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Master_Scythe

Calculating box size

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I'm on a bit of an addiction at the moment; I'm building portable stereo systems.

Why? Started as a hobby and something I could use scrap speakers I find on hard rubbish day for.

Now? After a camping trip with a bunch of mates; and many more to come, I found out how useful these things really are!!!!

 

I started with some 8" PA drivers with 93db sensitivity; but with only 5w power handling they maxed out pretty quick.

Ive since built one out of an old home stereo. Glued the speakers together, and put my Tripath Amp on top; Lasted 3 months between charges (1x 4 hr day a week from a 5.5Ah battery) Pretty happy.

 

So this time, I've decided to go all out and build something truly impressive when I take it out of the car.

 

I already know the front is going to be 'cheesy' aluminium checker plate at the end; but I'm a long way off that yet. Right now I'm designing enclosures; and that's where I need help.

 

So my internal dimensions are somewhere around 200x310x150; taking into account driver displacement, I guess about 0.25ft3.

Using 2" 2 way 91db car speakers with 25w RMS rating.

I have a 3" long, 50mm port on one; tuning this guy to about 84-89hz; and the other speaker is a 2.5" port; making for about a 91-95hz tune.

They're not equal on purpose, because its an outdoor boombox; exact audio symmetry takes a big step back for loud low-mid range.

 

What I'm trying to figure out is whether or not adding another identical speaker to my enclosure changes my calculations (besides the volume lost to the magnet).

 

Do I calculate as if each speaker has a 0.125ft3 of room; or does nothing change?

 

I'm still deciding what I want to add as a second driver; I'm thinking an 8ohm horn tweeter in parallel. Overwork the amp a little, seeing 2.66ohm,

but nicely balancing the power output because horns can be pretty damn harsh.

Wired in parallel, with a 20W Tpath amp.

8ohm horn = 6.66 Watts
4ohm Spkr = 13.33 Watts

 

The other option of course is something like a 3" full range;

http://www.jaycar.com.au/productView.asp?ID=AS3034

But at only 82db sensitivity, and an FS of 100hz, my cabinet tuning might be a bit low at 85ish hz....

 

This is why I need to know if my box calcs are affected.

 

In the end I'll probably just add a pair of 4" car speakers with 90db and call it a day; but I'm curious to know what people think.

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Hmmm

 

If I run two of these;

http://www.ebay.com.au/itm/4x-5-5-20WRMS-PA-DJ-Speaker-Subwoofer-Woofer-Sub-Driver-5-5-Inch-8-Ohms-Quality-/371026762252?pt=AU_Pro_Audio&hash=item5662eb1a0c

 

• Nominal Impedance: 8 Ohms
• Power Handling: 20WRMS, 50W Program Power (peak)
Test Parameter Data (T/S, Thiele/Small):
Rated Impedance: 8
Power Handling: 20WRMS
Program Power (peak): 50W
Nominal Diameter: 5.5 Inch
Sensitivity (1m/1w/2.38v): 97
Frequency Range: 130-13000Hz
At 4ohm in parallel; I'd get +3db for the extra cone, and +3db for a 4ohm load..... That's a nice 103db 1m/1w result there.... just no bottom end..... but outdoors most of it is lost anyway.....
Only problem is the 130hz resonance.... I suppose I could tune the cab to 100Hz, put a bass blocker capacitor in line at about 80Hz and hope for the best.... hmm.
I CAN try both, I'm building these boxes with a frame, and a screw on front and back; so I can remove the face and cut new speaker holes time and time again untill I like it.
Edited by Master_Scythe

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