Low-pass filter

A low-pass filter is an electronic filter that allows low frequencies to pass through, but attenuates high frequencies. Low-pass filters are used to block unwanted high-frequency signals, whilst passing the lower frequencies. They are the opposite of high-pass filters.

Examples of low-pass filters

A physical barrier acts as a low-pass filter for sound waves. When music is playing in another room, the low notes are easily heard, while the high notes are largely filtered out. Similarly, very loud music played in one car is heard as a low throbbing by occupants of other cars, because the closed vehicles (and air gap) function as a very low-pass filter.

Low-pass filters are also used in subwoofers and other types of loudspeaker, to block high pitches that they can't efficiently broadcast.

Radio transmitters use low-pass filters to block harmonic emissions which might cause interference with other communications.

Types of low-pass filters

There are a great many different filter circuits, with different responses to changing frequency. The frequency response of a filter is generally represented using a Bode plot.

A first-order filter, for example, will reduce the signal strength by half every time the frequency doubles. The magnitude Bode plot for a first-order filter looks like a horizontal line below the cutoff frequency, a straight line approaching zero.

A second-order filter will reduce the signal strength by three-fourths every time the frequency doubles. The Bode plot for this type of filter resembles that of a first-order filter, except that it falls off quadratically instead of linearly. Third and higher order filters are defined similarly.

The meanings of 'low' and 'high'--i.e. the break frequency--depend on the characteristics of the filter. A physical barrier acts as a filter at audio frequencies (between about 20 and 20000 Hz). Electronic circuits can be devised for any desired frequency range, right up through microwave frequencies (above 1000 MHz) and higher.

One simple electrical circuit that will serve as a low-pass filter 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, causing them to go through the load instead. At higher frequencies the reactance drops, and the capacitor effectively functions as a short circuit. The break frequency, also called the turnover frequency, is determined by the choice of resistance and capacitance.

A low-pass filter can also be realized as a digital filter.