Audio: band pass filter (0:03)
band pass filter plays band pass filtered pink noise to create the effect of an approaching storm. A Butterworth filter is used, a popular second-order band-pass filter, named after its designer, the physicist Stephen Butterworth. The centre frequency increases from 200Hz to 1000Hz and the bandwidth has a variable Q factor ranging from 10 to 1. The band pass filter figure shows the waveform.
Band pass filtering passes a band of frequencies and reduces the amplitude of frequencies above and below the band.
The centre frequency is the frequency at the centre of the passband. The passband is the band of frequencies to be passed and is more commonly called the bandwidth.
Quality factor (Q) equals the centre frequency divided by the bandwidth. The higher the Q factor, the narrower the bandwidth.
band pass filter has an initial centre frequency of 200Hz and a Q factor of 10. This means that the filter starts by passing a band of frequencies between 190Hz and 210Hz and reducing the amplitude of frequencies above and below the band. At the end of the sound the centre frequency has risen to 1000Hz and the Q factor is 1. This passes a band of frequencies between 500Hz and 1500Hz and reduces the amplitude of frequencies above and below the band.
A band pass filter is a combination low pass and high pass filter. Band pass filtering is exactly the same as low pass filtering then high pass filtering (or high pass then low pass). A band pass filter simply makes it easier to do both processes at once.
The versatile band pass filter combines the good points of low pass and high pass filters. It can simulate many natural sounds when it is used with a sound source such as white noise and pink noise. Band pass filtering is excellent at simulating motion: someone entering a room, a car passing, footsteps receding into the distance. It can also produce a wide variety of synthetic sound effects.