--lowpass number Use a lowpass filter when encoding
Enables a lowpass filter of the specified frequency when encoding the source. Range [0.001..50]kHz or [50..50000]Hz
Using a lowpass filter helps reducing the amount of data to encode. This is important in MP3 due to a limitation in very high frequencies (>16Khz). The default value depends on the target bitrate/quality. It is not recommended to change it as a general basis.