Check out "Audiocheck" online

Nice find. Thanks, Bob.

My computer sound is the original Cambridge Soundworks satellites with subwoofer - one of the better computer sets I've heard - but I didn't hear much below 40Hz. Now to burn the disk and test my audio systems.

Not hearing the low notes may not be your hearing. High yes but not low. For one thing a low note must be much louder for the human ear to hear it, speakers tend to roll the low notes off and the unknown factor of room size. My understanding is the loudest note you can hear in a given room is directly related to room size and ceiling height. I was told that a note had to make a full wave for to be heard. 1100 feet per second divided by 20 hz would be 55 feet my room isn't that big. You can feel it but you can't hear it. I can rattle the dishes and feel the pressure at 20hz but in my room I can't hear below 40hz.

I said full wave I guess it's half wave or 26.5 feet for a 20 hz. wave I tried to find more info on this but this seems to be a very controversial issue. One thing is for sure human hearing is down about 70 db at 20hz. See hearing chart in middle of the page. It's funny we work so hard to get sound level from 20 to 20K but our ears don't hear that way. It all depends on the level of sound. Again check out the chart at different sound levels. The louder the sound the flatter our ears hear.
http://en.wikipedia.org/wiki/Sound_pressure

I couldn't hear a thing except the floor joists wobbling.

I'd interpret what you hear with caution. According to the LF/Subwoofer test, my Harman-Kardon computer speakers, which came included with a Dell computer about eight years ago, are good to 30Hz, which is difficult to believe. Sure enough...I used a known microphone to capture the sound using another computer, then looked at the waveform. What was coming from the subwoofer was all harmonics.

Just came across the Audiocheck website independently, and saw that there was already a thread about it.
It really is a great resource for a wide range of tests that will help you set up your stereo etc.