[FFmpeg-user] How to make the audio louder (at least "visually")?

[Marcin Borkowski]

Hi all,

I am continuing my work involving waveforms.  So far it's great, and
thanks for your help!  But I need one more thing.  I'd like to make the
waveform better visible.  I noticed that most of the time, when the
characters speak on a recording, they do not speak very loudly, so the
bumps I can see (corresponding to syllables and words) have pretty low
amplitude.  Is it possible to make them louder before generating the
waveform so that they are better visible?  I don't care about any sound
distortions, since I only want the waveform image.  Right now, this is
what I am doing:

ffmpeg -accurate_seek -ss ... -to ... -i ... -loglevel 0 \
-filter_complex showwavespic=s=640x36:colors=white,negate \
-frames:v 1 -c:v png -f image2 -

I tried adding -filter:a "volume=4.0", but there seemed to be no effect.

What was I doing wrong?

TIA,

-- 
Marcin Borkowski
http://mbork.pl