Sound--Transmission

This research presents findings from an in-situ experiment utilizing a hydrophone line array to capture the sound production of the Goliath grouper. Analysis revealed that Goliath grouper calls exhibit multiple frequency components, including one high-amplitude component and 2 to 3 low-amplitude components. The primary high-amplitude component is concentrated in the 30 to 70 Hz band, peaking around 50 Hz, while low-amplitude components span 20 to 30 Hz, 70 to 115 Hz, and 130 to 200 Hz. Comparison between in-situ data and results from a normal modes transmission loss model identified regions where echo level increased with propagation distance. This suggests that the loudness of the call may not necessarily indicate proximity, indicating the Goliath grouper might rely on other cues for localization, such as changes in the frequency profile of its call. Two methods for estimating call distance are presented. The first method vi utilized a transmission loss model and measured transmission loss across a hydrophone line array. This method could also determine the source level of the calls, yielding source level estimates ranging from 124.01 to 144.83 dB re 1 μPa. The second method employed match field filtering, validating the accuracy of the transmission loss model. Both methods produced similar call distance estimations, ranging from 11.5 to 17.1 meters, placing the grouper inside or near its typical habitat.

A novel acoustic wave propagation model has been developed to determine the effects of the ocean variations on the acoustic propagation field, and to determine the signal measured by a receiver at any distance from an omnidirectional source. The model accounts for environmental conditions. First, a stationary estimate of the complex sound attenuation is computed as a function of frequency and location, using the parabolic equation numerical technique. For a given range, the vertical profile of the attenuation frequency spectrum is decomposed in the wave number domain. A specific Doppler shift is associated with each wave number. The space-frequency attenuation filter obtained is applied to the transmitted signal to create time-frequency selective fading. This model has been used to simulate the performance of the General Purpose Acoustic Modem, which transmits MFSK modulated sequences between 15.6 kHz to 32.1 kHz. The range of operation varies from 1 to 5 km, in 15 meters of water. Experimental data have been collected under sea-state 2 conditions. The model has been successfully validated when compared to experimental data and to the Crepeau model.

High-resolution sound propagation measurements were made on a 1/10000 th-scale model of the Santa Lucia Escarpment, located off the Southern California coast. The tank was modified from previous experiments using a rubber coating on the tank bottom. High frequency, high resolution, Transmission Loss measurements were made on the SFTF range, Dania Florida. The Parabolic Equation Model RAM was used to validate these measurement sets. A new approach to account for shear wave effects on the Transmission Loss for the RAM model was developed. Using this new approach, the scaled low frequency Santa Lucia measurements showed excellent agreement with the RAM calculated TL, but there were discrepancies in the predictions of the high frequency at sea measurements at ranges greater than 1 km.

The vibrational and acoustic characteristics of fluid-loaded, cylindrical shells with single or multiple, aperiodically-spaced ring discontinuities are studied using an approach based on the mobility power flow (MPF) method and a hybrid numerical/analytical method for the evaluation of the velocity Green's function of the shell. The discontinuities are associated with internal structures coupled to the shell via ring junctions. The approach is a framework allowing alternative shell and/or internal structure models to be used. The solution consists of the net vibrational power flow between the shell and internal structure(s) at the junction(s), the shell's velocity Green's function, and the far-field acoustic pressure. Use of the MPF method is advantageous because the net power flow solution can be used as a diagnostic tool in ascertaining the proper coupling between the shell and internal structure(s) at the junction(s). Results are presented for two canonical problems: an infinite, thin cylindrical shell, externally fluid-loaded by a heavy fluid, coupled internally to: (1) a single damped circular plate bulkhead, and (2) a double bulkhead consisting of two identical damped circular plates spaced a shell diameter apart. Two excitation mechanisms are considered for each model: (1) insonification of the shell by an obliquely-incident, acoustic plane wave, and (2) a radial ring load applied to the shell away from the junction(s). The shell's radial velocity Green's function and far-field acoustic pressure results are presented and analyzed to study the behavior of each model. In addition, a comparison of these results accentuates the qualitative difference in the behavior between the single and multiple junction models. When multiple internal structures are present, the results are strongly influenced by inter-junction coupling communicated through the shell and the fluid. Results are presented for circumferential modes n = 0 & 2. The qualitative differences in the results for modes n = 0 and n = 2 (indicative of all modes n > 0ified in the far-field acoustic pressure and velocity Green's function response with the characteristics of the shell and internal plate bulkhead. The results for the single junction model demonstrate the significance of the shell's membrane waves on the reradiation of acoustic energy from the shell; however, when multiple junctions are present, inter-junction coupling results in a significant broad acoustic scattering pattern. Using the results and analysis presented here, a better understanding can be obtained of fluid-loaded shells, which can be used to reduce the strength of the acoustic pressure field produced by the shell.

The response of fluid-loaded plates has been extensively studied in the past.
However, most of the work deals with either infinite plates or finite plates with particular
boundary conditions and the results are generally presented only in the limit of small
wavelengths compared with the dimensions of the plates. Furthermore, the problem of
coupled finite plates where both the acoustic interaction and structural interaction are
included in the solution has not been considered. In this dissertation the response of two
coupled finite plates set in two alternative configurations is considered. The plates are
simply supported on two edges, with arbitrary boundary conditions on the remaining two
edges. The solutions obtained for the response of the plates include both the structural
interaction at the common junction and the acoustic interaction due to the scattered
pressure from each of the two plates. The results are presented in terms of the vibrational
power flow into and out of each plate component. The solution is based on a formulation developed in the wavenumber domain
combined with the Mobility Power Flow method. Using this approach, different
substructural elements coupled under different boundary conditions to form a complex
global structure can be considered. The detailed spatial and temporal scales of the structure response are not lost when using this method.
In obtaining the solution for the scattering from the fluid-loaded plates, a modal
decomposition in the direction normal to the simply supported edge is used. A spatial
Fourier-transform decomposition is used in the other direction. Due to the finiteness of
the plate, eight unknowns parameters are obtained in the transformed result. The solution
for these eight unknown parameters is obtained from the boundary conditions and the
condition that the response must remain finite. Two analytical approaches are used to
solve the final plate integral equation. The first approach consists of an approximation
method which obtains a solution based on the solution of the corresponding infinite plate
problem. The second approach is a more accurate solution based on the Projection
Method for the solution of integral equations.
Both of the approaches used in the solution provide accurate predictions at high
frequencies. At low frequencies especially for low structural damping or for heavy fluid
loading, only the Projection Method gives reliable results. This is attributed to the fact
that at low frequencies, the influence of the edges of the plates on the scattering is
significant.
The overall results obtained from this analysis indicate that the fluid loading and
the plate characteristics have a significant influence on the acoustic scattering properties,
especially in the case of heavy fluid loading.
The application of the method to coupled fluid-loaded plates indicates that the junction
enhances the scattering properties. The acoustical interaction between the coupled plates
increases the contribution to scattering from subsonic wavenumber components. In the
absence of the interaction, only supersonic wavenumbers contribute to the scattering.
Inclusion of acousticlal interaction requires both supersonic and subsonic components.
The significance of the contribution from the subsonic wavenumber components is
dependent on the type of the fluid loading.