Cutoff Filters

What Is a Cutoff Filter?

A filter generally refers to a device that removes something from any given thing. In the audio world, a filter does a very similar job. A cutoff filter is a processor that can attenuate or completely remove certain partials from any given frequency spectrum when the signal is passed through it. The user sets the upper and lower frequency limit.  

Important Terms to Know

• Pass Band: The band of frequencies allowed to pass through the filter is known as the pass band. The frequencies in the pass band are also called ‘Shelf Frequencies.’
  For example, if the signal is sent through a low pass filter and cut off 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.

• Stop Band: The band of frequencies that get attenuated or removed from the spectrum is the stop band.
  For example, if the signal is sent through a low pass filter and cut off 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 through is called the stop band.  

• Cutoff Frequency: The frequency whose intensity is attenuated by the filter by one-half, relative to the un-attenuated pass band frequencies, is called the cutoff frequency. Depending on the type of filter, the frequencies above and below the cutoff frequency are also attenuated. It can also be defined as the frequency in the stop band, which is attenuated by 3dB.

Below is a diagram depicting the cutoff frequency, passband, and stop band of a low pass filter (LPF):

   

• Slope and Poles: Slopes and poles of a cutoff filter are counterparts of each other. A slope of a cutoff filter represents progressively greater attenuation of frequencies above and below the cutoff. A longer slope represents smoother attenuation. The 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 a 24dB/Oct slope. The more the poles, the steeper the slope.

Types of Cutoff Filters

• High Pass Filter (HPF): A filter that progressively attenuates frequencies below the cutoff 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 cutoff frequency is called a low pass filter. It cuts off the high end.  

• Band-Pass Filter: This filter has two cutoff frequencies. It allows the frequencies between these two cutoffs to pass while progressively attenuating frequencies outside the pass band.

• Band-Reject Filter: This filter also has two cutoff frequencies. It attenuates the frequencies between the two cutoffs while passing those outside the stop band. Basically, it is the exact opposite of a Band-Pass filter.