Low Pass Filter

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Low Pass Filter allows through (passes) frequencies below its cutoff frequency and attenuates frequencies above its cutoff frequency.
 
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Sound above the cutoff frequency is not eliminated but increasingly attenuated as the frequency rises further above the cutoff. The steepness of the attenuation is set by the rolloff value. For example, with a rolloff of 6 dB per octave, the sound decreases by 6 dB for each octave above the cutoff frequency (an octave above is double the frequency).

It is hard to design a pass filter with a sharp cutoff, so the cutoff or corner frequency chosen always defines the point at which the audio is reduced by 3 dB. Thus if a cutoff frequency of 1000 Hz is chosen with a rolloff of -24 dB, the 1000 Hz frequency will be reduced in volume by 3 dB and a frequency of 2000 Hz will be reduced by 24 dB.

Additionally there will always be a small and decreasing amount of attenuation just below the cutoff frequency.

250 Hz, 6 dB per octave low pass filter

To achieve more attenuation you can run the effect again or use a greater rolloff.

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