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Author Topic: Waves - Sound in Particular the Echoes of Ultrasound  (Read 2783 times)

Offline LukyTom

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Hey

Ive been reading about the physics behind echoes of sound, in particular the workings of the ultrasound. My question is, the books have stated that a sound wave will bounce of say a cliff, and the echo will be reflected back and heard. My question is, why dosent the echo travel through the "rock" medium, and just continue going forever? Is it due the accoustic impendence? If so, what is accoustic impendence (I have a rough idea, but not very clear)?

Is this the same physics utilised in medical ultrasounds? That ultrasounds bounce of the bone and reflect the wave. If so, how can one explain the cliched image of the fetus ultrasound image? From my basic biology, i dont recall a bone plate just beneath the uterus. Could someone please explain?


 

Offline techmind

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Waves - Sound in Particular the Echoes of Ultrasound
« Reply #1 on: 12/05/2009 16:44:10 »
Acoustic impedance is tricky to describe, but has to do with the stiffness of the medium through which the sound propagates. Air is quite compressible, so for a given amount of acoustic energy a given particle will move a long way, but with only a low force to act on anything which attempts to stop it. In contrast a solid, such as metal or rock, is pretty incompressible, so any sound waves will result in very little movement of a given particle (or region of material), but there will be much more force for the same sound energy (per cross-section unit area).

Whenever you get a change in impedance in a transmission system you get a mixture of reflection and transmission: in general the more similar the impedance match the more transmission (100% if no mismatch), and the greater the mismatch the greater the reflection (approaching 100%), with transmission falling to 0%.

In the case of air-to-water, air-to-metal, air-to-rock etc the impedance mismatch is so great that virtually all the sound is reflected off the boundary.


For ultrasound medical imaging, the various soft-tissues and fluids will have similar enough impedance for some of the wave to continue deeper (so the image can see beyond the first interface), but enough difference to make a reflection which can be detected by the sensitive receiver. Bone is probably sufficiently harder than the fluids for the ultrasound not to transmit into the bone very well(?). We know that ultrasound is predominantly used for soft-tissue imaging anyway.
« Last Edit: 15/05/2009 15:57:20 by techmind »
 

Offline LukyTom

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Waves - Sound in Particular the Echoes of Ultrasound
« Reply #2 on: 15/05/2009 11:02:59 »
Thank you. That was a great help!
 

Offline LukyTom

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Waves - Sound in Particular the Echoes of Ultrasound
« Reply #3 on: 15/05/2009 14:26:33 »
Can i also ask about ultrasounds; why when frequency is lowered, penetration is also lowered, yet better resolution?
 

Offline techmind

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Waves - Sound in Particular the Echoes of Ultrasound
« Reply #4 on: 15/05/2009 16:00:29 »
Can i also ask about ultrasounds; why when frequency is lowered, penetration is also lowered, yet better resolution?

In general higher frequency tends to lead to greater resolution but lower penetration. I would assume ultrasound is the same (but I'm no specialist, and there could be other factors which come in to play).


Higher frequency gives better spacial resolution because the wavelength of the waves gets shorter. I believe medical ultrasound-imaging systems use frequencies of a few MHz so in the body (water, where the speed of sound is 1500m/s) the wavelength will be around 0.5-1.0mm.

If you can correlate the timing of your 'echo' to (say) 1/10th of an acoustic cycle then that might give you ~0.1mm resolution in depth (Z).

Probably more importantly you also need to think about sweeping and focussing the beam for X and Y resolution - this relies on phasing (interference) effects and you require an extended ("phased array") source and/or detector which is many wavelengths long. For a given sized source/detector, this beam forming gets far more precise the shorter the wavelength.
« Last Edit: 15/05/2009 16:14:50 by techmind »
 

Offline LeeE

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Waves - Sound in Particular the Echoes of Ultrasound
« Reply #5 on: 15/05/2009 17:07:59 »
I have a friend who installs and maintains hospital ultra-sound imaging systems and while I've never asked him about specific resolution I do know that it's high enough to accurately measure and differentiate both mother and embryo blood flows using doppler techniques.
 

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Waves - Sound in Particular the Echoes of Ultrasound
« Reply #5 on: 15/05/2009 17:07:59 »

 

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