A8 port size

[aeyb701]

I've searched the archive but have found little about port size for the A8. The Altec specs call for a 1-inch by 20-or-whatever inches across sized slit, which is what I have now, but is that optimal? The reason I ask is that the consensus on the A7 port was that 75 square inches is optimal even though the official Altec port was much larger.
Does the port size-should-equal-radiating area of woofer apply only to horn loaded designs or is it applicable to BR as well?
If I have a 416-8B then that should be roughly 150 sq. inches, but at present the port is about 20 sq. in.
For sake of argument the A8's volume is 7.5 cu ft.
Basically I'm asking what is the optimal port size to give decent low bass in the 40Hz-or so region?
Thanks,
Jon Archibald

[LICORNE]

I have plans of A-8 with 2 5-1/4 " Dia hole.

I don't know how to translate this in square inch.

According to 1968 chart it should be 35 sq inch for a 7 cubic ft. cab at 25 HZ.
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<font color="#FFFFFF" size="1">[ November 11, 2007 04:55 PM: Message edited by: CONVERGENCE ]</font>

[bfish]

WinISD "optimal" 416-8B default is a 7.932cu/ft box tuned to 36.47Hz by a 3.23"x4.02" rect. vent in a 0.6" panel. Going to 7.5cu/ft thickens the slot to 0.72".

BR is actually where the 'vent area = Sd' thing came from. Worked pretty good till T/S found a better way.

GM's done some better sims on these if you do some searching.

[Ronald Lee]

Originally posted by CONVERGENCE:
I have plans of A-8 with 2 5-1/4 " Dia hole.

I don't know how to translate this in square inch.

According to 1968 chart it should be 35 sq inch for a 7 cubic ft. cab at 25 HZ.
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<font color="#FFFFFF"><font size="1">[ November 11, 2007 04:55 PM: Message edited by: CONVERGENCE ]</font></font>

<font size="2" face="Verdana, Helvetica, sans-serif">For both ports, 43.3 square inches.

[LICORNE]

RKLee

For both ports, 43.3 square inches.

<font size="2" face="Verdana, Helvetica, sans-serif">As per 1968 chart for one port.

http://www.lansingheritage.org/image...ure/page04.jpg

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<font color="#FFFFFF" size="1">[ November 11, 2007 09:18 PM: Message edited by: CONVERGENCE ]</font>

[aeyb701]

Thanks all:

so...from what I read from you all by increasing the slit size from 1x16 inches to 3x 16 inches I'll have an area of 48 sq in. which the '68 chart says is tuned to 40-ish Hz-more useful to me than the sub-audible resonant freq it must be at now.

BTW, BFIsH, the WinIsD said the port size is 4x3 inches in a .6-inch panel...does that mean a 12-sq in hole in 5/8" (or for sake of arg. 3/4")?

If that is what you mean then why such a small hole vs the '68 recommendations?

-Jon

[bfish]

Originally posted by aeyb701:
BTW, BFIsH, the WinIsD said the port size is 4x3 inches in a .6-inch panel...does that mean a 12-sq in hole in 5/8" (or for sake of arg. 3/4")?

If that is what you mean then why such a small hole vs the '68 recommendations?


-Jon

<font size="2" face="Verdana, Helvetica, sans-serif">You're welcome Jon.

Yeh, you have it right, ~12", one for .6" (5/8), and one for .72" (3/4, in 7.5 ft box)). I just posted the default selection, and didn't manipulate the vent area. I intend to try to verify the preloaded 416-8B T/S figures sometime, as something doesn't jive to my limited use.

Just get you some rigid sheet foam or real heavy cardboard and a roll of duct tape and go to town. It's pretty easy to make a temporary little hole from a big one. Just watch the excursion closely while you do. Raising Fs will quickly give you a boomy bump in a bad place, but you might like it. Use the lowest-Hz tone you can to test, once you commit to tuning, anything lower could ruin your drivers at higher levels.

[aeyb701]

Good suggestion Bfish-

I played a 30Hz test CD, as well as the stereophile test Cd and using freq's from 65-hz down to 20 Hz I covered the 1"x16" slot to varying degrees with a small hard-bound volume on diesel locomotive maintenance (I wonder if the subject matters...ha ha) and found that max. excursion occured in all cases, at least below 50 Hz anyway, at the full opening-i.e. 16 sq in. (There was the most drop-off in loudness between 30 and 20 Hz, as one might expect.)

That makes me wonder if increasing the port will allow more excursion (i.e. louder bass at say 30-40 Hz) ?

Thanks,
Jon

[aeyb701]

I think, as you say, I'll leave the port size as is since there is audible output in the 30-40 Hz region- not to mention cutting into an already finished cab.
By the way how big, or small, does a leak have to be to interfere with port tuning?
Thanks for your patience, as audio engineering isn't my strong suit.
-Jon

[bfish]

I should have been clearer. When I said watch the excursion, I meant the amount of travel (something that's really hard to do). When the cone moves beyond 0.13" peak, you get distortion. If it goes past 0.7", you get damage.

As you lower your test tone F, you should notice a point where the amount of excursion decreases, then rapidly increases, indicating you just past the (current) port tuning F. If not, you may have a leaky box. I can't imagine you'd want to enlarge the port any.

<font color="#FFFFFF" size="1">[ November 13, 2007 06:36 PM: Message edited by: bfish ]</font>