There's a formula for skin-depth, which is the basis of magnetic shielding. If your metal
is significantly thicker than its skin-depth at the frequency in question it attenuates well.
Skin depth decreases with sqrt of frequency and with sqrt of magnetic permeability, so
higher frequency more shielding, more ferro-magnetic(*), more shielding.
(*) has to be magnetically soft, ie magnetizable, a permanent magnet doesn't work so well,
so soft iron or mu-metal are better than steel.
thickness to make sense of that difference I think.
is significantly thicker than its skin-depth at the frequency in question it attenuates well.
Skin depth decreases with sqrt of frequency and with sqrt of magnetic permeability, so
higher frequency more shielding, more ferro-magnetic(*), more shielding.
(*) has to be magnetically soft, ie magnetizable, a permanent magnet doesn't work so well,
so soft iron or mu-metal are better than steel.
sqrt(20k/60) ~= 18, so about 18 times thinner skin-depth - but you need to know the material
thickness to make sense of that difference I think.
If aluminum was an effective magnetic shield then why are none of the CRT shields I've ever seen made of aluminum?
Skin effect depth at 60 Hz in aluminum is about 9 mm. So since it has 2 surfaces, that's 18 mm - nearly 3/4 inch. A 1 inch thick piece of aluminum will have some magnetic shielding ability but still have much less magnetic shielding effectiveness at 60 Hz than a 1/16 thick piece of mild steel.
1" aluminum plate is about $150 per sq. ft. from retail sources.
1/16" mild steel is about $18 per square foot, and is more effective than 1" thick aluminum at 60 Hz where most noise from power supply transformers and chokes occurs. Even more so for 50 Hz.
You want to try to use aluminum for magnetic shielding, go ahead. No electrical engineer would consider this sensible.
Skin effect depth at 60 Hz in aluminum is about 9 mm. So since it has 2 surfaces, that's 18 mm - nearly 3/4 inch. A 1 inch thick piece of aluminum will have some magnetic shielding ability but still have much less magnetic shielding effectiveness at 60 Hz than a 1/16 thick piece of mild steel.
1" aluminum plate is about $150 per sq. ft. from retail sources.
1/16" mild steel is about $18 per square foot, and is more effective than 1" thick aluminum at 60 Hz where most noise from power supply transformers and chokes occurs. Even more so for 50 Hz.
You want to try to use aluminum for magnetic shielding, go ahead. No electrical engineer would consider this sensible.
I have had good success with Giron from lessemf.com Applicable over 50Hz - 1kHz or so - I have used it to increase magnetic shielding around power transformers that were too close to other components.
Magnetic Shielding Materials
It is pretty expensive, but less expensive than mu-metal.
Magnetic Shielding Materials
It is pretty expensive, but less expensive than mu-metal.
Good point.
According Morgan Jones author in Building Valve Amplifiers/2004 page 85:
Steel is not suitable for the chassis of valve amplifiers. Steel is magnetic, and allows leakage flux from transformers to flowthrough the chassis and induce currents into the pins of the valves. If a steel chassis is unavoidable, induction into the chassis can be greatly reduced by fitting a non-ferrous gasket between transformers and the chassis; 1.5 mm Paxolin is ideal
I believe this above is valid for audio amplifiers only for other applications may be irrelevant otherwise so shielding of audio transformers with ferrous plate/cups must be avoided in view of the ferrous changing the magnetic field of the transformer, but ferrous plate is cheaper and plentiful than alu so it will continue to be used by certain builders.
Hope this help.
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Giron looks interesting but I wonder how it will look like when used on amp. Do you wrap transformer with it, or use is as a flat barrier?
A flat plate of aluminum, copper, silver, gold, etc. . . .
Put that flat plate perpendicular next to a strong AC magnetic field.
The plate will act as a shorted turn, and will produce a magnetic field in the opposite direction.
Therefore, the plate will be repelled away from the AC magnetic field.
Haven't you ever seen the AC solenoid with iron or steel core, that has a circular aluminum plate placed around the solenoid core (like a slice of pineapple with the core removed).
The aluminum plate is "shot" off of the solenoid core.
Go to any Science Technical Museum, or any High School Physics class and see it.
Now you remember, Right?
There is a reason for such lab experiments. It is easier to comprehend and remember than understanding and remembering pages in a Physics textbook.
In some regards, Physics never changes. Even using Einstein's Physics, you eventually come around to the beginning again.
Put that flat plate perpendicular next to a strong AC magnetic field.
The plate will act as a shorted turn, and will produce a magnetic field in the opposite direction.
Therefore, the plate will be repelled away from the AC magnetic field.
Haven't you ever seen the AC solenoid with iron or steel core, that has a circular aluminum plate placed around the solenoid core (like a slice of pineapple with the core removed).
The aluminum plate is "shot" off of the solenoid core.
Go to any Science Technical Museum, or any High School Physics class and see it.
Now you remember, Right?
There is a reason for such lab experiments. It is easier to comprehend and remember than understanding and remembering pages in a Physics textbook.
In some regards, Physics never changes. Even using Einstein's Physics, you eventually come around to the beginning again.
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I performed a quick test.
Placed 2 chokes next to each other as closest as possible. Connected one choke to 60Hz source, then I measured AC voltage from second choke.
- 100mV @0cm
- 10mV @70mm distance
- 3.5mV @140mm distance
- 7.2mV @70mm distance, a thin steel chassis (empty) between 2 chokes.
- 4.3mV @70mm distance, 100W large aluminum heatsink between 2 chokes.
*steel chassis is smaller than heatsink
Well, the least expensive option is obviously distance, and it's free.
I'll do the same test with shield choke later.
Placed 2 chokes next to each other as closest as possible. Connected one choke to 60Hz source, then I measured AC voltage from second choke.
- 100mV @0cm
- 10mV @70mm distance
- 3.5mV @140mm distance
- 7.2mV @70mm distance, a thin steel chassis (empty) between 2 chokes.
- 4.3mV @70mm distance, 100W large aluminum heatsink between 2 chokes.
*steel chassis is smaller than heatsink
Well, the least expensive option is obviously distance, and it's free.
I'll do the same test with shield choke later.
Shielded and potted choke (army surplus) at the same condition above (the same AC current to the first choke, about 530mA).
Distance is center to center of chokes, because this choke is a little smaller.
They share similar inductance, not exactly the same, though.
- 58mV @0cm
- 12mA @70mm
PS: Measured again, and the number is fixed. The distance is hard to set, and number is approx.
Distance is center to center of chokes, because this choke is a little smaller.
They share similar inductance, not exactly the same, though.
- 58mV @0cm
- 12mA @70mm
PS: Measured again, and the number is fixed. The distance is hard to set, and number is approx.
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Non magnetic materials will only give shielding to a changing "AC" magnetic field, there is no shielding effect from a constant "DC" magnetic field e.g. from a nearby loud speaker magnet
Brian
It will not shield against a STATIC magnetic field, correct.
It will not shield against a DYNAMIC magnetic field, wrong.
Dynamic magnetic shielding, skin, proximity effects a.s.o., all are do to a secondary field set up in a conductor.
The primary fields induces current in the conductor.
Those currents set up a secondary magnetic field that provides the "shielding".
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