Goodmans Axiom 10 reflex cabinets. - diyAudio
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Old 2nd May 2011, 11:26 AM   #1
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Default Goodmans Axiom 10 reflex cabinets.

Hey guys, I finaly found a plan for my widerange drivers.
A set of Goodmans Axiom 10's, sadly I don't know much about speaker design, just enough to know how little I know.

Can you tell me more about this reflex enclosure, like what its resonant points are and how low it goes etc... even if you think its still a good design by today's standards.

I intent to get the original tweeters/horns that came with these drivers too and has slots provided for in the box plan. it comes with a built in crossover and some sort of level control knob.

The enlosure is about 5000cubic inch (82litres), the port dimensions are as follows. 9" high, 7" wide and 4" deep, with a 4"x7" mouth on the side... would that 9" be reduced by the thickness of the face panel I choose?

I want to build the corner one...

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Last edited by digits; 2nd May 2011 at 11:53 AM.
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Old 3rd May 2011, 07:41 AM   #2
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Nothing guys?
As far as I can tell the box should be good for 40Hz, but I'm totaly lost on the port thing.

Just look how cool these ol things can still look today... I mad a picture in sketchup for inspiration...Oh and also to compare to the rectangular box, which looks horrible.
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Old 3rd May 2011, 08:45 AM   #3
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Hi digits,
the resonance frequency of the box is calculated as ( Helmholtz resonance - Wikipedia, the free encyclopedia ):

Fs = (v/2*PI)*sqrt(A/(V*L))

where:
Fs: box resonance frequency
v: speed of sound in air (at 20: 344 m/s)
A= section area of the reflex conduct port (4"x7")
V= box internal net volume ( internal volume - reflex conduct volume - speaker driver volume )

Hope this helps
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Old 3rd May 2011, 05:52 PM   #4
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Hi I'm trying to get my head around this.

A, is this in squared inch?
V is this in cubic inch?
and L is this of the port or the box, and in what units?

I can do the firt part at least....
FS=(344/2PI) * sqrt((4x7)/(5000xL))
FS=54.7493 * sqrt((28)/(V*L))?
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Old 4th May 2011, 10:31 AM   #5
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Hi,
all the units in the formula should be coherent, i.e. all expressed in inch or centimeters (or meters).

So, converting all units in centimeters (squared for areas, cubic for volumes):

V (box internal net volume) = Vbox - Vconduct
where, approximately (by defect, considering the box a triangular prism to semplify calculation (*) ):
Vbox = 1/2*B*H*L
B (triangle base)=18"=45.7cm
L (prism length, ie box height)=30"=76.2cm
H (triangle height)=45.4cm
Vbox=1/2 * 45.7 * 76.2 * 45.4 = 79050 cm^3
Vconduct = Throat area (4"*7") * Conduct length (9")= 180.7 cm^2 * 22.8 cm = 4130 cm^3
V = 79050 - 4130 = 74920 cm^3
In theory we should subtract also the volume of the speaker driver but I won't do it (having already approximated by defect the box volume).

Now to calculate Fs, we need to convert v in cm/s for dimensional coherence:
v = 344 m/s = 34400 cm/s

Fs = (v/2*PI)*sqrt(A/(V*L))
Recapitulating:
A (conduct throat area) = 4" * 7" = 180.7 cm^2
V (box net volume) = 74920 cm^3
L (conduct length) = 9" = 22.8 cm

Fs = 5477 * sqrt(180.7/(74920*22.8)) = 5477 * sqrt(0.000105785) = 5477 * 0.10285= 56Hz

Please, remind that this is only theory, in practice to calculate the resonance frequency of a bass reflex tube is normally used a correction factor for the tube length.
Furthermore, I don't know if in this case it's correct to consider the length of the conduct as 9", from the throat (the top of the conduct) to the mouth bottom, or if it's more correct to consider it 7" (from the throat to the mouth middle).
Moreover, keep in mind that I'm a novice in acustic, so it's possible that I've done some misassumption. It should be kind if someone could confirm what I wrote is fine.

(*) whereas in reality it should be seen as a composition of two scalene and one isosceles triangular prism, or three isosceles triangular prism plus a parallellepiped
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Old 4th May 2011, 05:36 PM   #6
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Oh man you are awesome!
I used sketchup to model the internal space and its something like 5002 Cubic inch...or (81.991654.53 mm3) according to the entity properties... but otherwise I think I can work the stuff out now.

Just one question remains, the port is a 9" long tube with a SIDE opening at the bottom, which is 4 inches of the tube's lenght, would it thus make sense to say the port is 5 inches long?
And lets say I add a 2" face to the box, should I take the 2" off the top of the port then to keep the original lenght?

Last edited by digits; 4th May 2011 at 05:47 PM.
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Old 4th May 2011, 07:05 PM   #7
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Ok, with the correct box volume (actualy still need to deduct the port panels) it comes to 54Hz, useing the same port lenght, and I see that the longer the port gets, the lower the frequency gets.

Now is this point the F3 point or the point at which the box will resist any lower frequencies..?

I 'd have imagined such a large box would go lower.
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Old 4th May 2011, 11:00 PM   #8
GM is offline GM  United States
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Back then it was common to tune a cab between Fs and a 1/2 octave away with a 1/4 octave being popular with some, which is close enough to what this one is based on a 45 Hz Fs. This tuning helps ensure that there's adequate acoustic [box] protection right above Fs.

Fb = F6, though the box doesn't resist a lower signal, it lets it through unabated, rapidly over-excursing the driver to destruction if there's no electrical means to protect it. Nifty animation of driver, vent action: Essay

Back then though, there were precious few recordings with enough content below 45-50 Hz to cause any problems except for TT 'rumble', so if you had a good quality TT, then typically you didn't need a 'rumble' filter.

Still, some manufacturers such as Altec, JBL used drivers with a much lower Fs with cabs tuned accordingly to ensure no such problems, so when the digital age came along they continued to do just fine except for movie soundtracks with < 20-25 Hz LFE content.

GM
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Old 20th May 2012, 09:15 PM   #9
toxaco is offline toxaco  United Kingdom
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Hello,

greetings, just wondering if anyone knows the value of the attenuator used with the trebax tweeters? I've got one but no attenuator.

thanks
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