There are many different types of spectral effects used in audio. These effects are typically used to change the amplitude of one frequency region in the spectrum by a different amount than another frequency region.
These spectral effects can be described as filters because one frequency region gets filtered out while the other region does not. This section will focus on some of the basic filter effects.
Consider the low-pass filter (LPF), sometimes also called the high-cut filter. In this effect, the low frequencies pass through the filter without being changed in amplitude. However, the high frequencies are reduced in amplitude; they are filtered out.
Another type is the high-pass filter (HPF), or the low-cut filter. As the name suggests, the high frequencies pass through unchanged, while the low frequencies are attenuated. Therefore, this effect conceptually performs the opposite of the LPF.
The last two types of filters we will look at in this section are the notch filter and the band-pass filter (BPF). In the notch filter, the low frequencies and the high frequencies pass though without being changed in amplitude. However, there is a notch in the middle of the spectrum where the amplitude is reduced. The BPF is the opposite. The low frequencies and high frequencies are reduced in amplitude, while a frequency band in the middle of the spectrum passes through without being changed.
Now that we have looked at what filters do to an audio signal, let’s look at how they can be created.