# I Why can ultrasound waves not travel through air well?

1. Aug 2, 2016

### RubinLicht

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 doesnt 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..

2. Aug 2, 2016

### davenn

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

Last edited: Aug 2, 2016
3. Aug 2, 2016

### davenn

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. Aug 2, 2016

### RubinLicht

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

5. Aug 2, 2016

### davenn

yes that is also correct

may well be, I am not sure

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

No, sound is a propagating mechanical wave, light is a propagating electromagnetic wave ... very different beasts
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. Aug 2, 2016

### nasu

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.
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.

7. Aug 11, 2016

### RubinLicht

Very cool, thank you.

8. Aug 31, 2016

### davenn

to revive a not too old thread

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 realised
They use between 5 and 8 MHz frequency. so attenuation through air pockets is very considerable

Dave

9. Aug 31, 2016

### nasu

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.

10. Sep 3, 2016

### olivermsun

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.