Water, water, everywhere

[Tomorrow]

I understand that sound doesn't travel further over water...but that it is amplified by water. Would one of you audio experts please 'splain this to an acoustics dummy?

T'anx in advance.

 

[Rob Babcock]

I'm no expert, but water is much, much denser than air.

 

[Tomorrow]

I'm no expert, but water is much, much denser than air.

What I meant was....when on open water, like a lake, sounds like voices seem to carry quite a ways. But I've been told that it has to do with the fact that the water is colder and actually amplifies the voice sounds by bending the sound wave. This I do not understand and need 'splainin' to.

 

[racquetman]

What I meant was....when on open water, like a lake, sounds like voices seem to carry quite a ways. But I've been told that it has to do with the fact that the water is colder and actually amplifies the voice sounds by bending the sound wave. This I do not understand and need 'splainin' to.

I'm no expert, but I don't believe temperature plays a role in why sound seems to travel further on water. Temperature does have an effect on sound waves, but the effect it has is velocity related, not intensity related.

My guess is that the water acts like a solid surface (water has a high surface tension), and the sound wave skips off the surface like a stone causing it to carry further. Less of the sound is absorbed compared to other surfaces, therfore more intensity is retained for longer distances.

Another possibility is that we just perceive that the sound is carrying a greater distance .

 

[Sheep]

I'd like to hear GreenJelly's opinion on the matter.

Hahaha, this will be my new BS forum saying.

SheepStar

 

[rtcp]

If I remember my physics class correctly, it's because the air temperature is lower on the surface, and causes refraction.
Same thing that causes that "wet-looking" mirage thing on a hot highway.
Feel free to entirely destroy what I just said.

 

[rtcp]

That is to say, the lower air temperature causes the soundwave to refract towards the surface, and makes it perpetually bounce-or skip, I suppose-off the water surface.

 

[f0am]

i dont know the correct answer but i would think the fact the surface is almost perfectly smooth, less the movment, would help as well.

 

[Tomorrow]

If I remember my physics class correctly, it's because the air temperature is lower on the surface, and causes refraction.
Same thing that causes that "wet-looking" mirage thing on a hot highway.
Feel free to entirely destroy what I just said.

I think you have hinted at the right course of thought. Sadly, it was back in the Dark Ages when I had my physics classes at UC Bezerkeley, but I know refraction is the essential element of this problem. I just can't figure out why the bending of sound adds amplitude...thus making the sound heard more clearly at distance. Accelerating waves above the lower, slower, cooled waves?

Dang, where are those acoustics gurus when you need them. I should have titled this thread Bass Traps For The Aquarium, lol!

 

[bandit]

Hi all,

I agree with the high tentsion surface etc. of smooth water. I could see how it would seem to allow sound to carry further. Another possibility has to do with what may be in the way when your not on water- such as grass - trees - cars- people - dust - etc.

Bandit.

 

[astrodon]

As somebody who has a doctorate in astrophysics and is a physics professor at a university, I will add my two cents. What has been described by some of the others, the water surface reflecting sound waves has some merit, there are two main ressons why sound seems to travel farther on open water than on land. The main reason is that background sound (i.e., "noise") is typically much reduced on a lake than on land where other people, animals, and birds like to hang out. Hence one can hear sound at lower decibel levels than when the background noise is higher (on land). The other reason has to do with obstructions (e.g., trees, buildings, hills, etc.) that are often plentiful on land. Obstructions will redirect (and also absorb) sound waves, once again reducing the decibel level at the position of the listener.

 

[Tomorrow]

As somebody who has a doctorate in astrophysics and teaches physics at a university, I will add my two cents. What has been described by some of the others, the water surface reflecting sound waves has some merit, there are two main ressons why sound seems to travel farther on open water than on land. The main reason is that background sound (i.e., "noise") is typically much reduced on a lake than on land where other people, animals, and birds like to hang out. Hence one can hear sound at lower decibel levels than when the background noise is higher (on land). The other reason has to do with obstructions (e.g., trees, buildings, hills, etc.) that are often plentiful on land. Obstructions will redirect (and also absorb) sound waves, once again reducing the decibel level at the position of the listener.

Thanks for the input, and I respect your credentials. Those are pretty common-sense applications of logic and I buy into some of them as causitive agents. But water can be pretty noisy (splashing/gurgling) and non-linear (waves), and terrestrial sounds abound in small lakes. And even on an unobstructed piece of land, voices will not be heard as far (stated another way... as easily heard via amplitude) as on water.

And there is still this nagging issue of the refraction-effect that needs 'splainin'. I believe the answer lies in the temperature differentials. (C'mon Ethan, Chris, etc. Clear this up. )

 

[racquetman]

And there is still this nagging issue of the refraction-effect that needs 'splainin'. I believe the answer lies in the temperature differentials. (C'mon Ethan, Chris, etc. Clear this up. )

You don't need those other guys!! I think I found the answer for you. Enjoy.

http://www.kettering.edu/~drussell/Demos/refract/refract.html

 

[mtrycrafts]

I believe the answer lies in the temperature differentials. (C'mon Ethan, Chris, etc. Clear this up. )

What would that do to sound waves if the surface of the water is 50F and the air is 65F at the height of the listener? The sound wave generated by a person on shore at the same height, while it expands in all direction, would still pass through that 65F layer. Granted, there is a boundary layer a few feet down at the surface whereas none above.

 

[Tomorrow]

You don't need those other guys!! I think I found the answer for you. Enjoy.

http://www.kettering.edu/~drussell/Demos/refract/refract.html

V e r y nice!! Excellent research, alandamp. This makes complete sense, now. I knew my memory hadn't taken a complete dump on me...(just a partial dump, lol).

Learning is such fun! Many t'anx for scratchin' my itch.

 

[f0am]

After reading the article they still dont mention anything about the dense smooth surface the sound is traveling across there for helping it travel...
This theory doesnt exactly work in the woods as well as it does water!

 

[Tomorrow]

After reading the article they still dont mention anything about the dense smooth surface the sound is traveling across there for helping it travel...
This theory doesnt exactly work in the woods as well as it does water!

F0am,

The issue here isn't reflections or even reflectivity, interferences, absorbsion, modes, etc. It is granted that those are all issues that prospectively affect sound propagation. But smooth surfaces of and by themselves do not amplify sound. That was the core of my curiosity and question....why is it that water can and does amplify sound?...that was the issue. That article provided a succinct answer...and it is because of a temperature differentials that cause sound refraction and velocity variance within acoustic waves. Theoretically, it would work in any environment that displays the temperature inversion described.

 

[f0am]

Heh not to mock this entire thread... the whole thing facinates me = )..
After reading the provided material and seeing the facts, I have decided the best next purchase for my home theater is ceiling mounts and a 12 foot sheet of dry ice. heh

 

[Ampdog]

I took part in such experiments - but most of the basics have by now been covered!

The explanation by Drs Dan Russel and "Astodon" is of course correct, but the matter mostly also includes other factors. Depending on the evaporation of the water surface the effect of temperature can vary. One must keep in mind that the differential can be greater in the day than at night, which will cause the opposite effect - less "amplification" at night! I have unfortunately no measured temperature data on this.

Also as someone remarked, a large surface of water is often choppy and some varying reflection will take place (keep in mind that the wavelength of 1 KHz is about 1,1 ft). Where we were also interested in the audibility of speech, a choppy water surface often resulted in a decrease in sound level with frequency, such that intelligibility of speech was negatively influenced. On natural terrain the "obstacles" were of dimensions sufficient to attenuate sound, often severely so. Also undergrowth plays an absorbtive role. In our case I cannot recall a very marked difference between the phenomenon at day or night (not however to negate the information by the above academics). We also found an increased sound propagation in daytime over water as compared to land - as said, in our case most of the difference was attributed to the difference in the terrain between test points.

But I enjoyed the "refurbishing" of my physics years - also I fear measured in decades ago rather than years!

Edit: I had the wrong name for Dr Russel!

 

[Ampdog]

Sorry folks, I was a bit greenjellied here.

I had the tempreture lapse the wrong way round. It will of course promote increased audibility at night, as I thought we found (tests also about two decades ago). But, as said, in our case more marked with respect to terrain - and air is also not stationary. There was wind blowing, which may or may not have come over hot terrain.

(It would seem that one must be careful of hot air. )