The Longer Reach of High Frequency Sound Waves

[daniel_i_l]

Why can a sound wave with a high frequency travel further and stay more concentrated (i guess there are basically the same) than one with a lower frequency?
Thanks.

 

[billiards]

i don't know where you got that idea, high frequency sound waves attenuate more quickly than low frequency ones. This can be understoond qualitatively because if it is higher frequency it is doing more work than a lower frequency wave of the same amplitude by virtue of the fact that, by definition, it is oscillating more quickly.

 

[haiha]

I think I have the same feeling to Daniel because there are a number of instruments using ultrasound waves to measure distance or in other purposes.

 

[daniel_i_l]

I got the idea from an article I read about hypersonic sound:
http://en.wikipedia.org/wiki/Hypersonic_sound
After reading it again I see that the ultrasonic wave doesn't necessarily travel further, it's just more concetrated into a narrow beam - sort of like a laser. So why does the higher frequency make the beam narrower?

 

[AlephZero]

If the wavelength of sound is larger than the source, the sound radiates equally in all directions. If the wavelength is smaller than the source, you get a more directional beam of sound, because away from the axis of vibration the sound from opposite sides of the source is out of phase and cancels out.

So the easiest way to create a directional beam of sound with a small transmitter is to use ultrasound (wavelength of the order of 1mm). To do the same at audible frequencies (wavelength of the order of 1 meter) you would need a transmitter several meters in diameter, which isn't very practical.

 

[daniel_i_l]

Cool, thanks.

 

[Danger]

I didn't check out the link, but I find it pecular that Wiki would have an article about 'hypersonic sound'. There is no such thing. Ultrasonics have to do with frequencies above human hearing level, and infrasonics are below audible.
Hypersonic and supersonic refer to objects that are moving faster than the speed of sound in the same medium.
Anyhow, longer wavelength (lower frequency) sound travels far more than higher frequencies.

 

[AlephZero]

I didn't check out the link, but I find it pecular that Wiki would have an article about 'hypersonic sound'. There is no such thing.

HyperSonic Sound is a trademark for a sound-beaming product (which is what the other wiki page is about) The upper case S makes all the difference, I suppose. http://www.atcsd.com/

 

[Danger]

Thanks, Aleph. Capitalization seems to be a somewhat random event around here, so I didn't think of that being a name.

 

[Chris Hillman]

I urge frequent discussion of the woes of WP--- somewhere else!

I didn't check out the link, but I find it pecular that Wiki would have an article about 'hypersonic sound'.

Indeed. If every physics graduate student in the world joined WP, they might just be able to maintain it despite the incredible inefficiency of its administrative/judicial/legislative procedures. Or it might become even more chaotic (due to the inefficiency of its administrative procedures, aka lack of any central vision, planning, or leadership). What to do? The only thing I can think of is to point out specific problems as you notice them here, and perhaps also by dropping by http://en.wikipedia.org/wiki/Wikipedia_talk:WikiProject_Physics to mention what you spotted. I couldn't advise trying to edit WP yourself (been there, done that, and I regret the time I wasted there). I've tried to discuss the perennial quality control problems of WP elsewhere, so anyone curious can look for that thread, and anyone who wishes to express a dissenting view should start a new thread in another forum, citing my discussion (I probably wouldn't join in though, since even well-reasoned and well-informed criticisms of WP seem to be highly inflammatory to the often ardent fans of that website.)

Daniel, the correct buzzword for low frequency sound is infrasound. This is a largely unaddressed social issue because of its role in the epidemic of hearing loss (because as already mentioned low frequencies attenuate less rapidly, and are produced at high power by various common consumer items). From http://www.cbc.ca/news/background/health/hearing-loss.html

Hearing loss is becoming an epidemic problem for today's adults – and it's happening earlier than ever. "People are starting to lose their hearing 20 years earlier than in the past," said Heather Ferguson, president of the Hearing Foundation of Canada in Toronto. "I believe we're on the verge of noise induced hearing loss becoming a public health crisis."

From http://www.bbc.co.uk/health/conditions/deafness1.shtml

Excessive exposure to noise is an important cause of a particular pattern of hearing loss, contributing to problems for up to 50 per cent of deaf people. Often people fail to realize the damage they're doing to their ears until it's too late. Although loud music is often blamed (and MP3 players are said to be storing up an epidemic of deafness in years to come) research has also blamed tractors (for deafness in children of farmers), aircraft noise, sports shooting and even cordless telephones.

Laws on noise pollution are highly outdated and have been rendered dangerously obsolete by the common availability of various consumer items which generate dangerous levels of infrasound, sufficient to cause permanent hearing loss with a minute or so at considerable distance.

 

[cesiumfrog]

Why can a sound wave with a high frequency [..] stay more concentrated[..?]

Is this a question about "diffraction"? (Other factors come to mind, like perceived directionality relating to the difference of scale between wavelength and the listener's head.)

 

[daniel_i_l]

I meant that it all goes in one direction, unlike normal sound that spreads out.

 

[billiards]

What happens, if this is anything like the air-guns used in seismic marine surveys (as seen here: http://www.fugro-robertson.com/default.asp?section=services DIRECTIONS: click 'preview it here', scroll down and click 'Offshore seismic sources', and then scroll down to a 3-D ball for graphic illustration), is that at higher frequencies, the energy becomes concentrated in narrow bands. The wavefront will still presumably be spherical, but most of the high frequency energy is concentrated so that it is not lost by geometric spreading.

Note that this mechanism is completely unrelated to the energy loss mechanism I mentioned in my earlier post.

 

[Danger]

Ah, yes... high frequencies are a lot easier to focus, and so can transfer more energy to a small target. The range, however, is severely limited. You can weld plastics and possibly some metals with ultrasonic pulses, but if you want to rattle somebody's teeth a couple of miles away, go for the 'thunder'.

edit: I just thought of a fairly apt example for anyone who hangs out at the track. A peroxide rocket or jet powered dragster has an aweful lot more horsepower than a 484 ci AA fueler. Most of the physical effect upon the observer, though, is serious ear pain. That digger with open headers, on the other hand, makes your ribs dance around like they're on springs.