IB15 dipole prototype

[clyde]

I couldn’t resist the urge to pick up a pair of the IB15 woofers that are on sale at Parts Express, with the intention of building an Infinite Baffle sometime in the future (assuming my wife approves of cutting a hole in the living room floor, of course.)

When the IB15s arrived, I immediately thought of building a prototype dipole subwoofer, to see how this highly touted design would actually sound and measure in my listening room. I found a piece of MDF, a Behringer active crossover and a Behringer DSP EQ laying around from an earlier project to use.

A day later, I had a 24” H baffle for the IB15 built and ready to go. I set the active crossover to 100hz and configured the DSP EQ for 6dB per octave dipole compensation. According to the dipole model at the Linkwitz web site (www.linkwitzlab.com), one IB15 should outperform a pair of Orions and even a pair of the Pyle dipole subs discussed earlier in this forum.

I used the Real Time Analyzer in my DSP EQ to check frequency response (nearly flat to 20hz at 90dB SPL) and adjusted system balance. I thought the IB15 could use some loosening up, so I ran 100dB SPL of pink noise for a day before sitting down to see how everything sounded.

The sound of this sub is very different from the monopole VMPS sub that I recently sold. It is tightly integrated and coherent, extending the bass response to 20hz without adding any noticeable artifacts. Mr. Linkwitz is dead right regarding the advantages of dipole bass.

I did a 1/10 octave measurement of the dipole sub, since the 1/3 octave resolution of my RTA is insufficient to clearly show room mode resonance. I found a sharp peak of 10dB at 30hz and a -8dB null at 54hz, easily corrected with parametric filters in my DSP EQ. Using two dipoles would undoubtedly reduce room mode problems.

I like the dipole sub prototype so much that I plan to finish it and keep it in my system. The IB15 driver is excursion limited to about 90dB SPL at 20hz and 100dB at 30hz in a 24” dipole, more than adequate for the music that I listen to. The Infinite Baffle version will be about 10dB louder, but that project can wait for the urge for full reference level LFE to develop.

[jjdche]

Nice, sounds like a fun little project. I've really been chomping at the bit to build an IB sub. Of course, thats a problem when you live in an apartment! Soon enough though.

I might have to look into dipole.

[clyde]

Here's a picture of the IB15 dipole prototype. The H baffle is 24" wide, 24" deep and 16" high.

Sorry, the uploaded image is not displaying for some reason.

[clyde]

Here's a link for the picture: http://pages.sbcglobal.net/cmwooten/IB15sm.jpg

[catapult]

one IB15 should outperform a pair of Orions

Better check your math (net Vd not to mention the relative distortion/noise levels of the drivers as you start to push them beyond 1/2 Xmax.)

[clyde]

The Linkwitz dipole spreadsheet takes into account Xmax, Sd and baffle size, giving the following projections for excursion limited SPL into half space:
--------------------------20hz----30hz
Orion - 4 drivers ------- 83dB----93dB
IB15 proto - 1 driver -- 88dB----99dB

I listened the Orion at SL's house and posted a rave review in another forum. The Orion's bass response is qualitatively excellent but its output below 40hz is limited by the relatively small 10" drivers and small dipole baffle.

My point is that a single larger driver and dipole baffle can match or exceed other published dipole designs with multiple drivers. I have been listening to the IB15 prototype all weekend and continue to be stunned by the quality and quantity of bass output.

[clyde]

I figured out what to do with the second IB15 driver that I have. I ran the Linkwitz dipole spreadsheet for two IB15 drivers in a 32" H baffle (end table size) and got the following excursion limited SPL projections:

20hz 97dB
28hz 106dB

Harmonic distortion will also be reduced with two drivers.

It looks like I won't have to cut a hole in the floor for an Infinite Baffle after all.