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Tom Loizeaux
08-05-2003, 07:02 PM
I have a 5.2 cu.ft. cabinet that I'm tuning by adjusting the length of the 26 sq.in. rectangular port. I'm putting in a vintage Gauss 4580 and using a sweep generator to find the frequency where the cone stops moving. What frequency should I tune this cabinet to to best extend the low end of this speaker? I only need to get to 40Hz, since the 4 string electric bass only goes to 40Hz.
The Gauss 4580 specs are: fs 31Hz, Qms 2.3, Qes .26, Qts .23, Vas 7.0

Thanks,

Tom

(P.S. Gauss WAS owned by Altec in it's last form)

Mr. Widget
08-05-2003, 07:22 PM
Since you are making a music producer rather than reproducer, your playing style and taste will determine what is best for you.

I would start with tunings in the 40-45Hz range. That would be a 26 sq in port of 5-3 inches deep.

See if you like that. The 3" port will give you an F3 of about 45Hz. These figures are based on a model assuming the cabinet is lined. In this application you may prefer an unlined box.

After thinking about this a second, you probably should tune it to 40Hz, the 5" port. Since everytime you played your lowest note with a 45Hz tuning the speaker would be unloaded. (Below it's tuning frequency.) Tuning to 40Hz gives you an F3 of around 47 Hz.

boputnam
08-05-2003, 09:10 PM
Mr. Widget beat me too it (and with much greater experience...)

Quoting:

"One further improvement results from deliberately raising the resonant frequency of the enclosure to a point higher than the free-air cone resonance of the speaker. If the driver has a free-air cone resonance of 30Hz, (and if) the enclosure resonance is raised to 45Hz ... the high-frequency (resonance) peak is (resultantly) quite small and the bass response is smooth and efficient all the way to the cutoff point."

For the idealized case discussed, the LF doesn't roll-off until ~20Hz.

from: Badmaieff, A. and Davis, D., 1988, How to build speaker enclosures, p65 (Tuning the Port) and Fig. 4.5

;)

GordonW
08-05-2003, 10:11 PM
Couple of things:

First off, the recommendation of tuning higher than resonance :banghead: It really depends TOTALLY on the Qts of the speaker! So many people put forth these general rules-of-thumb... coincidentally enough, it works in THIS particular case... but I've had LOTS of cases, where the best bass came from tuning BELOW resonance, sometimes WAY below resonance. For an example, my home speakers consist of a 6.5" woofer with an Fs of about 48 Hz, with a port frequency of 27 Hz! And it sounds GREAT, and the bass goes down to the low 30s! However, the Q of the speaker is fairly high (.49)...

Secondly, that's an awfully BIG box for that woofer, unless you're using a tube amplifier (lower damping factor, raises the Q). Usually, even in a ported box, you don't want to go MUCH bigger than the air volume that if sealed, would give you a Qtc (woofer in box) of .5. In this case, that's about 1 3/4 cubic feet. YOu can use a larger box, to extend the rolloff of the speaker (in 1 3/4 ft^3 the f3 is in the mid 60s), but you will have to make sure that it doesn't wanna bottom out.

However, if you wanna proceed with the big box, I'd probably tune it to about 37-38 Hz... that will give a reasonably flat response, with a rolloff of about 39 Hz... should do OK for open-E's...

Regards,
Gordon.

boputnam
08-05-2003, 10:28 PM
"So many people put forth these general rules-of-thumb... coincidentally enough, it works in THIS particular case... That's why it was properly referenced. It's not my "rule of thumb", but the finding of some very thorough and "sound" ;) research that I was recently referred to. :D

Earl K
08-06-2003, 04:46 AM
Hi All

FWIW,,,this is from some mid 1980s' cut sheets, Gauss shows 3 tunings in 5 cu' boxes. The 35 hz (Fb) tuning is a 20 sq" port that is 5" deep. The 45 hz (Fb) tuning is a 25 sq" port that is 2.5" deep. The third was a sealed box of 5 cu ft. My experience with Gauss speakers and tuning software is they don't "tune nicely" on paper, ie they always droop in the mid bass area with a bump around the Fb area. In practice this isn't a bad curve to work with. My guess is that their operational (Ts) parameters were composed almost empirically - certainly not influenced by my software . As such, I'd go with what the manufacturer suggests works. In this case - 5 cu ft - at one of those 2 tunings.

regards <> Earl K

Tom Loizeaux
08-06-2003, 07:06 PM
Thanks guys for the responses and suggestions.
Since the port was already built in, I had to cut sections out in order to shorten it. I didn't want to cut too much away, so I stopped at 5.5", cleaned it up and put the 4580 driver back in. It certainly has a stronger low end then it did with the 10" port that I started with. When I play the open E string (40 Hz) the port opening is blowing serious air!
I suspect I could shorten it a little more and move the cabinet tuning up to pick up from the cone a little sooner, but I'll wait a bit before I get into it again.
This speaker sounds very rich and deep, though lacking a little on the top end compared to my JBL K140s, but it has a nice thick character that does nice things sonically to Fender Basses.
Thanks again,

Tom