R
Russell Griffiths
Guest
Greetings.
"Notes on the Troubleshooting and Repair of Audio Equipment and Other
Miscellaneous Stuff"
Author: Samuel M. Goldwasser
found on sci.electronics.repair
6.10) Comments on speaker shielding
When loudspeakers - even those little speakers that came with your PC
- are
near TVs or monitors, there may be problems with the fringe fields of
the
powerful magnets affecting color purity, convergence, or geometry.
Speakers
designed to be used with PCs in close proximity to their monitor will
likely
include some internal shielding. This may even be effective.
However, the
large powerful loudspeakers used with high performance stereo systems
will
likely not have such shielding. The best solution where display
problems have
been traced to the loudspeakers is to move them further away from the
TV or
monitor (and then degauss the CRT to remove the residual magnetism.
Where
this is not possible, shielding of the speakers may be possible:
(Also see the document: "TV and Monitor CRT (Picture Tube)
Information".)
(From: Lionel Wagner (ck508@freenet.carleton.ca)).
**********************************************************************
Put a Tin can over the magnet. This will reduce the external field by
about 50%. If more shielding is desired, put additional cans over the
first, in layers, like Russian dolls. (Note: a Tin can is actually
made
nearly entirely of steel - the term 'Tin' is historical. --- sam)
(From: Nicholas Bodley (nbodley@tiac.net)).
While both electrostatic and electromagnetic (E/M) fields can affect
the paths
of the electron beams in a CRT, only E/M fields are likely to be
strong enough
to be a problem.
Magnetic shields have existed for about a century at least. Some
decades ago,
a tradenamed alloy called Mu-Metal became famous, but it lost its
effectiveness
when bent or otherwise stressed. Restoring it to usefulness required
hydrogen
annealing, something rarely done in a home shop (maybe one or two in
the USA).
More-recent alloys are much less fussy; tradenames are Netic and
Co-Netic.
Magnetic shields don't block lines of force; they have high
permeability,
vastly more than air, and they guide the magnetism around what they
are
shielding; they make it bypass the protected items.
I have been around some shielded speakers recently, but never saw any
disassembled. They looked conventional, must have had the "giant thick
washer" (my term) magnet, and seemed to have a larger front polepiece
than
usual.
They had a shielding can around the magnet; there was a gap between
the front
edge of the can and the polepiece. I suspect that a second internal
magnet was
placed between the rear of the main magnet and the rear (bottom) of
the can,
so there would be minimal flux at the gap between the can and the
front
polepiece. Holding pieces of steel close to the gap between the can
and the
polepiece showed very little flux there.
Modern magnets are not easy to demagnetize, in general.
(From: Dave Roberts (dave@aasl.demon.co.uk)).
The *good* so-called magnetically screened speakers rely on two means
of
controlling stray flux. The static field from the magnet on the
speaker
(which would cause colour purity problems) is minimized by the design
of the
magnet. This is often at the expense of gap field linearity, leading
to
greater distortion - not that most users seem to worry about that...
The mains varying field is minimized by use of a toroidal mains
transformer,
but the more recent mains powered speakers seem to be coming with
*plug top*
PSUs, which take the problem further away.
Bye for now,
Russell Griffiths.
"Notes on the Troubleshooting and Repair of Audio Equipment and Other
Miscellaneous Stuff"
Author: Samuel M. Goldwasser
found on sci.electronics.repair
6.10) Comments on speaker shielding
When loudspeakers - even those little speakers that came with your PC
- are
near TVs or monitors, there may be problems with the fringe fields of
the
powerful magnets affecting color purity, convergence, or geometry.
Speakers
designed to be used with PCs in close proximity to their monitor will
likely
include some internal shielding. This may even be effective.
However, the
large powerful loudspeakers used with high performance stereo systems
will
likely not have such shielding. The best solution where display
problems have
been traced to the loudspeakers is to move them further away from the
TV or
monitor (and then degauss the CRT to remove the residual magnetism.
Where
this is not possible, shielding of the speakers may be possible:
(Also see the document: "TV and Monitor CRT (Picture Tube)
Information".)
(From: Lionel Wagner (ck508@freenet.carleton.ca)).
**********************************************************************
Put a Tin can over the magnet. This will reduce the external field by
about 50%. If more shielding is desired, put additional cans over the
first, in layers, like Russian dolls. (Note: a Tin can is actually
made
nearly entirely of steel - the term 'Tin' is historical. --- sam)
(From: Nicholas Bodley (nbodley@tiac.net)).
While both electrostatic and electromagnetic (E/M) fields can affect
the paths
of the electron beams in a CRT, only E/M fields are likely to be
strong enough
to be a problem.
Magnetic shields have existed for about a century at least. Some
decades ago,
a tradenamed alloy called Mu-Metal became famous, but it lost its
effectiveness
when bent or otherwise stressed. Restoring it to usefulness required
hydrogen
annealing, something rarely done in a home shop (maybe one or two in
the USA).
More-recent alloys are much less fussy; tradenames are Netic and
Co-Netic.
Magnetic shields don't block lines of force; they have high
permeability,
vastly more than air, and they guide the magnetism around what they
are
shielding; they make it bypass the protected items.
I have been around some shielded speakers recently, but never saw any
disassembled. They looked conventional, must have had the "giant thick
washer" (my term) magnet, and seemed to have a larger front polepiece
than
usual.
They had a shielding can around the magnet; there was a gap between
the front
edge of the can and the polepiece. I suspect that a second internal
magnet was
placed between the rear of the main magnet and the rear (bottom) of
the can,
so there would be minimal flux at the gap between the can and the
front
polepiece. Holding pieces of steel close to the gap between the can
and the
polepiece showed very little flux there.
Modern magnets are not easy to demagnetize, in general.
(From: Dave Roberts (dave@aasl.demon.co.uk)).
The *good* so-called magnetically screened speakers rely on two means
of
controlling stray flux. The static field from the magnet on the
speaker
(which would cause colour purity problems) is minimized by the design
of the
magnet. This is often at the expense of gap field linearity, leading
to
greater distortion - not that most users seem to worry about that...
The mains varying field is minimized by use of a toroidal mains
transformer,
but the more recent mains powered speakers seem to be coming with
*plug top*
PSUs, which take the problem further away.
Bye for now,
Russell Griffiths.