fb below f3? - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-16-12, 03:44 PM Thread Starter
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rob

Join Date: Feb 2010
Location: ephrata pa
Posts: 8
fb below f3?

hi guys, i had a question, can you have an fb lower than f3? If so how much lower would you want to go?
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Old 01-16-12, 05:07 PM
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Join Date: Nov 2011
Location: St. Louis, MO
Posts: 147
Re: fb below f3?

Per Ray Alden's "Speaker Building 201," the formula for F3 is F3=.26 x Fs/ (Qts)^1.4 , and the formula for the ideal Fb is Fb=.42 x Fs/ (Qts)^0.9. In his example, Fb is slightly higher than Fs, "a typical result of using a woofer with a relatively high Qts." This implies that with a driver of lower Qts, Fb could be lower than Fs, and I think that also implies less peaky response curves, a Qtc closer to .5 - .7.
I think... but now you've got me thinking, so I'll noodle it around some more.

Tom
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Old 01-16-12, 06:33 PM Thread Starter
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rob

Join Date: Feb 2010
Location: ephrata pa
Posts: 8
Re: fb below f3?

cool tom ,thanks, reason i ask is my new pa cabs have 4 triangle shaped "ports" on the front baffle, 3/4" long, and there is a sharp peak at 60hz, about were my box seems to be tuned. I blocked the two lower holes and i have much better low end now, but still that peak. I have in room of 31.5hz. The stated low end is 40hz(no ref given). So i measured the box and used a port calculator, but i was just wondering if i could use a fb of say, 30/35 hz? I will block off the other triangles and replace them with 2 conventional ports.
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Old 01-16-12, 10:49 PM
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Re: fb below f3?

Again, according to Alden: "The vented box is more sensitive to "misalignments" than a closed box. Thus, when using a vented box design, it would be wise to personally measure the parameters of the driver..."

Do you have a WT3, or a way to measure the driver? Sealing ports and redesigning the cab are pretty hit-or-miss without some measurements and planning. Not to say the system was well designed in the first place, since it sounds like you've already made some improvements. And then to add in the room nodes, well, that's just plain complicated.
Are you using WinISD or some modeling software? How big is the cab you measured, how big are the ports, what driver are you using (maybe the parameters are on the web-site)? We can work up some models with some measurements, if you have them.

Tom
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Old 01-17-12, 03:49 PM Thread Starter
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rob

Join Date: Feb 2010
Location: ephrata pa
Posts: 8
Re: fb below f3?

no i do not, it is on my wish list though!! That would make it easy, eh. But the box is gross 5.05ft, say 4.4 loaded. Specs 40hz-19khz, no ref given. I just thought after running some port calculators, that, boy could i make these cabs hit harder, and go a bit lower. 4 triangle ports, two now blocked, 3/4" long. std 2 way pa. the 15" woofers are rated 40-2500khz, 1.75 titanium comp driver. speakers up on 11" blocks,(ie:milk crates) I think a pair of 2" ports will work, but i thought about tuning the box below the woofers f3. with the current 2/4 triangles in the baffle i get a box tuning of around 60hz. let me know tom, thanks!
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Old 01-17-12, 04:37 PM
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Re: fb below f3?

No disrespect intended, and my experience with pro equipment is limited, but that sounds like one overstretched 15" driver trying to go up to 2500hz. Vented designs roll off 24db per octave below F3; if the box is now tuned to 60, you're out-of-gas at 30, and I don't know that that woofer will take much more tuning. The neat thing about vented enclosures is that the woofer doesn't really have to move around the box tuning freq, since the sound is coming from the port. Unfortunately, woofers loose control and definition quickly below Fs, so tuning too low will sound boomy and muddy. It takes a low Qts driver to tune Fb below F3 ( I'm still thinking on it), and I don't think that would lend itself to a high crossover freq. I'm going to look at the specs of some pro woofers and play with some models, but my initial thinking is that you're asking a mid-woofer to go sub, and that won't be pretty. Might be better to add a dedicated sub or bass guitar box rather than ask too much of the original set-up.
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Old 01-17-12, 06:11 PM Thread Starter
Shackster

rob

Join Date: Feb 2010
Location: ephrata pa
Posts: 8
Re: fb below f3?

none taken tom, I would just like the 30-40 range bumped some, (or 35-45) and the peak at at 60 dropped off, we can do that at least, i hope, i want to try anyway! later i can try replacing drivers and such. I know it wont go sub, I have two other cabs that go sub, but while one is getting repaired i was trying out my new cabs. they are a little light, as would be expected, but, i fiddle, and like i said after i found out what the "ports" were, i started measuring and thought i could enhance the lows a bit, not trying to rewrite physics!
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Old 01-17-12, 08:08 PM
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Re: fb below f3?

Okay, I modeled a Celestion TF 1525 in your 4.4 foot cubic cab, with 2 2 inch vents .12" long at 240 watts (picked a power), gave a nice 3-4 db boost between 30 and 100 hz, F3 below 20hz, Fb at 40. (The driver is on sale from Parts Express; seemed like a good example to model.) Problem is, excursion exceeds Xmax at 38hz, so I hope that your driver has better Xmax, and/or your amp less power. No peaks other than the few db rise between 30-100. Does this help?
Tom
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Old 01-17-12, 08:43 PM Thread Starter
Shackster

rob

Join Date: Feb 2010
Location: ephrata pa
Posts: 8
Re: fb below f3?

yes, what if you backed off on the boost, would it still bottom? this driver really has a small xmax!! wow. am really just trying to tune this enclosure as is to better take advantage of the 35-45 hz range, while eliminating a bump at 60. this is DYI!! we can do better!!
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Old 01-18-12, 10:43 AM
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Join Date: Nov 2011
Location: St. Louis, MO
Posts: 147
Re: fb below f3?

I didn't apply any boost; that's the natural lump of running this woofer in a box bigger than it likes. This thing just runs out of excursion at any power greater than 10 watts. Assuming that your woofer is in a box size appropriate to its parameters, and the amp is properly sized, I think the 2 2" ports which tunes the box to about 40hz is the best you'll get without some more measuring equipment to better spec what we're working with. I'm assuming you're using WinISD or something similar to tune your cab, so what driver parameters are you using? Maybe the Celestion isn't the right driver to be modeling... I just picked a mid-quality pro 15" at random after looking at a bunch of parameters to get a feel for the range. As in all things, your mileage may vary... can we get more specific?
Tom
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