Originally Posted by Bill Fitzmaurice
But it is 1/2 wavelength at 160Hz, and there's plenty of second harmonic content in subwoofer output. ....
You're calculating for the 2nd harmonic directly as the onset of HD-driven standing waves. As you said: "The highest one. 1/4 wavelength at 80Hz is 3.5 feet..."
I was thinking along siimilar lines, but discounted 2nd in favor of 3rd, thinking it's easier to fit a higher harmonic. The problem I had, looking at Ricci's distortion data
, was the low level of 3rd harmonic... below 1% by 40Hz in both sealed and tuned alignments and 120Hz still too low. There is no shortage of 2nd harmonic... both alignments are ~5% well past a normal sub LP cut-off.
(Note this is all at a 113dB output level from a TC Sounds LMSR-12, save for the horn, which has less distortion at 6dB higher SPL.)
5% is -30dB, and it's quite possible for a standing wave resonance to build that high... talk about a localizable sub!
Thanks for the inisight, Bill.