Dipole Subwoofer DIY (1 Viewer)

[Travis_G]

I am considering building a dipole subwoofer; possibly using a single 15" Tempest. I am wondering what kind of output I could expect at ~ 20 Hz.

I don't have the know how to build active filters so I am also wondering if using an EQ would work well (like the Art 351).

How do I calculate the amplifier power required to reach reference levels for this type of subwoofer?

Travis

 

[Ryan Schnacke]

I'm one of the least knowlegable about dipole subs but still I'm thinking maybe your expectations may be off a bit. As I understand it, a dipole sub involves mounting the driver(s) in a baffle that does not completely separate the front wave from the back wave.

In this respect, the woofer is playing free air and will be excursion limited rather than thermally limited. In other words, you should be more concerned with how much power a single Tempest can handle free air before it bottoms out.

Also I would expect that at the very best, a dipole could only achieve the same SPL as a sealed sub (given that both are excursion limited). And I'm thinking it would take a minimum of 4 sealed Tempests to reach Reference level at 20Hz (before considering room gain). With room gain it might only require 2. But that's sealed subs and that's also if you are not redirecting the bass from your front, center or surround speakers.

I'm sure theres a calculator around somewhere. I'd search for Linkwitz Lab (sp?) since I recall a lot of dipole info on that site. But the fact that you're asking about "Reference Levels" and "20Hz" tells me that a single Tempest is likely to be inadequate.

For equalizers, I think the Behringer Feedback Destroyer would be much better suited to the task.

 

[ThomasW]

It's fundamentally impossible for a driver mounted in free air to reproduce reference level output unless you consider nearfield measurements.

It takes 2-3 times more displacement for drivers in a IB or dipole to equal the output of single driver mounted in a box.

 

[Mark--M]

How would an IB require 2-3 times the excursion of a sealed box? I was thinking it would be basicly the same.

 

[ThomasW]

The simple answer is that drivers mounted in free air are limited by the amount of power needed to drive them to the limits of their excursion. A driver mounted in a 'box' is power limited by the thermal heating of the VC.

The rough difference is that the free-air mounted drivers can handle about 40% of the power that a driver mounted in a box can take.

 

[Ryan Schnacke]

Mark, I agree with you. In the case of both sealed and IB you've got a woofer cone that must move to displace air. The back wave is (ideally) completely seperated from the front wave so that SPL comes only from cone excursion. If the cone is moving X distance at Y frequency then you get Z SPL.

But Thomas is correct in that a sealed subwoofer's excursion will flatten out at low frequencies (for a input level) because of the box whereas the IB subwoofer's excursion will just continue to climb at lower frequencies. So if you're shooting for max SPL at a specific frequency with an IB sub you might consider a high pass filter set a little below the frequency of interest. That way you wouldn't have to fear over-excursion with even lower frequencies.

Reduced power handling in an IB is simply a function of the higher sensitivity at lower frequencies.

And I'll say this again because I think its important to understand. 40% is NOT some sort of power standard for IB vs. box subs. As far as I can tell, this number comes mainly from the fact that PE derates their 295-455 IB15 woofer from 350 watts (thermal) to 140 watts (mechanical, free air). But there is no reason to believe that other woofers will follow this relationship where free air mechanical excursion will be limited to 40% of the thermal power rating of the coil. I think this would vary quite a bit from one woofer to the next. In fact, the Dayton IB15 is not very representative of most of the woofers I see used in IB installs. They usually have much lower Qts which completely changes woofer excursion on the very low end.

So power handling in an IB is dependent upon what frequencies you're concerned with. This, too, could vary quite a bit from one application to another. One person may be music only, another may be HT only with no rumble filter and yet another may have a rumble filter to protect from the ultra-lows.

Thomas, I mean no disrespect. If there really is something I'm missing here, let me know. But my unsolicited opinion is that power handling of an IB is application specific.

 

[Ryan Schnacke]

Travis,
I apologize for the sidetrack. Here's the website I was recommending for dipole subwoofer info:

http://www.linkwitzlab.com/woofer.htm

Specific to your SPL question:

http://www.linkwitzlab.com/faq.htm#Q5

 

[ThomasW]

Ryan,

I used the PE warning as a quick and dirty example. I wasn't in the mood to go into a long and complicated explanation regarding this.....

 

[Travis_G]

Thanks everyone.

It seems like it would only be possible to produce refrence levels down to about 40 Hz with a single tempest. Maybe I will just build a ported tempest for now.

I might eventually build a dipole speaker to cover the 40-80 Hz range.

Thanks Ryan for the links

Travis

 

[Ryan Schnacke]

Travis, you're welcome.

Thomas, I understand. And BTW, until just now I'd never read a single post on the AVS forum. But after seeing the fuss about it on the IB Cult, I read through all 8 pages of "that" thread. Makes my little rant here seem kinda silly by comparison.

 

[ThomasW]

Ryan,

No problem :wink:

 

[Jack Gilvey]

Thank you. As such, it's a good thing.

The only reason that more drivers are "required" in an IB is that there's no box acting as a high-pass filter. While most subs won't even hint at what's way down there, an IB will give it a shot, and have the excursion to prove it.

A dipole is the only sub type that has sounded fundmentally different to me. Competent monopoles sound more alike than they do a dipole, as good as the room will allow.

Here's mine in its simplest form, a couple of DPL-12's in an axe-hewn U-frame:



I wired them for a Qts ~.8, so I was able to get away without eq to extend response.

 

[ThomasW]

Not sure if I've posted this info before.

I have both sealed and ported subs at the house, in addition to the 2 IB subs. There are 2 BFDs that have separate EQ bands dialed in, to optimize all the exiting subs, IBs included. When people come over to audition it's simple to have them pick any of the box subs and create an instant A/B comparison with either IB. So far anyone choosing to have an A/B comparison has found a significant difference between the SQ of either IB and any of the box subs.

When I had the 8-SPL12's up and running in a dipole, they had the best SQ I've heard, but the lowest octaves were lacking in output. To get more low octave output would mean adding in probably 4 more drivers and additional EQ. That just wasn't a workable situation. So I'll stick with the IB's .... :wink:

 

[Jack Gilvey]

Yessir. As you might imagine, a pair of DPL-12's in dipole does not a HT sub make. Really, really nice music bass, though, 40Hz or so. I think my almost-square listening room makes dipole's advantages much more obvious than they'd be under more favorable acoustic conditions.