Relaxation times of molecules during sound propagation

[TP9109]

So i understand completely what the first source is saying i.e. the longer the relaxation time, the more absorption of the ultrasound beam. The first paragraph of the second source however says the opposite- "very long" relaxation times mean the sound wave passage is unaffected?
My attempt at a solution is that the second source first paragraph is referring to a "very long" relaxation time where the molecule for example takes a long time to transfer energy from vibrational to translation and when it does, it translates in phase with a future compression? Therefore it isn't moving in the opposite direction so no attenuation, is that correct?

The first source comes from https://radiologykey.com/ultrasound-imaging-3/ "Ultrasound imaging"
"Relaxation time refers to the length of time for particles within a medium to revert to their original positions after being displaced by an ultrasound pulse. A longer relaxation time means that displaced particles have a higher probability of encountering the next ultrasound pulse before fully relaxing. The particles may be moving in an opposite direction than the new compression pulse, which results in increased dissipation of energy from the ultrasound beam. Therefore, increases in all three factors—frequency, tissue viscosity, and tissue relaxation time—lead to increases in heat generation, and hence, absorption of the ultrasound beam."

The second source of info is shown below, it's a book called "Sound Attenuation" from Architectural Acoustics, 2014.

"If the relaxation time is very long—that is, if it takes a long time to transfer energy back and forth between translation and vibrational motion—then a sound wave, which generates rapid increases or decreases in sound pressure and therefore sound temperature, is unaffected by energy transfer to other modes that take place too slowly to influence its passage. The fluid is said to be in a “frozen” condition insofar as this energy transfer mechanism is concerned. Similarly if the relaxation time is very short—that is, much shorter than the time for changes in pressure due to the sound wave to occur—then the energy transfer back and forth between translation and vibration happens so quickly that the fluid is in a state of thermal “equilibrium” between the various energy modes, and again, there is little effect on the passage of the sound wave.

If the relaxation time is just the right value, then when a pressure wave passes by, the increased translational energy is converted into vibration and then back into translation coincident with the arrival of the low-pressure region. The wave amplitude is attenuated since the acoustic energy is converted either to random molecular motion (heat) or to pressure that is out of phase. When the acoustic frequency is of the same order of magnitude as the relaxation frequency (1/2 π τ) of a particular vibrational mode, air can induce significant sound attenuation"

Any help appreciated! Thanks

 

[hutchphd]

I do not understand your first source at all...I would simply ignore it. The second source makes sense (and agrees with the first source for short relaxation times). Go with that one.

 

[.Scott]

The first article is application-specific (medical ultrasound) whereas the second appears to be more generic.
There are two differences:
1) The first article is discussing a train of ultrasound pulses. The second article is describing a continuous wave. Medical ultrasounds can operate with either continuous or pulsed, but pulsed it much more common - it allows for the selection of a range of depths.
2) There is likely a domain difference. The second source is describing the results from a very wide domain of frequencies and relaxation times. The first is describing the much narrower domain seen by a radiologist.

 

[TP9109]

Thanks all for the replies, that makes sense now

The first article is application-specific (medical ultrasound) whereas the second appears to be more generic.
There are two differences:
1) The first article is discussing a train of ultrasound pulses. The second article is describing a continuous wave. Medical ultrasounds can operate with either continuous or pulsed, but pulsed it much more common - it allows for the selection of a range of depths.
2) There is likely a domain difference. The second source is describing the results from a very wide domain of frequencies and relaxation times. The first is describing the much narrower domain seen by a radiologist.

Thank you that makes more sense now, I never thought about ultrasound imaging using pulses