[ubuntu] warn me before system suspends?

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http://ubuntuforums.org – I am using Lubuntu, I set the system to suspend when I'm idle for 20 mins (in Preference -> Power Manager -> Xfce Power Manager -> On AC power supply). But I want the system to make a sound during the 19th minute to warn me that it is soon going into suspension. What is the simplest way to do that? Thanks! :D (Hardware)