Patent Grant
8689935
1. Field of the Invention
The invention generally relates to a device capable of abating noise. More specifically, the device relates to reducing low frequency noise in an aquatic environment.
2. Description of the Relevant Art
Noise abatement techniques are often employed to satisfy environmental regulations, which are in place to protect marine life and habitat. For example, underwater acoustic noise from drilling ships in the Arctic is known to adversely affect the migratory patterns of marine mammals. Much of this noise occurs at low frequencies between 10 Hz and 200 Hz. Governmental environmental regulations related to underwater noise limit the oil exploration and drilling season in this region to a small fraction of the year. The current strategy for dealing with these regulations is a passive one in which biologists and other experts are employed by the oil companies to survey large areas in the vicinity of operations for these animals. Once their presence is detected, communications are sent back to the ship and operations are halted, making this strategy quite expensive and further reducing the amount of time spent exploring and drilling. Thus, there is an industry-wide need for an active noise abatement solution.
Underwater sound abatement technologies include either the use of freely rising bubbles or the deployment of air-filled, hard spherical shells. Systems that use freely rising gas bubbles generally require the continuous supply of compressed air, which in turn requires operation of an air compressor, thus consuming energy and also radiating its own noise. If the compressor is powered by a combustion engine, air pollution is created. Furthermore, air supply lines are typically run from the compressor to the location of deployment, thus increasing capital and deployment costs. Meanwhile, the use of air-filled, hard spherical shells has proven to be acoustically unsatisfactory for frequencies below 1000 Hz. Also, due to their physical dimensions, air-filled hard spherical shell systems are expensive to transport and deploy in the field.
As described herein and in the accompanying materials, the inventors hereof have discovered that encapsulated bubbles may be used to abate, mitigate, or attenuate low-frequency, anthropogenic underwater noise in various applications and configurations. For example, in some embodiments, an encapsulating material, shell, container, or capsule may hold a first fluid or medium (e.g., air, gas, etc). The container may be sufficiently thin and flexible to achieve desired levels of sound attenuation or abatement (e.g., 10 dB, 20 dB, or more, depending upon the application). For example, in some cases the shell may include a flexible membrane constructed with latex, vinyl, rubber or other suitable materials, and may have a wall thickness of approximately between about 0.5 mm to about 5 mm. The gas-filled container may have a non-spherical or a substantially non-spherical wall (e.g., a toroidal shape or spherical cap geometry), and may have a physical characteristic designed to confer a selected resonance frequency to the shell upon immersion into a second fluid or medium (e.g., water, freshwater, saltwater, mixtures of water and hydrocarbons, etc.) at a predetermined depth. In some cases, the physical characteristic that at least in part determines the resonance frequency of the gas-filled container may include an effective spherical radius, an effective spherical diameter, or an effective spherical volume of the container or membrane.
A plurality of gas-filled shells may be coupled, attached, or connected to a support. For example, a support may include a network of lines, cables, pipes, beams, etc. forming a mesh, net, framework or the like. In some embodiments, the support may be provided in the form of a spool. A cable may be a metal, rope or polymeric cable. Further, the apparatus may be configured or adapted to attenuate sound emitted by a sound source. To that end, the apparatus may be positioned near the sound source in a curtain configuration or a cloud configuration. For example, a network of gas-filled containers may be deployed in the form of dome, cube, etc. encompassing the sound source. Additionally or alternatively, a network of gas-filled containers may be interposed between a sound source and a region, underwater, that is in need of protection from sounds emanating from an underwater sound source to act as a wall, barrier, or the like. In some embodiments, two or more such networks may be used together (e.g., in parallel with each other or side-by-side).
Containers coupled to an array or network may be separated from one another by a selected distance. In some applications, a sound field generated by the sound source has one or more components with a frequency between approximately 10 Hz and 1000 Hz, and the resonance frequencies of one or more gas-filled containers in the array are selected to approximately match the frequencies of the one or more components. In some embodiments, the level of abatement is proportional to the number density of gas-filled containers or the void fraction occupied by gas.
In a non-limiting scenario, an array of gas-filed containers may be deployed such that the effective spherical radius, an effective spherical diameter, or an effective spherical volume of the containers follow a distribution (e.g., a Gaussian distribution) designed to attenuate a particular frequency range. In another non-limiting scenario where a sound source produces signals components (e.g., harmonics) at two or more distinct frequencies, an array of gas-filled containers may be designed such that a first set of containers may have a first resonance frequency that approximately matches a first one of the distinct frequencies, a second set of containers may have a second resonance frequency that approximately matches a second one of the distinct frequencies, and so on. The number of gas-filled containers in the various sets of gas-filed containers may be proportional to the desired attenuation for each corresponding frequency. In a more general case, any number of signal components and corresponding sets of gas-filled containers may be used. Furthermore, the effective spherical volume of the gas-filled containers in each distinct set may have its own distribution. As such, the various sets of differently designed gas-filled containers may independently control the attenuation in a particular frequency band, and therefore “filter” the spectrum emitted by the sound source as desired. In addition, when the sound source has directional components, differently designed gas-filled containers may be appropriately positioned around the source so that their resonance frequencies match corresponding directional components. In some embodiments, two or more networks of gas-filled containers may each be designed to address a particular frequency band, and thus facilitate an appropriate distribution of different gas-filed containers around the source (e.g., a directional source).
In various embodiments, the use of thin-walled, flexible encapsulation, may allow an enclosed bubble of any size to be formed. Further, non-spherical shapes (e.g., toroidal shape, similar to tire inner tubes) may allow for easy attachment of the bubbles to noisy structures or machinery, and may include a gas valve or the like suitable for underwater operation.
In some embodiments, the level of noise abatement may be proportional to the number density of gas-filled containers and hence the cost of the network, array, mesh, or net; therefore, the level of abatement may be dictated by the financial constraints of a particular project, and not by the techniques disclosed herein. In some embodiments, a noise abatement system may utilize inexpensive, readily available, mass-produced, off-the-shelf components, to offer considerable flexibility in deployment on or around underwater noise sources. Once deployed, at least some of these systems may require little or no power to operate.
Illustrative applications for the systems and methods described herein include, but are not limited to, the abatement of underwater noise radiated by oil drilling ships, drilling rigs, underwater construction, pile driving, shipboard machinery and engine noise, marine wind turbine installations, underwater seismic surveying operations, or any other source of anthropogenic underwater noise. In other applications, various embodiments described herein may also be used to abate underwater noise radiated by military vessels, reduce detectability by sonar systems, etc.
Advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description of embodiments and upon reference to the accompanying drawings in which:
While the invention may be susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
It is to be understood the present invention is not limited to particular devices or methods, which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include singular and plural referents unless the content clearly dictates otherwise. Furthermore, the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not in a mandatory sense (i.e., must). The term “include,” and derivations thereof, mean “including, but not limited to.” The term “coupled” means directly or indirectly connected.
In some embodiments, the term “approximately” may refer to a value that is within 1% of another value. For example, a shell, container, or capsule having a resonance frequency of 101 Hz may be deemed to approximately match the frequency of a sound component at 100 Hz. In other embodiments, the term “approximately” may refer to a value that is within 10% of another value, in which case a resonance frequency of 110 Hz would be deemed to approximately match the frequency of a sound component at 100 Hz. In yet other embodiments, term “approximately” may refer to a value that is within 25% of another value. For example, a resonance frequency of 125 Hz may be deemed to approximately match the frequency of a sound component at 100 Hz. Also, in some embodiments the term “substantially non-spherical” may be used to refer to features that are largely non-spherical. For example, a sufficiently flexible spherical feature, when immersed in a particular medium, may be subject to compression and/or other forces that may alter its largely spherical shape, even if only slightly (e.g., a sphere may be transformed into an ovoid, or the like). This is in contrast with a “substantially non-spherical” feature such as, for example, a toroid, which is naturally non-spherical.
The strategy described herein involves the use of air bubbles to reduce radiated acoustic noise. The acoustic effects of air bubbles in water are well-known and have been studied extensively for at least 100 years with many documented results. One key aspect of bubble acoustics is that an air bubble in water behaves as a simple harmonic oscillator. A layer of water that surrounds the bubble acts as an effective mass, the compressibility of the air inside the bubble behaves as an effective spring, and the bubble will resonate when excited. An acoustic wave that encounters a collection of bubbles experiences significant attenuation due to energy lost through a variety of mechanisms, and the sound speed in the bubbly water is significantly altered compared to bubble-free water. Both of these effects can be potentially used to abate noise radiated from a drilling ship.
Previous examples of air bubbles in underwater acoustic screening have primarily exploited the acoustic impedance contrast between bubble-free and bubbly water. This mechanism has been shown to result in the reduction in the amplitude of transmitted sound with some success. A “bubble curtain” has been used to abate noise from an underwater pile driving operation, however, its effectiveness was limited likely due to sound transmission through the seafloor. Bubbles have also been employed on naval ships to abate both machinery and propeller noise at higher frequencies with a system called Prairie-Masker, although the technology is not available for commercial applications.
The devices described herein exploit both the bubble resonance and acoustic impedance mismatch mechanisms to reduce the radiated sound from an underwater device. In embodiments, the decibel level of sound emanating from an underwater device may be reduced by:
Because the experiments were performed in a lake, which is in essence a large acoustic waveguide, it was necessary to take into account the modal structure of the lake itself when analyzing the data. The observed behavior is spatially and temporally dependent, and while the time-dependent effects can be partially removed when looking at measurements averaged over time, an observer will still experience the spatial structure of the sound pressure field. Thus, the measurements were made at enough receiver locations to uncover this some of this structure and the effects that the bubbles have on it. Measurements at a single position or even a handful of positions would not be sufficient to accurately describe the pressure field, even in the case of a shallow water waveguide at sea where drilling operations might take place. For these tests two receivers were positioned at 10 m and 65 m horizontal distance from the source with measurements made on each at water depths ranging from 2 m to 20 m.
A set of encapsulated bubble screen configurations were chosen to cover a representative portion of the pertinent parameter space. In general, the main parameters governing both encapsulated bubble screen systems are:
The initial test matrix for the inner tube configurations to be used is shown in Table 1. Here, three inner tube sizes are referred to: large, medium, and small, with encapsulated air volumes of 1879.4 cm3, 654.9 cm3, and 185.2 cm3, respectively. For our frequency band of interest, the corresponding wavelengths, λ, range from roughly 1.5 m to 150 m. Because these wavelengths are much larger than the dimensions of the inner tubes, the inner tubes can be considered as effective spherical volumes of air with radius defined by:
where V is the volume of air inside the inner tube. Each inner tube size has a different spherical bubble resonance frequency, which is approximately given by the Minnaert frequency:
where p0 is the hydrostatic pressure outside the inner tube, γ is the ratio of specific heats of air at constant pressure to constant volume, and ρ is the density of water. The predicted zero depth individual bubble resonance frequencies are 42.9 Hz, 61.0 Hz, and 92.9 Hz for the large, medium, and small sizes, respectively. Because of the variation of hydrostatic pressure with depth, the resonance frequencies take values up to 56.4 Hz, 80.1 Hz, and 122.0 Hz at a depth of 4 meters for each of the three sizes. In general, the actual resonance frequencies of the encapsulated bubbles are modified from the shell-less values depending on both the thickness and stiffness of the walls and the surface-area-to-volume ratio. In the case of the inner tubes, the walls are fairly thin and elastic, allowing for sufficient resonant motion of the encapsulated air volume for the absorption mechanism to occur. Additionally, the less contact the air volume has with the rubber walls, the more bubble-like it behaves, making a smaller surface-area-to-volume ratio more desirable. At the mean deployment depth of 2 meters, the predicted resonance frequencies become 44.3 Hz, 63.0 Hz, and 96.0 Hz, respectively. Note that future references in this paper to the predicted individual bubble resonance frequencies will quote these mid-depth values.
The void fraction is defined as the ratio of the volume of air, Vair, to the total volume of water and air, Vtotal=Vair+Vwater, in the bubbly water region:
VF=Vair+Vtotal
The initial inner tube configuration matrix examines not only the effect of changing the void fraction, but also adding more than one inner tube size for a given void fraction, or using polydisperse as opposed to a monodisperse size distributions. As used herein the term “polydisperse” refers to an apparatus that includes gas-filled containers having two or more different volumes. As used herein the term “monodisperse” refers to an apparatus that includes gas-filled containers that all have about the same volume. The left column lists total (or global) void fraction while the right column lists the number of inner tubes needed to obtain that void fraction.
A second set of inner tube configurations was added to look at the effects of changing the inner tube volume and using equal void fraction polydisperse distributions, shown in Table 2. Here, a larger inner tube size, called jumbo, is added with an encapsulated air volume of 7763.2 cm3 and a predicted individual bubble resonance frequency ranging from 26.1 Hz at zero depth to 35.1 Hz at 4 meters. The resonance frequency at the mean deployment depth of 2 meters is 27.7 Hz.
The sub-resonant bubble cloud configuration matrix is displayed in Table 3. Here, the left column lists void fraction, which was estimated from the air flow rate to the diffuser hoses. The right column lists the diffuser hose pressure needed to obtain a particular air flow rate. For the two lowest hose pressures, the flow was too small to be measured so there was only an upper bound on the void fraction. In the case of the bubble clouds, only the effect of void fraction on the acoustic behavior is examined.
Finally, the combined effect of using both an inner tube array and a sub-resonant bubble cloud were examined. These configurations are shown in Table 4, where the void fraction is listed in the left-hand column, the diffuser hose pressure in the middle column, and the inner tube number in the right-hand column. Equal void fractions for both the bubble cloud and various inner tube arrays were used.
For each case, measurements were made at both ranges from the sound source. Additionally, for each range, measurements were made at depths ranging from 2 m to 20 m in 2 m increments. The specific types of acoustical measurements made are briefly discussed in the following sub-sections.
Transfer function measurements were made between the source and receiver. The transfer function is defined here as a function of frequency:
Y(f)=H(f)X(f)
where Y is the power spectrum of the system output or received signal, X is the power spectrum of the system input source signal, and H is the transfer function. Because these quantities are in general complex, the transfer function is usually represented in terms of its amplitude and phase:
In this investigation, the transfer function was measured using a vector signal analyzer (VSA). The source and received signal were acquired by the VSA, where they were digitized and transformed to the frequency domain using a fast Fourier Transform (FFT). Each FFT had 1601 frequency bins in a frequency range of 60 Hz to 2 kHz. The FFTs were used to compute the transfer function onboard the VSA, and the amplitude and phase were recorded. Typically, the data was averaged over 30 consecutively-acquired spectra. The coherence spectrum was also monitored to ensure the quality of the data. This is given by:
where the asterisk denotes the complex conjugate. For γ=0, the two signals are incoherent while for γ=1, they are coherent. Values in between indicate partial coherence. Typically, the data was considered to be good if the coherence is close to unity (>0.8).
The basic measurement set-up for the transfer function data collection is shown in
In some instances, it was preferable to collect data in the time domain as opposed to the frequency domain. In these cases, the ambient sound level of the lake environment was such that the low-frequency part of the periodic chirp used for the transfer function analysis was obscured by the noise, even when running the J-13 at full power. Therefore, to obtain data at frequencies lower than 60 Hz, it was necessary to use time-coherent averaging of pure sinusoidal tones. Sources of the noise are wind, breaking waves, boat engines and propellers on the lake, and changes in hydrostatic pressure from passing wakes, among other things.
The experimental set-up which accomplished this technique is shown in
The bubble screen apparatus, in one embodiment, uses a steel frame with netting to which the various gas-filled containers (e.g., inner tubes) were attached using cable ties. An exemplary apparatus is shown in
A quantitative comparison between the spectra of the underwater sound source with no inner tubes, referred to as the “reference case”, and the sound source surrounded by inner tubes is shown in
Comparison of the ambient noise and the received signals are shown in
In an attempt to better extract the signal from the ambient noise, measurements were made using single-frequency sinusoidal source tones. The received waveform was acquired 64 times, and time-coherent averaging was performed. The results of this analysis are shown in
To isolate the effect of altering the void fraction, only the large inner tube size was used, and the number of inner tubes attached to the frame was varied. As the void fraction is increased, the received level decreases at both locations, thus reduction in radiated pressure occurs over all receiver depths. The greatest reduction for any particular case occurs in the frequency range from about 70 Hz to just above 500 Hz.
In
The average sound pressure level (SPL) reduction was computed in the frequency band from 60 Hz to 200 Hz by averaging over the measured sound pressures in that frequency range for both the reference and inner tube cases and then taking their difference.
To isolate the effect of inner tube size on the radiated spectrum, the void fraction was fixed at VF=0.005, ensuring that the received signals had a great enough amplitude such that they overcame the ambient lake noise level. Three inner tube sizes were used in monodisperse distributions. These were jumbo, large, and medium, which had predicted individual bubble resonance frequencies of 31.0 Hz, 49.7 Hz, and 70.7 Hz, respectively, at the mean deployment depth of 2 meters. The observed dip in the measured spectrum is interpreted to correspond to the individual bubble resonance frequency, thus, the dip should shift left or right along the frequency axis for an increase or decrease in encapsulated air volume, respectively.
Comparison of measured transfer functions for separate monodisperse distributions of the three inner tube sizes is shown in
In
To map out the sub-60 Hz of the jumbo inner tube array, the time-coherent averaging technique was used with single-frequency sinusoidal tones ranging from 30 Hz to 100 Hz in steps of 10 Hz. This tone data is overlaid on top of the transfer function in
Inner tube distributions combining multiple sizes were employed to determine if attenuation over a broader range of frequencies could be achieved. Two possibilities considered for constructing a polydisperse distribution out of discrete inner tube sizes were to use either equal numbers of each size or equal void fraction for each size.
Although a Commander and Prosperetti model predicts that the range of high attenuation ought to extend to a greater number of frequencies when adding multiple bubble sizes, there are some complications that can arise when considering multiple discrete bubble size populations. As a simple case, consider a bubble size distribution that consists of two Gaussian distributions centered about spherical bubble radii a1 and a2. These radii are such that a1 is greater than a2 and their resonance frequencies are f1 and f2, where f1<f2. For frequencies below f1, the Commander and Prosperetti model predicts that the attenuation is very low because all of the bubbles oscillate in phase with the incident sound wave. Above f1 there is significant attenuation due to the bubble population centered around a1, which oscillates out of phase with the sound wave; however, because the population centered around a2 is still below resonance, this group of bubbles oscillates in phase with the wave. These in-phase oscillations can reduce the amount of attenuation observed in the frequency band between f1 and f2. These “short-circuiting” effects were observed in the data although they could potentially be overcome by increasing the void fraction either globally or for the various sub-populations.
For the first series of polydisperse distribution tests, equal numbers of each inner tube size were used. For a fixed global void fraction of VF=0.01, three distributions were employed: 70 large inner tubes; 52 large and 52 medium inner tubes; and 50 large, 50 medium, and 50 small inner tubes. Measured transfer functions for each of these cases are shown in
An additional set of experiments on polydisperse inner tube distributions was performed using an equal void fraction for each inner tube sub-population. In these cases, the global void fraction is not fixed, but increased from 0.005 to 0.015. The void fraction for each sub-population was VF=0.005. Also, to extend the attenuation to lower frequencies, the jumbo, large, and medium sizes were used. The different cases were: 10 jumbo inner tubes, 10 jumbo and 35 large inner tubes, and 10 jumbo, 35 large, and 87 medium inner tubes. The transfer functions for each of these cases are shown in
The global void fraction has the primary effect on the amount of observed attenuation, and the combination of multiple inner tubes sizes has a less significant influence on the radiated spectrum. This is illustrated in
The bubble screen apparatus only required slight modification to incorporate the generation of a cloud of freely-rising bubbles. Two cloth-covered ceramic diffuser hose rings were attached to the steel frame approximately 0.5 meters below the location of the J-13 projector and approximately 3.5 meters below the surface of the water. Continuous air flow was delivered to the diffuser hoses by a low-pressure, high flow rate, diesel-powered air compressor. The flow rate for each diffuser hose ring was regulated manually by an adjustable flow meter, which also served the purpose of monitoring the air flow rate. The regulator assembly also included a pressure gauge for each ring to monitor the air pressure as well as valves for shutting off the air flow to each ring. Additionally, a submersible electronic pressure sensor was attached to one of the diffuser hose rings to measure the air pressure on the hose at depth. The mean radius of the bubbles produced in this manner was previously determined to be approximately a=0:25 cm.
The bubble cloud void fraction was essentially the only controllable physical parameter for the system. Estimates of the void fraction in the bubble cloud were obtained using the measured air flow rate and the initial rise time of the bubble cloud for a given set of operating parameters. The flow rate was varied from 22 cfm to less than 5 cfm, which was the lower limit of the scale on the flow meter used. These flow rates corresponded to void fractions ranging from less than 0.006 up to 0.026.
A quantitative comparison of measured transfer functions with and without a bubble cloud enclosing the sound source is shown in
To determine the effect of void fraction on the performance of the bubble cloud modality, the air flow rate to the diffuser hoses was varied. The corresponding air pressure on the hoses was measured with the submersible electronic pressure gauge and recorded so that the operating conditions could be reproduced in later tests. Higher measured pressure corresponds to a higher air flow rate, which is equivalent to higher void fraction within the bubble cloud. Comparisons of the received level for various void fractions are shown in
The band-limited SPL reduction from 60 Hz to 200 Hz due to the bubble clouds was computed in the same manner as for the inner tube data. The results of these calculations are plotted for all five values of void fraction in
Due to the disparity in individual bubble size between the inner tube and bubble cloud modalities, there are different frequency ranges over which the bubble resonance mechanism dominates the attenuation. Note that the acoustic impedance mismatch mechanism plays a role in attenuation over the entire range of frequencies for both modalities. The relative effectiveness of each modality over a given frequency band can be illuminated by looking at the transmission loss for each as a function of frequency and comparing them. Here, the transmission loss is defined as:
TL=|H|ref−|H|bub
where |H|ref is the measured transfer function for the bubble-free case and |H|bub is the measured transfer function for either the inner tube or bubble cloud case.
The transmission loss for both the bubble cloud and inner tube modalities are plotted in
For frequencies below the transition to bubble cloud dominance, the relative performance of each modality can be quantified by looking at some of the low-frequency lake resonances.
Comparison between bubble cloud and inner tube modalities of band-limited SPL reduction in the 60 Hz to 200 Hz further illustrates this difference. The band-limited SPL reduction is plotted for the five bubble cloud cases and a representative sample of inner tube cases in
Although the inner tube modality consistently outperforms the bubble cloud at attenuating low frequencies, the bubble cloud modality could be used to augment attenuation from a few hundred hertz up to the kilohertz range, serving as motivation for testing a combination of the two modalities.
Selected inner tube configurations were combined with the bubble cloud modality to determine if the performance of the bubble screen system could be enhanced by using such a mixed modality. The 10 jumbo inner tube configuration was selected as the monodisperse inner tube distribution for the comparison because this configuration displays the highest attenuation below 100 Hz. Here, the void fraction is 0.005. Acoustic data was collected for this configuration with and without the presence of a roughly equivalent void fraction bubble cloud, which was generated using an air flow rate of 5 cfm.
A comparison of the transfer functions for each of these cases is plotted in
Two other mixed-modality cases are plotted in
Testing has generally focused on constant sound sources. In some embodiments, the sound source producing the underwater noise is an impulsive noise generated by a sudden event (e.g., a pile driver).
In another embodiment, a noise reducing apparatus was prepared to reduce noise produced by a pile driving device. The noise reducing device includes 24 lines having gas-filled containers coupled to the lines.
The lines were arranged to partially surround the receivers, as shown in
The pile driver sound output was determined prior to testing. The pile driver has a measured peak-to-peak SPL of 210 dB @ 1 m; 185 dB @ 112 m; and 150 dB @ 2660 m. The sound produced by the pile driver varied from day to day by as much as ±10 db. Thus, the set up described above was used to obtain simultaneous measurements.
In the particular location used to test the device, a nearby dam produces a reflected sound wave that creates two distinct sound events during each cycle of the pile driver. The direct and reflected paths are predicted to travel through the sound reducing device in different directions. An algorithm was written to find and separate the two sound events.
During the course of our tests, several inner tube and bubble cloud modalities were employed to determine the parametric dependence of the attenuation on the various bubble screen configurations. The primary conclusions from these experiments are:
1. Surrounding the sound source with inner tubes was demonstrated to provide levels of attenuation at low frequencies of 40 dB or more due to a combination of bubble resonance and acoustic impedance mismatching mechanisms. The amount of attenuation was shown to depend primarily on the total void fraction.
2. The addition of multiple discrete inner tube sizes seems to have only a second-order effect on the radiated levels in comparison to the effect of global void fraction.
3. Using larger volumes of encapsulated air, the bubble resonance mechanism can be used to reduce the radiated level of lower frequencies. The results suggested that the simplest and possibly most effective solution would be to use a high void fraction of very large inner tubes to provide the best low-frequency attenuation.
4. Surrounding the sound source with a cloud of small freely-rising bubbles was shown to provide attenuation, the amount of which was also highly dependent on the void fraction. For frequencies below the bubble resonance, attenuation of as much as 20 dB was observed due to impedance mismatch effects for high void fraction bubble clouds. For frequencies extending from a few hundred hertz up to one kilohertz, an increase in absorption was observed, which was aided by bubble resonance absorption. It is possible that for some applications, the use of a high void fraction bubble cloud would provide the required reduction in radiated sound.
5. Tests with both inner tubes and bubble clouds suggest that combining the modalities has the potential to provide increased attenuation across a broader range of frequencies, although some subtle effects must be considered. Due to their disparity in size, the constituent bubble sub-populations can have opposing interactions with the radiated sound, possibly leading to less attenuation in certain frequency bands. Thus, care should be taken when determining the void fractions of the various sub-populations in the mixed modality case to minimize these effects.
6. Broadband transfer function measurements are useful for a complete understanding of the sound field, but the current regulations rely on sound pressure level measurements which are a time-domain average measurements. An approximation of the average sound pressure level in the 60 Hz to 200 Hz frequency band was computed from transfer function measurements. Inner tubes were shown to provide up to 35 dB of attenuation in this frequency band while bubble clouds provided up to 20 dB of attenuation for comparable void fractions.
Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/478,172 filed on Apr. 22, 2011.
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