tsarkar wrote:If Issac Newton was alive, he would have been so proud of you!
That by every class in vibrations and acoustics that you sat through, you not only picked up every pebble & shell on the beach but you also swam across the vast ocean of knowledge to the other side and said, "bunkum" to others. Well done, boy!
While, I could have started with a retort of " Old Man", I am not going to. Why ? None of what I write here or read here is personal, nor do I care if it is personal against me. I care two hoots. I don't know anyone here , haven't met anyone neither does anyone here know who I am or have met me. That is the way it will remain as well as I have no plans to meet anyone. And no, nor do I intend to insult you or show you in poor light as you seem to assume, though I do admit to be part of at least two of the "cliques" you suggest seem to exist and possibly could apply for more
That said, what I DO care about is what is presented here , especially if it concerns areas I know pretty well about . Now the trouble as always is for anyone to accept, nay even contemplate ,especially in areas where one considers themselves as experts/authority figures that there might be others who actually have the background and knowledge, more so in a board like this,( in fields ranging from science, math,engg of nearly every specialisation, comp sci, medicine, finance, management, business, and of course, defense , foreign policy, public policy.. you name it) to be challenged when you extrapolate what you know and is correct and true, into areas which you don't based on a set of assumptions and could possibly be very wrong.
Surely, the Science of Physics exceeds beyond Impedance and the Science of Acoustics exceeds beyond-
That despite impedance, the low frequency signal remains, propagates in multiple ways as Urick discovered and can be detected..
Let us address the more substantive points. Impedance is fundamental to many branches of Engg. For electrical enggs and communication enggs, it is vital. Most Elec engg guys would instinctively know about impedance matching and transfer functions. You cant be an Elec Engg without knowing voltage, current and resistance. So for someone studying acoustics without knowing impedance is like someone studying electricity without knowing what resistance is.
While you may have possibly heard of Acoustic Impedance for the first time , lots of people from different backgrounds know about it. Enggs, doctors, physicists to name a few. I happen to know it too. So let me explain to you the physics behind it and why your assumptions, experiences (self reinforced by the papers you Googled up) lead you astray.
1. Air- Water boundary is characterized by huge acoustic impedance mismatch and that boundary is a nearly (99.9%) perfect reflector. (yes, that applies to sea water too, we are not talking electrical impedance here). So, nearly all the incident sound energy is reflected and a very small % goes into the water.
2. I know you think I am probably B.S ing when I put out all the analogies, fish, and earlier whale etc. But no, I am deadly serious. The speeds of sound in air and water are different, so knowing that, you can calculate the air to water refractive index for sound and from that, you can calculate the critical angle for sound beyond which there is Total Internal Reflection
and the sound wont penetrate. This for the air/water boundary is a cone with a 13 deg half angle.
3. So really, for all the huge sound that the helicopter puts out, only a minute fraction (ie. 0.11% in the max case) and that too within a very small cone of 13 deg half angle can penetrate.
4. Now, you dig up the Ulrik and Brar_W posted IEEE reports and say, okay, I can detect that, and I know sound waves propagate across large distances, so I can hear it from afar. You also seem to understand that , it really is low frequency sound that propagates substantially and you in turn seem irritated when I write that high frequency sound dies down very quickly and dont talk about the low frequency sound necessary to confirm your assumptions.
5. Of all the ways sound can propagate , the directly refracted sound is the strongest signal. With the already very low signal strength, actually hearing direct sound is the best bet. If it is high frequency, the range is very limited. The sonar has to be directly under the sonic cone
6. What of low freqeuncy ? Why not talk about that as the papers discuss those are the ones that really propagate? Trouble is in the impedance mismatch, it is the highest frequency that penetrates best. The low frequency ones are lost.
When you are in a plane and your ears pop, what you lose MOST is the low frequency noises. That happens because of impedance mismatch due to pressure differences.
Check out this Audiology Text Book
on middle ear, under the section Eustatian Tube and Pressure equalisation.
the Eustachian tube, which connects the middle ear cavity to the nasopharynx, normally opens and closes periodically thereby insuring that the static pressure in the middle ear will remain the same as atmospheric pressure. When the Eustachian tube fails to open, a negative pressure immediately begins to build in the middle ear cavity due to absorption of the trapped air by the middle ear mucosa. There is an increased stiffness in the middle ear mechanical transmission system. The transmission loss is greater for low frequencies and has been observed to be of the order of 20 dB for frequencies below 1000 Hz
In case you dont believe that this is the way impedance mismatch would behave when sound bounces off water, check the
wiki entry for underwater acoustics in Sound Reflection and Scattering
in the section "Speed of Sound, Density, Impedance"
The large impedance contrast between air and water (the ratio is about 3600) and the scale of surface roughness means that the sea surface behaves as an almost perfect reflector of sound at frequencies below 1 kHz
7. So what that means, is that out of the sound energy that penetrates (the 0.11%), the bulk of the energy is concentrated , in the higher frequencies, which unfortunately die out very quickly. So the actual low frequency spectrum energy
is a very small portion of the already small (ie. fraction of 0.11%) signal that the helicopter /aircraft radiates.
8. Together 6 and 7 mean that the best chance for a hydrophone to register aircraft noise is when it is directly overhead. Transmission is highly improbable.
9. Therefore the already highly improbable task of hearing sounds underwater is very nearly impossible.
10. Same reason why machinery, men peeing, flourscen lights etc cant transmit to water via air be detected. Practically non existant signal strength.
So let me do the menial work and email Dr Rao and get more illumination on Hudimac, Urick & Ferguson's work, and how good a student you were in Structural and Acoustic Engineering, and whether Newton's Ocean of Knowledge ends at Vina & Impedance.
I would never consider any sort task associated with learning "menial", after all Gurukul in earlier days was being fully menial in pursuit of knowledge .Please do whatever it takes to satisfy your curiosity, maybe being it from just picking up a standard acoustics text book ( by Ulrick himself, his underwater acoustics is one of the best) , talking to your fellow service folks who probably are specialized in this, and of course with any one else ,including Dr Rao. In case you are writing to him, please do mention that one of his former students fondly remembers him , his classes , wishes him very well and that is how you came to know of him.