OmniTop12 tuning frequency

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Gbone
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Joined: Thu Mar 26, 2020 10:07 am

OmniTop12 tuning frequency

#1 Post by Gbone »

Hi all ... can anyone help identify the tuning frequency of the OmniTop 12 cabinet ?

I own the plans, calculated approximate cabinet volume and the two handle cutouts for venting.
I'm thinking its in the 90Hz range.
Horn loading looks like it starts to effect around 200Hz and up.

My plan is to experiment with this concept on a larger scale in upcoming months and looking to understand more about this design/concept.
I have no experience with front loaded vented cabinets. Any comments or information is appreciated.

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Bill Fitzmaurice
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Re: OmniTop12 tuning frequency

#2 Post by Bill Fitzmaurice »

It's closer to 75Hz. In a standard bass reflex with the drivers used tuning would be in the 45-50Hz region, for maximally flat response to the lowest possible frequency. By tuning it higher, along with a smaller than optimal rear chamber, the port output is higher, at the expense of low frequency extension, so that the port output matches that of the front wave. This also increases the effective driver resonant frequency, enhancing the horn sensitivity at the lower end of its range. There's no need to experiment, you model it in HornResp. That saves a lot of time, effort and plywood.

Gbone
Posts: 6
Joined: Thu Mar 26, 2020 10:07 am

Re: OmniTop12 tuning frequency

#3 Post by Gbone »

Very informative Bill, thank you.
I do have HornResp and with my limited experience played with conical front loaded horn, vented chamber scenario. After inputting cabinet dimensions and TS parameters the response looked different from your SPL charts. The venting parameters tried were incorrect (90Hz). Chose conical because I could play with the Loudspeaker wizard in that scenario. Also chose quarter space. Definitely going to continue to learn how to use HornResp.

"This also increases the effective driver resonant frequency, enhancing the horn sensitivity at the lower end of its range."
I need to understand this in greater depth. How much is the lower range of the horn enhanced in the OT12?
Compared to the old JBL 4560 the OT12 horn is much smaller but response around 80Hz and up isn't too far off.

I acquired a huge vintage fiberglass flare 28.5" x 23" with a little compression cap ("hard hat") on the rear. Cap is probably only 0.3 - 0.4 cuft and I want to try to build a vented chamber around the flare instead, essentially a large version of your OT12 design without the HF portion. Target range 80-85Hz to about 500-600Hz.

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Bill Fitzmaurice
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Re: OmniTop12 tuning frequency

#4 Post by Bill Fitzmaurice »

Look at the response of the Jack 12. Even with venting the ports into the horn it doesn't have the sensitivity of the OTop 12 at 100-200Hz. Don't model in quarter-space, unless that's how you're going to use it. Even half-space is somewhat optimistic at the lower end of the band width, since you won't get half-space loading all the way down with the cab elevated to where it needs to be. I doubt your fiberglass flare will equal a DR250, and I'd not go to only 500-600Hz.

Gbone
Posts: 6
Joined: Thu Mar 26, 2020 10:07 am

Re: OmniTop12 tuning frequency

#5 Post by Gbone »

Thanks Bill. Much appreciated.
DR250 would be great but I don't have the ability to build that on my own at the moment.

To fellow readers... my system is not for playing live events or large events ... it's a 4way active system at home in the basement. I'm ready to try something new for midbass/lower mid part of my system. I keep reading and coming back to the OT12 concept after listening to my direct radiating midbass for years. Also like the idea of the 12" midbass positioned further back on the front loaded horn so the vertically stacked drivers can be physically time aligned instead of using DSP delay. The front loaded horn with a vented chamber would cover the target range. Looking forward to learning how to use HornResp.

Lower crossover point of 600Hz really improved details and dynamics when I finally tried a horn/driver combo (BRH90 + M200) that can actually play that low and sound good.
Previous crossover was 1 octave higher with smaller horn and different 2" driver.

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