About EMF Shielding
First, let's understand that the magnetic fields from a wire emanate from that wire in a pattern that could be described as concentric cylinders. The image at right represents a cross section view of a current carrying wire. Notice the concentric circles of magnetic field lines around the wire. Notice also, that the magnetic field lines are more concentrated near the wire, and less concentrated as the distance to the wire increases.
Now, understanding that magnetic shielding "works" because it is a better "conductor" of magnetic field lines than air or just about any other material, let's see what happens with 2 different shield designs. First, let's make a shielding cylinder around the wire. In the cross section image at right, we see that the magnetic field lines that would have occurred at the radius of the shield will exist INSIDE the shield. However, magnetic field lines at all other radii will not be affected. Net effect: no shielding.
But what happens if we use a flat shield? As you can see from the image below, the magnetic field lines which intersect the flat shield will be compressed into the shield, leaving less magnetic field on either side of the flat shield.
But also, note the following:
If the edges of the shield are bent slightly TOWARDS the source, the high field area at the edge of the shield will move further away from the "shielded area".
In conclusion, flat (or nearly flat) shielding is very effective for fields from wiring in the area adjacent to the shield. The wider the shield, the larger the shielded area. Contact us if you have specific questions about your shield design.
The sad short answer is: there is no such thing as a safe distance.
Here are the reasons:
While there are official standards for exposure to electric and magnetic fields, they are based on the amount of field needed to cause immediate harm. There is plenty of evidence to show that biological effects occur at levels well below the standard limits. In the end, we are each left to decide how much exposure we are willing to accept. One rule of thumb that is used by some experts is that you should limit your exposure to 60 Hz magnetic fields which are in excess of 2.5 mG. There is not a lot of scientific evidence to support this recommendation, but it is based on the Swedish recommendation for exposure to ELF fields from computer monitors.
You should get a gaussmeter and make some measurements. At least find out if the fields from the powerline exceed the 2.5 mG guideline.
In general, there are 5 ways to reduce your exposure to magnetic fields:
When it comes to powerlines, the options are limited as you do not have control over the powerlines themselves. The first step should always be to record readings of the magnetic field strength over a period of a few days using a reliable AC Gaussmeter to find out if you truly have a problem. Remember that the field will vary according to how much current (not the voltage) if being carried by the powerline. Also, remember that the only relevant readings are those taken where people actually spend time. High readings up close to the powerline are meaningless if the field inside your house is low.
Armed with this documentation, your next step should be to contact the utility company that owns the powerlines. Explain your concern and ask for their help in reducing your exposure. If the utility company wants to, they can do several things to lower your exposure:
Should you fail to get assistance for the power company (likely), you may be tempted to consider shielding. Naturally, the most effective shielding approach would be to shield the wires. Unfortunately, this is also impossible as the power company would never permit it. Shielding your home is possible, but not very practical. To achieve a reasonable degree of shielding, you would have to create a metal vault around your house, using thick metal plates with no windows. It would also be very expensive. Placing magnetic shielding material around your body is possible, but again not very practical.
Moving your house further back from the powerlines may be an option, but certainly not a very easy one. Make sure you carefully survey every proposed location for your house to make sure the fields are actually sufficiently lower at the new location that you are considering. Selling your home and moving to another location also comes under this heading. Make sure to use your gaussmeter to survey all homes you are considering, to avoid jumping from the frying pan into the fire.
Finally, there is the possibility of installing an active cancellation system. This is a device which constantly monitors the incoming field and produces an equal and opposite cancellation field. While it is not a do-it-yourself project, it may actually be your most practical solution. For each active cancellation system, an engineer must visit your location and custom design and install a system that takes into account the size, location, and strength of the offending source(s) and the dimensions of the area you want protected. Costs can vary from $10,000 to $50,000 or more. If this option appeals to you, contact us. We will be happy to answer your questions and refer you to a qualified engineer.
Our experience in measuring monitors of all kinds is that one cannot make generalizations about which type or which brand has higher or lower emissions.
A few years ago, we took our meters to a large electronics store to try to settle this question. We measured dozens of different types of TVs and monitors, including CRT, LCD, and plasma. Some where high, some were low. Some were high on E but not M, some were high on M but not E. Size had no correlation either. We found one unit which was the lowest on both. It was a Motorola product. The display model had a black bezel (plastic frame), but we wanted the gray one so we took a boxed unit OF THE SAME MODEL, but with gray bezel. When we got it back to the office, we set it up and tested again. It was worse than the worst unit in the store!!
Furthermore, a few months later, we went back and found that almost all the units available were different models.
From this, we have learned:
Compared to magnetic field shielding, shielding a home from cell tower radiation is reasonably straightforward. In theory, you want to create a continuous, highly conductive enclosure around the home. Any areas that are not conductive, even cracks under a door, will allow radiation to leak in. Perfect total shielding requires a perfect total enclosure. However, in a home environment, total radiation elimination may not be required. For example, perhaps 90% reduction is adequate.
There are several materials you can use to create the conductive enclosure, depending on your needs and your budget. Some materials are more appropriate for walls and ceilings, while other are better for windows. The higher the conductivity of the material, the better the shielding it will provide. Keep in mind such additional factors such as: durability, corrosion resistance, toxicity, ease of installation, appearance, and size.
For doors, walls, floors and ceilings, CuPro-Cote or Y-shield conductive paints offer very good shielding and are very convenient. Apply like ordinary paint on interior surfaces. You can paint over the conductive paint with a standard latex paint to achieve the desired color and to protect the conductive surface.
You can also cover the walls with a conductive fabric such as Pure Copper Polyester Taffeta or Soft&Safe. Apply the fabric as you would a wallpaper, remembering to overlap slightly at the seams to avoid leakage. You can cover over the fabric with a standard wallpaper, paneling or drywall.
Remember to treat openings such as switch plates, outlet covers, dryer vents etc. But because shielding materials are conductive, be very careful to avoid allowing them to come into contact with electric wires to avoid a shock hazard. Also remember to provide proper grounding to each component which is not in contact with the others.
There is only one important 1 key to successful RF shielding: control leakage.
Remember that the attenuation spec for a shielding material is how much radiation penetrates through the shield. Let’s look at an analogy:
In many ways, RF behaves much like visible light, and RF shielding materials behave much like two sided mirrors. Image that you are outdoors on a sunny day. You set a large mirror on a stand above your head. The attenuation specification for the mirror is very high, perhaps 120 dB or more, so basically no light comes through the mirror. If leakage was not an issue, you would be in total darkness. We all know this is not the case. Leakage from the sides easily illuminates the shaded area. Granted, the amount of illumination is less than standing in full sunlight, but the attenuation is no where near 120 dB. Maybe more like 20 dB. Furthermore, using a shield with even better attenuation will not yield any more benefit.
Now imagine you are in a small room with only one window. Bright sunlight comes in the window and illuminates the room. When you place your mirror shield over the window, you get a dramatic attenuation of the light. But the extent to which leakage of light occurs around the perimeter determines how far from total darkness you will achieve in the room. We have all experienced this when trying to draw a curtain over a bedroom window. You must control the leakage to get it really dark. There is nothing wrong with the shield, light is leaking around the shield.
Because RF shields are reflective on both sides, radiation which does leak in will be reflected by the inner surface of the shield, effectively amplifying the amount of radiation in the room. Even the tiniest leakage at a seam can reduce attenuation by many dB. The obvious solution is to pay serious attention to leakage points. A great how-to book is available which describes materials and methods for controlling leakage in detail.
There are several ways to interpret this question.
First, let's look at it
from the perspective of the cellphone owner:
Now, what's so hard about
blocking a cell phone signal completely?
1- Therefore your shielding materials must provide very high attenuation levels. Typically, 80 to 100 dB attenuation materials are required.
2- You must control leakage point VERY carefully. Gaps under doors, joints between shield sections, and even pinholes from sewing shielding material can permit these high frequency signals to penetrate. You need to create a "complete" enclosure. Any part that is not shielded is a leakage point.
A small pouch is not that difficult to make with the proper material. Shielding a whole house, or even a whole room is a more difficult challenge... if you want to completely kill the signal.
Conventional speakers incorporate both a permanent magnet and an AC magnetic field to produce sound. The field from the permanent magnet is present whether the speaker is active or not. The AC magnetic field is only present when the speaker is activated, and varies in frequency and strength with the pitch and volume of the sound produced. The magnetic field from the two sources can deflect the electron beam in a cathode ray tube monitor (TV) causing distortion of the image, sometimes called jitter (and possible damage to the equipment).
You will have to use magnetic shielding alloys
to shield these magnetic fields and you have a choice of several
methods. Keep in mind that with magnetic fields, you can either shield
the source of the offending field, or shield the thing(s) that you wish
1] Method for maximum aesthetics
To achieve maximum aesthetics you will need to be able to open the speaker cabinet and get access to the back of each speaker. There, you will find a donut shaped magnet, proportional in size to the size of the speaker, over which you will place a cup shaped shield.
Because you will be placing the shielding material in close proximity to this strong magnetic field, you will have to take saturation into account. This means using at least 2 layers of shielding.
For the layers closest to the magnet, choose a high saturation material such as MagnetShield. This material has the ability to "absorb" the initial blast of the field without saturating and becoming useless, but it will only give a limited attenuation. It is very low cost, so 2 or 3 layers are practical.
The outermost layer should be a high permeability material such as Joint-Shield. This outer layer will "absorb" much of the field which has evaded the first layer and yield a very high degree of attenuation. Note that attenuation will be greatest close to the speaker magnet, where the field is strongest (most interfering) anyway.
MagnetShield and Joint-Shield are both offered in a convenient 4 inch wide strip. The material is thick enough to provide good shielding, but still can be cut with a scissors and shaped by hand. For especially strong magnets, you may need more than one layer of each material.
Here is how you do it:
Wrap the MagnetShield around the speaker magnet (notice that it is attracted to the magnet) in a cylinder shape. Cut it so that you have about 1" of overlap at the seam. Use duct tape to tape the seam securely. Cut the material which extends backward at several locations so you can bend these "tabs" inward to form the "bottom of the cup" shape. Leave this layer in place.
Joint-Shield is provided with a peel-and-stick adhesive on one side. Before removing the adhesive backing, cut and shape the material just like the first layer, but on top of the first layer. Remove the adhesive backing and press the second layer onto the first layer. You are done!
Just be careful not to disturb or allow the shield to touch the electrical contacts on the speaker.
2] The Quick and Easy Way (and Maximum Field Reduction!)
If you need maximum field reduction, or cannot open the speaker cabinet, or you simply want to take the easy route, you can simply place flat magnetic shielding alloy between the speaker and the TV.
The magnetic fields at the side of the speaker magnet have different characteristics compared to at the back of the magnet, and different shielding materials are required. Take this into account when considering where your speakers will be positioned relative to the TV.
Shielding the side of the speaker:
Shielding the back of the speaker:
3] The Third Alternative
You can always place the TV or monitor inside a shielded enclosure. This will protect the monitor from external fields produced by the speakers and any other sources.
The most direct way to reduce your exposure from a laptop is to increase your distance from the device. Use a remote keyboard (not a wireless type!!) and place the laptop as far away as you can while still being able to view the screen. You can increase the text size on the screen if needed.
Laptops produce at least two types of electromagnetic fields: AC electric fields and AC magnetic fields. You can either shield the laptop (source) or shield yourself.
To shield the electric fields from the laptop, use ClearShield or VeilShield Fabric to cover the screen. Be sure to attach a ground cord. Covering the keyboard area of the laptop with a shielding fabric such as High Performance Silver Mesh will reduce electric fields from these areas while still allowing you to see the keyboard.
To shield the magnetic fields we recommend that you form a tray under the laptop with Magnetic Shielding Foil if you will have the laptop near you. It is not necessary to ground the foil. If you are using a remote keyboard, you can achieve much higher reduction of magnetic field by making a 5-sided box from Magnetic Shielding Foil. The base of the laptop is inserted into this box. The open end of the box faces away from the user.
A SafeGuard Apron worn on the body will help block the electric fields. Shielded Gloves can be used to protect the hands. Another option is to use a remote keyboard and increase the distance from the laptop to your body.
In the strictest sense, magnetic shielding is not truly shielding at all. Unlike the way a lead shield stops X-rays, magnetic shielding materials create an area of lower magnetic field in their vicinity by attracting the magnetic field lines to themselves. The physical property which allows them to do this is called "permeability".
Unlike X-rays, sound, light or bullets, magnetic field lines must travel from the North pole of the source and return to the South pole. Under usual circumstances, they will travel through air, which by definition has a permeability of "1". But if a material with a higher permeability is nearby, the magnetic field lines, efficient creatures that they are, will travel the path of least resistance (through the higher permeability material), leaving less magnetic field in the surrounding air.
Air ........... 1
Nickel .................. 100
Magnetic Shielding Alloys* ....... 20,000+
Now it is easier to
see why a magnetic shield in the shape of an enclosure (sphere, box,
tube, etc.) offers much better shielding than a flat shape or partial
enclosure. A source within the shield will produce field lines
which will travel through the air immediately surrounding the North
pole until they reach the shield. Then traveling through the shield,
they will emerge into the air surrounding the South pole and back to
the source. Traveling through the low permeability air outside the
shield does not offer any efficiency advantage! (Notice that the
diagram to the right is a cut-away view of a tube shaped shield.)
Similarly, if the source of the field is outside of the enclosure, the magnetic field lines will travel through the material of the enclosure on their way back to the source, never finding it more efficient to permeate the air space inside the enclosure. For these reasons, enclosing either the source of the field, or the thing(s) that you wish to protect from the field, offers the most effective use of the shielding material, and is usually the most cost efficient as well!
An important consideration when shielding magnets is that the magnets will be attracted to the shielding material. There are no magnetic shielding materials that will not be attracted to a magnet.
Shielding repulsion between 2 magnets
Shielding attraction between 2 magnets requires that each magnet have its own shield. The shield does not need to be in contact with its respective magnet, but it must be held fixed in position relative to its magnet. Again, the proper number of layers will depend on the strength of the magnets, the distance between them, and the size and shape of the shield. Simply add one layer at a time until the two magnets drop away from eachother.
Finally, a word about shielding just one
pole of a magnet:
Magnetic shielding is the basically the same no matter where you use it:
1] Start by using a gauss
meter to determine IF you have high magnetic fields where
the people are. In a car, the magnetic field profile will be
different at highway speed compared to idling. Check both ways.
Just be aware of a few points:
a) as in all magnetic shielding applications, you need a
fairly wide shield to prevent fields from coming around the edge of the
shield. In a car, there are many size limitations, so you may not
always be able to get as large a piece of shield in place as you would