While the lowpass filters are the most important in synthesizers, highpass, bandpass and bandreject (aka notch) filters are nice to use as well. Here are some examples. The horizontal red bars stand for the partial tones in a waveform. The gray area emphasizes the partials that are allowed to pass the filter. The light red bars show the partials being filtered out.
The angle of the filter near the cuttoff frequency is called edge steepness. This means that the filter does not cut off the frequencies above (or below) the cutoff frequency abruptly. Instead, it damps the frequency the more the far they are away from the cutoff frequency. In synthesizers, we often find filters with an edge steepness of 24 dB per octave. Sometimes 12 dB per octave filters can be found. The first ones will sound harder, which is good for lead and drum sounds, while the latter ones will sound softer, being useful for pad and string sounds.
At the beginning of each recording, the cutoff frequency is at the bottom line of the graphics. It then traverses the spectrum of partial tones to the top (higher frequencies) and back to the bottom (lower frequencies). I applied some reverb to the recordings to make the sounds more beautiful :) :
Lowpass Filter
Highpass Filter
Bandpass Filter
Notch (aka band reject) Filter
