Audio: band stop filter (0:03)
band stop filter plays band stop filtered white noise to simulate a centrifuge sound effect such as a washing machine. A Butterworth second-order band stop filter is used in which the centre frequency varies from 250Hz to 2500Hz and back again twice a second. The Q factor is one. The band stop filter figure shows the distinctive spectrogram of a band stop filter with a notch in the spectrum at 1325Hz.
A band stop filter, or band reject filter, stops a band of frequencies and passes frequencies above and below the band.
A band stop filter does the exact opposite of a band pass filter. It stops a band of frequencies and passes frequencies above and below the band. The stopband is the band of frequencies to be stopped, the centre frequency is the frequency at the centre of the stopband and the quality factor (Q) equals the centre frequency divided by the stopband. The higher the Q factor, the narrower the stopband.
band stop filter has an initial centre frequency of 250Hz and a Q factor of 1. This means that the filter stops the band of frequencies between 125Hz and 375Hz and leaves the amplitude of frequencies above and below the band untouched. At the end of a sweep up the centre frequency has risen to 2500Hz. The filter stops a band of frequencies between 1250Hz and 3750Hz and leaves the amplitude of frequencies above and below the band untouched.
A notch filter is a band stop filter with a very narrow stopband. A notch filter is primarily used to repair a sound. It is typically used to eliminate mains hum from a 50Hz mains power supply (60Hz in the US) that is sometimes present in a sound recording.
The sound in band stop filter has no known name. Essentially it is the opposite of a wah wah which is a band pass filter with a variable centre frequency and a narrow passband. A wah wah sound that repeats itself on a regular basis produces an effect known as auto wah. band stop filter has a variable centre frequency, the same as a wah wah, and repeats itself regularly, like an auto wah, but it has a wide stopband, not a narrow passband. Band stop filtered sound produces an effect that sounds like an inverted auto wah, therefore, the sound effect is duly christened the auto yam.
Band stop filtering is much less used than low, high and band pass filtering. This is a surprise, as it creates a rather pleasant hollow sound. There is plenty of scope to use the filter to create unusual sound effects.