[Vorbis] ultra-low bitrate stream?

[Qiangning Hong]

I am writing a long-run sound recording application using OggVorbis. 
The user can adjust sound quality parameter to make balance between 
storage space and sound quality.

The input sound is 22050Hz, mono.  To maximize the recording capability 
on a given storage volume, we want the result bitrate as low as possible.

I use -q0 parameter and it produces a 32.0kbps stream.  It is too high 
(our competitor using Microsoft WMA can produce 16kbps streams at the 
lowest quality setting).  I tried -q-1 and the bitrate is still 
24.2kbps.  -q-2 and -b16 both produce error messages.

How can I get lower kbps using oggvorbis?