Have 2 Fi Q18s - can I use them for PA subs?

ebr

Member
2010-12-05 1:49 am
New here and have done some searching but can't find any specific answers and only one hit on my specific driver. So, if my question is obvious, I apoligize and ask that you simply point me in the right direction. Thanks.

I built a custom sub "wall" for my dedicated theater in my last house with 4 Fi Q18s. When I sold the house I took two of the drivers out and brought them with me. I am running sound now for a local amatuer band and we need some subs for kick and bass reinforcement. Can I use these drivers or am I better off with something else? If I can, what kind of design should I be looking at? We don't play huge venues obviously but we need something to give us that punch. Needs to be somewhat portable and easy to build :).

Suggestions?
 

ebr

Member
2010-12-05 1:49 am
Thanks for the reply. Fi, I think, specializes actually in car woofers but I did have these made for my HT install. I found the specs from when they made them:

Re 2.8 Ohms
Fs 22.0 Hz
Qms 5.44
Qes .56
Qts .51
Mms 345
Cms 1.52X10^-4 m/N
Vas 310.2 l
SD 1210 cm^2
Bl 15.41
SPL 89.6 1W/1m

What kind of specs should I be looking for?
 

18Hurts

Member
2010-11-21 11:20 pm
You could use them,

It will take a LOT of power to get high sound pressure levels though. Generally speaking, PA speakers have very light weight cones and a shorter stroke since they tend to cut off at 35 to 45 Hz. The light weight gives much higher sound pressure levels in the "music band" or around 40 Hz.

Back in my PA days, I ran Cerwin Vega W15P woofers that had a sensitivity when tuned to 40Hz (SPL 1 watt/1 meter) of 100 dB. They would punch out over 120 dB with the rest of the box at 100 to 200 watts with ease.

If you slapped those Fi subs in a sealed box (what the numbers indicate you should do...look at the Qts of .51) it will be a HUGE box and you'll need 1000 watts to get 120 dB. Run two of them and that is 2,000 watts and blown breakers all around.

You could model them ported at 30 to 35 Hz and see what you get or maybe they can be horn loaded? The horn loading will give you at least a 8 dB boost in sensitivity so that might be your best bet. They will be BIG though... maybe if you tuned them for 35Hz to get the horn loaded box down to a manageable size?

Since the Fi subs seem to prefer sealed boxes--would the "scoop" style subwoofer design be his best bet? There is a solution out there--it might be rather large/heavy but something should work.
 

HaoDaMao

Member
2009-08-23 7:36 pm
what's wrong with the Fis for PA bass use? don't you just need to move air? It seems like the Fi's would have a higher ability to move air than a PA woofer.

PA "punch" is about 80-160 Hz output. PA subs rarely dip below 50 Hz; it's just not used in live sound. So you want a sub capable of a LOT of output in what most car guys call the midbass area - the impact of a kick drum frequency range.

To get a good amount of stroke, you need high efficiency or LOTS of power. Lots of power is an option if you have dedicated wiring; otherwise you're pretty much limited to ~1500W (in the US) for your subwoofers (one, typical 15A circuit), and a low efficiency 18" with 1500W will simply not have the stroke or output in the 100-200 Hz range than a high efficiency subwoofer will.

The Qs, like most car and home audio subwoofers, excel at the first octave (20-40 Hz) stuff that simply doesn't matter for prosound use.
 

ebr

Member
2010-12-05 1:49 am
Yeah, these things were designed for an IB or large sealed enclosure, so I understand the mismatch. I imagine the size of the box and amount of power needed for them will make them unmanageble. I'll keep them for a better use. Thanks everyone for the input.