Im not sure I can agree or not. Having some back ground in this this is what is called a beamforming function where in simple terms you compare the signal difference in the left and right ear and turn your head untill it sounds the loudest and that is the direction it is coming from. It is a matter of signals being in phase or not. You mind is use to receiving in air where the speed is what 1300 fps water being 4800 fps messes the minds processing of what is comeing in the ears. not to mention in the water sound tends to vibrate the entire body of water and your ears is not used to working with that like it does with air where the obvious greatest volumn is more directional. Its like taking your finer on a body of water and touching it and you see the ever growing circles expanding. try that on a pan of jello and it just sits there and wiggles.
Quiz - Physics - Sound
- Thread starter Pedro Burrito
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Yes and in addition to that is that when a train goes by and is blowing the whistle there is a comparable function of frequency that is drastically different. if a train has a 200 hz wihstle there will be sufficient frequency change for the ear to notice. if you have a propeller with 3 blades and it is radiating a blade rate of 200 HZ you would have to have a prpopeller speed of 60-70 revolutions per second to equal the 200 hz whistle. The engine however has its own noises that allow the doppler effect to work much better. Then again I am used to listening to such things.
Yes, the point is that the Doppler shift is a fractional change in the frequency you are listening to. If you are listening to a higher frequency the fractional shift is more Hertz, so you have a chance to hear it. But the BIG effect in water vs air is that the fractinal shift is smaller than in air, by the amount that the sound speed is larger. So a shift in air of 4% in frequency becomes just 1% in water. This is further confounded by the speed of objects in water vs air....boats are slower than trains, for example.
Some people, of course, have extreme sensitivity to frequency (pitch) so can hear really small shifts. Most of us can't.
I have done a lot of work inthis area as a job. the % of shift in both air and water is the same eh wave leignths are different but are proportional to eh speed of sound. It cant work any other way. You are adding volocity in a direction. In the water If i remember 300 hZ sees a .1 hz change in frequency for every 1 knot of speed that is applied +/- in the direction of the receiver. its a sine cos function. The speed in the water does not change but the wavelength does because of the relative movement of he sender and receiver. the wave length differences are canceled out with the same changes in Speed of sound. such as in the water a 1k tone will have a wave length of 4.8 ft (4800 fps) in the air it will have a length of 1300 ft (1300 fps) the dpppler inwater as a functionof wave is about 4 times larger fut in apparent frequency is the same. You and I will both agree it is not a simple 50 word concept. either wave speed relationship is much like on an Oscope if you have a 1k sweep rate and you input a 1k signal you see one wave on the crt but reduce the sweep rate to 100hz and you will see 10 waves on the crt. I dont know if we will come to a written discription agreement but I am sure we both are right from our viewed perspectives.
IMO the reason it is difficult to detect the doppler shift in the water is not a matter of physics it is rather the source generator that is different. A boat passing by has so little freq shift because of the low frequencies it is generating as compared to the pitch of a whistle of a train in the air. the boat however has other sounds other than the prop like bearings etc that is much higher freqs such that the tone change is now noticable. With out any fact base. Lets say you can detect a freq change of no less than 10 hz. a low freq cource and a doppler altering the rediated freq of 100 hz by 4 hz would not be noticable. but from a source of 1k it would generate per ratio a change of 40 hz and the ear would under that hypothetical assumption could detect that. Reality is different of course but it functions t or should I say produces the same type of results. I think we dont necessarily hear specific changes but rather hear some sort of change and a direction of change. I hear a sound drop pitch when it passes but I cant tell how much. unless there is a significant change in process a diver would have no clue as to what shifts were in play. Again it is a topic that to understand takes a lot of class room time. After teaching it no one really references itany more it becomes an assumed standard like working ohms law and the effects of changing values. once understood n o one gets out a calculator to figure the precise compunents. a short is by function a 0 ohm connection and is bad no matter what color the wire is.
Back to this topic it is not difficult to determine if you hear a boat prop when the boat passes by you at any threatening distance and is going away vs coming at you to decide when to come up to SS or the surface.
It's unfortunate that PADI doesn't allow qualified physics people to review their physics curriculum. I'm a physics professor (and yes... it was a little uncomfortable during my IDC, watching my CD try to teach the class the physics section.) This statement, with the "correct" answer of "d", is technically incorrect.
The speed of sound in a fluid is the square root of the ratio of the bulk modulus of the fluid to the density of the fluid. Which means sound waves travel SLOWER in denser fluids. This is simple mechanics.
But sound waves travel faster through fluids with a higher bulk modulus, which is the inverse of its compressibility. Water is about 800 times denser than air... which would case sound to travel about 2.7 times SLOWER in water. But the bulk modulus of water is about 13,000 times greater than that of air (i.e. water is about 13,000 times resistant to compression than is air.)
The ratio of these two, 13,000 to 800, is about 16, and the sq root of that is 4. So the ratio of the speed of sound in water to speed of sound in air is about 4.
But the density of water does not make the sound travel faster.
Ahhh... you beat me to it. I replied after reading the first post, just now started reading the rest of the thread.
Which bolsters my argument: it would be nice if PADI (and other agencies) consulted qualified people on even the simple technical stuff. There's enough people available for the task (heck, even if they just posted their questions on Scubaboard, they'd get instant valuable feedback.)
Now that we have all learned something new today, can we get back to our standard sport of making fun of people with dangling consoles and fins in the sand?
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