Introduction: Audio Filters With Audacity

This presentation will explain to you how to use filters to affect the music that you listen to, and also what is happening when it is used properly.



Step 1:

Before creating a filter to affect the way your music sounds, it is important to recognize what is actually happening in your music.

The audio files that you listen to are actually made up of various sine waves, each representing different amounts of air pressure that are generated when played through speakers. As seen in the image above.

Humans can realistically hear between 20Hz and 20,000 Hz without any assistance.

Step 2:

The music that we listen to is made up of sine waves with various frequencies. See the image above for reference.

When we add a filter to the music, we can subtract some of the frequencies so that the speaker that we play the signal through only receives the ideal frequencies.

Step 3:

The image above shows a clip of an audio file before any filters are applied. For this scenario, we want to apply a low pass filter to the file.

The audio file is also available to download.

Step 4:

A low pass filter allows for frequencies lower than your cutoff frequency to pass through, while blocking out the frequencies that are higher than the cutoff. In this case, the passband includes the lower frequencies, while the stopband includes the higher frequencies. The roll-off is the slope of the signal at the cutoff frequency. By increasing the order of the system, the slope will also increase, allowing for a more precise cutoff.

Step 5:

The audio file in the picture above is from the same timestamp as the original, however this one has a low pass filter at 120 Hz with a roll-off of 48 dB per octave. You can see that a large chunk of the signal has been blocked due to the filter, and due to such a steep roll-off, there is actually too much that is not allowed to pass. Next, we will keep the desired frequency cutoff, but decrease the roll-off.

When listening to this audio file, it is almost impossible to hear anything other than a slight sound at some very low frequencies.

Step 6:

By keeping the cutoff frequency at the same level, the majority of the signal stays consistent with the original attempt. However, by decreasing the roll-off to 6 dB per octave, the filter does not completely block out the signal at desired frequencies, and this results in being able to hear the proper frequencies that we would want to be passed to a subwoofer using a low pass filter.