Very interesting test.. 20 Hertz signal, driven to Bassmaxx ZR18s, SPL of 110dB as measured on SPL meter. Behringer ECM8000 omni capsule mic measuring the sound, fed to Behringer MX1804X mixer, output of which drives Tektronix AA501 Distortion Analyzer and HP 3580A Audio Spectrum Analyzer.
Not taking into account hum and noise, the distortion reading (unweighted) is 0.4%. On the spectrum analyzer, with the test signal stopped, there is 60Hz hum at -60dB, accounting for at least 0.1% of the total distortion reading. Of course there are many other factors, including room boundaries vibrating and re-radiating their own distortion, so this test is not completely accurate and probably contains much noise which adds to the THD % figure. I suspect as such, because distortion seems to go down as I increase level, up to a point.
The measurement mic has a distortion rating of 1% at full input at mid frequencies, so it's probably higher at very low frequencies and only kept down because the level at which the test is conducted is below the mic's max SPL. 110dB seemed to be the sweet spot between noise floor and room/mic distortion. It is safe to say that any distortion produced by the subwoofer is way below 1%.
If I could test in an anechoic chamber with a mic that has zero distortion, I could get a better picture of what the ZR18s are doing, but even with the distortion introduced by the environment and test setup, the figures look pretty darned good for a subwoofer.
Even with the sound system shut off, there is still a big spike at 60Hz on the spectrum coming into the distortion analyzer. No doubt that is contributing to the reading as it shows up as a third harmonic. Ground loops place a lower limit on THD tests because the noise is counted as distortion.
Not taking into account hum and noise, the distortion reading (unweighted) is 0.4%. On the spectrum analyzer, with the test signal stopped, there is 60Hz hum at -60dB, accounting for at least 0.1% of the total distortion reading. Of course there are many other factors, including room boundaries vibrating and re-radiating their own distortion, so this test is not completely accurate and probably contains much noise which adds to the THD % figure. I suspect as such, because distortion seems to go down as I increase level, up to a point.
The measurement mic has a distortion rating of 1% at full input at mid frequencies, so it's probably higher at very low frequencies and only kept down because the level at which the test is conducted is below the mic's max SPL. 110dB seemed to be the sweet spot between noise floor and room/mic distortion. It is safe to say that any distortion produced by the subwoofer is way below 1%.
If I could test in an anechoic chamber with a mic that has zero distortion, I could get a better picture of what the ZR18s are doing, but even with the distortion introduced by the environment and test setup, the figures look pretty darned good for a subwoofer.
Even with the sound system shut off, there is still a big spike at 60Hz on the spectrum coming into the distortion analyzer. No doubt that is contributing to the reading as it shows up as a third harmonic. Ground loops place a lower limit on THD tests because the noise is counted as distortion.