TOO SIMPLE ?? Oval port area = effective cone area

[bluesfendermanblues]

I'm building a couple of cabinets for my ODS clone, and have been tryin to figure out, how HAD have arrived at the port sizes on his Combo, and 2x12 cabinets.......

I found a combo cabinet design in the files section, stating a port size of
6" x 13" (15.4 cm x 33 cm)
AND a small "port" between the amp chassis and the back baffle og 0.5"x 19.5" (1.2cm x 49 cm)

This gives an area of (the oval port + the small chassis "port") 80 inch2 (or 516 cm2)

SO FAR SO GOOD

We all know that a 12" speaker is - suppriiiiiiiiise 12" - which gives a cone area of 113 inch2

This gives a ratio of 80/113 = 0.7

We all know that HAD is a genius (not kidding), so I figured that this ratio was probably the "golden angle" of speaker designs, untill............

NOT, if you calculate the EFFECTIVE cone area of a 12" speaker , which is typically 10" (you can find this info for most speakers units, e.g. all Eminence speakers, Or measure the speaker cone for yourself)......Guess what?? the cone area of 10" is 78.6, which gives a port/speaker area ratio of 80/78.6 = 0.98 ALMOST IDENTICAL AREA SIZE!

So I ask you - could this solve the puzzle????????

[jaysg]

There are other threads quoting 9" x 11" or 11.5" for the real oval on a 1-12". Does that change anything? Just wondering, as I'm ponding an HD run. I want to change the rear panel on my ancient 2-12" for an oval...gotta keep up with the kids.

[briane]

hey jaysg,

I did some experimenting with this and found it did matter...Though I only did a 1/2 oval on an open back cab (a couple of inch slot above the back panel). I found the difference substantial. Stronger bass response, and a hair more articulate. Its like the frequency response is slightly wider, and a hair cleaner.

This is a technology that has been around for a long time. You can look up Karlson cabinets. It was a frequency response 'extenuator' or 'resonator' if thats the right word. In a way its really just a technology from the 30's,50's, whatever, that HAD readapted.

Its a subtle difference, but to me very noticable. The half oval is so easy to make, and so wonderful, I dont think I would use any other back shape on a new cabinet myself, aside from the leap to full ovall. I imagine with a properly sized box, a proper front port tube(which I still have not done), and the oval back opening would be the penultimate of design.

Of course, in my mind HAD reversed the design, from an edge cut to an inner oval, and back mount rather than front mount, but I think there are a lot of parallells in the technology (though I am sure a few differences).

check it out:
http://home.planet.nl/~ulfman/theory.htm

and I just came across a karlson newsgroup here(but have not read much there):
http://gainclone.com/Karlson/

[bluesfendermanblues]

Hi Brinae and JaySG,

What is you opinion on the port size - is it by any chance related to the effective size of the used speaker unit(s)?

[briane]

Well like I say there are a few criticals, and essentially it is based on the relationship between (in my meager mind)...
-box volume & shape - this includes where and orientation of the amp in the combo.
-speaker size(and possibly response). note box above is based on this. also where speaker is mounted relative to box
-back side porting. the oval shape induces a (yet unknown strength) karlson response in my view
-front porting. I understand LC has one with a front tube port, which to most audiophiles is considered superior to just a simple front port hole.

I think jaysg is on it with what we know was done by HAD. For best results you might even want to copy shape, dimensions, and amp mounting as the master did it. I dont think anyone has any info on the front tube porting, but theres lots of info to work with.

Personally I have been meaning to try the front tube port with a 4" or so sonotube, somewherearound 8-10" long, but note in the front tube, how deep it extends into the cab is also critical. I'm not sure of the exact math on these, though Ive seen the equations on other sites. It seems simple enough that it might be faster to make some good guesses, then farm the rest out to your ears.

It would be really interesting is your math did indeed prove the associated volumes and frequency response relationships, but my feeling is theres also a lot to the shaping of the edge the the audio signal trails over, and how that affects a humans percieved frequency response of a given box.

anyays, so much for the mumbo jumbo. Maybe someone who has a full oval will chime in on before and after, or maybe Ill get motivated and fire up the saw and make a second back for one of the combos....

[skyboltone]

I horsed around with this a couple of years ago and posted my theory on here. Some engineer fellers hurt my feelings about it so I shudup. (just kidding) If you take Vas, which is expressed as cubic feet, convert it to cubic inches, then multiply by .55 and just arbitrarily call them square inches it turns out to be the port size of the speaker cab he started useing in the '70's with an Altec 417-C. The reason this is interesting is that Vas changes from speaker to speaker and is arguably the most important variable in cabinet and port size when using Thiel-Small parameters in design.

As before, no science, nothing empirical, not supportable, totally crazy, ignorant, it's just an observation. Can't possibly mean anything.

[Aurora]

If one tries to apply the calculations normally associated with loudspeakers, nothing fits!
The only resemblance a cab like this can have, is to a reflex design - and none of the well established theory fits.....
One of the reasons is that the typicall guitar speakers have very high Qts parameters, which fits badly in a reflex design....
The closest one gets, is to an OB- open baffle- design..which traditionally was done by " touch and feel"....
Obviously the size of the opening have some effect, but based on the available formulae and theory, it is mostly unpredictable.

[Aurora]

If one tries to apply the calculations normally associated with loudspeakers, nothing fits!
The only resemblance a cab like this can have, is to a reflex design - and none of the well established theory fits.....
One of the reasons is that the typicall guitar speakers have very high Qts parameters, which fits badly in a reflex design....
The closest one gets, is to an OB- open baffle- design..which traditionally was done by " touch and feel"....
Obviously the size of the opening have some effect, but based on the available formulae and theory, it is mostly unpredictable.

[bluesfendermanblues]

Hmm interesting.

I know a little bit, very basic, about HiFi speaker design.

The two cabinet designs that we have seen with D-style amps is

1. the Larry Carlton, "Bass Refleks" which is identical to the EV design pages. Only the LC cabinet uses two round port (like 1000's of HiFi speaker cabinets) instead of a square hole like Mesa's Thiele cab or EV's original dravings.

2 the Oval port design resembles the "infinite baffle" design and is a little more anvanced. We already know this design from the ordinary Fender combo design, but Dumble chose a design with a reduced size of the rear port. The oval shape, as opposed to a square hole, is a natural shape choise, because its secures the farest distance to the speaker unit front.

What puzzles me is the relation between
speaker unit used - cabinet size - and port size.

I can see that your guys have already been there, som perhaps its just trial and error that made HAD arrive at the design. Hmm

[Aurora]

The EV bass reflex cab most probably follows general design factors for reflex cabs, and its response is therefore predictable. Design parameters for EV units are also available.

As for most of the old Celestion units used, there are almost no T/S parameters available, except for Fs and Vas. A cab like this should normally be seen either as an open baffle design ( with a fairly long front-back distance), or a sort of bass reflex. If it is designed as a reflex cab, the shape of the port have little influence, besides maybe looking more interesting, - only port length/ area matters. I actually tried simulating it after your first post, with the few parameters I could find, but the results were totally inconsistent. If one looks at all the other open back guitar cabs. combos in particular, there's no way to treat them as reflex cabs. While the rather smaller oval hole ( as compared to regular open back) most probably changes the loading of the element, so far I'd argue it's purely empirical, - or based on hearsay. However, there is a small chance it is based on the vintage design formulae used for reflex cabs in the 50's, - before T/S ( Thiele-Small) parameters became the established route. -but I'll have to dig out my antiques library to check upon this......

[bluesfendermanblues]

The EV bass reflex cab most probably follows general design factors for reflex cabs, and its response is therefore predictable. Design parameters for EV units are also available.

As for most of the old Celestion units used, there are almost no T/S parameters available, except for Fs and Vas. A cab like this should normally be seen either as an open baffle design ( with a fairly long front-back distance), or a sort of bass reflex. If it is designed as a reflex cab, the shape of the port have little influence, besides maybe looking more interesting, - only port length/ area matters. I actually tried simulating it after your first post, with the few parameters I could find, but the results were totally inconsistent. If one looks at all the other open back guitar cabs. combos in particular, there's no way to treat them as reflex cabs. While the rather smaller oval hole ( as compared to regular open back) most probably changes the loading of the element, so far I'd argue it's purely empirical, - or based on hearsay. However, there is a small chance it is based on the vintage design formulae used for reflex cabs in the 50's, - before T/S ( Thiele-Small) parameters became the established route. -but I'll have to dig out my antiques library to check upon this......

Which program do you use to simulate?
I don't think we should perceive the oval port cabs as bass refleks cab. I think the hole in tha back serves to adjust how freely the speaker unit is allowed to move - and there producing more bass than a simular sized, but fully closed cab. What do think about that?