'Tech Talk' with Dr. Mike Traweek
The following is a transcript from the Office of Naval Research's online series, Tech Talk. On Sept. 29, Dr. Michael Traweek answered questions from audiences on Facebook and Twitter. Traweek is a program officer for the Maritime Sensing Team, part of the Office of Naval Research's Ocean Battlespace Sensing Department. He has developed and managed projects ranging from mine countermeasure and anti-submarine warfare future naval capabilities to advanced submarine combat systems development. Traweek's research portfolio broadly addresses the design and utilization of complex systems supporting undersea warfare.
ONR: Thanks for joining our Tech Talk with Dr. Mike Traweek! Please continue to submit your questions for the next 30-40 minutes and we'll answer them in the order they appear.
Shelley Hanes: If I remember correctly, the ONR was studying the hammer head shark sensing capabilities too for this same reason. What attributes of underwater creatures such as the bluefin tuna and hammer head shark are of interest for classifying underwater sounds and understanding flow noise mitigation? How is this behavior related to acoustic propagation? Can these concepts be applied to other media besides water? What physiological regions of the bluefin tuna are of most interest in relation to these attributes? What structure-to-function relationships does ONR want to mimic from sea creatures and for what applications?
Shelley Hanes: Dr. Traweek, I cannot participate at 11:00am because I am busy in lab so unfortunately I will not be online at this time for your "live interaction" answers. For this I apologize, but I will try my best to attend ONR's November Naval Science & Technology Partnership Conference.
Chris Harding: I recently read a document on mitigation of sound created by "pile drivers". During the reading, I refreshed my knowledge on sound and how it interact with materials. I also learned a great deal as well. For instance, I learned that rubber is not nearly as efficient at sound reduction as foam, and, when a bubble curtain is used, the size of the bubble significantly affects sound attenuation and scatter.
Chris Harding: When considering flow noise, do the fish vary the aperture to their "ear" to mitigate certain frequencies? Do the fish tighten certain muscles that affect the rigidity of membranes that effect sound? Although fish are quite aerodynamic, I would believe their body structure may change to affect stagnation points and laminar boundary layers. Turbulence creates higher pressure but reduces the stagnant film thickness in the boundary layer. The higher pressure and turbulence, which may create bubbles, may affect propagation of sound. Is this true? Have you studied this? Pressure will affect sound and bubbles can absorb or scatter since the medium of air is so much different than water. Laminar flow, in contrast, has streamline properties that may allow a different frequency to propagate. Also, the "film" in the boundary layer may be "adjusted" by speed.
ONR: We'll take the first question from Shelley Hanes.
Chris Harding: Here is the article. I have a question regarding a statement in the article as well but the authors may be more appropriate. The article describes an equation using "N" and an extinction coefficient related to bubble size in bubble curtain. Later, the article mentions the "bubble curtain thickness" has an effect, but a later reference in section 3.3.3 mentions that thickness has "no" effect on attenuation because the value is not a part of the extinction coefficient.
Chris Harding: Question: Isn't the "thickness" of the curtain affected by the bubble diameter and "N"? In other words, the "effective" size a of bubble can increase as bubbles "overlap" in the "thickness of the curtain. Similar to "hole" reduction. As we know, the more "holes" the more db that are allowed to propagate through a material.
Actually, upon a second reading of section 3.3.3 I realized the sentence states: "If the number of bubbles remain the same." My discussion in the above question considered the "size" and "number" which is the actual equation. They had "set" a constraint. Although a second reading may have answered my own question, any input would be appreciated. :•) Underwater Noise Mitigation: Nehls, George; Betke, Klaus; Eckelmann, Stefan; Ros, Martin. Assessment and costs of potential engineering soultions for the mitigation of the impacts of underwater noise arising from the construction of offshore windfarms. Cowrie [online]. 2007. pp. 55. Available from: [http://www.offshorewindfarms.co.uk/Assets/COWRIE-ENGFinal270907.pdf]
Dr. T: Shelley Hanes, thanks for your question. Quite simply: Can it be shown conclusively, by observing behavior or perhaps by other objective means, that a fish can hear a far field signal at relatively low frequency for speeds corresponding to high levels of flow induced near-field noise?
Dr. T: The initial focus of this effort is intended to be upon an objective, quantitative evaluation of the ability of a fast swimming fish to hear sounds as a function of speed and frequency. This type of objective analysis must necessarily include a hydrodynamic / hydro-acoustic model for flow noise as a function of speed and frequency appropriate to the physical form of the fish.
Dr. T: ONR sponsors basic research that provides the foundation for future naval capabilities. First and foremost, the recurring lesson in sonar engineering is understanding noise in a physical, quantitative sense. This is particularly true in a challenging context like the detection, localization and classification of a very quiet signal. There are many variables which must be managed in the design of a sonar system with such a purpose.
Dr. T: Specifically, the Atlantic bluefin tuna is mentioned because it swims very fast for extended periods of time. So, while a slower moving fish may provide an opportunity to test the initial hypothesis that fish can hear through flow noise, a faster moving fish might offer an opportunity to empirically validate a model of hearing performance over a wider range of conditions. This does not, however, exclude other fish from study.
ONR: Thanks Dr. T. Next, we'll take a question from Chris Harding.
Chris Harding: Yes, the energy associate with a very quiet signal would have to be amplified. As such, a different medium would have to be used. Is that true? That medium would be a sensor :•) Of course, I asked an obvious question. :•) LOL
Dr. T: ONR is focused on basic research that provides the foundation for future naval capabilities. The purpose here is not to design such a system. Instead, we seek to avoid issues of classification guides and export controls and focus upon an objective, scientific characterization of a biologic system to contend with a very problematic source of acoustic sensor noise.
Chris Harding: So you are concentrating on the physiological and anatomical relationships of the fish more so than the "fluid dynamics" and laminar profiles.
Dr. T: There is a very large body of research on fish in the public domain and it's very interesting, but the purpose here is not to determine exactly how fish process sound, but how well they do it at speed. If they do it well, then we may begin to research the many factors in how they might do it.
Chris Harding: Understandable. So you invoke speed and then use known sound energies that the fish can here when stabilized. As such, the sound energy may be fixed while speed is variable. How do you control the point source so that the fish can migrate to a certain location? Speakers with different intensity and different locations?
Dr. T: Coincidentally, regarding bubbles and the attenuation of sound, Dr. Ralph Goodman, a leading expert on the effect of bubbles in the water column, recently passed away. At the 2009 Acoustical Society Meeting in Paris. a sessioin was devoted to that subject and as a tribute to Dr. Goodman' work.
Chris Harding: stabilize-I should have said "zero velocity" but I realize fish are almost always moving!
Chris Harding: I am sorry to hear that! I have never read Mr. Goodman's work. I base my questions off the above document and my education. It is quite interesting though.
Dr. T: Chris, we are focusing on measuring performance rather than the many factors that you've mentioned that may be involved.
Chris Harding: Performance? In terms of the consistent ability of the fish to respond to a sound at high velocities? Accuracy versus precision and repeatability?
Dr. T: Going back to Shelley's questions about sharks and fish in general, there is a well-documented set of measurements for the hearing performance of fish across a wide range of frequencies under static conditions.
Dr. T: It's remarkable that a fish the size of a goldfish with a sensing organ even smaller can detect and correctly estimate the arrival and direction of a signal at low frequency, with a wavelength much longer than the length of a fish.
Dr. T: To clarify, by performance, we're talking about the measured ability of a fish to detect known signals.
Chris Harding: Really! That is quite amazing and seems to contradict physical reality. My physics is a bit rusty but that is the reason "light" microscopes don't work for atoms. The smaller the "particle" the more energy the "particle". As such, equipment... like cat scans and NMR take advantage of high energy wave-particle duality to measure small quantities. I get your point!
Dr. T: If the Navy is moving toward small autonomous undersea vehicles, like the goldfish, they must have compact, effective, power-efficient and cost-effective sensors.
Chris Harding: Have you ever heard of "holosonics"? The use of ultra-sound to transmit human voices? Of course, this may mean that the fish are capable of reading reflected sound from a rock and "correlating" it with a larger object.
Dr. T: Incidentally, along with two ONR colleagues, I am currently sponsoring the doctoral research of a student at the Naval Postgraduate School that is related to hearing in pinnipeds (e.g., walruses and seals) specifically for the purpose of creating a sonar equation-like model of their hearing performance in the presence of noise.
Chris Harding: I liked the Robotic Fish!
Dr. T: My department is currently accepting white papers for compelling research. Visit our web site for more information: http://www.onr.navy.mil/. ONR's research partnerships are vital to equipping the fleet and forces with every possible advantage.
Chris Harding: Quite complicated and mathematical, I'm sure! :•) I have always appreciated most research that comes from the government. You choose the best and brightest. I had a peer from OSU who went to your Post-Doc program and he was quite impressive.
Dr. T: More specifically, my department web site is: http://www.onr.navy.mil/obs
ONR: Great! Thanks to all of you for participating in this week's online Tech Talk. Please join us at 11 am, next Wednesday, Oct. 6, for a talk with Dr. Paul Armistead of ONR's Sea Warfare and Weapons Department.
Tamra Temple: Thank you, too.
Shelley Hanes: For Dr. T: Thank you for answering my questions by clarifying ONR's research focus and intent on this topic. I have accessed your ONR webpage for learning -- very advanced applications. I will study more on these subjects in my leisure.