- by Kevin Deal
Audio pass filters are used to attenuate frequencies. These are simple audio tools which are always present in any audio engineer’s toolbox. The two types of audio pass filter include high-pass filter and low-pass filter. The filter attenuation you can do with either of them is quantified by decibels (dB) per octave.
Filters are used as corrective equalisers. Unlike creative equalisation, these have steeper slopes and are used to cut off high or low frequencies, depending on which type you use. They can’t boost frequency range, though. Most audio filters available in the market have a slope that ranges from 12 dB to 24 dB.
High-pass Vs. Low-pass Filters
High-pass filters (HPF) allows frequencies higher than the cutoff pass and those below are blocked. For instance, if the cutoff is set at 40Hz, you will hear refined frequencies from 40Hz and above. On the other hand, low-pass filters (LPF) do the exact opposite. They will only allow frequencies below the set cutoff to pass. For instance, if the cutoff is set at 40Hz, frequencies higher than 40Hz will be attenuated.
Between the two filters though, it is said that high-pass filter is more useful as it can also be used to clear away unnecessary background noises that the microphone usually gets from the background.
Noise vs. Signal
How can a pass filter determine a signal from a noise? It is true that it is hard to distinguish noise from signal as it is part of a waveform. Therefore, if you try to eliminate noise, you may end up eliminating the signal, too. However, there are noises that are separate from a frequency range. In this case, you can easily remove them by using pass filter.
Signal-to-noise ratio is used to compare the desired signal to that of the unnecessary background noise. If you aim to have an accurate result in removing noise, then you can employ the equation for SNR.
Eliminating Rumbles vs. Eliminating Bandwidth
If you want to eliminate rumbles which are in low frequencies, then you can easily use high-pass filter. It will easily block the frequency range below your set cutoff. However, if there are counter-productive bandwidth present in the audio, then you can try using low-pass filter and set the desired bandwidth. If you don’t specifically know which type of filter to use, then you can experiment and listen. When neither of them works, then it’s time for you to use other noise reduction plug-ins.
Tone Vs. Proximity Control
Limited bandwidth can lead to achieving harmonic content and get a high-quality tonal output. To do this, you can use low-pass filters to fix an d modulate the tone by changing its cutoff frequency. You can also try placing low-pass filters after boosting high frequency signal so you can have more control with the tonal effect of your audio.
To sort the proximity of frequencies, you can use a low-pass to make one signal seem like it’s far from the other. Do this on a delay’s output. Once the frequency within the cutoff is reduced, you can use the high-pass filter. The low-pass filter adds a realistic separation between the delay and signal while the high-pass exaggerates the audio’s spatial effect.
Compressor vs. HPF
Audio compressors are used to reduce the dynamic range by attenuating some bits of the signal. It is used to create clarity in the audio. For example, your audio has soft and loud parts. If you boost the frequency of the whole clip, then you’ll get a good signal for the soft parts but distorted one for the loud parts. What the compressor does is reduce the signal in specific ratio whenever it’s strong while letting the audio signal below the threshold be.
However, if you track the problem to the compression of the audio itself, then you can use high-pass filter (HPF) instead. To do this, set the cutoff frequency. Then, move the filter towards the sidechain of the compressor. Adjust until you can no longer hear the signal pumping.