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How to process audio files from the command line with SoX

Describing sound manipulation without the material available at hand is not very fun. So if you want to practice with SoX by yourself, all the original .wav files used in this article are available to download free of charges on Noiiz.com. Thanks to Dave Rose and the whole Noiiz team for their great work in making quality sound accessible to a broad range of creators.

Before going into the heart of the topic, let’s introduce the basic SoX syntax:

It takes an input file, processes it through zero, one or several filters, then write the result to the output file. Input and output files can be encoded in any SoX supported audio format. You can specify the exact encoding on the command line, but most of the time this is not required since SoX will guess it based on the filename extension.

Without any filter, SoX copy the input file to the output file—​so this may be useful if you have to re-encode some sound to a different format:

As it is often the case with similar tools, SoX uses a lossless intermediate representation of your sound during the processing. That means if you use a lossy format for both the input and the output, it will first decode your sound and then re-encode it, with a possible loss of quality. Even if both formats are the same:

Even if this is not its primary purpose, SoX can play and record sounds too. In the simplest use case, this is achieved by using some options instead of the input (respectively output) filename:

For convenience, the SoX package also provides the play and rec tools. So, to play a sound you can use either:

It is not difficult to deduce from that the command you can use to record sound with SoX. So, I let that for you as an exercise. But as a hint, I would suggest taking a look at the sox(1) manpage…​

Before looking at SoX filter options to transform a sound, we may take a look at the pseudo-transformation stat, allowing to obtain information about a sound file:

You may notice here the use of the -n option instead of the output file because we are not interested in the result of the transformation chain, but only by the file statistics.

SoX also provides the soxi command. But this one only displays the information found in the file’s header. So you may sometimes see differences between that and the result of sox …​ stat, which is based on the file’s content:

Worth mention this is a true stereo sound—​even if this would not be obvious if you listen to it. However, we can confirm that by plotting the difference between the channels.

Why would someone want to add some silence at the end of a sound file? Well, I had that exact need this week: I downloaded a couple of sound effects (SFX) to enrich my video’s audio tracks, some of them being pretty short "pops" or "cracks." On that occasion, I discovered my video editor was unable to process sound files whose length was shorter than one frame (1/25th of second in my case).

The solution? Pad the sounds with some silence, so they reach the minimum length for my video editor to accept them. Let’s examine first the actual length of the problematic SFX file:

Simple math shows that to reach a duration of 1/25th of a second, we need to pad that sound with about 0.027s of silence:

You may have noticed that pad command takes two arguments. The first one is the amount of silence to add at the start of the sound stream. The second one the amount of silence to add at the end of the stream. Since I only want to add silence at the end of my file, I set the start pad length to 0. In fact, the pad command can add silence at an arbitrary position in the file. But I let you check the SoX manual by yourself to see the actual syntax.

The SFX tracks above are stereo (two channels). For some special treatments, I will describe later, I may need to mix them down to mono (one channel). Something quite easy using the remix effect:

The remix effect takes a list of channel mapping as the argument. Here "1,2" means the average of channels 1 and 2. Since there is only one channel specification as the argument to the remix command, the output is single-channel (mono).

You may also want to use the pseudo-channel 0 to create a silence output channel:

Finally using a similar solution, you can create a dual mono track:

Something I could have written like that instead (but at the expense of creating the mono file first):

I encourage you to compare the result obtained by plotting the dual-mono file with the plotting of the stereo file we have seen earlier to be sure to understand the difference:

Until now we used only one input file. But the sox command allows you to load several of them. In that case, you may need to specify how SoX should combine them. The most useful combiners are the concatenate (--combine concatenate) and the merge (--combiner merge) combiners.

You may remember when mixing my effects down to a mono file, I said I would need that later. Now is the time: I would like my SFX to move in a circle in the stereophonic space just like when a very skilled sound engineer is using the manual pan slider on a mixing console.

For that, I need first a low-frequency sin wave:

Since I start with the null input, I had to manually specify the sample rate for the generated sound. That synth effect will produce an 8-second sound file containing two channels:

The -n option after the synth keyword is used to instruct SoX we want a full-scale output. Otherwise, SoX will reserve some headroom so subsequent transformation will not clip. You can achieve the same effect by adding a gain 0 stage at the end of the filter chain:

Formally we now have a sound file whose left and right channels are in opposition of phase. You may try to play that sound if you want, but at such low frequency, you will not hear anything. However, interesting things will happen if we try to multiply that sound channel by channel with the dual-mono file produced earlier:

Formally speaking, we are modulating the amplitude of the double-mono input file with the sin wave. As a consequence, you may now hear the sound going from left to right and back in the stereo space.

You can achieve a similar result in one pass using the amod (amplitude modulation) mode of the synth filter. Please note, you do not need to change the DC offset of the sin wave here since in that case SoX seems to modulate between 0 and 1 and not between -1 and +1 as it does by default.

I now have an 8-second sound going in a circle in the stereo space. If your DAW or video editor cannot loop regions automatically, you may need to repeat that sample in a loop file:

If you play the output file, you would hear nine times the same sound. That is the original one, plus eight repeats.

Those were all the tricks I wanted to share with you today. But by exploring the SoX documentation, you will find many other effects. If most of them are really convincing, I cannot leave you without a word about the noise reduction effect. I discovered SoX while searching a tool to remove from my recording the quasi-white noise caused by the constant humming of my laptop fan. Unfortunately, for that task SoX was not satisfactory. Its noise reduction algorithm creates too many artifacts and is easily outperformed by the Audacity noise reduction plug-in, or by the even better Noise-Repellent LV2 plugin for Ardour.

But, just like I did, don’t reject SoX just because of that: as we have seen, it can perform many other tasks, and it will perform them well. Finally, if you see possible improvements in SoX, please join the project: the latest stable release dates back to 2015, and if a couple of new developers could contribute I am sure it could be even better!