[Speex-dev] How to use aec correctly?

[keith]

Hi,all
  I have tested AEC on files, it works well.I have some files,one is echo
file, others are echo-added files(an origin file adding echo at different
delay,such as 20ms,40ms...120ms,140ms).AEC do wonderfully on those files
except echo added at 140ms-delay.
  But ,when i use AEC in my voip project, it does feebly. Who can give me
some hints why caused this.How long can  sound be picked up by mic after it
plays out.This is the main problem to align echo.Give me a hint.
Best regards
keith