Quiz - Physics - Sound

[Joneill]

and so it begins...

Exactly - only on SB will we find endless posturing on how the possible answers to a simple question in the BASIC section are all wrong. Geez! ...though, in this case, Angelo is correct!

 

[Steve_C]

Faster does not necessarily means that sound travels further. .

I ever said why it why it went further. Just important to know that it does. Appreciate your detailed explanation.

 

[Scared Silly]

Type louder I can't hear the replies.

 

[TMHeimer]

Knowing it travels faster and further in water, salt or fresh, is the useful information.

Yes, but it's not about useful information, but about various ways we can kill time dissecting questions.
I too appreciate his detailed explanation. Too early in the day to digest it. Maybe tonight.

 

[Altamira]

@Angelo Farina. Thank you for your posts. I always learn something from them.

 

[Angelo Farina]

@Angelo Farina. Thank you for your posts. I always learn something from them.

Your are welcome.
But I am just doing my duty, I am paid for doing that by the Italian public instruction system...

 

[tursiops]

Actually all answers are wrong.
The speed of sound in a liquid, c, is given by the square root of the bulk modulus (aka stiffness coefficient) K divided by the density ρ, as described by the Newton-Laplace equation:
View attachment 581401
Hence increasing the density makes the sound speed TO DECREASE!
The sound speed in water is larger than in air because water is much more stiff than air (which is highly compressible), not because water is more dense...

True.

Said that, one could expect that the sound speed is lower in salt water as it is more dense than fresh water. And instead, the sound speed in salt water is larger...
So what?
Read here for an explanation, but making it short, the effect of salt is larger on increasing the bulk modules K than on increasing density ρ!
Why does sound travel faster in Salt water then fresh water? : askscience

True.

Faster does not necessarily means that sound travels further. It travels in water for hundredths of kilometres not due to the large sound speed, but due to two concurring factors:
1) The mechanical losses are very very small, so a very tiny amount of acoustical energy is converted to heat.
2) Sound does not spread vertically, as the vertical gradients of temperature and salinity create what is called a "sound channel" (SOFAR channel), so the energy is confined at intermediate depth without ever hitting the surface or the sea bottom.
View attachment 581437
More info: NOAA Ocean Explorer: Sounds in the Sea 2001: diagram of how sound travels underwater

Knowing that the sound speed in water is larger than in air is important for the diver for another reason: we localise the direction of arrival of sound hitting our ears by means of two "cues", inter-aural time difference (ITD) at low frequency and inter-aural level difference (ILD) at high frequency.

View attachment 581438
ITD effect (left), ILD effect (right)
​

If sound travels faster, of course the inter-aural time difference between the two ears is strongly reduced. Furthermore, if the sound speed is larger, also the wavelength becomes larger, hence the human head does not provide anymore a significant shadow effect, and also the inter-aural level difference is much smaller than in air.
So we lose both cues, when being underwater, and we cannot localise anymore where the sound is coming from.
There are technologies which allow to retrieve directional hearing underwater, but I suppose that presenting them here would be overkilling, as we are in the "basic" section...
Instead all what I wrote in this thread is something which has been taught to my sons at the beginning of secondary school, when they were eleven or twelve, hence I consider this to be "basic science".

More context is needed....
(1) is only true for lower sound freqs (perhaps below 10kHz?); for high freqs the absorption is very large, so propagation distances are quite limited. As divers we sense this when hearing a beeping alarm from someone's computer; that high-freq sound attenuates quickly with distance, thank heavens!
(2) is only true (at most) in deep water where a sound channel can exist...see his figure. Even if deep water, sound channels are not everywhere. Divers are not usually in such deep water, so we would not normally be affected by or perceive any long-range propagation of this type. Rather, divers are mostly in shallow water (relatively speaking) where the surface and the bottom interact strongly with the sound as it propagates, causing much scattering and confusing arrivals at a receiver, and pretty much eliminates very ong-range propagation.

I know Angelo knows all this, but my preference is to deal with situations in which divers find themselves, not the general open-ocean deep-water situation. If you are curious about really long-range sound propagation, take a look at The Heard Island Feasibility Test and http://brigus.physics.mun.ca/~zedel/P6317/papers/heard_island.pdf.

 

[Vitesse2l]

Is there a practical application of this knowlege? For scuba, that is.

Bear in mind this should have been the first day of my club's LOB out of Hurghada. I may be... touchy. Today's lockdown substitute for Red Sea diving = rendering concrete block retaining wall.

 

[rhwestfall]

Is there a practical application of this knowlege?

Yes, so you can have an idea of how really bad (and soon) this is going to get (happen)...

 

[tursiops]

Is there a practical application of this knowlege? For scuba, that is.

Yes, several, but you appear to be (justifiably) in such a bad mood that I'm hesitant to provide any info!

Never the less, one example is acoustic location underwater, for example an acoustic beason at an exit point so in low-vis water a diver can find his way back to the exit. Several system exist to do this. Here is the manual for one. https://desertstarsystems.nyc3.digitaloceanspaces.com/Manuals/DiveTracker™ Sport.pdf Among other things, the manual say, "Measuring distance by looking at signal strength is a rather inaccurate and sometimes misleading science. For this reason, distance is indicated on the scale only in rough numbers. In most cases, you will be able to determine a doubling or halving of your distance (every two LED’s indicate a double or half distance). However, in other cases you may find that Mother Nature has forgotten to read the science books and the signal stays at the same strength for a long time, actually gets greater with growing distance or drops of far more rapidly than expected. All this may cause you to appear to be at a greater or lesser distance than you really are. Be careful! Do not take the distance readings as gospel but rather put about as much faith in them as you might in a long-range weather forecast. Over time, you will be able to interpret the numbers with growing confidence."