Calculations help for SB acoustics SB15NRXC30-8; ported or selaed? - diyAudio
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Old 10th January 2015, 11:48 AM   #1
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Default Calculations help for SB acoustics SB15NRXC30-8; ported or selaed?

Ok, so my preference would be for sealed as I haven't made many enclosures (about 4 so far, 2 of which have been sealed) and to be honest calculating alignments etc isn't something I've spent much time on, as I love filter design and crossover networks have enough to keep me learning for years anyway... problem is that's only one part of the solution.

A while ago I purchased 5* SB acoustics SB15NRXC30-8 mid woofers.

My original plan was to use these in 5 2 way bookshelves however my situation has changed and I now only listen in stereo, I also now no longer have a subwoofer.

I've been considering buying a pair of sb12mnrx25 midrange units and building these into WWMT floorstanders instead of the sealed TM units I currently have.

So my questions are as follows:

>according to online calculators, these drivers are better ported, can someone explain to me if this is the case - what advantages/disadvantages and reasons there would be to conclude from these calculators, and then should I port the 7ltr 2-way WT enclosures I currently have? tuning is kindly already provided by madisound and fitting a suitably sized port would be easy. I'm very happy with the sound of them sealed but bass response is definately lacking.

>If I used the SB15NRXC in a standard 3-way enclosure I would aim to BSC at xo freq, the crossover models very nicely (though in fairness I'll probably buy a dlcp and go active from now on) however I was really hoping to achive enhanced bass response from two drivers in a sealed enclosure - is this plain wrong, is it possible with a large enough enclosure or will I encounter other problems.... or should I just port it?

If someone who is 'good at these things' could help I'd be so grateful!!
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Old 10th January 2015, 12:16 PM   #2
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Quote:
Originally Posted by nannoo View Post
... however I was really hoping to achive enhanced bass response from two drivers in a sealed enclosure
You won't get that. For lower bass, you'll need a ported box of 10L ~ 12L. From modeling, F3 is 50Hz.

Since you have 5 units, you may want to consider a 2.5 design. When done right, it'll beat a 2-way.

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Mike
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Old 10th January 2015, 02:24 PM   #3
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Hi mike,

was what I was suspecting
I did consider 2.5 way (WTW or WWT) but decided in the end I'd prefer the phase alignment of a 3-way. Plus going 3 way allows me to account for baffle step with the enhanced effieciency of the parallel woofers, remove the L-pads from the tweeter, use relatively cheap inductors, achieve a nearly perfectly flat impedance (5 to 6 ohms throughout, excluding woofer resonance imp) and very flat FR with no peaks... so most of my design goals for the crossover/baffle side of things, just wasn't sure of ported alignment. The cabinet would likely be about 30L though... I could of course fill the bottom with ballast if that can't be tuned nicely.
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Old 10th January 2015, 06:01 PM   #4
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IMHO a 5" midwoofer mates easily with a tweeter, adding a dedicated mid for a true 3 way design will only complicate it, adding unnecessary expenses (big coils and caps). It is doable however. If you opted for a 6.5" or larger midwoofer then things change drastically.

If your idea is to low pass the "woofer" at the baffle step frequency, to low pass the mid (not band pass) at the crossover frequency to the tweeter, you are doing a 2.5 way and not a true 3 way. In this case the design is easier with two identical woofers (as Mike suggested).

Ralf
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Old 10th January 2015, 10:42 PM   #5
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Yeah I know 5" drivers aren't ideal for bass duties but they're what I have "in hand" so I thought is should be worth a try. The model I've been working on is two 5" wired parallel with the 12m and a seas tweeter.

Crossovers are 4th order electronic
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Old 10th January 2015, 10:55 PM   #6
system7 is offline system7  United Kingdom
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I don't know if you have seen SB Acoustic's own Elok design.

SB Acoustics :: Elok

It uses your bass drivers in an 20L or so MTM reflex.
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Old 10th January 2015, 11:38 PM   #7
Lojzek is offline Lojzek  Croatia
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I would go with MTM 30 lit vented, Fb=40 Hz,
F3/F6/F10= 40/35/31 Hz. Sensitivity about 87 dB
with full BSC. A lowish XO point, depending.

http://www.tb-speaker.com/detail/1230_04/25-1933s.htm
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Old 11th January 2015, 01:31 AM   #8
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Many favor real woofers for real bass. 8" or 10" woofer? Side-Side counter force pair?

Have you found any clever T + M + real_W speakers that fit your room, fashion and sonic goals?

A sealed SB15NRXC30 with an F3~85Hz from a clever cabinet construction has:
--the best sound
--the best transient response
--the lowest intermodulatino distortion
--the lowest total distortion
--the best power handling

Selecting the proper tweeter, cabinet construction, and T-M crossover is required for:
--the best sound
--the smoothest controlled directivity function
--the least beaming and no lobes
--the best imaging from diffraction control
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File Type: jpg Elok MTM Lobes.JPG (93.7 KB, 136 views)
File Type: jpg Narrow TM_W.jpg (64.3 KB, 133 views)
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Old 11th January 2015, 08:13 PM   #9
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Quote:
Originally Posted by LineSource View Post
A sealed SB15NRXC30 with an F3~85Hz from a clever cabinet construction
Can you elaborate on what you mean by a clever cabinet construction?

Also I'm still a bit lost on the "best alignment" front... if a speaker is better suited to a ported design does that mean I wont gain as much of the advantages as from a sealed alignment as are potentially there?


I know nothing about alignments in terms of what happens if a speaker isn't used with the ideal.

In an ideal world I'd probably build build larger woofer 3 ways, but as I have to limit the bass within reasonable levels in my new flat I guess I want flat response down to about 100hz, followed by the shallowest possible roll off... which I hope to be able to do with sealed and 5" woofers...That said I'm really hoping for "useable"bass down to at least 40Hz... My last 3 ways were pretty much flat 30-16k, and I was astonished by how the extra bass extension really seemed to bring recordings to life, it really was a different listening experience
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Old 11th January 2015, 08:35 PM   #10
Lojzek is offline Lojzek  Croatia
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You could have your bassmids installed in a sealed box and enjoy
thin sounding speakers or you could have them installed in a vented
design and enjoy decent bass performance.
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