I used the built in microphone of a laptop computer to record the sound of striking a spoke with a small metal bar.

After damping most of the spokes with a cloth, it sounded like this.

Then I gave the nipple a quarter turn and it sounded like this

I next plotted the spectrum:

The red line is after giving a twist to the nipple.

The fundamental around 600hz is in good agreement with my ear.

The increase from around 580hz to 610hz is easy to see.

I tried this a couple of times with differant spokes and it seems repeatable.

A quick and dirty filter in C or matlab would probably make this a viable method of

measuring relative tension without buying any equipment.