Can neodymium magnets be used to shield against ultrasound?

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Offline mriver8

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Like say if you attached them to sheet metal? I'm looking for an alternative to a polyhedral structure of vacuum cells.
« Last Edit: 17/04/2015 08:07:07 by chris »

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Offline Colin2B

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Like say if you attached them to sheet metal? I'm looking for an alternative to a polyhedral structure of vacuum cells.

I'm not aware of any research which shows that ultrasound in air is influenced by magnetic fields
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

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Offline alancalverd

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No.
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Offline mriver8

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[/quote]

I'm not aware of any research which shows that ultrasound in air is influenced by magnetic fields
[/quote]

This guy on a blog wrote-

"Using magnets from old computer drives attached to steel sheet metal roofing material was something I came up with to use as shielding from scalar waves. More magnets are are better than a few. I am still testing but am using 5 inch spacing so the magnetic fields overlap. Even so, there is still some of the beam that gets through. However, the difference between no shielding and just one sheet with magnets is significant for me and very noticeable relief."

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Offline RD

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This guy on a blog wrote-

"Using magnets from old computer drives attached to steel sheet metal roofing material was something I came up with to use as shielding from scalar waves ... "

"scalar waves" are pseudo-science ...
Quote from: rationalwiki.org/wiki/Scalar_wave
... the term "scalar wave" is used exclusively by cranks and peddlers of woo.
http://rationalwiki.org/wiki/Scalar_wave

« Last Edit: 18/04/2015 04:10:21 by RD »

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Offline Colin2B

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"I'm not aware of any research which shows that ultrasound in air is influenced by magnetic fields"

This guy on a blog wrote-
" ........ However, the difference between no shielding and just one sheet with magnets is significant for me and very noticeable relief."

Your question was about magnets and ultrasound. My statement still stands.

Any dense material will reduce the level of sound transmission compared to free air. This seems to be the comparison here. Heavy magnets will increase the density of the sheet. Concrete and brick would work better with less chance of unwanted resonance.
You need to look carefully at how he is measuring the reduction, doesn't sound very scientific to me.

If you are troubled by HF sound, possibly just above your hearing range, then you are better looking for the source. HF range in free air is quite short, so the source is most likely to be inside the house. Look for CH pumps, fans, computer and TVs, even some phone chargers. Try switching off one at a time. Problem is some sources can be intermittent and switching off can interrupt the steady state, eg by cooling, and the sound goes away for a while, so switching off everything and then back on one at a time might not work.
I know some people are troubled by HF noise, but if you approach it scientifically you have a chance of finding it. If you look to pseudoscience, you have little hope!

« Last Edit: 18/04/2015 10:51:18 by Colin2B »
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Offline chiralSPO

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The answer is simple: Go to a doctor and explain the situation. They will give you pills that prevent the government's ultrasound weapons from interfering with your head.

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Offline Colin2B

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The answer is simple: Go to a doctor and explain the situation. They will give you pills that prevent the government's ultrasound weapons from interfering with your head.

How long have you had this feeling that the government is interfering with you head?
There was me thinking you were sane, come lie on this couch while I charge you money  [;)]
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

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Offline mriver8

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The answer is simple: Go to a doctor and explain the situation. They will give you pills that prevent the government's ultrasound weapons from interfering with your head.

How long have you had this feeling that the government is interfering with you head?
There was me thinking you were sane, come lie on this couch while I charge you money  [;)]

I didn't mention the government you did.

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Offline mriver8

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"I'm not aware of any research which shows that ultrasound in air is influenced by magnetic fields"

This guy on a blog wrote-
" ........ However, the difference between no shielding and just one sheet with magnets is significant for me and very noticeable relief."

Your question was about magnets and ultrasound. My statement still stands.

Any dense material will reduce the level of sound transmission compared to free air. This seems to be the comparison here. Heavy magnets will increase the density of the sheet. Concrete and brick would work better with less chance of unwanted resonance.
You need to look carefully at how he is measuring the reduction, doesn't sound very scientific to me.

If you are troubled by HF sound, possibly just above your hearing range, then you are better looking for the source. HF range in free air is quite short, so the source is most likely to be inside the house. Look for CH pumps, fans, computer and TVs, even some phone chargers. Try switching off one at a time. Problem is some sources can be intermittent and switching off can interrupt the steady state, eg by cooling, and the sound goes away for a while, so switching off everything and then back on one at a time might not work.
I know some people are troubled by HF noise, but if you approach it scientifically you have a chance of finding it. If you look to pseudoscience, you have little hope!


What distance can ultrasound be projected through air? Would a bat detector pick up the sound and if so what would it sound like to human ears? Like the testing from emergency broadcast system alerts?

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Offline chiralSPO

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #10 on: 18/04/2015 21:02:42 »
You mentioned the government first. For instance:

No I don't have the genes for it. No men in my family on either side were bald for as far back as was recorded. I was not bald at all until around 33 while under federal fraud investigation the FBI began to use a DEW to burn my hairs in unatural patterns and they refuse to stop. I wanted to get the the testing done along with video, and photographic evidence that I have hair in one spot on the 1st, and a few days later after an intense tuning fork itching sensation it is gone. I wish to present it to my lawyer, and judge along with assault records, and other violation's evidence to sue them if they keep trying to push unfair issues, punishment on me due to them mishandling my case when I was initially put under a fraud probe.

Because from their point of view I've been uncooperative but we don't share the same opinion. DEWs are handheld, and satellite microwave, and acoustic. non lethal weapons. They use them to try to force people who don't have to cooperate by law to cooperate for whatever they want them to. Say testify against your mob boss cousin for example, if they don't have enough charges to force you to legally. It's not legal for that type of usage but they do it anyway. Yeah men on my mother's sode were never bald at all not her father, grandfather, or brothers at all, at all.

No I think the U.S. Government may have had sasers, and tech capable of doing yhis for decades, and the tech is now declassofied but hard to find info on. I'm guessing this because the US Goverment usually has tech for several decades before they declassify it. Usually because they now have stuff way more advanced like the Blackbird.

I'm interested in methods of deflecting/shielding ultrasound up to 3GHz traveling through air as a medium. My search lead me to the study of creating impedance mismatches to affect sound wavess. I'm looking specifically for the acoustic impedance neccessary to act as a shield of sorts for materials with an acoustic impedance around one and a half. Some of my searches have said an impedance of around 7 would suffice however I can not find any papers on such tests, and how various materials affected ultrasound travelling through air as a medium at different frequencies. I'm guessing that there have been tests in this area with military applications which would suffice if readily available. To my knowledge this is the best way to affect sound waves, if there are any other methods please feel free to share.

To clarify I mean once their locale is known how can they shield themselves long enough to escape? Say they are in a building and the government knows.

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #11 on: 19/04/2015 01:22:00 »
I was refering to this thread. In any event bat detector good for detection? And what's the distance ultrasound can be projected through the air? I was told it could travel miles and even underground tunnels where not even Rambo is safe. 😃

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Offline alancalverd

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #12 on: 20/04/2015 12:37:35 »
The dispersion of ultrasound depends on its frequency. Bat detectors probably work well up to about 40 kHz and a few meters from the bat, but bats aren't interested in long-range detection, and any frequency above 100 kHz will be very strongly attenuated by air. 

ymk.k-space.org/Ultrasound_HO.pdf contains all you will ever need to know about ultrasound. Or at least all that They will allow you to know - Hollywood prefers you to be ignorant and thus gullible.
helping to stem the tide of ignorance

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Offline PmbPhy

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #13 on: 20/04/2015 16:32:03 »
Like say if you attached them to sheet metal? I'm looking for an alternative to a polyhedral structure of vacuum cells.
What ever made you even think of such a thing? There's nothing in physics even remotely related to magnetic fields effecting ultrasound.

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #14 on: 20/04/2015 22:43:55 »
Like say if you attached them to sheet metal? I'm looking for an alternative to a polyhedral structure of vacuum cells.
What ever made you even think of such a thing? There's nothing in physics even remotely related to magnetic fields effecting ultrasound.


I was told magnetic fields can change the path of sound waves somehow but they need to be very intense in air. I read it on a forum post which is why I asked here to check it's validity.

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Offline PmbPhy

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #15 on: 20/04/2015 22:52:29 »
Quote from: mriver8
I was told magnetic fields can change the path of sound waves somehow but they need to be very intense in air. I read it on a forum post which is why I asked here to check it's validity.
Nope. That's total garbage. A magnetic field can act on only one thing and one thing only - Charge. And by extension to that, current (i.e. charge in motion but still possibly having zero net charge). Air is typically uncharged and has no electric current in it so its impossible for a magnetic field to alter it.

Where did you read this? I'd love to see who said it and what they said about it and why. Thanks.

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Offline chiralSPO

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #16 on: 20/04/2015 23:06:40 »
Pedantic note: magnets can also interact with other magnets, even when everything is neutral, and no electric currents involved. Oxygen is paramagnetic, and would react (ever so slightly) to (very strong) magnetic fields. But this is not what OP is referring to.

Many nuclei also have magnetic moments, and would interact even more weakly with magnetic fields (basis of NMR, which can be performed on gases, by the way)
« Last Edit: 20/04/2015 23:08:31 by chiralSPO »

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Offline yor_on

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #17 on: 21/04/2015 00:00:34 »
Sometimes fact is stranger than fiction.

" We offer experimental proof that sound waves do interact with external magnetic fields.

The experiment was carried out on a large, single crystal of a very pure semiconductor, indium antimonide, which had been cut into two unequal sections and then cooled to about -445F (-265C). A controlled amount of heat was made to flow in each section separately. At these temperatures, the phonons can be thought of as individual particles, like runners on a racetrack each carrying a little bucket of heat.

In the small section, the phonons often run into the walls, which slows them down. The small section is used as a reference, to make the experiment independent of the other properties of the solid that might interfere. In the large section, the phonons can go faster, and they don’t run into the walls as much as into each other. When we apply a magnetic field, they tend to run into each other more frequently. Because the magnetic field increases the number of collisions, it also slows the phonons down and lowers the amount of heat they carry by 12%.

We think this is due to the electrons that rotate in orbits around each atom in the solid. The orbital motion of these electrons emits a very small intrinsic magnetic field that interacts with the externally applied field – an effect called “diamagnetism.“ This property exists even in substances we don’t traditionally think of as magnetic, such as glass, stone or plastic. When the atoms vibrate due to the passing of the phonons, this interaction creates a force on the atoms that makes the phonons collide with each other more often. "



http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4247.html

And don't get your hopes up too high here mriver8.
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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #18 on: 21/04/2015 00:29:59 »
The dispersion of ultrasound depends on its frequency. Bat detectors probably work well up to about 40 kHz and a few meters from the bat, but bats aren't interested in long-range detection, and any frequency above 100 kHz will be very strongly attenuated by air. 

ymk.k-space.org/Ultrasound_HO.pdf contains all you will ever need to know about ultrasound. Or at least all that They will allow you to know - Hollywood prefers you to be ignorant and thus gullible.

Good read. What non biological materials do you think could mimmick the attentuation, propagation speed, and scattering characteristics of bone?

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #19 on: 21/04/2015 00:34:51 »
Sometimes fact is stranger than fiction.

" We offer experimental proof that sound waves do interact with external magnetic fields.

The experiment was carried out on a large, single crystal of a very pure semiconductor, indium antimonide, which had been cut into two unequal sections and then cooled to about -445F (-265C). A controlled amount of heat was made to flow in each section separately. At these temperatures, the phonons can be thought of as individual particles, like runners on a racetrack each carrying a little bucket of heat.

In the small section, the phonons often run into the walls, which slows them down. The small section is used as a reference, to make the experiment independent of the other properties of the solid that might interfere. In the large section, the phonons can go faster, and they don’t run into the walls as much as into each other. When we apply a magnetic field, they tend to run into each other more frequently. Because the magnetic field increases the number of collisions, it also slows the phonons down and lowers the amount of heat they carry by 12%.

We think this is due to the electrons that rotate in orbits around each atom in the solid. The orbital motion of these electrons emits a very small intrinsic magnetic field that interacts with the externally applied field – an effect called “diamagnetism.“ This property exists even in substances we don’t traditionally think of as magnetic, such as glass, stone or plastic. When the atoms vibrate due to the passing of the phonons, this interaction creates a force on the atoms that makes the phonons collide with each other more often. "



http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4247.html

And don't get your hopes up too high here mriver8.

He meant air.

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #20 on: 21/04/2015 00:37:02 »
Quote from: mriver8
I was told magnetic fields can change the path of sound waves somehow but they need to be very intense in air. I read it on a forum post which is why I asked here to check it's validity.
Nope. That's total garbage. A magnetic field can act on only one thing and one thing only - Charge. And by extension to that, current (i.e. charge in motion but still possibly having zero net charge). Air is typically uncharged and has no electric current in it so its impossible for a magnetic field to alter it.

Where did you read this? I'd love to see who said it and what they said about it and why. Thanks.

Probably a prankster maybe.

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Offline PmbPhy

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #21 on: 21/04/2015 00:56:30 »
Quote from: chiralSPO429567600
Pedantic note: magnets can also interact with other magnets, even when everything is neutral, and no electric currents involved.
This is a common misunderstanding. The force exerted on one magnet by another is due to the magnetic field generated by the source magnet exerting a force on the currents that give rise to the magnetic field in the object magnet itself. The currents are not the ones you might have been thinking about. They're atomic currents, i.e. the current created by a charged particle orbiting a nucleus. You'll find this explained in a a good (perhaps advanced) text on EM and a few of articles in the American Journal of Physics.

Think about it like this. The only force that a magnetic field can exert is the force on a moving charged particle. Since the force on the charged particle is perpendicular to the velocity of the charged particle. This means that the magnetic field cannot do work on a charged particle and thus it can't do work at all.

That said, not consider this; suppose there is a magnet sitting on your desk and you hold another magnet near it. As soon as you release the magnet it will rotate so that the poles are aligned to minimize potential energy and then the magnets will accelerated towards each other, the kinetic energy increased in the process. Since the only way for kinetic energy to increase is for work to be done on the particle, what is doing the work if not the magnetic field? :)
« Last Edit: 21/04/2015 01:05:46 by PmbPhy »

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #22 on: 21/04/2015 01:13:57 »
I found this which talks about mimicking the properties of biological materials. Has nothing really to do with magnetism, altough magnets can change the density of a metal correct?

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #23 on: 21/04/2015 01:38:53 »
And I see the attentuation coefficient for bone is 20 db/cm at 1MHz, what non biological materials would have around the same db/cm at 1MHz?

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Offline RD

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #24 on: 21/04/2015 01:57:59 »
... magnets can change the density of a metal correct?

Not by much ...

Quote
On magnetization, a magnetic material undergoes changes in volume which are small: of the order 10−6.
http://en.wikipedia.org/wiki/Magnetostriction

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Offline chiralSPO

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #25 on: 21/04/2015 02:46:41 »

This is a common misunderstanding. The force exerted on one magnet by another is due to the magnetic field generated by the source magnet exerting a force on the currents that give rise to the magnetic field in the object magnet itself. The currents are not the ones you might have been thinking about. They're atomic currents, i.e. the current created by a charged particle orbiting a nucleus. You'll find this explained in a a good (perhaps advanced) text on EM and a few of articles in the American Journal of Physics.


I'll have to read up on this a little more to be certain, but I don't like the explanation of "atomic currents."

As you know, the electrons aren't actually orbiting the nucleus. And while the angular momentum ml of an orbital can contribute to the overall magnetic moment, an electron residing in an s orbital (l = 0, therefore ml = 0), or any other orbital in which ml = 0, still has a magnetic moment due to the electron's own "spin" quantum number, ms. I think we can also both agree that the electron is not "actually spinning". These are merely quantum phenomena that give rise to the interactions that an electron in an atom can have with the magnetic field.

Similarly with nuclei: all nucleons have a spin of ±˝, so any nucleus with an odd number of nucleons, and (many with an even number) interact with magnetic fields. Perhaps one could think of this as a result of tiny currents within the nucleus (or even with a neutron), but that doesn't sit right with me.

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Offline PmbPhy

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #26 on: 21/04/2015 03:58:31 »
Quote from: chiralSPO
I'll have to read up on this a little more to be certain, but I don't like the explanation of "atomic currents."

As you know, the electrons aren't actually orbiting the nucleus. And while the angular momentum ml of an orbital can contribute to the overall magnetic moment, an electron residing in an s orbital (l = 0, therefore ml = 0), or any other orbital in which ml = 0, still has a magnetic moment due to the electron's own "spin" quantum number, ms. I think we can also both agree that the electron is not "actually spinning". These are merely quantum phenomena that give rise to the interactions that an electron in an atom can have with the magnetic field.

Similarly with nuclei: all nucleons have a spin of ±˝, so any nucleus with an odd number of nucleons, and (many with an even number) interact with magnetic fields. Perhaps one could think of this as a result of tiny currents within the nucleus (or even with a neutron), but that doesn't sit right with me.
Yes. I'm very well aware of the problems you mention but that's actually the way it's done.

Let me give you an example; Consider spin magnetic moment. Perhaps you typically think of it in terms of non-relativistic quantum mechanics. However this subject requires quantum field theory for an appropriate treatment. If you talk to a particle physicist about the magnetic moment of a particle then you're going to hear an explanation in terms of current. For example; consider an article that a friend of mine wrote, i.e.

What is spin? by Hans C. Ohanian, Am. J. Phys. 54, June 1986
Quote
Abstract - According to the prevailing belief, the spin of the electron or of some other particle is a mysterious internal angular momentum for which no concrete physical picture is available, and for which there is no classical analog. However, on the basis of an old calculation by Belinfante [Physica 6, 887 (1939)], it can be shown that the spin may be regarded as an angular momentum generated by a circulating flow of energy in the wave field of the electron. Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field. This provides an intuitively appealing picture and establishes that neither the spin nor the magnetic moment are ‘‘internal’’—they are not associated with the internal structure of the electron, but rather with the structure of its wave field. Furthermore, a comparison between calculations of angular momentum in the Dirac and electromagnetic fields shows that the spin of the electron is entirely analogous to the angular momentum carried by a classical circularly polarized wave.

The important comment here is Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field.
« Last Edit: 21/04/2015 04:18:05 by PmbPhy »

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Offline Ethos_

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #27 on: 21/04/2015 05:27:35 »


What is spin? by Hans C. Ohanian, Am. J. Phys. 54, June 1986

Abstract - According to the prevailing belief, the spin of the electron or of some other particle is a mysterious internal angular momentum for which no concrete physical picture is available, and for which there is no classical analog. However, on the basis of an old calculation by Belinfante [Physica 6, 887 (1939)], it can be shown that the spin may be regarded as an angular momentum generated by a circulating flow of energy in the wave field of the electron. Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field. This provides an intuitively appealing picture and establishes that neither the spin nor the magnetic moment are ‘‘internal’’—they are not associated with the internal structure of the electron, but rather with the structure of its wave field. Furthermore, a comparison between calculations of angular momentum in the Dirac and electromagnetic fields shows that the spin of the electron is entirely analogous to the angular momentum carried by a classical circularly polarized wave.
This is important information and frankly a complete surprise to me. These moments when my understanding takes a paradigm shift is why I find physics so interesting. From this moment forward, my view of subatomic structure has been changed. This interpretation of particle spin is indeed, very revealing. Thanks for posting this abstract Pete!










« Last Edit: 21/04/2015 06:00:39 by Ethos_ »
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Offline alancalverd

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #28 on: 21/04/2015 08:15:33 »
What non biological materials do you think could mimmick the attentuation, propagation speed, and scattering characteristics of bone?

If only we knew - orthopedic surgery would be revolutionised! The nearest thing I know is glazed brick, or the ceramic tiles used to cover the space shuttle.
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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #29 on: 21/04/2015 09:14:34 »
What non biological materials do you think could mimmick the attentuation, propagation speed, and scattering characteristics of bone?

If only we knew - orthopedic surgery would be revolutionised! The nearest thing I know is glazed brick, or the ceramic tiles used to cover the space shuttle.

I was thinking http://acoustics.co.uk/products/acoustic-absorbers-syntactic-foams/anechoic-absorbers/aptflex-f28/


I wonder what happens to the attenuation coefficient of bones that are no longer in living animals because of the spongy layer in bone. Like antlers, or even left over steak bones. Lol. From my understanding of what I read the hardness of the outer layer of bone combined with the spongy layer inside gives it a higher attenuation coefficient than other biological materials due to scattering, and absorption properties. I don't exactly understand how acoustic impedance affects the difficulty ultrasound has going through bone uneffected if at all. I guess the structure/composition somehow contributes to this.? So then I skimmed through a PDF briefly about ultrasound absorbing materials and from what I gather it's not just the material it's made of that contributes to absorption but the structure/shape of the cells that make up the ultrasound absorbing foam provide scattering at freq above 1MHz that gives the foam better ultrasonic absorption qualities then regular acoustic foam or things like fiberglass used to reduce sound in airports. Also why would the surgery be revolutionized? Couldn't they just use bone if the reason the material was for its' ultrasonic absortion coefficient? Note I was asking what non biological material could mimmick the ultrasonic absorption qualities of bone. I'm confused I think you were referring to a synthetic bone replacement method correct?
« Last Edit: 21/04/2015 09:33:17 by mriver8 »

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #30 on: 21/04/2015 09:22:11 »
And then I wonder if it's possible for bone to absorb ultrasound or most of it, do you think there are any materials that totally reflect it? That deals with acoustic impedance and I don't quite understand the ways in which ultrasound passing through 2 mediums with differing acoustic impedance would be affected based on what the numbers are, or the gap between them, etc..

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Offline PmbPhy

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #31 on: 21/04/2015 16:43:55 »
Quote from: Ethos_
This is important information and frankly a complete surprise to me. These moments when my understanding takes a paradigm shift is why I find physics so interesting. From this moment forward, my view of subatomic structure has been changed. This interpretation of particle spin is indeed, very revealing. Thanks for posting this abstract Pete!
You're welcome, buddy. My pleasure. I too found if quite interesting and surprising as well when I read it.

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Offline Colin2B

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #32 on: 22/04/2015 00:19:05 »
Quote from: Ethos_
This is important information and frankly a complete surprise to me. These moments when my understanding takes a paradigm shift is why I find physics so interesting. From this moment forward, my view of subatomic structure has been changed. This interpretation of particle spin is indeed, very revealing. Thanks for posting this abstract Pete!
You're welcome, buddy. My pleasure. I too found if quite interesting and surprising as well when I read it.

Interesting ideas appear in the most unlikely places. Never expected to find one of this magnitude in this thread.
I rate it 5 stars
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

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Offline chiralSPO

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #33 on: 24/04/2015 15:51:15 »
Quote from: chiralSPO
I'll have to read up on this a little more to be certain, but I don't like the explanation of "atomic currents."

As you know, the electrons aren't actually orbiting the nucleus. And while the angular momentum ml of an orbital can contribute to the overall magnetic moment, an electron residing in an s orbital (l = 0, therefore ml = 0), or any other orbital in which ml = 0, still has a magnetic moment due to the electron's own "spin" quantum number, ms. I think we can also both agree that the electron is not "actually spinning". These are merely quantum phenomena that give rise to the interactions that an electron in an atom can have with the magnetic field.

Similarly with nuclei: all nucleons have a spin of ±˝, so any nucleus with an odd number of nucleons, and (many with an even number) interact with magnetic fields. Perhaps one could think of this as a result of tiny currents within the nucleus (or even with a neutron), but that doesn't sit right with me.
Yes. I'm very well aware of the problems you mention but that's actually the way it's done.

Let me give you an example; Consider spin magnetic moment. Perhaps you typically think of it in terms of non-relativistic quantum mechanics. However this subject requires quantum field theory for an appropriate treatment. If you talk to a particle physicist about the magnetic moment of a particle then you're going to hear an explanation in terms of current. For example; consider an article that a friend of mine wrote, i.e.

What is spin? by Hans C. Ohanian, Am. J. Phys. 54, June 1986
Quote
Abstract - According to the prevailing belief, the spin of the electron or of some other particle is a mysterious internal angular momentum for which no concrete physical picture is available, and for which there is no classical analog. However, on the basis of an old calculation by Belinfante [Physica 6, 887 (1939)], it can be shown that the spin may be regarded as an angular momentum generated by a circulating flow of energy in the wave field of the electron. Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field. This provides an intuitively appealing picture and establishes that neither the spin nor the magnetic moment are ‘‘internal’’—they are not associated with the internal structure of the electron, but rather with the structure of its wave field. Furthermore, a comparison between calculations of angular momentum in the Dirac and electromagnetic fields shows that the spin of the electron is entirely analogous to the angular momentum carried by a classical circularly polarized wave.

The important comment here is Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field.

Ok, Pete. After further reading, I accept that this is a reasonable way of thinking about the magnetism of atoms and subatomic particles.

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Offline alancalverd

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #34 on: 25/04/2015 00:38:11 »
And then I wonder if it's possible for bone to absorb ultrasound or most of it, do you think there are any materials that totally reflect it? That deals with acoustic impedance and I don't quite understand the ways in which ultrasound passing through 2 mediums with differing acoustic impedance would be affected based on what the numbers are, or the gap between them, etc..

Ultrasound, just like any other compression wave, is transmitted by elastic materials (generally, those we consider fairly incompressible) and dispersed by energy-absorbing materials such as foams and gases. Reflection occurs at interfaces between materials whose speed of sound differs.

As this and similar threads seem to be about seeing inside someone's brain, or controlling the workings thereof, ultrasound is pretty useless. There is a huge attenuation at the interface between the skull and the brain but with luck you can pick out some gross anatomy, particularly of the fetal brain, and in principle you can use doppler ultrasound to measure blood flow in the major vessels, but MRI is better and digital subtraction x-ray is definitive. As for covert surveillance, an ultrasound probe has to be tightly coupled with liquid or gel (which is why it works well for a fetus) and you'd really know about it if someone was pursuing you with an x-ray or MRI machine - they'd need a truck and a lot of cooperation, for a start.
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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #35 on: 27/04/2015 04:18:09 »


Ultrasound, just like any other compression wave, is transmitted by elastic materials (generally, those we consider fairly incompressible) and dispersed by energy-absorbing materials such as foams and gases. Reflection occurs at interfaces between materials whose speed of sound differs.

As this and similar threads seem to be about seeing inside someone's brain, or controlling the workings thereof, ultrasound is pretty useless. There is a huge attenuation at the interface between the skull and the brain but with luck you can pick out some gross anatomy, particularly of the fetal brain, and in principle you can use doppler ultrasound to measure blood flow in the major vessels, but MRI is better and digital subtraction x-ray is definitive. As for covert surveillance, an ultrasound probe has to be tightly coupled with liquid or gel (which is why it works well for a fetus) and you'd really know about it if someone was pursuing you with an x-ray or MRI machine - they'd need a truck and a lot of cooperation, for a start.

I wasn't refering to ultrasound being used on the brain. Lol. Take a look at these tables. According to them the intensity the freq is being projected at affects impact so I wonder what the relationship between intensity of a sound beam and absorption capability of the material is.

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #36 on: 27/04/2015 04:29:50 »
I know ultrasound is above 20 kHz so I don't understand what Frequency/kHz in the first table is stating.

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Offline alancalverd

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #37 on: 27/04/2015 06:50:53 »

I wasn't refering to ultrasound being used on the brain. Lol. Take a look at these tables. According to them the intensity the freq is being projected at affects impact so I wonder what the relationship between intensity of a sound beam and absorption capability of the material is.

Negligible up to the point where the sound pressure permamently distorts the absorber.

Quote
I know ultrasound is above 20 kHz so I don't understand what Frequency/kHz in the first table is stating.

exactly that - the frequency, measured in kHz. It's a weird sign convention that probably looks logical to someone on a publishing committee somewhere, but I've never liked it.

Anyway the good news from that table is that ultrasound is unlikely to have any effect on hearing as the ear, by definition, doesn't respond to it.
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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #38 on: 28/04/2015 04:53:27 »
So when they state sound from .19 to 8 is capable of causing damage is that a typo? Because that's within the normal sound range no? Do they mean 8 kHz?

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Offline alancalverd

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #39 on: 29/04/2015 08:12:53 »
Yes.

And there's nothing magic about it. Sound is just a series of longitudinal pressure waves, so at very high intensities (as listed in your reference) the effect is the same as being hit repeatedly with a boxing glove: at some point, you can expect to suffer damage. 

A little more thought may help explain the nature of the damage. At low frequencies, i.e. with a long interval between impacts, you are going to distort large organs like the brain and gut. At high frequencies, where the wavelength is short, you are more likely to damage small structures like the inner ear.
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Offline PmbPhy

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #40 on: 29/04/2015 15:37:30 »
Quote from: mriver8
So when they state sound from .19 to 8 is capable of causing damage is that a typo? Because that's within the normal sound range no? Do they mean 8 kHz?
I got lost. Who are "they" and what is meant by ...sound from .19 to 8...? I don't know what "sound from" means. What does the numbers represent, frequency or decibels?

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #41 on: 04/05/2015 00:05:45 »
I'm refering to the numbers in the first table under Freq/ kHz. It just has numbers that say .19-8 is capable of causing said structural damage. But 8kHz is within normal range so how can that cause damage? I was thinking it has to either be a typo or I am reading it incorrectly. I would think 8MHz could cause cell damage but not 8kHz unless I'm misreading something in the first table of the picture I attached.

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Offline alancalverd

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #42 on: 04/05/2015 00:10:27 »
Cells are disrupted by gross mechanical shear of the organ. Ask any boxer, or a kid with a grazed knee.
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Offline chiralSPO

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #43 on: 04/05/2015 01:36:35 »
I'm refering to the numbers in the first table under Freq/ kHz. It just has numbers that say .19-8 is capable of causing said structural damage. But 8kHz is within normal range so how can that cause damage? I was thinking it has to either be a typo or I am reading it incorrectly. I would think 8MHz could cause cell damage but not 8kHz unless I'm misreading something in the first table of the picture I attached.

Sound at any frequency can cause damage as long as the amplitude (volume) is great enough.

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Offline PmbPhy

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #44 on: 04/05/2015 03:47:19 »
I'm refering to the numbers in the first table under Freq/ kHz. It just has numbers that say .19-8 is capable of causing said structural damage. But 8kHz is within normal range so how can that cause damage? I was thinking it has to either be a typo or I am reading it incorrectly. I would think 8MHz could cause cell damage but not 8kHz unless I'm misreading something in the first table of the picture I attached.
That sounds wrong. I can't see how that makes sense of it's supposed to hold for any amplitude.

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Offline chiralSPO

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #45 on: 04/05/2015 17:43:28 »
The chart is discussing a range from 125 to 170 dB (!!VERY LOUD!!)

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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #46 on: 04/05/2015 20:25:30 »

[/quote]
That sounds wrong. I can't see how that makes sense of it's supposed to hold for any amplitude.
[/quote]

I beleive you are correct and it's a typo. I think 1-8 MHz or GHz makes more sense. One last question. How far can a low orbit satellite project ultrasound? Miles at strong intensities? Or would satellites triggering devices attached to towers or undergound make more sense?

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Offline chiralSPO

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #47 on: 04/05/2015 21:16:29 »

That sounds wrong. I can't see how that makes sense of it's supposed to hold for any amplitude.
[/quote]

I beleive you are correct and it's a typo. I think 1-8 MHz or GHz makes more sense. One last question. How far can a low orbit satellite project ultrasound? Miles at strong intensities? Or would satellites triggering devices attached to towers or undergound make more sense?
[/quote]

Again, don't confuse frequency with amplitude. Sound is not like light, so increasing the frequency doesn't increase the energy content in the same way (10 mW high frequency light like UV or x-ray is potentially dangerous, 10 mW low frequency light, like visible or infrared is harmless)

Satellites would be very ineffective at projecting sound of any frequency because they are in space... Projecting ultrasound for miles in air would also be very difficult... (I'm certain both of these points have been made already, in this thread or others of yours)

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Offline alancalverd

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #48 on: 04/05/2015 23:48:51 »

Quote
That sounds wrong. I can't see how that makes sense of it's supposed to hold for any amplitude.

I beleive you are correct and it's a typo. I think 1-8 MHz or GHz makes more sense. One last question. How far can a low orbit satellite project ultrasound? Miles at strong intensities? Or would satellites triggering devices attached to towers or undergound make more sense?

(a) You are both wrong! The chart clearly specifies the amplitude (in decibels) at which damage has been detected

(b) A satellite cannot project ultrasound. Sound is a series of compression waves in a medium. Space is not a medium!

(c) High frequencies are strongly attenuated underground and even in air. There are reports of a 20 kHz interferometer being used to distract a racehorse at a few yards but compression nonlinearities strongly limit the projection of intense high frequency sound waves in air. Listen to a marching band approaching: the first thing you hear is the bass drum, then the tubas: the fifes and clarinets, though quite painful close up, are almost inaudible at half a mile. That's why ships' horns, fog horns, and whale song, are all at low frequencies.
« Last Edit: 04/05/2015 23:58:04 by alancalverd »
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Offline mriver8

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Re: Can neodymium magnets be used to shield against ultrasound?
« Reply #49 on: 04/05/2015 23:55:45 »
I'm also wondering if some combo of piezo material on the outside of a barrier, with an inner layer of say a conductive plastic or something might work. I don't know much about engineering.