Bass roll off vs box size

tezza10

Member
2012-10-24 3:50 am
I have 4 x 15" woofers that I had wanted to use in a large pair of speakers, and was hoping for a max output of around 130dB ( Tri-amped). After using Jeff Bagley's modelling software it appears that even though these drivers are rated at 250wrms they are limited by xmax issues being fed just 125 watts. The problem occurs at 50-60Hz. It would appear that I'm better off halving the size of the cabinet (490 litres down to 260 litres for 2 drive units wired in paralell) to deliberately lessen the lower roll off and use a sub. Are there any adverse effects in doing this? It's not practical for me to ditch these drivers in favour of expensive JBL's or similar. I just need the best outcome from these. Help appreciated!
Drive unit specs:
Fs 47
Re 5.5
Qms 3.51
Qes 0.65
QTS 0.55
Vas 141
mms 81
cms 0.14
BL 14.3
Le 0.42
xmax 4.1
nO 2.6%
sd 855
 
Vented systems have an increased output capability around the tuning frequency. Unfortunately the bandwidth of the improvement is rather small. Above the tuning frequency the maximum spl is some dBs reduced and below the tuning frequency a real disaster takes place.

A maximum spl simulation of your Jamo R909 clones (single driver in 245 litres):

MaxSPLJamoClone.jpg

Of course you can add subwoofers to your system. However, having four 15" drivers and needing additional subs is somehow absurd. Please imagine what size of drivers you need in the subs to achieve an spl of 130 dB down to 30 Hz.
 

tezza10

Member
2012-10-24 3:50 am
Vented systems have an increased output capability around the tuning frequency. Unfortunately the bandwidth of the improvement is rather small. Above the tuning frequency the maximum spl is some dBs reduced and below the tuning frequency a real disaster takes place.

A maximum spl simulation of your Jamo R909 clones (single driver in 245 litres):

View attachment 337451

Of course you can add subwoofers to your system. However, having four 15" drivers and needing additional subs is somehow absurd. Please imagine what size of drivers you need in the subs to achieve an spl of 130 dB down to 30 Hz.

This is the very point I'm struggling with. 4 x15" and I still need subs! I may have to settle for less than 120dB spl in order to retain low end (without troubling excursion limits). I want a powerful kick drum and chest thump, and I'm getting somewhat dispondent trying to find ways to achieve that with these drivers. They may have to go afterall!
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the difference between 125w & 250w is 3 dB, in a domestic situation, is it going to make that much of a difference? I can p off the neighbours with only 20w.
However, the idea of 2x15" is appealing - to me the only practical way to do this without having fridge size boxes is to use them with a Linkwitz Transform in sealed boxes
http://sound.westhost.com/linkwitz-transform.htm
 
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How about a dual chamber reflex or tuning the 15's to different fundamentals (each 15" would require its own subvolume for this) to make up some of the slack, or even both? The dual chamber approach may even let you increase enclosure volume for more total output at the paired BR tuning points.
 
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Placing the four 15"s arround the room in seperate closed boxes may get you to 120 dB (130 dB would actually be dangerous) if there are some modes below 50 Hz to help out. You have to drive the subs seperately and EQ the final result in-situ, but four 15"s should be able to do this unless they are very wimpy power handling and excursion. You can get a lot of gain from a few modes, but doing things this way requires EQ or the whole thing will sound boomy - because it is!

No "box design" is going to circumvent your problem however. Only matching the subs to the room with measurements and EQ are going to achive something like what you are looking for.

Don't rely too much on "free field" simulations because in the room all of that is meanigless. Its how the subs work in the room at the right locations that is the key. Just go with closed boxes, lots of power and EQ. This WILL get you great bass at some SPL level and below and I think that you will find that a 120 dB target might be more than you need.
 
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