I recently started using LAME 3.90.3 via iTunes-LAME on my G5. I have it set at --alt-preset-extreme. I am seeing quite a range of bitrates for the files I am encoding. Most of them are between 200 to 250, which I understand is normal. There are some, however, that are as low as 150 or 160. These songs, btw, are mostly old Jazz recordings that have a limited fidelity range. Can I assume that the low bitrate is due to the sonic simplicity of the song? Can I also assume that adding a switch for a minimum bitrate would just be wasting space as it would not provide an audible difference in sound quality? Thanks in advance.

There are several possible reasons; near-mono sound, high noise level, low recording volume, little high frequency content... And i doubt there will be an audible difference if you increase the minimum bitrate.

Yes and yes.

There seems to be broad consent in that the --alt-preset switches can be trusted whithout further ado... distribution of bits will vary a lot (according to many factors) but the result should be satisfactory in 99,99% of cases... and as youre using --alt-preset extreme you should be pretty safe... (If you need more security regarding fidelity than that maybe you should consider MPC or even lossless...)

Thanks a lot for your replies. I did not think that I had anything to be concerned about, but I wanted to be sure. Thanks again.