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Kyle

How strong can a magnetic field be before you get hurt or die?

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Kyle    62

Not exactly on topic but I thought this was a cool read and is at least, in the spirit of science

 

https://gravityandlevity.wordpress.com/2015/01/12/how-strong-would-a-magnetic-field-have-to-be-to-kill-you/

 

 

Speakers typically have about 0;5 to 1T in the gap so a field of 100,000T would be insane! Neutron starts do have these types of fields and I would imagine one would be pulverized if you got near one in more than one way. The Hadron Collider (from what I have read) only gets the field up to about 8 or 9Tesla... still nothing approaching atomic distortion levels :)

 

 

quick and dirty calculation: If you're speaker produce 90dB at 1 watt with a 1T in the gap then if you increased that to 100,000T

 

log base 2 of (100k) is about ~16.

16*6dB = 96dB (rough dB conversion)

 

90+96 = 186dB @ 1 watt...

 

...You would tear the the cone right off the spider and surround with just 1 watt :)

in fact it might become a weapon or projectile of sorts, haha

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Infrasonic    150

 

...You would tear the the cone right off the spider and surround with just 1 watt :)

in fact it might become a weapon or projectile of sorts, haha

 

Excellent. :D

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Kyle    62

Now that I think about it, this type of motor might be able to push the cone faster than the speed of sound and create a pulsing series of shockwaves at low frequencies. That would be fun to see!

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Kyle    62

Neutron motor does have an intimidating sound to it :)

 

steps 1 and 2: We need to start with a star and then make it go supernova

 

I'm stuck on steps 1 and 2 :(

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shadyJ    19

If you can put a ring of conductive and non-ferrous material around a magnetar, with a massive way to charge it, you are on to something. Maybe use a pulsar somehow to charge it? The problem is you need a medium to put it in to conduct sound. Anyway, there are already ready-made cosmic subwoofers that are very powerful which use black hole motors.

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Infrasonic    150

Neutron motor does have an intimidating sound to it :)

 

steps 1 and 2: We need to start with a star and then make it go supernova

 

I'm stuck on steps 1 and 2 :(

 

Step 3: Step back, way back

 

giphy.gif

 

tumblr_lyw2m7RNMd1qzguyto3_r1_250.gif

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SME    208

Most of the strongest usable electromagnets rely the phenomenon of superconduction.  A superconductor is a material with effectively zero electrical resistance.  Superconductors typically require operation at cryogenic temperatures and are limited with respect to maximum current and magnetic field or else the superconduction is lost.

 

Here is a claim to the highest human made magnetic field at 91.4 Tesla ( Oddly enough, they did not use a superconducting coil.  However, the field lasts for less than 0.02 seconds to not damage the coil.

 

In terms of how strong a magnetic field must be to be unhealthy, I don't know if there is a good answer to that question.  The strongest MRI scanners subject the human body to 3 Tesla.  I'm not aware of any negative side effects of that kind of exposure.

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Kyle    62

91T is insane. Image an electric car motor made with those magnets.

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Ricci    647

91T is insane. Image an electric car motor made with those magnets.

 

90 Tesla might be a bit much but what if we lower the goals a bit? Say 5T through the gap? Design us a 5T sub motor Kyle!

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shadyJ    19

What about a field coil type driver? Could charged copper coils have a more powerful field than conventional permanent magnets?

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SME    208

What about a field coil type driver? Could charged copper coils have a more powerful field than conventional permanent magnets?

 

Sure.  Build a superconducting electromagnet, and you can definitely get more magnetic field strength than from a permanent magnet.  While it's superconducting, the magnet coil itself will consume very little power, but of course you need a way to cool that coil to cryogenic temperatures, at which point the exercise begins to look pretty ridiculous.

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shadyJ    19

Sure.  Build a superconducting electromagnet, and you can definitely get more magnetic field strength than from a permanent magnet.  While it's superconducting, the magnet coil itself will consume very little power, but of course you need a way to cool that coil to cryogenic temperatures, at which point the exercise begins to look pretty ridiculous.

Nothing is ridiculous when it's in the name of SCIENCE!

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andy497    6

Back in college I got to spend a fair bit of time with an NMR (analytical chem version of an MRI, except the center diameter is super focused at maybe an inch or two across).  I don't recall the field strength for sure, but I seem to remember 7 teslas, and it needed to be cryogenically cooled 24/7.  It was strong enough that the CRT monitors in adjacent rooms had their displays color smeared and contorted permanently.  

 

We were required to empty our pockets next door and carefully inspect clothing for any ferrous bits, else we couldn't go near it, although I don't think anyone ever tested that.  You had to climb the thing and get right near the meat of it to drop a sample in the top, and that was always a little scary.  I thought my glasses were going to tear off my head or something, but there was no perceptible sensation.  

 

The cooling was actually hard work, so that might get tricky when retrofitting for voice coil duty.  It was double vacuum-walled with an outer chamber containing liquid nitrogen and inner with liquid helium, and they're always slowly boiling off, so you need to keep replenishing or it will destroy itself.  Liquid nitrogen is pretty common but helium was crazy expensive.

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If you go with an ironless motor you can exceed the normal iron/steel limits of saturation which are near 2 Tesla.  Then a field coil motor could push motor force up considerably.

 

But.

 

The first motor design I did was for a very high efficiency fullrange.  We got it up to 108 db/watt on a 6.5 inch cone.  The greater the motor strength the lower the Qts and the greater the damping on the motor.  That meant that we were sacrificing low end for efficiency.

 

Neutron star motor.

 

Listening to it might be a little death dealing.  But an interesting concept.  Listen to tines while you are taken apart molecule by molecule.  I guess it depends on the tunes you are listening to!

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SME    208

The first motor design I did was for a very high efficiency fullrange.  We got it up to 108 db/watt on a 6.5 inch cone.  The greater the motor strength the lower the Qts and the greater the damping on the motor.  That meant that we were sacrificing low end for efficiency.

 

By "low-end", you mean low-end sensitivity right?  However, sensitivity depends on DC resistance, which can usually be manipulated independently from efficiency.  Thus, I would contend that the real problems with very strong motors are: (1) They need signal shaping to flatten their response; however, this is almost always the case anyway for optimal performance in different acoustic environments.  (2) They need amps that can maintain stable operation and deliver close to their rated power into impedances that may drop very low.  These amps don't need to *increase* their power output into these lower impedances because the sensitivity loss due to the stronger motor is greatest where the impedance is actually very high.

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By "low-end", you mean low-end sensitivity right?

 

I mean a decrease in the drivers Qts.  More or less an increase in motor damping so you end up with less low frequency output without EQ.  And the EQ is also a diminishing return rather rapidly as you will end up with power compression due to voice coil heating very quickly.  Balance in the key to getting the greatest output from a system.  A motor and the enclosure it sits in are a system.  And that is inextricably linked together.  A larger enclosure will almost always give you greater low frequency output when you are using a high BL motor.  Larger enclosures are the area where a skilled DIY build can produce greater output than something available commercially.

 

Very high BL and thus very low Qts has a knee point where you are not getting anything for the greater motor strength versus the box size.  You get no more output in SPL in the low end for the stronger motor.  A very high BL driver is a poster child for that very situation.  Too strong of a motor and no greater low frequency output for the outlay in cost to make the motor stronger.  As with X-max BL has become a number to tickle the desire of people looking for something different.  The truth is quite different from what is being marketed.

 

As a simple math reality check.  You have a great driver that has 25mm true X-max in a sealed enclosure so you can EQ the snot out of it.  To get 3 db more what do you need?  Another 25mm.  If you had such a driver capable of the 50mm X-max in the same sealed enclosure you will not be able to get the full excursion before you massively exceed the drivers safe operating area.  So to get the desired X-max you nuke the driver.  Not so good for warranty purposes.

 

So buying a driver that for instance has 4mm more X-max looks enticing.  But in reality you are looking at maybe another db in output.

 

Last but not least lets look at amp output.

 

Many are rated at their peak output capability.

 

Even PASCAL Audio genuinely high fidelity units which I have used and recommended to quite a few clients, have a rated output of 2400 watts into 4 ohms and peak output of 4800 watts into 4 ohms.  But a quick look at the data sheet page 6 will show you that pink noise RMS power into 4 ohms is 680 watts.  Not 2400 watts, not 4800 watts.

 

And this is from a quality manufacturer.  The same is said of Powersoft and IcePower.  They actually have considerably less pink noise output capability.

 

All is not what we are led to believe.

 

Last but not least.

 

Motor parts.  Not just the magnet and the metal.

 

The spiders and the surrounds.

 

Few companies are actually tooling and doing the R&D for the spiders and the surrounds in their drivers.  They simply buy what is available.

 

I've worked with a few companies that do exactly that. 

 

The reality is that you need to do the math, and the engineering to get your drivers parts to work as a system.

 

An example.  Many companies are using dual spiders.  You stack two spiders spaced apart from each other at an arbitrary distance and everything should work out right.  Or you could do a little thinking and design the top and bottom spiders as mirrors of each other.  When one spider has the curve going up the other spider has the curve going down.  So one is the reverse of the other in terms of rolls and valleys.  The benefit?  2nd order distortion drops as you have a truly balanced restorative force.  The spider on all larger high X-max drivers are the dominant part in the suspension of the voice coil and the centering of the voice coil. 

 

TO get this to work properly you have to do design math that will prove out that you have the correct valley lengths and flats on the valleys and peaks.  Grabbing open tooling and hoping that it will work is not what pushes the limits in driver design.  It is the status quo.  What everyone else is doing. 

 

I just dropped a little over a grand in spider tooling.  And it's not open!  It's mine.  Same goes for surround tooling.  You want to push the limits in driver design you have to actually do the design work!

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SME    208

I mean a decrease in the drivers Qts.  More or less an increase in motor damping so you end up with less low frequency output without EQ.  And the EQ is also a diminishing return rather rapidly as you will end up with power compression due to voice coil heating very quickly.  Balance in the key to getting the greatest output from a system.  A motor and the enclosure it sits in are a system.  And that is inextricably linked together.  A larger enclosure will almost always give you greater low frequency output when you are using a high BL motor.  Larger enclosures are the area where a skilled DIY build can produce greater output than something available commercially.

 

Yes, I also mean decrease in Qts and increase in motor "damping".  The quotes around damping clarify that it looks like a damping term in the mechanical equation of motion but the energy is not lost to heat as is the case with Qms or mechanical damping.  Instead, the energy is transformed back to electrical energy in the coil and can be recovered by the amp, depending on the design.  This actually allows one to make the system very power efficient.  The EQ is needed to correct the response shape, but even with the necessary boost, actual power consumption will still be *lower*.  You have to look at the impedance curve in addition to the frequency response curve.

 

Very high BL and thus very low Qts has a knee point where you are not getting anything for the greater motor strength versus the box size.  You get no more output in SPL in the low end for the stronger motor.  A very high BL driver is a poster child for that very situation.  Too strong of a motor and no greater low frequency output for the outlay in cost to make the motor stronger.  As with X-max BL has become a number to tickle the desire of people looking for something different.  The truth is quite different from what is being marketed.

 

You keep using the term "low frequency output" where I think you mean "low frequency sensitivity".  There is a big difference.  Actual low frequency output depends on a lot of possible factors.  Increasing BL or Qts could affect output in either direction depending on which factor is responsible for limiting performance.  If the limiting factor is thermal power handling and compression, then the higher BL and lower Qts system will usually *increase* output.  If, however, the limiting factor is amplifier voltage, then the reduced *sensitivity* (output per volt vs. frequency) around the resonance frequency (or the higher of the two impedance peaks in a ported system) will lead to *decreased* output there.  As you say, it could go either way, and it depends on the system.

 

But to repeat what I already said.  You can almost always increase sensitivity by reducing the coil resistance or running two or more coils in parallel in a MVC design.  Or if you are also excursion limited, you can add drivers and run those in parallel.  You just need an amp that can keep it together when presented with a lower DCR.  Those spots where you need the EQ boost are typically where impedance is high and where not much power flows to the coils despite the higher voltages required.

 

As a simple math reality check.  You have a great driver that has 25mm true X-max in a sealed enclosure so you can EQ the snot out of it.  To get 3 db more what do you need?  Another 25mm.  If you had such a driver capable of the 50mm X-max in the same sealed enclosure you will not be able to get the full excursion before you massively exceed the drivers safe operating area.  So to get the desired X-max you nuke the driver.  Not so good for warranty purposes.

 

So buying a driver that for instance has 4mm more X-max looks enticing.  But in reality you are looking at maybe another db in output.

 

Last but not least lets look at amp output.

 

---cut---

 

Forgive me, but last I checked, doubling excursion gets you +6 dB not +3 dB.  But anyway, I agree that excursion is also important, but I don't think this has much to do with motor strength and the efficiency benefit that it brings.  If I can wring all the available excursion out of a driver with less power, that's a *good* thing.  That means my driver will see less power throughout its operation than another driver with the same displacement capability that needed more power to get it there.

 

I'm aware of the limitations you speak of with real world amps.  Indeed, they are all over the map in terms of what they can deliver and for how long, especially in the lowest subwoofer frequencies.  If you haven't seen it already, a few members have done some tests to try to assess the real world performance of various amps.  See, e.g. Luke's basic Amplifier Tests and Amplifier Comparison SpeakerPower SP2-12000 and Powersoft K20-DSP-Aesop.  Both of the amps compared in the latter thread are serious performers, even if they can't sustain their rated power indefinitely.  What's important is how long and to how low a frequency the amp can sustain high power output, and how impedance effects its behavior.  Unfortunately, this kind of information is almost never available from the manufacturer, at least publicly.

 

Motor parts.  Not just the magnet and the metal.

 

The spiders and the surrounds.

 

Few companies are actually tooling and doing the R&D for the spiders and the surrounds in their drivers.  They simply buy what is available.

 

I've worked with a few companies that do exactly that. 

 

The reality is that you need to do the math, and the engineering to get your drivers parts to work as a system.

 

An example.  Many companies are using dual spiders.  You stack two spiders spaced apart from each other at an arbitrary distance and everything should work out right.  Or you could do a little thinking and design the top and bottom spiders as mirrors of each other.  When one spider has the curve going up the other spider has the curve going down.  So one is the reverse of the other in terms of rolls and valleys.  The benefit?  2nd order distortion drops as you have a truly balanced restorative force.  The spider on all larger high X-max drivers are the dominant part in the suspension of the voice coil and the centering of the voice coil. 

 

TO get this to work properly you have to do design math that will prove out that you have the correct valley lengths and flats on the valleys and peaks.  Grabbing open tooling and hoping that it will work is not what pushes the limits in driver design.  It is the status quo.  What everyone else is doing. 

 

I just dropped a little over a grand in spider tooling.  And it's not open!  It's mine.  Same goes for surround tooling.  You want to push the limits in driver design you have to actually do the design work!

 

No doubt, these are all very important design details.  To the extent that one may have to compromise suspension performance (or excursion) to accommodate a stronger motor, I can understand where "bigger" may not always be better.  Of course, stronger motors effectively give the amp more control over the driver, so the relative influence of soft parts will tend to be reduced, at least as long as the driver is within its excursion limit.  Small boxes also tend to reduce the influence of the suspension because the air spring is a lot stiffer than the suspension is.  (Again, this is only true within the excursion limit.)

 

Anyway, I understand there are many practical situations in which one would want a driver with less BL than one with more.  But high BL (all else the same) does generally improve efficiency, that is, output per watt of power.  It just comes with side-effects that must be taken into account in the design of the overall system.  The amp is a critical part of that system, and it's a shame that so little information is available concerning their behavior.

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Ricci    647

Well said SME.

There are a number of parties who have realized what advantages can be had by increasing motor strength and decreasing qts in combination with modern dsp and digital amplifiers. I'm certainly one of them. It offers a number of performance advantages.

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Well said SME.

There are a number of parties who have realized what advantages can be had by increasing motor strength and decreasing qts in combination with modern dsp and digital amplifiers. I'm certainly one of them. It offers a number of performance advantages.

+1

 

Sometimes I type when I'm tired.  And SME corrected most of my booboos thanks!

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"  Of course, stronger motors effectively give the amp more control over the driver, so the relative influence of soft parts will tend to be reduced, at least as long as the driver is within its excursion limit.  Small boxes also tend to reduce the influence of the suspension because the air spring is a lot stiffer than the suspension is.  (Again, this is only true within the excursion limit.)"

 

 

This statement is not true in many different ways.  I don't have time to type aa thorough answer this morning but I'll return to this this evening.

 

Some basic laws of physics are against what is perceived to be the case.

 

As a starter.

 

A loudspeaker motor is driven by the amplifier and the amplifier is driven by the motor via back EMF.  An amplifier does not control a coil in a loudspeaker motor.  It is a very different type of a relationship.  And a high BL motor means a much more difficult relationship.

 

Back in a few hours.

 

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shadyJ    19

 

"  Of course, stronger motors effectively give the amp more control over the driver, so the relative influence of soft parts will tend to be reduced, at least as long as the driver is within its excursion limit.  Small boxes also tend to reduce the influence of the suspension because the air spring is a lot stiffer than the suspension is.  (Again, this is only true within the excursion limit.)"

 

 

This statement is not true in many different ways.  I don't have time to type aa thorough answer this morning but I'll return to this this evening.

 

Some basic laws of physics are against what is perceived to be the case.

 

As a starter.

 

A loudspeaker motor is driven by the amplifier and the amplifier is driven by the motor via back EMF.  An amplifier does not control a coil in a loudspeaker motor.  It is a very different type of a relationship.  And a high BL motor means a much more difficult relationship.

 

Back in a few hours.

 

 

I am interested in seeing these statements expanded, so I am looking forward to your post on this subject.

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So now that I have time I have been thinking of the natural progression of questions that would follow if I state that a coil in motion in a magnetic field will produce a back Electro-Motive Force.  How much force?  How much voltage, current.

 

It's got me a thinking.

 

And a looking. 

 

It's one thing to know the concepts.  But I'm looking for a good explanation and calculation of the induced EMF off of a coil in motion versus applied voltage and acceleration.

 

I think I have found a few sources for the proper calculations.  And I'm looking.  So answer is not dead.  I'm simply trying to get my head around it from every angle.

 

Teaser number two.

 

An amplifier must both source and sink voltage.

 

Almost all amplifiers behave before clipping of either voltage or current as a constant voltage source.  That is they try to send a given voltage into a loudspeaker as a constant.  Say one watt.  Into 8 ohms that is 2.83 volts.

 

Fool around with this for a bit.

 

http://www.anycalculator.com/ohmslaw.htm

 

Near a drivers resonance the current into a driver is pretty low as the impedance is pretty high.

 

I'll be back!

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