2-way: Waveguide + Cardioid-like
After dipoles it seems waveguides are the new craze. In spite of all the differences between the two, the concepts share a focus on controlling directivity Ė which was also the main criterion in the design of my Unbaffled Dipole. Therefore, for me it wasnít much of a stretch to design a speaker with a waveguide.
The concept is a 2-way speaker with a 12Ē woofer and a 1" compression driver with a 12Ē waveguide, crossed passively at 1 khz. A miniDSP is used for amplitude corrections and the crossover to subwoofers below about 100 hz.
I considered many drivers, but I ultimately settled for a Selenium D220Ti compression driver, a Dayton H12RW Waveguide and a A&D R1230 midwoofer. The Seleniums are relatively cheap and they are quite popular, so I thought those would be a good starting point. I bought both the Dayton JBL-clone (the one used in the Econowaves) an the Dayton H12RW. I chose to use the H12RW because it is axi-symmetric and thus has more constant vertical control. The rectangular waveguide allows better vertical alignment of the acoustic centres, which helps to minimize lobing at the crossover, but I donít believe that to be as relevant as some others do. With a crossover at 1 khz, with this speaker the acoustic centres are spaced less than one wavelength apart, which is much better than many a conventional 2-way box speaker.
Initially I intended to put the drivers in a closed box, but in order to get the smoothest possible powerresponse through the crossover-region a simple box seemed not to be the best solution. Although others have already shown that the closed box format is very workable if you cross at the frequency where the directivity of the woofer and waveguide match, below the crossover frequency directivity usually decreases quite fast. Therefore I decided to experiment with an open box Ė a box with holes in it. The box is stuffed to the brim with rockwool (I intended to use glasswool, my favorite, but I didnít have it at the time) and I use carpet without backing to finish the sides.
I think it is difficult to predict the exact acoustic behavior of this concept. My expectation was that the directivity pattern would be more or less dipolar at low frequencies and that it would lie somewhere between that of a conventional box a cardioid at the top of the intended passband. Such behavior would be more or less ideal, as this would help to match DIís at the crossover and it would not let directivity drop too much at lower frequencies.
Typical measurement setup (measurements of the speaker with crossover are taken about 1.5 meters higher than this) :
Letís see some measurements!
http://img40.imageshack.us/img40/133...gadwoofer0.jpg A&D R1230 Horizontal dispersion from 0 to 90 degrees in 15 degree steps (gated measurement).
http://img692.imageshack.us/img692/1...sionnogate.jpg A&D R1230 Horizontal dispersion from 0 to 90 degrees in 15 degree steps (un-gated measurement 1/3 oct. smoothing).
These measurements show that practice approximates the anticipated behavior pretty well. All curves look pretty smooth and there is good directivity around 1 khz. Below about 300 hz the radiation pattern looks like a dipole, above that frequency there is much greater directivity Ė in the transition region some widening is apparent. What happens below 100 hz is unclear.
Now what does it look like at the rear?
http://img833.imageshack.us/img833/9...wooferrear.jpg A&D R1230 Horizontal dispersion from 180 to 90 degrees in 15 degree steps (gated measurement).
It looks a bit messy, but at least all curves are a lot lower in level than those at the front. Interesting to observe is that the 135 degree curve overall looks to be lowest in level, which is a bit like a super-cardioid. When you look at the non-weighted average of all these curves you get a pretty smooth looking curve.
http://img814.imageshack.us/img814/9...nseaverage.jpg A&D R1230 Average of horizontal dispersion from 180 to 90 degrees in 15 degree steps (gated measurement).
In the next picture you get to see no new data, but it's still informative: A&D R1230 comparison of 0 degree and 180 degree response:
http://img824.imageshack.us/img824/3...uizingvolm.jpg A&D R1230 Comparison response at 180 and 0 degrees (gated measurement).
At low frequencies the difference between the curves is greater than it would be with a closed box, at higher frequencies itís less.
Letís have a look at what the Selenium-Dayton combo looks like in comparison!
http://img221.imageshack.us/img221/1...enadr1230p.jpg Comparison on-axis response A&D R1230 and Selenium D220Ti on Dayton HR12W
The first thing I noted was the ragged response above 5 khz. Letís hope it doesnít sound as ugly as it looks! The second thing that struck me as odd is that except for the 2khz bump the curve overall is very flat! I expected to see a downward trend. Letís see how constant directivity is.
Selenium D220Ti on Dayton HR12W Horizontal dispersion from 0 to 90 degrees in 15 degree steps. Measurements taken at approx. 2 meters.
http://img708.imageshack.us/img708/1...opwg0tot90.jpg Selenium/Dayton Horizontal dispersion from 0 to 90 degrees in 15 degree steps (gated measurement).
In comparison to my dipoles, this is not very constant directivity, however it does look very controlled! When you look at greater off-axis angles you see increasingly downward sloping curves. It doesnít look bad, but making this flat on-axis might lead to a sound that is not very bright. The Daytons are axi-symmetric. Therefore I expected to see a diffraction dip in the treble region, but strange enough there appears to be nothing there! Ignoring the increasing downward slope, the off-axis curves track the on-axis curve very well. I would say this is exactly what you want, but not everyone agrees: http://www.diyaudio.com/forums/multi...ml#post2146754. I wonder if HOM are the Ďcauseí of this slightly unexpected yet welcome behavior. Too bad I donít know how to measure HOM.
The passive crossover was a bit more difficult to get right than Iím used to. The acoustic centre of the tweeter lies a bit behind that of the woofer. This causes some excess phase. To work around this, I had to cross steep and I had to do a few tricks to get the right amount of relative phase rotation.
http://img221.imageshack.us/img221/273/filterw.jpg Crossover Filter
Some inductors have unnecessary low Rdc, but thatís because I used components I already had. When I build the crossover into the speaker Iíll use more logical component values. The impedance correction is optional.
http://img3.imageshack.us/img3/8059/...htsfunctie.jpg Transfer function
http://img808.imageshack.us/img808/6...se15graden.jpg Response at 15 degrees
http://img97.imageshack.us/img97/4775/fase15graden.jpg Phase at 15 degrees
EQ is done with a miniDSP. After some tweaking I ended up with this result:
http://img684.imageshack.us/img684/4...gwgspeaker.jpg 2-Way dispersion 0 to 90 degrees (gated measurement)
http://img33.imageshack.us/img33/869...averageron.jpg [2-Way comparison between on-axis response and the non-weighted average response from 0 degrees to 180 degrees [/I]
Unfortunately, then it started to rain, so I couldnít do more measurements. Indoors I later lifted the highs a little to make the response flat on the listening axis of about 20 degrees. Also I EQíed a few of those nasty looking peaks. In the range below 500 hz I do a little room-EQ. Subwoofers are used below about 100 hz.
The design is not 100% ready yet. Iíll probably keep tweaking the EQ for some time. Iíve noticed that EQ is very critical with this speaker. When itís ready, I may do a short subjective review. Thus far I am very pleased with the sound. All in all I think it is better than my dipoles. It all just sounds so effortless and very detailed. I tend to listen to them at a much higher sound level than other speakers. At normal sound levels they sound very clean and clear. At higher levels the midrange becomes a bit shouty on some material.
Iíd like to hear what you guys think!
Some observations that I find interesting and which might be worth a discussion:
- The ugly looking high frequency response doesn't sound half as ugly as it looks. This may have to do with the critical bandwidth of the auditory system.
- The unfiltered response of the CD-waveguide combo doesn't have the expected downward trend. A downward trend corresponds with more or less constant directivity. Personally I wouldn't call the directivity of this combo constant (at least not as constant as is attainable with dipoles with minimum baffle), but it's not really less constant than that of other speakers that their designers do call Constant Directivity. Why is the on-axis response relatively flat?
- There is no on-axis dip anywhere. Does this mean this combination creates a lot of HOM? If so:
- Don't HOM primarily harm the highest frequencies? If so, then I don't understand, because the highs sound very clean. If there is one thing I don't like about them, it is perhaps that they sound a bit less spacious than the highs of wider dispersion speakers.
- At higher levels the midrange becomes a bit shouty on some material. Do you guys things this is caused by some kind of horn distortion, or maybe be the rather lively room they are in? Another possibility is the EQ that may not yet be top notch.
Can you explain some more about your intention of having carpet over the holes? A very "lossy box" perhaps? I'm experimenting with fish net material behind a solid baffle, so the sides of the "box" are nothing more than glass wool suspended behind the drivers. Automotive air filters are another way of attacking this as they create very lossy sides, close around the driver. The midrange is phenomenal.
At the sides there is only a minimum of rockwool, so damping is also limited. I needed something to finish the sides anyways (the MDF panel with holes in it doesn't look very nice) and I reckoned the carpet may provide some extra damping / flow resistance.
In case you are interested these people make a number of speakers and subwoofers with a cardioid characteristic. Not sure how but I guess it involves clever port tuning and it appears to be working very well. Owners report that there is hardly any bass audible directly behind the speakers.
Thanks! I'll have a look at their products. True cardioid wasn't the goal here though; it's primarily about matching directivity at the crossover and keeping the powerresponse / directivity more constant or at least smoother.
I'm doing outside measurements right now. There's some sunshine, most dark clouds have moved on, but there's some wind too. Anyways, for those of you who like 'm, a directivity plot:
That 150-500 Hz increase in pressure is likely all baffle related.
Look at MBK's experiments (and the difference in baffle/pressure release) here:
Fantastic Keyser! Excellent job. Thank you for posting and having the insight to do this.
Very Nice!!!! :D:D:D
Look up aperiodic enclosures - it is similar to the method you are using on your enclosure and might help out with the crossover.
There are several websites that discuss the theory - here is one that shows a practical application: WD25A Design Pg2
Great job indeed! :D
It's not entirely the same situation as yours. But the throat is worth investigating and some improvement.
The entrance of this Dayton WG is not exactly 1", it's slightly bigger. So there'd be a step from the driver to the WG.
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