Low-pass filter

A Low-pass filter passes 'low' frequencies fairly well, but attenuates 'high' frequencies. Hence it is useful as a filter to block any unwanted high frequency components of a complex signal whilst passing the lower frequencies.

Typical sittuations that shows low-pass behavior is if you play music loud in another room, you will almost only hear the low frequensies while the higher ones will be attenuated.

Another good example is the heat in a room. If you turn the heat upp and then down in fairly fast on a radiator the temperature in the room won't have time to get warm before you turn it down again and when you turn it down i will take a while before the air cools down again, hence the dynamic of the room attenuates hig frequensies (in this case fast alterations in the thermostat on your radiator). Now if you repeat the last procedure much slower the temperature will follow the temperature, set on te thermostat much better.

Of course, the meanings of 'low' and 'high' frequencies are relative in the case of loud music in another room we are talking about a cutoff at appr. a few hundred Hz whilst in the case of the radiator is a matter of appr. 0.00015 Hz if you turn the temperature up during 30 min and then turn it down for 30 min.

One simple electrical circuit that will behave like a low-pass filter consists of a resistor in series with the signal path in conjunction with a capacitor in parallel with the signal path. The combined values of RC determine a turnover frequency. This is one way to do it but any combination that you can think of or put together at random that will attenuate high frequensies whilst leaving lower frequensies unattenuated is in fact a low-pass filter.

Such a circuit would be used in combination with a Subwoofer, Loudspeaker.