4

I've just used sox to reduce the amplitude by 3dB on an mp3 file, because the song was playing back too loudly.

I used

sox input-file.mp3 output-file.mp3 gain -3

Does this processing alter the signal content in output-file.mp3 in any way*, or perhaps cause it to deteriorate slightly? (*Apart from changing the amplitude of course.)

2

Yes, sox uncompresses and re-compresses lossy formats while processing (see man sox), and iterated compression will cause slight detoriation of quality.

To change the gain in an mp3 file, it's not necessary to re-encode it, because the gain values are separate from the encoded data. You can use applications like mp3gain to change the gain values directly, without causing detoriation in audio quality.

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