Fun with Dipoles & Waveguides

[matevana]

As is sometimes the case, what started as a bit of a goof, materialized into a <somewhat> legitimate project. I had been rapping up a fairly traditional open back build, when a trip to Ikea yielded an unlikely suspect. My friend Wade and I both spotted it at the same time, propped on it's side between a leather ottoman and some Swedish chocolate bars. We looked at each other, looked back at the object, then back at each other and and blurted out quite loudly, "WAVEGUIDE!"

Made of bamboo, a mere two inches deep and available in both 12" and 18" diameters. Hmmm... bamboo could be cut, sanded and shaped fairly easily, at least compared to the more traditional plastic guides. Surprisingly it had a contour reminiscent of some legitimate guides, was fairly smooth, consistent, and yes... somewhat attractive in a Geddes-esque kinda way. The price? $5.99. I'll take two please.

Marketed by Ikea under the Swedish name "Hultet" which I believe translates roughly into English as "Earl Geddes".

Prior to delving into the waveguide concept, I had some success with Peerless 3" aluminum drivers as the proverbial icing on the dipole cake. While smooth directivity isn't their strong suit, they were fairly polite up high. And unlike domes, maintained an unimpeded backside that sounded nearly as good as the front (gotta love a nice rear end). Seemed like an appropriate candidate to test the guide as it would play pretty low without straining if called upon to do so. The "guide", btw, has a nice flat surface on the bottom, which clearly lends itself to rear mounting. There is even enough bamboo "meat" to screw mount the driver w/o breaking through the other side, provided small wood screws are used. A hole saw followed by some sanding and shaping completed the test project. 5 minutes well spent!

My 3 way dipoles are actively crossed, eq'd and biamped. The low end comes courtesy of a pair of Peerless 10" xxls drivers; the mids are 8" Usher honeycombs which are passively crossed at 2k to the Peerless 3" drivers up top. The low to mid xo point is 118 Hz, at a 24dB/oct slope. EQ compensates for dipole roll-off as well as flattening the in-room response.

How do they sound? Well, in my somewhat biased opinion, good. Damn good! Much better than they probably have a right to sound. I still need to improve upon the alignment, possibly by reshaping the baffle as well as some point-source concerns in the near field. But directivity is much improved. There are no audible resonances that I can hear. Both the constrained layer baffle construction and magnet mounting of the waveguide assembly probably speak to this issue. Horns and woodwinds sound great, airy, natural. Dianna Krall sounds amazing, as do piano and strings.

Next steps? Rearranging the drivers orientation to minimize spacing, aligning acoustic centers and possibly improving upon some minor diffraction issues. Not too shabby for a spur of the moment impulse buy. Thanks Ikea.

[gainphile]

Someone must try to convince Ikea that a 90deg bowl holds more fruit !!...

[matevana]

Originally Posted by gainphile

Someone must try to convince Ikea that a 90deg bowl holds more fruit !!...

Hey Andi. I still haven't been able to find the 2x2 CD by James Morrison & Joe Chindamo. Bet it would sound good on the IKEAbowl! Have you done any more work with the Dayton guides?

[matevana]

I thought it might be interesting to see exactly how the guide affected the 3" Peerless wide range driver. The driver is crossed at 2k, LR 12dB/oct. In terms of SPL at various frequencies, this is what I observed vs. the previous more traditional flush baffle mounting.

1. 1.6kHz +1dB
2. 2kHz +1dB
3. 2.5kHz +1dB
4. 3.15kHz 0
5. 4kHz 0
6. 5kHz +1dB
7. 6.3kHz 0
8. 8kHz +1dB
9. 10kHz +2dB
10. 12.5kHz +2dB
11. 16kHz +1dB

Not too surprising due to the relatively shallow profile of the guide; not a lot of compression going on here. Not a bad thing either, as the primary goal is improved directivity w/o any evidence of honk.

It was a simple matter to EQ back to flat. Now, the more complex task of measuring at 15 and 30 degrees.

[Wavebourn]

Funny thing, about a month ago I was looking at big square wooden plates in Grocery Store thinking that I have all needed speakers at home, otherwise I would buy something similar.

[dantheman]

Originally Posted by gainphile

Someone must try to convince Ikea that a 90deg bowl holds more fruit !!...

Sounds like a signature line.

Dan

[matevana]

The off axis measurements are very promissing. At certain frequencies I can recover 2-3 dB at 15 and 30 degrees off axis, to more closely parallel the on axis measurements, but other frequencies are unchanged. I suspect this in part speaks to the differences between a random "bowl" guide vs. a professionally designed waveguide.

I do have a bit of a perplexing question. Even after EQing the driver back to flat, it has taken on a new persona. Instruments clearly sound more lifelike and airy, yet I am unable to account for this change in simple SPL measurements. I am sure it's more than a perception on my part, just knowing the guides are installed. What might cause such a change in character of the sound, that would not be evident in simple measuremnts?

[454Casull]

Off-axis response

[matevana]

Originally Posted by 454Casull

Off-axis response

Even though my listening position hasn't changed? is it the change to the ambient sound I am hearing off-axis as its being reflected into the listening position?

[ra7]

Yes, you are giving your brain more information about the location of instruments through off-axis reflected sound.