[LAU] Gain and clipping wav -> lame

Does anybody know why wav files that do not have clipping would clip when encoding them with lame? I have a bunch of self-made clips that for various reasons don't have enough gain. I wrote a wav->wav conversion that determines maximum gain (as in highest or lowest absolute value) and uses sox' volume argument to boost. Inspecting it with same gain tools and listening to it shows the expected gain raise, but not clipping. Then, when encoding in lame it gets clipped. Lame warns about it and the values have been boosted to the max. When doing the boost, with the same factor, directly in lame (feeding in the unboosted wav) I get the same problem. Very roughly a clip with 3% headroom ends up being too high by 16%, or about a 20% boost. What's going on? I assume there is something about mp3 encoding I don't quite understand. Is there anything more efficient that I can do about that doing one lame run to get an appropriate boost value and then doing another encoding pass? Here are the stats of a boosted wav file and the resulting lame: t02c02.tmp.wav_maxgain_tmp.wav: Samples read: 36900960 Length (seconds): 384.385000 Scaled by: 2147483647.0 Maximum amplitude: 0.969208 Minimum amplitude: -0.970673 Midline amplitude: -0.000732 Mean norm: 0.213396 Mean amplitude: 0.000000 RMS amplitude: 0.274775 Maximum delta: 1.675323 Minimum delta: 0.000000 Mean delta: 0.193639 RMS delta: 0.245325 Rough frequency: 6820 Volume adjustment: 1.030 mp3: Samples read: 36907776 Length (seconds): 384.456000 Scaled by: 2147483647.0 Maximum amplitude: 1.000000 Minimum amplitude: -1.000000 Midline amplitude: -0.000000 Mean norm: 0.203464 Mean amplitude: 0.000010 RMS amplitude: 0.262007 Maximum delta: 1.636044 Minimum delta: 0.000000 Mean delta: 0.185782 RMS delta: 0.235408 Rough frequency: 6863 Volume adjustment: 1.000 /opt/good-sox/bin/sox WARN sox: `-' input clipped 725 samples /opt/good-sox/bin/sox WARN sox: `/tmp/cracauer/l.wav' output clipped 360 samples; decrease volume? Encoding output: lame -h -b 160 --replaygain-accurate --clipdetect t02c02.tmp.wav_maxgain_tmp.wav t02c02.tmp.wav_maxgain_tmp.mp3_tmp '&&' mv t02c02.tmp.wav_maxgain_tmp.mp3_tmp t02c02.tmp.wav_maxgain_tmp.mp3 + sh LAME version 3.96.1 (http://lame.sourceforge.net/) Using polyphase lowpass filter, transition band: 18000 Hz - 18581 Hz Encoding t02c02.tmp.wav_maxgain_tmp.wav to t02c02.tmp.wav_maxgain_tmp.mp3_tmp Encoding as 48 kHz 160 kbps j-stereo MPEG-1 Layer III (9.6x) qval=2 Frame | CPU time/estim | REAL time/estim | play/CPU | ETA 16015/16018 (100%)| 0:22/ 0:22| 0:22/ 0:22| 17.121x| 0:00 average: 160.0 kbps LR: 2551 (15.93%) MS: 13467 (84.07%) Writing LAME Tag...done ReplayGain: -9.1dB WARNING: clipping occurs at the current gain. Set your decoder to decrease the gain by at least 1.3dB or encode again using --scale <arg> (For a suggestion on the optimal value of <arg> encode with --scale 1 first) -- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Martin Cracauer &lt;cracauer(a)cons.org&gt; http://www.cons.org/cracauer/