The Skin Game

[sully]

fde..

Why skin depth?

 

[fdegrove]

Hi,

Why skin depth?

Electrons tend to travel on the outside diameter of a conductor, hence the term skin depth which refers to the depth of conductor penetration.

In the context of oxides on the surface of conductors it should be obvious that this will increase resistance at high frequencies as higher frequencies occupy the outside diameter and are said to not even penetrate the conductor but pass as an electron cloud at the outside....which in turn brings up the issue of DA in insulators.

For indepth reading on this topic I'd recommend papers by Dr. Malcolm Hawksford, a British scientist who publicised well founded papers on this phenomenon.

Cheers,

 

[sully]

Oh..ok

I asked about skin depth because I wasn't sure if you were referring to the typical exponential depth profile used in the literature, or the complete bessel function analysis required when the skin depth for flat infinite width plates is invoked.

The flat plate exponential functions are inaccurate for round conductors when the skin depth approaches 1/5th of the conductor diameter, so there you gotta switch to bessels..they are a p.i.t.a..

BTW..When oxidation prevents interstrand conduction, they interrupt the radial eddy currents that are responsible for skinning..So, when a stranded conductor oxidizes, the skin depth increases..that is typical of lower resistivity materials..Superconductors have very high skinning, for example..

Hawksford.. If you examine the inductance formula, there are three components..geometry related, you know, the diameter/spacing log ratio thing..a loop length compensation component part, which is really small for typical lengths, and the self inductance component..which is Mu time delta..delta is .25 at dc, and goes to zero at hf..end result, internal energy storage is based on .03 microhenries per inch for copper twisted pair..That is approximately 18% of a #18awg zip cord..

Hawksford, of course, did analysis on copper wire...then used steel for his test...steel has mu's up to 1000, even to 10,000. Meaning the use of steel hugely compromises the results..as, the internal self inductance of the steel can exceed the parallel wire geometric inductance by two to three orders of magnitude..

If you wish more details, perhaps a new thread? I apologize for keeping it so non technical, but it's not a skin thread..

Sully

 

[millwood]

Electrons tend to travel on the outside diameter of a conductor, hence the term skin depth which refers to the depth of conductor penetration.

Cheers,

I am afraid that this is one of those "scientific" discussions that turned out to be nothing but discussions. Yes, skin effect is real. But it is only real at high frequencies (shall we say over 100Khz?) and on bare wires.

I challenge anyone to find measured increase of skin-effect at audio frequency (say 20Khz tops) that is more than 10 or 20%.

and I challenge anyone to find the same thing to be even remotely material for multi-strand wires at audio frequency.

anyone having worked on an AM receiver will be able to tell you how thin those wires are and how many strands they are. for a good reason.

On a 2nd thought, I have seen people here spending far more resources on less useful things so maybe being concerned about skin effect at audio frequency isn't bad, relatively speaking.

My next project is to build a neutrino-shielf for my amps and enjoy neutrino free sound,

 

[sully]

sorry

I said: ""or the complete bessel function analysis required when the skin depth for flat infinite width plates is invoked.""

I meant..or the bessels when the skin depth compared to conductor diameter does not allow the flat plate approximation equations..

Sully

 

[millwood]

Re: Oh..ok

If you wish more details, perhaps a new thread? I apologize for keeping it so non technical, but it's not a skin thread..

Sully

Sully, I would love to learn more. the more math and the merrier.

thanks.

 

[fdegrove]

Hi,

If you wish more details, perhaps a new thread? I apologize for keeping it so non technical, but it's not a skin thread..

As I don't think this topic was really discussed indepth before ( pun intended) then yes, please do.

I'm by no means a scientist so don't expect major scientific contributions from me but it could make for very interesting reading/discussions nonetheless.

If you like it could even extend to DA issues, cable geometry, whatever you fancy.

Cheers,

 

[sully]

Luckily, even with 3 billion neutrinos passing through every square inch of your body every second (solar nutrinos, of course)... none interact with your mass.

I worry bout the inductive energy storage and current profile..as skinning is current slew dependent..

Course..I still use my #12 extension cord wire for my speakers..at least they don't go up in flames at 1500 w/ch..

Sully

 

[sully]

skin thread

New thread on skinning...soon

going through an abode change, so my test setup is in total disarray..

The Hawksford article does have some interesting things which are inconsistent with electromagnetic field theory, as that subject is understood spanning the last five decades (plus 6). I love the switching of steel for copper, and the TEM wave propagation theory that "magically" goes right by the amp and speaker binding posts without reflections...and people who believe that transport current signals travel at 2.93 meters per second along the wires...

Ah, I digress...

Another thread...another time..soon, hopefully


Sully

 

[millwood]

Re: skin thread

transport current signals travel at 2.93 meters per second along the wires...

Sully

even a dumb banker can tell that's not right.

Through electrons do travel at fairly modest speed. It is just the EM signal the electrons "carry" goes much faster.

Either way, I would love to see a real discussion on skin effect.

 

[fdegrove]

Hi,

Millwood,

Not that I want to kick you in the shins but the only math that has been useful to me in designing audio has been Ohms law and Kirchoff.

Other than that there just are too many unknown parameters to make blanket statements as you seem to prefer to do.

This is audio after all, not just physics class.

Oh, and yes it is physics and science but so much is entwined that it's really hard to master the wood, mastering a tree is just not enough.

No matter how hard we try to grasp what exactly is happening I accepted ages ago that my mind isn't just Einsteinian enough to understand it all.
Having the ability to lose everything we've been spoonfed in class and take abstraction is exactly what made Albert Einstein such an extraordinary character.

If the forum interaction helps people to understand the complexity of this science then that may be a help to them...

So, can we please lose the negative attitude?

Thank you,

 

[SY]

Skin thread

Sully, that would be great. There's a lot of misinformation here that ought to be hashed out.

 

[sully]

I am confused..

Millwood attitude? I guess I haven't perused the thread far enough back.. I figured millwood was the banker..

I am constantly amazed at the piezo stuff...the motor/generator stuff..the crystal boundary stuff..the skin stuff...diode rectification...

Thankfully, I hate the math...math is for people who don't really understand...

Sorry..had to have some fun..

In reality..the math is great..but, I know too many who know the math, but not the actuals..

And, I know the best..

Another time...another thread..

Sully

 

[mrfeedback]

Skin Deep............

Artist: The Stranglers Lyrics
Song: Skin Deep Lyrics

Many people tell you that they're your friend
You believe them
You need them
For what's round the river bend
Make sure that you're receiving the signals they send
'Cause brother you've only got two hands to lend
Maybe there's someone who makes you weep
And some nights loom up ahead
When you're asleep
Some days there's things on your mind you should keep
Sometimes it's tougher to look than to leap
Better watch out for the Skin Deep
Better watch out for the Skin Deep
Brother watch out watch out for the Skin Deep
Brother watch out watch out for the Skin Deep
Better watch out for the Skin Deep
One day the track that you're climbing gets steep
Your emotions are frayed
And your nerves are starting to creep
Just remember the days
As long as the time that you keep
Brother you better watch out for the Skin Deep
Better watch out for the Skin Deep
Better watch out for the Skin Deep
Brother watch out watch out for the Skin Deep
Brother watch out watch out for the Skin Deep
Better watch out for the Skin Deep

Eric / - In Groman Mode.

 

[sully]

Wow..

you either type fast, or you have a lot of time on your hands..

Sully..

Ps..I prefer Cate brothers..

Hey hey, Mr. Union Man...How'm I gonna pay my dues..

Or my landlord...or the doctor...

How'll I get...new shoes???

18"" horn loaded...101 db spl, 1w/1m..1500 w/ch..4 channels..living room 12 by 15.

Fun..neighbors no like me..

 

[fdegrove]

Hi,

Fun..neighbors no like me..

Tell me about it...

Cheers,

 

[sully]

Ahhhh, spoken like a person who had to pay for the knick knacks that fell off the shelves because of your dismal attitude at the volume control...shame on you..(yah, I know the feeling)

Own a condo..3 intimate neighbors..sucks..

Changing abode as we speak..

Course..nothing to do with IC vs speaker length..major hijacking of the thread..

Use short IC's..long speaker...you will regret the IC loop intercept, the thumps whenever the HVAC fan kicks in..I know I do..

Sully

 

[Steve Eddy]

Audio cables typically use copper wire as a conductor because copper is inexpensive, pliable, and a good electrical conductor. However, copper oxidizes when exposed to air, forming copper oxide. This material is a poor conductor, which can cause a cheaply made cable or connection to degrade in performance over time.
http://emusician.com/ar/emusic_good_connections/

I wasn't talking about bare copper in mechanical connections (though that can work fine too if you do it right and don't mind a little regular maintenance). I was talking about bare copper between crimped or soldered connections. More specifically, internal point-to-point wiring.

Now if you'd care to explain to me how an exceedingly thin film of oxide is going to degrade performance in any significant way, I'm all ears.

"A simple twisted pair is only helpful if the interface is balanced. If it's unbalanced, then about all the twisted pair can do is not add any further imbalance."

Reducing the area of the antenna loop is a very effect method of reducing magnetic pickup and there are many ways in which it may be achieved. The simplest and most effective loop area minimization strategy is to use twisted wire pairs. If the field is localized the pitch of the twist need to be fine (say one twist per centimeter) and conversely for wide area fields the pitch can be coarse (one twist permeter). Twisted wires works by local cancellation. The wires form a local antenna loop, but after the next twist the loop is inverted so a voltage with the opposite polarity is generated. This will cancel the voltage from the first loop.

How does a twist constitute a local loop when there's no conductive path between the two wires making up the twist? Last I looked, a loop constituted a complete circuit path. Where does such a loop exist within a single twist?

It's always been my understanding that twisting works by causing the two wires to effectively occupy the same physical space on average over a given length such that each wire is on average exposed to the same magnitude of magnetic field so that the gradient between the two conductors is ideally zero. In other words, the induced voltages are the same in each conductor, i.e. common-mode.

Whereas in a parallel pair, one conductor will always be some distance closer or farther away from the source of interference relative to the other conductor causing there to be a gradient between the two and a differential voltage between them.

By the way, I don't see any physics, engineering, measurements, or anything else of any substance on that web page supporting their local loop claim. Am I supposed to swallow it simply because it's on some company's website?

Varying magnetic fields will induce a varying current into a measuring circuit. These are far more troublesome than electrically coupled voltages because the current tends to flow regardless of the circuit impedance and magnetic shielding is very expensive. The magnitude of the induced current is proportional to the magnetic field strength, the rate at which it changes and the area of pickup loop in the measuring circuit.

Huh? How are they any more troublesome than electrically coupled voltages? They seem to be implying here that with electrical coupling, the current is dependent on the circuit's impedance but with magnetic coupling, the current is irrespective of circuit impedance and a function of the magnetic field strength.

If that's the case, then for a given magnetic field strength, increasing the impedance of the circuit would increase the power in the circuit which seems to be a violation of the law of conservation.

Care to 'splain that one, Fred?

Noise induced by varying magnetic fields will be differential and not rejected by a balanced circuit, which rejects common mode noise.

If that's true, then there would be no point whatsoever in using balanced interfaces. Balanced interfaces would pass along any induced noise the same as unbalanced interfaces.

So tell me, Fred, what's the source of all that induced common-mode noise that all those balanced interfaces out there are supposed to be rejecting? Are balanced interfaces just a big conspiracy? Are all those papers on Jensen's website that Jocko recommended amt go read full of lies?

"What added capacitance?"

For the same length cable, twisting the wire will require a great length of the straight wire before it is twisted, than a straight run of wire for the same finished cable length. The shortest distance between two point is a straight line. If you have X pF per foot times a greater length, the capacitance increases.

When I asked what added capacitance, I meant compared to what? In other words, what are people typically using that has such low capacitance that solid state amps don't have a problem with it but a simple twisted pair presents such an increased capacitance that it can be problematic.

se

 

[piertr]

Yes, skin effect is real. But it is only real at high frequencies (shall we say over 100Khz?) and on bare wires.

I challenge anyone to find measured increase of skin-effect at audio frequency (say 20Khz tops) that is more than 10 or 20%.

and I challenge anyone to find the same thing to be even remotely material for multi-strand wires at audio frequency.

EM theory will tell you that skin depth at 20kHz is roughly equal to the radius of an 18AWG wire. Stranding doesn't help as much as you might think, even with litz wires.

 

[Steve Eddy]

In the context of oxides on the surface of conductors it should be obvious that this will increase resistance at high frequencies as higher frequencies occupy the outside diameter and are said to not even penetrate the conductor but pass as an electron cloud at the outside....

Huh? If you decrease the diameter of the wire, you increase its resistance at ALL frequencies and the difference in resistance between low frequencies and high frequencies due to skin effect will be LESS.

So unless you're concerned about increasing simple resistance, reducing the diameter of the wire is a PLUS in terms of skin effect.

And for crying out loud, we're talking about reducing the diameter of the conductive portion of the wire by something on the order of less than a micron. Do you know of anyone producing wire to such tolerances?

For indepth reading on this topic I'd recommend papers by Dr. Malcolm Hawksford, a British scientist who publicised well founded papers on this phenomenon.

Well founded papers? They were nothing more than articles published in a couple consumer audio magazines. They're about as well founded as if he'd have just tossed them up on a website.

And don't get sully started on Hawksford.

se