Cutoff Frequency (Low-pass filters)

This equation is used to compute the break frequency, also called the turnover frequency or cutoff frequency (in hertz).

One simple low-pass filter circuit consists of a resistor in series with a load, and a capacitor in parallel with the load. The capacitor exhibits reactance, and blocks low-frequency signals, forcing them through the load instead. At higher frequencies the reactance drops, and the capacitor effectively functions as a short circuit. 

The combination of resistance and capacitance gives the time constant of the filter `tau`=RC. 

Reference
Electronic Low-pass filters. https://en.wikipedia.org/wiki/Low-pass_filter#Electronic_low-pass_filters [Retrieved January 13, 2016]