Measuring speed with acoustics and a tape recorder...
That accuracy can be improved as one gets more comfortable with the technique. Also, the results are almost always a little less than the radar because the farther the plane is from the mic when it passes by, the calculated speed is reduced.
I've noticed that by tweaking the settings on the spectrograph program I can get graphs that are much easier to analyze. Ideally, you want to see a clean consistent approaching freq and a consistent departing freq. Sometimes the graph is hard to read, but by adjusting the record volume and lowering the sampling rate I can get clean graphs.
The biggest increase in accurach will be when I get the recorder mounted on a pole so someone doesn't have to stand in the field ready to dodge out of the path of a speeding plane. Once it's on a pole I'll be more comfortable flying closer to it.
Jason