Tensioning spokes by "tone"

Tension by tone has good theory behind it. It is difficult to apply
where the nipples meet the rim, as it were. The tone is not very
clean, as you get a cord from the crossing spokes. We have tried
music tuners here on spokes, and they did not prove consistent.

John Allen has a web site describing the process of truing by tone.

Tension by tone is good for relative tension, but for absolute tension
is become more difficult. If you do decide to get the Park Tool TM-1,
see the excel spreadsheet,
http://www.parktool.com/repair_help/TCC.shtml