Format Options

Sox detects format from file extensions. When that isn’t possible — raw PCM files, pipes, unusual encodings — you provide it explicitly.

Recall from chapter 2: format flags describe the next filename. Put them in the wrong section and they apply to the wrong file.

The four core flags

These describe the audio itself. Sox will resample, convert, or remix as needed to meet them.

Flag	Meaning	Example
`-r`	sample rate (Hz)	`-r 44100`
`-b`	bit depth	`-b 16`
`-c`	channels	`-c 1` (mono), `-c 2` (stereo)
`-e`	encoding	`-e signed-integer`

Common encodings: signed-integer, unsigned-integer, floating-point, a-law, u-law.

-t is different: it names the container format (WAV, AIFF, FLAC, raw, and so on) rather than a property of the audio. Sox normally infers it from the filename extension, so you rarely set it. Reach for -t only when there’s no extension to read (pipes with -, headerless raw files) or the extension lies about the content.

-t raw      # headerless PCM
-t wav      # force WAV regardless of extension

Resampling

sox input.wav -r 8000 telephone.wav    # downsample to 8 kHz
play telephone.wav                     # noticeably lo-fi
sox input.wav -r 48000 hq.wav          # upsample to 48 kHz

The format flag before telephone.wav describes the output. Sox resamples automatically.

Changing bit depth and channels

sox input.wav -b 24 output.wav    # convert to 24-bit
sox stereo.wav -c 1 mono.wav            # stereo → mono (averages channels)
play mono.wav
sox mono.wav -c 2 stereo.wav            # mono → stereo (duplicates channel)

-c uses sox’s default algorithm: averaging when going down, duplication when going up. For anything more specific — dropping a channel, swapping L and R, custom mix weights — use remix (chapter 8).

Fully-specified output

Sometimes you want to know exactly what comes out: a specific sample rate, bit depth, channel count, and encoding. This matters for archival (so the artifact doesn’t drift with the default audio config), for interop (another tool expects 16-bit 44.1 kHz stereo signed-integer and nothing else), and for pipelines that hand audio to downstream processes with narrow assumptions.

The recipe: specify all four flags on the output.

sox input.wav -r 44100 -b 16 -c 2 -e signed-integer output.wav

That produces a WAV with exactly those properties regardless of what the input looked like — sox resamples, converts bit depth, remixes channels, and re-encodes as needed. Verify with soxi output.wav.

The same four-flag pattern works for raw output — just add -t raw:

sox input.wav -t raw -r 44100 -b 16 -c 1 -e signed-integer output.raw

Reading raw PCM

Raw files have no header, so you must describe them completely:

sox -r 44100 -b 16 -c 1 -e signed-integer input.raw output.wav
play output.wav

Writing raw output (see “Fully-specified output” above):

sox input.wav -t raw -r 8000 -b 8 -c 1 -e unsigned-integer output.raw

Piping

Use - for stdin or stdout, with -t to specify the format:

# Two sox processes in a pipeline
sox input.wav -t raw - | sox -t raw -r 44100 -b 16 -c 1 -e signed-integer - output.wav

For piping between two sox processes specifically, the -p flag emits sox’s own internal format, which avoids specifying all those flags manually:

sox test.wav -p trim 0 5 | sox - output.wav norm -3

Taming sox