THD sweeps Round 3 ->
The THD was measured at the same time with max level sweeps. The frequency resolution is around 1.4 Hz. Note that the SPL isn’t absolute but follows the frequency response and possible power compression instead! One should also notice that if the amplitude of the fundamental goes very low, the accuracy of the THD measurement is reduced. For example if the fundamental is at 70 dB, the random background noise at around 30 dB causes 1% THD. Measuring THD on the fly is more demanding for a subwoofer than measuring with single 2-3 second sine waves. Therefore the results are not perfectly comparable with each other.
Round 1 and Round 2
Subwoofer’s THD levels measured with single sine waves. We adjusted the gains on subwoofers so that we got 90 dB (+/- 0,5 dB) at 50 Hz - this was our starting point. Then we run the “sweeps” using 5 dB increments up to 105 dB. The program measures the THD level at 10 frequencies between 100 Hz - 16 Hz. The program interpolates the values in between. Each tone plays around 2.5 seconds, so the whole ”sweep” takes around 25 s. Notice that the absolute SPL is correct only at 50 Hz. For other frequencies one must look at the power compression sweeps and find the correct SPL.
THD levels should be as low as possible over the whole frequency range. THD at low frequencies (below 40-50 Hz) isn’t as harmful or audible as THD at high frequencies (above 40-50 Hz).
