shielding

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

radiance

Well-known member
Joined
Jun 4, 2004
Messages
3,228
Location
the Netherlands
Let's talk shielding. I'll start of with a couple of questions.

- what material is best for "audio frequency range"  shielding?
- We use to shield audio cables in tube equipment. Would it an idea to shield heater PSU cables as well?
      ( ....keeping the interference within the braid around the heater PSU cables....)


At RS they sell this  (264-9310) expensive but interesting metal braid

A sheath manufactured from silver plated nylon fibres
Offers effective shielding for cable diameters from 8 to 20mm

Technical specification
Electric field shielding effectiveness: 64dB @ 1MHz
 ---------------------------------- 55dB @ 10MHz
 --------------------------------- 44dB @ 100MHz
Magnetic field ---------------------- 8dB @ 1MHz
Plane wave ----------------------- 43dB @ 400MHz
  ----------------------------------40dB @ 1GHz

Can't find any info on shielding quality in the audio frequency range though...
 
yes, it is spiral wrapped around the wire(s) to be shielded and thus provides 100% coverage. Usually the finished cable is much stiffer though, if that matters.  Check out Canare L4E6AT (foil) and L4E6S (braid).
 
foil shield cables are intended for fixed installations generally - the foil can develop cracks if it's flexed too much. braided shields are much tougher, which is why they are used for iggy pop's mic cables.

i believe this topic has had a thread or two in the past? i think there is two types of shielding we need to talk about with audio electronics... electro-static and electro-magnetic.

as far as i know you can make an electrostatic shield using any conductive material. this is the type of shielding used on cables and also standard enclosures for gear. aluminium, copper and steel are common here.

electromagnetic shields require a metal that responds to magnetism (iron/steel/mu-metal/nickel? etc) and is used to keep stray fields from being picked up by magnetic components like transformers and inductors. perhaps this kind of shielding is used in hard drive construction too? perhaps this could be a source of electro magnetic shielding material? perhaps someone knows about this...

someone please correct me if any of this info is wrong!

haima
 
i have no idea! it just crossed my mind so i thought i'd mention it.

i doubt there'd be mu-metal in there... also i believe mu-metal quite is sensitive to being bent and cut, so it might be hard to salvage anything useful. i do wonder if anyone knows about this though.
 
Many foil shields are actually foil on a mylar or plastic film which provides extra strength that the foil alone doesn't have.  The foil sometimes has a "shorting" fold down one edge so that the mylar layer doesn't insulate the foil where it overlaps itself.  the reason being that gaps in the shielding degrade it's quality.
 
  I know that this thread is about cable shielding, but I'm going to check in to the hard drive thing. I'm also a computer hack/builder, and I've taken apart a few hard drives in the past, the top outer cover is held on by just a few screws, I never thought that it might be mu-metal, and it does make sense that it might be to shield the platters and read/write heads..... this is so cool.... ;D
 
Very cool indeed. I'll have some daed ones laying around. Will check this for sure.

Anyone has any thoughts on the question below?

radiance said:
- We use to shield audio cables in tube equipment. Would it an idea to shield heater PSU cables as well?
      ( ....keeping the interference within the braid around the heater PSU cables....)

Shielding your PSU cable as well as your audio cables...hm..overkill?
 
radiance said:
- what material is best for "audio frequency range"  shielding?
Almost impossible to answer this question because shielding is a matter of attenuating an interference; a stupid answer would be: "the one that has the best attenuation", because the "best" shield in the world can be shielded, or to put it differently,
Q: what is better than a shield?
A: Two shields
Shielding is a matter of compromise; you have to draw a line whre to stop and weigh the disadvantages (bulk, weight, cost, practicality)
- We use to shield audio cables in tube equipment. Would it an idea to shield heater PSU cables as well?
Yes, it's been done. The reason heater wires are generally twisted is that it provides some shielding. But when you really want to make sure there is no interference from heater supply, you really want to go DC. Shielding the wires is not enough because you cannot shield the internal connections in the tube itself (although it's supposed to be done)
expensive but interesting metal braid
A sheath manufactured from silver plated nylon fibres

Technical specification
Electric field shielding effectiveness: 64dB @ 1MHz
 ---------------------------------- 55dB @ 10MHz
 --------------------------------- 44dB @ 100MHz
Magnetic field ---------------------- 8dB @ 1MHz
Plane wave ----------------------- 43dB @ 400MHz
  ----------------------------------40dB @ 1GHz
Can't find any info on shielding quality in the audio frequency range though...
Probably because they would be extremely difficult to measure accurately. The electrostatic "shielding effectiveness" is probably around 100+dB in the audio range and the magnetic shielding close to 0.01dB. At audio frequencies, the electrostatic shielding efficiency is more or less equal to the ratio of conductive area vs. holes in the surface; in that respect, foil has almost infinite protection, and braid has less (95% coverage braid offers ca. 26dB shielding, 99% gives 40dB). I would say that in most circumstances, any well built shielded cable is good enough in the audio frequency domain. But also you must consider the effects of HF and RF interference, and, depending on the environment, you may have to consider those MHz performance.
On a different subject: No mu-metal in HD's, at least not in any significant quantity. HD covers are made of plain old donkey iron (with a nice plating though).
As haima said: "electromagnetic shields require a metal that responds to magnetism" and that's the most challenging part of interference. magnetic interference is to shielding what low frequencies are to acoustic isolation. You need mass and distance, in that case a lot of iron (or Ni or Mu...) and good physical separation.
 
abbey road d enfer said:
Q: what is better than a shield?
A: Two shields

Q: What is better than two shields?
A: Two shields and more distance from the interferer.

Q: What's better than two shields and a lot of distance?
A: Two shields, a lot of distance and attacking the interference at the source

Q: What is even better than that?
A: Making the receiver as noise-immune as possible.

Shielding is only one means to an end. ARd'E already mentioned physical separation, but you can also consider ways of reducing the interference or making the victim more immune. As pointed out before lots of RF shielding tricks do little against LF magnetic interference.

If your mic pre is humming at 50Hz, consider rotating or removing the power transformer before messing with Mu-Metal. If it's humming at 100Hz, investigate other rectifiers or better supply line filtering first. If your mic pre is receiving Talk Radio from the 40kW transmitter on top of your building, add RF filtering on inputs and outputs, and maybe add some shielding.

(I know this thread is about shielding. The main reason I'm pointing this out is that there are enough professional (and DIY) products out there where the designer apparently applied a shitload of shielding to fix a real or perceived problem, without addressing other ways).

JD 'holistic' B.
 
Thanks for all comments!  This metal braid I found seemed like a nice shielding solution but as it turns out I can better wrap some aluminium foil around whatever it is I want to shield, probably wires ;-)
 
I'm wondering why transformer manufacturers are still making mic inputs with wires instead of pcb pins. It would be much easier to shield this sensitive connections if we could choose the cables that we want and need.
I hate it when i take care of grounding, buy nice toroid for minimum interference...but then have to run to-from input tx all around the chasis. That is a lot of wire when you take in account connections for phantom, pad, etc.

What do you guys do about this, use a foil or something?
 
The usual way is connecting the xfmr to the PCB, then a shielded cable to the switches, and another shieded cable back to the PCB. But there are alternatives, like shielding the PCB tracks, either with one or more PCB copper layers (depending on number of layers) and/or copper foil.
 

Latest posts

Back
Top