Ken, precisely. The beat frequency method counts the discreet explosions over a known time interval. You have to use the same section of road to normalize the elevation changes, etc. The program then counts the beat frequency and compares it to the next test. The differences in the rate of change is then calculated to a difference in HP. The weight of the vehicle, CD, the gear ratio, tire diameter, are all entered into the program. You need a computer, a microphone, and a digital recording device.

There is another post that mentions a similar method that measures spark impulses.