If an EVP researcher has any qualms about distinguishing valid EVPs from occasional broadcast or other electronic interference, one way to lessen the chances of electronic contamination is to build a Faraday Cage. 

This is easy to do and can be made from materials found around the home.

    Find two small "nesting" boxes -- cardboard (or wood) boxes where a smaller one will fit comfortably into a larger one. 

The boxes need only be as large as required to house your digital voice recorder or other audio recorder.

    When you have acquired the two boxes, cover both in a DOUBLE wrapping of aluminum foil.  While aluminum is not the best material that can be used, it is adequate to the purpose and readily available.  Wrap the foil around the boxes in such a way as to allow the cover flaps to still be opened and closed.  You may wish to add a bit of padding (paper towels, old dishtowels, rags) to the bottom of each box to reduce the sliding movement of the inner box within the outer box and the recorder inside the inner box.  Ensure that you use ample sticky tape (duct tape is quite strong) to fix the foil in position.

    Some instructions call for you to add a grounding wire with an alligator clip on the end so that you can easily ground the box and drain off excess electrical energy.  This, however, is probably only needed for massive EM bursts such as from nuclear detonations and solar flares.  Common, everyday broadcast energies will probably not require such grounding.

    In this Faraday Cage -- or in this case, Faraday boxes -- arrangement, the outer box catches electronic interference and insulates the inner box from it.  In addition, the inner box further insulates the recorder from the energies coming into contact with the outer box thus creating a relatively shielded environment for the recorder to operate in.

    Please also note that your common household microwave oven will also produce the Faraday Cage effect. 

The drawback to this, naturally, is the lack of portability which the Faraday boxes will provide.