Electromagnetic Interference, otherwise known as "EMI", is a naturally occurring phenomenon when the electromagnetic field of one electronic device disrupts, impedes or degrades the electromagnetic field of another device. A good example of this would be to place an electric fan close to your monitor. After doing this you would notice your monitor flickering or jumping from the interaction of the EMI fields.
In computer technology, computer devices are susceptible to EMI because electromagnetic fields are a byproduct of passing electricity through a wire. Data lines that have not been properly shielded are susceptible to data corruption by EMI. Simply stated, EMI causes a piece of electrical equipment to malfunction due to unwanted electrical energy being in the wrong place at the wrong time.
PCs generally do not cause very much interference with other devices; they are required by the Federal Communications Commission (FCC) to be certified as Class B devices. This certification is used to show that the PC conforms to standards that limit the amount of EMI that a PC can produce. But those of us in the modding community are never happy with the status quo of a mass produced piece of equipment. We are drawn to create something more unique then what is simply given to us. This is where EMI can become a problem.
All computers use digital signals, some of these are square waves rich in harmonics. These signals can be generated by the several oscillators found in most computer systems. In addition to the oscillators, all computer circuits sub-divide these oscillators into signals that are sub-multiples of the oscillator frequencies. Computers use switching power supplies that, after a modification, can become a source of EMI. The monitor has a separate power supply, plus sweep and high-voltage circuits that can also generate EMI. When you put them all together, a computer system can generate RF signals from below the HF band well into VHF.
For example, lets say that a person undertakes a project to modify their power supply by replacing the metal case with a colored acrylic case. If the maker of the power supply was using the case as a ground to help in reducing these emissions and by replacing the metal enclosure with a none metal enclosure, these waveforms will now be free to “interact” with the closest electronic devices to it. For most of us without a server case, that would be our processors, RAM, or whatever may be in our 5 ¼ drive bays.
If you have recently installed a window to your all-metal case and have started to notice performance issues, keep EMI in mind, though not very common, this can become an issue if you attend LAN parties. As all of us know space is at a premium at these functions and our computers can reside very close to our monitors or speakers in these situations. By creating an opening in the case you may be letting EMI waves come into contact with the internal workings of your computer. An aperture is defined as any opening to the outside of an enclosure that contains or is susceptible to EMI. The maximum allowable size of the opening is largely a function of the EMI frequency to be contained. A general rule of thumb the area of the aperture is less important than the longest dimension, as in seams and joints, which should not exceed 1/20th the waveform of the highest frequency of interest.
If you suspect EMI interference, one quick and easy way to verify may be to isolate the computer from the suspected source. For example if you have recently put a new side window in your computer and now your monitor is looking fuzzy, try separating the two on either side of the desk and see if your picture clears up. By doing this you would be removing each of the components from the offending magnetic field.