What is a Cutoff Filter?
A filter generally refers to a device that can remove something out of any given thing. In the audio world a filter does a very similar job.
A cutoff filter is a processor, which can attenuate or completely remove certain partials out of any given frequency spectrum, when the signal is passed through it. The user sets the upper and/or lower frequency limit.
Important terms to know
The band of frequencies, which is allowed to pass through the filter, is known as the pass band. The frequencies in the pass band are also called ‘Shelf Frequencies’.
If the signal is sent through a low pass filter and cut of is set to 700hz. Then the frequencies below 700hz will be allowed to pass through the filter hence falling under the category of pass band.
The band of frequencies, which gets attenuated or removed from the spectrum, is defined as the stop band.
If the signal is sent through a low pass filter and cut of is set to 700hz. Then the frequencies above 700hz will be attenuated or removed completely hence they will fall under the category of stop band.
In simple words, a band of frequencies that is stopped by the filter to pass though is called the stop band.
The frequency whose intensity is attenuated by the filter by one-half, relative to the un-attenuated pass band frequencies is called the cut off frequency. Depending on the type of filter the frequencies above and/or below the cut-off frequency are also attenuated.
It can also be defined as the particular frequency in the stop band, which is attenuated by 3dB.
Below is a diagram depicting the cutoff frequency, pass band and stop band of a low pass filter (LPF):
Slope and poles
Slopes and poles of a cut-off filter are counterparts of each other.
A slope of a cut-off filter represents progressively greater attenuation of frequencies above and/or below the cut-off. Longer slope represents smoother attenuation. Slope is quantified using ‘decibels per octave’.
A pole is nothing but increments of 6dB attenuation. So a four-pole low pass filter has 4 poles in series and hence has a 24dB/Oct slope.
More the poles, steeper the slope.
Types of Cut-Off Filters
High Pass Filter (HPF)
A filter that progressively attenuates frequencies below the cut-off frequency is called a high pass filter.
It cuts off the low end, hence making the sound thinner.
Low Pass Filter (LPF)
A filter that progressively attenuates frequencies above the cut-off frequency is called a low pass filter.
It cuts off the high end.
This filter has two cut-off frequencies. It allows the frequencies in between these two cut-offs to pass while progressively attenuating frequencies outside the pass-band.
This filter also has two cut off frequencies. It attenuates the frequencies in between the two cutoffs while passing those outside the stop band. Basically it is the exact opposite of a Band-Pass filter.
Additional Resources & Source Texts
Sound syntheses and sampling – Martin Russ