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Author Topic: Can neodymium magnets be used to shield against ultrasound?  (Read 26916 times)

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.
 

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 »
 

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_ »
 

Online 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.
 

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 »
 

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..
 

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.
 

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
 

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.
 

Online 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.
 

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.
 

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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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.
 

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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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.
 

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?
 

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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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.
 

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.
 

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.
 

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!!)
 

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?
 

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)
 

Online 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 »
 

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.
 

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Re: Can neodymium magnets be used to shield against ultrasound?
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