One common application in AF and RF alike is the need for a low pass filter. Motors and fluorescent lighting can result in a lot of audible noise in the power lines. There is a low cost way to eliminate the problem.
The frequency of the filter is given by w=1/RC which is easier than it looks. For filtering the power line, often all that is needed is a capacitor as the line becomes the impedance itself.
Power lines run at 60 Hz or 50 Hz depending on the country a person lives in. Usually 80µF is enough and make sure is rated at 4x the line voltage to be safe. For 120V system use a 400V capacitor for safety. For EU with 220V use a 600V capacitor.
This approach can save a lot of money on power by blocking unwanted frequencies from getting into the power grid.