Why do ultrasound waves not travel through air well?

In summary, ultrasound waves do not travel well through air due to the high impedance mismatch between the air and the transducer, causing most of the waves to be reflected instead of transmitted. This is why ultrasonic gel is necessary to prevent air from coming between the scanner and the human body, allowing for better transmission of the waves.
  • #1
RubinLicht
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I'm at an internship and I saw a container labeled ultrasonic gel, which is used to prevent air from coming between a scanner and the human body. This is necessary because apparently ultrasound doesn't travel through air well. Why is this? (don't say because the density is low, because I will reply by asking why low density means ultrasound waves can't travel far)

Also, I'm not sure if it's that the waves spread out in air, or just that they don't propagate as far in air. The website were fairly obscure since they were targeting their customers, not curious high schoolers who want to learn physics..
 
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  • #2
RubinLicht said:
This is necessary because apparently ultrasound doesn't travel through air well. Why is this? (don't say because the density is low, because I will reply by asking why low density means ultrasound waves can't travel far)

because the lower the density the higher the attenuation

EDIT: to expand on that ...
for a given density, as the frequency increases, the attenuation increases
eg so for a given density that audible sound frequencies say 300Hz to 5kHz,
travel through with minimal attenuation, as the frequency increases to ultrasonics
20, 30, 50 kHz the attenuation will increase proportionally
 
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  • #3
consider why sound doesn't travel through a vacuum ... same thing

To quote a movie line ...
"In space, no one can hear you scream"Dave
 
  • #4
davenn said:
consider why sound doesn't travel through a vacuum ... same thing

To quote a movie line ...
"In space, no one can hear you scream"Dave
I went and read a bit on attenuation and also talked to the main researcher at the company (dads friend who knew me as a child so i could approach him), He gave an explanation that was slightly different (because he was addressing ultrasound in ultrasonic machines specifically), he said that when the waves hit a boundary, the majority of it is reflected since air/skin is a pretty high density change. and then if you further on try to scan for bones and blood with this significantly weaker wave, the return signal will be barely visible. (is this called boundary attenuation?)

more questions that came up during research:
attenuation occurs as a net effect of absorption and scattering, absorption causes the energy of the wave to turn into other forms of energy. could a soundwave (of a frequency of your choice) somehow produce light? (sorry i have strange thoughts...)

ps nice quote from alien
 
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  • #5
RubinLicht said:
he said that when the waves hit a boundary, the majority of it is reflected since air/skin is a pretty high density change. and then if you further on try to scan for bones and blood with this significantly weaker wave, the return signal will be barely visible.

yes that is also correct

RubinLicht said:
(is this called boundary attenuation?)

may well be, I am not sure

RubinLicht said:
attenuation occurs as a net effect of absorption and scattering, absorption causes the energy of the wave to turn into other forms of energy.

yes that and the fact that in a lower density, there are less particles(molecules) of the medium with which the sound wave cam be propagated

other forms of energy --- really only one other form = heat

RubinLicht said:
could a soundwave (of a frequency of your choice) somehow produce light? (sorry i have strange thoughts...)

No, sound is a propagating mechanical wave, light is a propagating electromagnetic wave ... very different beasts :smile:
They are produced by different mechanisms and as such their propagation is different
light will travel through a vacuum, sound never will regardless of its frequency

cheers
Dave
 
  • #6
RubinLicht said:
I went and read a bit on attenuation and also talked to the main researcher at the company (dads friend who knew me as a child so i could approach him), He gave an explanation that was slightly different (because he was addressing ultrasound in ultrasonic machines specifically), he said that when the waves hit a boundary, the majority of it is reflected since air/skin is a pretty high density change. and then if you further on try to scan for bones and blood with this significantly weaker wave, the return signal will be barely visible. (is this called boundary attenuation?)
The main reason for using coupling gel is the one given by the researcher in the lab.
It is true that sound attenuation is higher in a gas like air than in water or body tissue. But this will have a small effect for the small gap (mm or less between the transducer and skin). However, the reflection coefficient at the interface transducer-air is almost 1 and there is almost no ultrasound leaving the transducer. This is due to the huge miss-match between the so called acoustic impedance of the air and the transducer. It is like having two materials with a very large difference in the index of refraction, in optics.
The acoustic impedance is the product between the speed of sound and the density of the medium. Air has density about 1000 times lower than the ceramic of the transducer and speed of sound about 10 times lower so the impedance of the air is bout 10,000 times lower than that of the transducer.
If you want to learn more, see here, for example.
https://www.nde-ed.org/EducationRes...ltrasonics/Physics/reflectiontransmission.htm

There are ultrasound transducers designed to match the impedance of air and they can sent significant ultrasound power into the air.
 
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  • #7
nasu said:
The main reason for using coupling gel is the one given by the researcher in the lab.
It is true that sound attenuation is higher in a gas like air than in water or body tissue. But this will have a small effect for the small gap (mm or less between the transducer and skin). However, the reflection coefficient at the interface transducer-air is almost 1 and there is almost no ultrasound leaving the transducer. This is due to the huge miss-match between the so called acoustic impedance of the air and the transducer. It is like having two materials with a very large difference in the index of refraction, in optics.
The acoustic impedance is the product between the speed of sound and the density of the medium. Air has density about 1000 times lower than the ceramic of the transducer and speed of sound about 10 times lower so the impedance of the air is bout 10,000 times lower than that of the transducer.
If you want to learn more, see here, for example.
https://www.nde-ed.org/EducationRes...ltrasonics/Physics/reflectiontransmission.htm

There are ultrasound transducers designed to match the impedance of air and they can sent significant ultrasound power into the air.
Very cool, thank you.
 
  • #8
to revive a not too old thread

nasu said:
The main reason for using coupling gel is the one given by the researcher in the lab.
It is true that sound attenuation is higher in a gas like air than in water or body tissue. But this will have a small effect for the small gap (mm or less between the transducer and skin).

Having just spent another week in hospital and having had a couple of ultrasounds done
I was able to ask some questions

In fact it has a HUGE effect ... the gel is used specifically to remove air pockets / bubbles between the transducer and the skin
This is because the frequency of the ultrasound is significantly higher than anyone in this thread ( myself included) at the time realized
They use between 5 and 8 MHz frequency. so attenuation through air pockets is very considerableDave
 
  • #9
Yeah, it's a huge effect. But is due to impedance miss-match and not to attenuation. You should not quote out of context. In the following paragraph I mentioned how significant may be the effect of reflection at the interface air-transducer.
 
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  • #10
nasu said:
Yeah, it's a huge effect. But is due to impedance miss-match and not to attenuation. You should not quote out of context. In the following paragraph I mentioned how significant may be the effect of reflection at the interface air-transducer.
Yup, the impedance mismatch is the key here. This is similar to the reason for horn assemblies in (audible) acoustic sources such as musical instruments and loudspeakers.
 
  • #11
Rubin, to try to simplify it and to put an image to it, imagine you are at a concert and it's crowded. Now another concert where you have been, was way more crowded where you almost stepped on your next fellow's foot. At that first concert someone tripped and fell onto someone, but nobody was behind that someone so only that someone fell as well. At the second concert, someone tripped and fell on two persons, and those two persons fell on someone else too. It doesn't make it an infinite wave of people falling it just increases the depths of the wave by a LOT (The higher the density, the more crowded the concert is, and the fact that it's liquid as well has to play a big role in it).
 
  • #12
Given all of this, is it safe to assume that the gel is engineered to have the same impedance as the body, as a means of illuminating the boundary between the two? And if so, is it made to match the impedance of the skin, or of the tissue underneath (or are they the same)? This all sounds a lot like the semi-elliptical can full of salt water used in lithotripsy.
 
  • #14
Cool link! My son and I are not making ublec this weekend, I guess.
 
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  • #15
This is in the same ballpark as the reason why our ears have all that complicated arrangement of small bones (ossicles) which act as levers and Transform the (low) impedance of sound in air to the (high) impedance of sound in the watery environment of the inner ear.
On similar lines, if you tap lightly on a table, you can hardly hear it but, if you put your ear (skull bones, mainly) in contact, the tapping is much louder. Impedance matching again. Then there's the cowboy trick of laying down and putting your ear onto the railway line. . . . . . And many more
 
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1. Why do ultrasound waves not travel through air well?

Ultrasound waves do not travel through air well because air molecules are too far apart to effectively transmit the vibrations of the ultrasound waves. This results in a loss of energy and a decrease in the intensity of the waves as they travel through air.

2. Can ultrasound waves travel through any medium?

Yes, ultrasound waves can travel through any medium, including solids, liquids, and gases. However, they travel most efficiently through denser materials such as water or tissue, and are not able to travel as well through less dense materials like air.

3. Why are ultrasound waves used for imaging if they don't travel through air well?

Ultrasound waves are used for imaging because they are able to travel through and reflect off of different densities of tissue in the body, allowing for the creation of images. Although they don't travel through air well, they are still able to produce high-quality images when used in conjunction with a specialized probe and gel.

4. How do ultrasound waves travel through the body if they can't travel through air well?

As mentioned, ultrasound waves are able to travel through denser materials such as water or tissue with greater efficiency. In medical imaging, a specialized probe is placed on the skin and emits ultrasound waves, which travel through the body and are reflected back to the probe. The probe then measures the return time and intensity of the waves to create an image.

5. Can the ability of ultrasound waves to travel through air be improved?

There is ongoing research to improve the ability of ultrasound waves to travel through air, as this would have practical applications in fields such as non-destructive testing and remote sensing. However, due to the physical properties of air and the nature of ultrasound waves, it is not currently possible to significantly improve their transmission through air.

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