When the Use Variable Bit Rate Encoding option is enabled in the MP3 Encoder preferences, iTunes automatically raises and lowers the compression bit rate in order to keep a consistent level of quality. For example, if there is a point in the source music that contains a wider range of frequencies, iTunes increases the bit rate. Conversely, if there is a point that contains a narrower range of frequencies, the bit rate is decreased. iTunes does not allow the bit rate to go lower than the Bit Rate settings you've chosen. Click the checkbox to enable Variable Bit Rate
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