file FSP-MAGICBOX

12 years 2 months ago #3685 by deadbeat
Replied by deadbeat on topic FSP-MAGICBOX
You know, if we make the port long enough, it would exhibit quarter-wave resonator effects...which could be a bonus depending on how you look at it (would throw off the tuning, but if we modeled it as a tline...)

So, double or single?
Single 18?
Double 15/12?
Or even Double 18?
My favourite is the one in the middle.

I should have been clearer with the specs - flat as high as humanly possible, yet ideally crossed over at 120-50hz (numbers pulled out of my hat).

Edited by: Deadbeat

[url=\"http://users.ece.gatech.edu/~mleach/beraneklaw.html\"]Beranek\'s law[/url]
\'bits of ply round a driver\'

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12 years 2 months ago #3689 by jake_fielder
Replied by jake_fielder on topic FSP-MAGICBOX
You stated that you want high efficiency, how high? 103/104? that could be doable, but what would the power handling of the cab be like? should we say it has to reach X watts with out over excursion, or do we want to aim for an overall dB value from one cab @ max power?

(did u miss a "1" off the "120/150hz" bit?)

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12 years 2 months ago #3693 by deadbeat
Replied by deadbeat on topic FSP-MAGICBOX
I think an overall dB value (without overexcursion of course) would be great, and that that sensitivity sounds right (though it would matter whether we're doing a double or a single...)

I guess you could look at it being 120/50hz.

[url=\"http://users.ece.gatech.edu/~mleach/beraneklaw.html\"]Beranek\'s law[/url]
\'bits of ply round a driver\'

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12 years 2 months ago #3694 by jsg
Replied by jsg on topic FSP-MAGICBOX
Wow, this thread is really picking up.

As for throat porting, think of it like this:

Bass horns generally exhibit resonant pipe behaviour at low frequencies. This is because the impedance match between the horn mouth and airload degrades, permitting reflections. This effect is incorporated in most bass horn designs already - theresponse peakresulting from the quarter-wave resonance is designed to occur just as the horn gain is rolling off and acts to extend the flat response down a bit (with a steeper rolloff below).

One can liken such a bass horn to a fourth-order bandpass in its low-end behaviour - it only differs above about the half-wave frequency. The so-called "bandpass horn" just pulls down the resonant frequency by adding additional air volume at the throat.

A typical ported horn that is ported to the outside can be likened to a parallel or type A 6th order bandpass. With the port being like the lower tuned chanber and the horn like the upper tuned chamber.

A throat-ported horn simply resembles the series-tuned (type B) 6th-order bandpass. The benefit is that the resonant-pipe behaviour of the horn provides additional gain over what the port produces alone. The disadvantage is that some of the higher frequencies will tend to disappear through the ports back into the cab instead of coming through the horn. So you get more output at the low end in exchange for less output higher up.

I also believe it should be easier to get port output in phase over a greater range of frequencies. This is because standard ported reflex theory applies when the port output is adjacent to the driver. If ported to the outside, you have to compensate for the horn path delay.



Ars est celare artem

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12 years 2 months ago #3695 by jsg
Replied by jsg on topic FSP-MAGICBOX
I'll wade in on the two-driver question:


2 drivers need twice the volume in ported chambers and twice the horn throat area (hence twice the volume in the horn). So the cabinet gets to be twice as big. But it will be 3dB more efficient, so you *may* be able to claw back some volume by reducing horn gain, port resonant gain etc.

Don't worry about the port dimensions at this stage. The design should confer a resonant frequency for the port, and max power output considerations should confer a minimum cross-sectional area. The length should be found by experimental tuning on a prototype.
Edited by: jsg

Ars est celare artem

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12 years 2 months ago #3698 by deadbeat
Replied by deadbeat on topic FSP-MAGICBOX
Thanks for the reply jsg, this thread may actually bear fruit!

On horns as BP, just a side note but I like to think of normal horns as 4th order BP, with tapped horns (or throat ported horns) as 6th order series (replace all the chambers and ports)...

[url=\"http://users.ece.gatech.edu/~mleach/beraneklaw.html\"]Beranek\'s law[/url]
\'bits of ply round a driver\'

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12 years 2 months ago #3701 by strapping young stu
Replied by strapping young stu on topic FSP-MAGICBOX
Looking at this IMO any horn is out.

Straight horn - too big
BP or Tapped Horn or Scoop - wont go high enough
Bandpass - as above
Folded Horn - wont go high or low enough without being massive
Ported BP or Folded Horn - As above

So it leads me to this

http://www.voidaudio.com/news/news_item.asp?NewsID=35

Whatever the resonant chamber technology is it obviously has the effect of extending the low end of a reflex, while this might not be the most efficient setup in the world it fulfills the initial criteria of

36-300Hz -3dB

Without honk or colouration.

StuEdited by: Strapping Young Stu

Power = Versatility x Intelligence

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12 years 2 months ago #3714 by wafflesomd
Replied by wafflesomd on topic FSP-MAGICBOX
If you can make this a single 18, I'll be happy to build it.

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12 years 2 months ago #3717 by deadbeat
Replied by deadbeat on topic FSP-MAGICBOX
Still trying to be positive and have made max size:

(inches) 24x48x(32-6)

should try to keep it under that, but it makes me a bit happier.

HF cutoff changed to around 130hz.

djk's tapped horn.
30hz 2pi 2cabs.
http://www.speakerplans.com/forum/forum_posts.asp?TID=18434&KW=tapped+horn&PID=185613#185613

Edited by: Deadbeat

[url=\"http://users.ece.gatech.edu/~mleach/beraneklaw.html\"]Beranek\'s law[/url]
\'bits of ply round a driver\'

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12 years 2 months ago #3722 by jake_fielder
Replied by jake_fielder on topic FSP-MAGICBOX
OK -

103/104dB efficient (although the more the merrier!)
30/35Hz to ~130Hz
61cm x 122cm x 81.3cm

Do ya know... If I knocked 20litres off my ported horn design then it would be exactly half the size you specify, (its probably quite doable, just cut 5.5 cm off the front) and it could well be efficient enough and maybe (with a more hardcore driver) drop low enough.....

Im off to do some modeling

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