how to set the bitrate when use libfaac

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I use libfaac with mencoder,and i found it can't set the bitrate of audio.
My Command :
mencoder input.avi -ovc copy -oac lavc -lavcopts acodec=libfaac:abitrate=32:aglobal=1 -o abc.flv

When finished,i check the abc.flv file :
Stream #0.1: Audio: aac, 44100 Hz, stereo, s16, 58 kb/s

But my setting is 32bit. Anyone who can help me to set the bitrate to 32kbps,thanks