subwoofer suggestion for OB

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi All

I've been lurking here for a while and was playing with the idea of building an open baffle 'sub' for low end. I always thought the Pure Audio Project speakers were kind of cool. I like the idea of modular.

Anyway, previously I had another idea and picked up a pair of Eminence 12cx's with ASD:1001 tweeters and stock crossovers designed for the pair (Using coaxial products for DIY monitors and home hi-fi applications | Eminence Speaker). I had read that if I put a low pass on the 12cx's at 125Hz they would perform decently in open baffle format (Qts 0.48).

I was toying with the idea of using the Eminence 15As for the 'woofer' part on the open baffle but saw that the Fs is 41Hz (vs the 12cx 47Hz).

Is there a point in using the 15As with the 12CXs since the Fs is so similar?

I know that the extra 15As would add to the low end (with a 125Hs crossover (?)) but should I look for a driver with a lower Fs? (which seems to make sense to me, but as it is open baffle....???)

I'm a total newb at this stuff so any advice is appreciated!

Thanks!

:)
 
Thanks for the suggestions everyone!

@woody - I did read that document earlier on... tbh most of it made my head spin, but on reading it again it seems to make more sense...

Because the sensitivity of the 12CX and 15A drivers are almost the same, the roll-off will also be the same. But because of placement of the 15A on the lower part of the baffle the loss of output can be made up by the floor reflection (which would make the speaker appear more efficient (?)) and extend the bass output of the 15A, whereas the output from the 12cx would roll-off higher and more because of placement and the TS characteristics of that particular driver... (?)

I'd still have to figure out the crossover point of between the two sets of speakers (making this effectively a 3-way system)... I am guessing this is where measurement equipment comes in handy? (I have none...). Other than buying more equipment (I know it will happen sooner or later).. any suggestions on how I could tackle this with TS info and existing graphs? The MJ King article gives info on the 15A, but the 12cx+ASD:1001...?

@Octavia - I had looked into those earlier but wanted to keep this a 'budget' build as I am trying this out for the first time... (I have been playing around with some woofers, an existing crossover (4way), and a bunch of VG9s I picked up on sale (right now I am playing around with 2 8" woofers in an h-frame with 2 VG9s in (technically) dipole configuration up top (also brings the speakers up to 8ohms)...still playing, but I digress)

Thanks again everyone! Much appreciated!

:)
 
I would not do a standard OB subwoofer unless you can accommodate a seriously LARGE baffle. You might consider a U-frame or H-frame with two drivers, one mounted the reverse of the other, to cancel some distortion. Remember the drivers will require a lot of excursion capability if you are really wanting to end up with something that can reproduce "subwoofer" frequencies (below 40Hz) loudly enough to hear and feel.
 
Because the sensitivity of the 12CX and 15A drivers are almost the same, the roll-off will also be the same. But because of placement of the 15A on the lower part of the baffle the loss of output can be made up by the floor reflection (which would make the speaker appear more efficient (?)) and extend the bass output of the 15A, whereas the output from the 12cx would roll-off higher and more because of placement and the TS characteristics of that particular driver... (?)

Bingo.

The T/S parameters will determine the low-frequency rolloff. Now, the Alpha-15A has a very high Qts*. High enough, in fact, that its tendency to ring a little around its resonant frequency (~40Hz) provides enough output to prop up the falling low-frequency response of a driver in a dipole system.

The Beta-12CX has a lower Qts, and will therefore begin rolling off at a higher frequency. A rough guide is to take resonance (Fs), and divide by Qts - this is a very rough rule of thumb to determine how low the driver can go without assistance.

* Physics lesson: the Q-factor of a resonance describes how long it'll ring for before dying away. For instance, take a ruler and put half of it over the edge of the table. "Twang" the end, and it'll vibrate. Put the palm of your hand on the ruler just over the edge of the table, and you'll notice the vibrations die down much faster - your hand is providing damping. Q-factors are defined such that a high-Q system will take a long time for the vibrations to decay, and low-Q systems die down quickly.
Very very low-Q systems actually take a while to allow the moving part to get back to its rest position - imagine the ruler moving through custard - its gonna take a while to make its way back through to its unbent position, but it will get there.

Q=0.5 is defined as the fastest decay time without any overshoot. That is, the amount of damping that will allow the moving part to head back to its rest position, but not so fast that it'll carry on out the other side.
Q=0.7 gives the fastest decay time IIRC, but has a slight overshoot.

Higher-Q systems tend to have large vibrations at their resonance, with relatively small amounts of vibrations at other frequencies, for a given driving force.
Lower-Q systems will usually have most vibration at resonance (the exception is very-low-Q), but other frequencies are given more consideration.


Chris

PS: FWIW, 40Hz is useful extension for most music. If you're on a budget, the Alpha-15A will get some low end on OB without much help (EQ, etc). Better drivers are available, and I'd recommend them where possible. For a short while, I used a pair of nice Beyma 15" drivers on OB, but the 30mm p/p travel needed for <30Hz content got scary. 2x15" a side would be a minimum for going below 40Hz at decent levels.
 
Last edited:
thanks everyone for their suggestions and info! I'm in Canada so am looking for something that is preferably already north of the border (shipping, duties, taxes, exchange being the big factors), thus the choice of the 15As. I am open to other Canuck-friendly suggestions for the woofers as well....!

@chris661 - thanks for the lesson and confirmation! based on the 'rough guide' it looks like i'm looking at getting into ~98Hz for the 12cx and ~32Hz for the 15A, before rolloffs, baffle size/shape, and floor boundary. This gives me a reference to work from. I'm trying to keep the budget down so I don't know that I would do 2 x 15" a side to start... also WAF would be negative a billion^2... ;) Since the potential space is quite small I may be able to get more out of the 15" with room boundaries. As I am planning to build the baffles in a modular fashion I can scale the project up at a later time...

That said, any suggestions on dialing in a crossover? I am leaning towards, and like the idea of, passive...

Thanks again and much appreciated!

:)
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.