Hello. I'm trying to concatenate two OPUS audio files (from WhatsApp) into an MP3. On my first try without specifying output bitrate I found that the MP3 file is far too big (10 MB) than the sum of the input OPUS files (1 and 2 MBs). Then I figured it was because of the output bit rate not being set. But even with using -b:a to set the bitrate to the same as the input, I find that for some reason the output seems to use a higher bitrate and the output is still bigger than the sum of its inputs. Please guide me as to how I can get my desired effect of getting an output MP3 file that is comparable in size to the sum of its input OPUS files. $ ffmpeg -f concat -safe 0 -i <(printf "file '$PWD/%s'\n" *.opus) -b:a 18k -c libmp3lame output.mp3 Input #0, concat, from '/dev/fd/63': Duration: N/A, start: -0.002167, bitrate: N/A Stream #0:0: Audio: opus, 48000 Hz, mono, fltp Stream mapping: Stream #0:0 -> #0:0 (opus (native) -> mp3 (libmp3lame)) Press [q] to stop, [?] for help Output #0, mp3, to 'output.mp3': Metadata: TSSE : Lavf57.71.100 Stream #0:0: Audio: mp3 (libmp3lame), 48000 Hz, mono, fltp, 18 kb/s Metadata: encoder : Lavc57.89.100 libmp3lame size= 5319kB time=00:22:41.61 bitrate= 32.0kbits/s speed= 103x video:0kB audio:5319kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.004351% $ for x in *.opus output.mp3 ; do ffprobe $x ; done Input #0, ogg, from 'AUD-20170705-WA0000.opus': Duration: 00:07:41.82, start: 0.002167, bitrate: 18 kb/s Stream #0:0: Audio: opus, 48000 Hz, mono, fltp Input #0, ogg, from 'AUD-20170705-WA0001.opus': Duration: 00:14:59.78, start: 0.002167, bitrate: 18 kb/s Stream #0:0: Audio: opus, 48000 Hz, mono, fltp Input #0, mp3, from 'output.mp3': Metadata: encoder : Lavf57.71.100 Duration: 00:22:41.64, start: 0.023021, bitrate: 32 kb/s Stream #0:0: Audio: mp3, 48000 Hz, mono, s16p, 32 kb/s $ ls -l *.opus output.mp3 1096672 Jul 5 06:30 AUD-20170705-WA0000.opus 2124414 Jul 5 06:30 AUD-20170705-WA0001.opus 5446797 Jul 5 07:01 output.mp3 -- Shriramana Sharma ஶ்ரீரமணஶர்மா श्रीरमणशर्मा