Need Help for DIY Sub (Dayton Audio HO Reference 18") - Page 2

i zero out the inductance (le) because winisd does not do an accurate job of modelling that parameter.

others may have different views, but in my estimation no need to overcomplicate minor details, so:

1. no adjustment necessary
2. yes, but you can wing it by assuming the whole driver is about 0.2 cubic feet for a 15"
3. calculating the precise volume of the bracing is not necessary either. just try to get reasonably close.

"1.When we add driver displacement (Vd) into the net volume, do we use 2-way Xmax mutiplied by Sd?"

there is no need to adjust the cabinet volume based on how much the driver will move back and forth.

when "driver displacement" is used in the context of adjusting the volume of the enclosure, it is the volume of the magnet, basket, and so forth...not how much displacement in terms of sd and xmax.

i can see how that could be confusing as the same terminology is used to mean two different things.

you do not

eggcrate foam won't do anything in a subwoofer anyway.

u don't need to worry about it really . heavily stuffed lightly stuffed lined with foam are all so close to the same when measured its within the realm of being errors in testing. i wouldn't recommend lining a sub with any foam though it just makes the box smaller less air and u lose low end. i would stuff lightly with polyfill as most do.

It will damp internal reflections of above bandwidth harmonics. Do you have panel to panel distances of at least 1/4 wavelength within the speaker passband? If not then there are no standing waves of major consequence, though some in the midbass above the passband could be excited by the harmonic content in the signal if the low pass filter lacks adequate slope to filter them out. Even if there are no harmonics in the signal the driver will create them, but a couple of inches of foam, polyester battting or type 700 fiberglass will take care of that. I'm afraid that none of that is true.

The highest one. 1/4 wavelength at 80Hz is 3.5 feet, at 20Hz 14 feet. You can see why with modern subs standing waves tend to be moot. They will arise in bookshelves, but in the midrange frequencies where they do they're easily damped. They can be an issue in full range towers, so with those care must be taken to use adequate damping at the top and bottom if they're not sealed stuffed enclosures.

"It will damp internal reflections of above bandwidth harmonics."

there is no practical benefit to that.

If you don't think so then by all means leave your subs empty. I've measured the practical benefits, so I'll continue to recommend it.

It is. I'd use four inches of foam or glass on the panels at either end of that span.

Bill,
To my understanding, standing waves in a closed box are integer 1/2 wavelength multiples... making 80Hz a 7' half wavelength that won't fit in 50" rendering braveheart's concerns moot.
http://en.wikipedia.org/wiki/File:OpenCylinderResonance.svg.

1/4 wave odd-integer multiples apply with one end open and the other closed - a Coke bottle. I don't see a port changing this...
http://en.wikipedia.org/wiki/File:ClosedCylinderResonance.svg

I know of no downside to (sanely) lining the interior of a ported sub, and Bill's suggestion for end damping fits this scenario. The fact is that subs produce far higher levels of non-linear distortion than we'd ever accept in the vocal range, for example. Ricci's data doesn't include a vented box (used a passive radiator design instead) so his vented data is optimistic... but you can see why Bill can measure a benefit!

But I don't see a standing wave scenario here.

Have fun,
Frank

But it is 1/2 wavelength at 160Hz, and there's plenty of second harmonic content in subwoofer output. True you don't get the 1/4 wavelength resonance of an open ended pipe, but still once you get to where there are 1/4 wavelength pathways panel to panel you're best off to be sure there's sufficient damping to take care of what may arise, because even if it's a third harmonic that can be pretty significant.

You can over-damp a cab, especially with thick foam, and with a ported cab you have to be careful not to have any damping near the port entrance, as that will throw the tuning off. I've seen guys use memory foam and that will kill the midbass big time, it's just too dense.
OTOH it could have been that the box was just wayyy too big for that driver. I didn't model it myself, but Alpine recommends 1.5 cu ft.
Edited by Bill Fitzmaurice - 4/16/13 at 4:34pm

"If you don't think so then by all means leave your subs empty. I've measured the practical benefits, so I'll continue to recommend it."

of egg crate foam lining in a subwoofer?

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.

HAve fun,
Frank