Conversions and Anatomy

Your first conversion

Sox infers format from file extension. Converting is often just:

sox test.wav test.mp3
sox test.wav test.flac
sox test.wav test.ogg

Use soxi to verify the output matches your expectations.

The null output -n discards the result entirely — useful for checking that sox can read a file without writing anything:

sox test.wav -n

The anatomy of a sox command

Every sox command follows this structure:

sox  [global opts]  [input opts]  infile(s)  [output opts]  outfile  [effects...]
     ────────────   ──────────────────────   ────────────────────    ──────────
     globals        input                    output                  effects

• globals — options affecting the whole run (-V, --buffer, …)
• input — format options for the input(s), placed immediately before the filename
• output — the output file, with its own optional format options before it
• effects — the effects chain, applied left to right

The -v flag from chapter 1 lives in the input section: it scales the input amplitude as the file is read, before any effects run. The vol effect (chapter 3) does the same arithmetic but in the effects chain. These two commands produce identical output:

sox -v 0.5 test.wav out.wav   # scale at input
sox test.wav out.wav vol 0.5  # scale in effects chain

-v is input-only — sox will reject it as an output flag. To scale the output, use vol or gain in the effects chain.

play and rec are just sox with one section missing. play has no output section (the speaker is implicit). rec has no input section (the microphone is implicit). Any effect chain you’d write after the output filename in sox comes directly after the input in play:

sox  test.wav out.wav highpass 300 norm -3
play test.wav            highpass 300 norm -3

Format flags are positional

A format flag describes the next filename in the command — input or output, depending on where you place it:

sox -r 8000 test.wav out.wav   # override input's declared rate; output inherits 8000 Hz
sox test.wav -r 8000 out.wav   # resample output to 8000 Hz; input rate unchanged
sox -r 16000 test.wav -r 8000 out.wav  # both specified explicitly

All three are valid and mean different things. Placing a flag in the wrong section will silently produce a different result than you intended, which is the most common source of bugs in sox commands. Format options are covered fully in chapter 5.

Effects come last

Effects go after the output filename. This surprises most people once and never again:

sox test.wav out.wav trim 5 10 reverse
#                    ───────────────── effects
play out.wav

Multiple effects are applied left to right: first trim, then reverse on the trimmed result.