Saturday, February 14, 2015

Acoustat vs Sub Impulse Response
Above is the graph made by Tact when I was time aligning an Acoustat panel with an SVS sub.

Notably they both initially seem to respond out-of-polarity initially, and the sub for a longer period.  But taken as a whole, the impulse has correct polarity in both cases.  I have in other ways verified that the Acoustat polarity is correct, such as with an old "SoftPolarityTest" signal, which I can view on Android oscilloscope.  The sum total of an audio signal is correct, but a tiny leading transient (exaggerated in the Tact HF measurement shown above) is out-of-polarity.

Today I added one more verification, using the Android PolarityChecker app.  I copied the mp3 files from the phone storage (where the App puts them) onto my Mac, and then updated my Sonos libraries. Then I was playing the polarity test in living room and bedroom.  Both systems show correct polarity, and the Acoustats individually.  This is clearest full range, where I get the green signal more than 4 times out of 5.  With the 4kHz test, about 1/3 of the time it shows incorrect polarity.  I believe this is because, as the plot above suggests, there is significant phase shift, sufficient to cause an effective out-of-polarity initial transient at the higher frequencies, or at least look that way to a test comparator a significant fraction of the time.  The acoustic transient response shown above suggests serious phase shift above 10kHz, perhaps as low as 3kHz.  But this is also where audibility of phase shift is lowest.

Likewise the sub has phase shift in it's initial response due to roll off above 300 Hz in the speaker itself, and 80 Hz in the very steep crossover.  That could explain it's long initial out-of-polarity component.

My guess for now is that the Tact image is roughly correct.  But it has also occurred to me that the picture shown might be of the Tact transient itself, which might have a significant leading out-of-polarity component.  That was why I wanted to resurrect my old LAUD 3 measurement tool, which computes a very clean transient response (from a MLS signal).  But so far that effort has been hampered by the old computer with ISA bus that it requires for the vintage DSP card from Turtle Beach.  Some time ago the Fiji card was removed, perhaps it had issues.  I tried installing a spare Pinnacle card, but the floppy drive wasn't reading the drivers.  I later found I had disabled the floppy drive because it was stuck and preventing the ATX power supply from ever turning off.  So it needs a new floppy, or I could burn the drivers to a CDROM, which I may do soon.

No comments:

Post a Comment