-b

-b n Minimum allowed bitrate

When used with variable bitrate encoding (VBR), this specifies the minimum bitrate to be used. The allowable values for n depend on the MPEG version and layer being encoded to or decoded from:

MPEG-1 Layer I

n = 0, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448

MPEG-1 Layer II

n = 0, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384

MPEG-1 Layer III

n = 0, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320

MPEG-2 Layer I MPEG-2.5 Layer I

n = 0, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256

MPEG-2 Layer II MPEG-2.5 Layer II MPEG-2 Layer III MPEG-2.5 Layer III

n = 0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160

When encoding, MPEG version depends on the output sampling frequency selected:

**Table 2.1** MPEG versions and sampling frequencies.
Version	Sampling Frequencies
MPEG-1	32 kHz, 44.1 kHz, 48kHz
MPEG-2	16 kHz, 22.05 kHz, 24 kHz
MPEG-2.5	8 kHz, 11.025 kHz, 12 kHz

Notes

LAME always encodes to layer III (MP3).
When decoding, the MPEG version and layer is obtained from the file.
When used with variable bitrate encoding (VBR), -b specifies the minimum bitrate to be used. However, in order to avoid wasted space, the smallest frame size available will be used during silences.