outlines a less expensive binaural dummy head built using ANSI/ASA S3.36-2012 standard as a reference as part of an independent coursework. A hard plastic mannequin was used as head and torso, and the two ears were real human ear replicas casted out of water-based alginate gel. The complex Head-Related Transfer Functions (HRTF) of our dummy head were measured in an anechoic chamber to evaluate its spectral and directional properties and were compared to the same properties of the standard commercial dummy head.

Effects of ground characteristics on wind-seismic coupling

In seismic surveys, wind noise at low frequency seriously degrades seismic data quality. In order to find a solution for this problem, the driving pressure perturbations on the ground surface associated with wind-induced ground motions were investigated in a previous work [Naderyan, Hickey, and Raspet, J. Acoust. Soc. Am. 136, 2139 (2014)]. Multiple triaxial 10 Hz geophones were deployed at different depths to study the induced ground velocity as a function of depth. The displacement amplitudes of the ground motions as a function of depth and frequency were predicted, where the ground was modeled as a homogeneous half-space elastic media. In the current work, the predictions are extended for inhomogeneous ground. The measurements are conducted at a different site with different characteristics. Since wind noise is dominant at lower frequencies, the triaxial 4.5 Hz geophones were used to investigate a lower frequency range. [This work was supported by USDA under award 58-6408-1-608.]

An investigation of wind-induced and acoustic-induced ground motions

Wind noise at low frequency is a problem in seismic surveys, which reduces seismic image clarity. In order to find a solution for this problem, we investigated the driving pressure perturbations on the ground surface associated with wind-induced ground motions. The ground surface pressure and shear stress at the air–ground interface were used to predict the displacement amplitudes of the horizontal and vertical ground motions as a function of depth. The measurements were acquired at a site having a flat terrain and low seismic ambient noise under windy conditions. Multiple triaxial geophones were deployed at different depths to study the induced ground velocity as a function of depth. The measurements show that the wind excites horizontal components more than vertical component on the above ground geophone due to direct interaction with the geophone. For geophones buried flush with the ground surface and at various depths below the ground, the vertical components of the velocity are greater than the horizontal components. There is a very small decrease in velocity with depth. The results are compared to acoustic-ground coupling case. [This work is supported by USDA under award 58-6408-1-608.]

Experimental verification of computer modeled loudspeaker sound level performance based on excitation signal

An investigation of a loudspeaker sound level performance in a simple geometry to verify the EASE sound level definition comparing pink noise and multi-tone signal utilizing field tests of loudspeakers.