DM-4 Reference Upper Cabinet 2-way Speaker Build Part IV

cad_front_upperThe speaker project continues! I managed to find a few hours in the last couple of weeks to finally start the upper cabinet speaker portion of my sweet DM-4 Reference Speaker build. If you haven’t read my original introduction to these speakers, then you might want to start over there first. The inspiration for this design originates from both the Wilson Audio Watt and the Von Schweikert VR-5 hi-fi speakers. It’s sort of a marriage of the two designs, taking the best of the aesthetics of both and incorporating them into one totally awesome speaker system – and then ditching the passive crossover for a fully active DSP-based crossover with all the bells and whistles. It’s going to be my greatest achievement yet!

The dimensions of the upper cabinet are 12-1/2″ at the bottom with a 7° slope on each of sides ending up at a width of 9″ at the top. The speakers are 21″ deep at the bottom and 18-3/16″ at the top. The back baffle is perpendicular to the bottom but the front baffle is tilted back 14° to physically time align the woofer and the tweeter. The very top front edge has a 3″ chamfer cut at 14° (which mimics slightly the VR-5) which also reduces the effects of edge diffraction. There are 1/4″ chamfers around all exterior edges which finish off the cabinets (similar to the VR-5) which also aid in edge diffraction. These are the basics of the design which gives it that distinct look that I was shooting for with this whole speaker project. The sloped sides, the sloped front, the chamfered top edge all give the speaker that classic “Watt/VR-5″ look.

So just a few words on how I designed some of the shape and dimensions for this speaker because I think there are important factors with each parameter. This is not a just carbon-copy/knockoff speaker but is truly designed for the drivers I have selected. I came up with 14° sloped front baffle by drafting up side views (or cutaway views) for the 7″ Dayton woofer and 1-1/8″ Dome tweeter in Delta CAD using the .pdfs from Parts Express’s website and then played around with the angle to align the drivers until they were just right. Phase/time alignment is achieved by placing the acoustic centers of each speaker in the same vertical plane. It is believed that the acoustic center of a speaker sits roughly at or around the location of the voice coil. Since the voice coil location is not well defined in the datasheets, it can also be approximated by assuming the voice coil is centered within the top plate of the magnet assembly (not the magnet itself). I considered alignment was met by setting the baffle back such that the centers of each voice coil were aligned vertically. In the case of the Dayton RS-28F tweeter and RS-180P woofer, this turns out to be about 14° assuming the drivers are 1/2″ apart on the front baffle. This suits me just fine since most off-axis measurements are taken in 15° degree increments, so when seated directly in front of the speakers, you will be listening to both the woofer and tweeter at about 14°-15° off axis. The tweeters will sit at about 44” from the floor when resting on the lower cabinets. Based on the FR plots of both drivers, they both have a very well-behaved response at 15° off axis with only slightly more high-frequency roll-off than when listening to them head on. The exact baffle angle probably doesn’t matter too much as far as timing goes since the crossover is going to be MiniDSP-based (active), so I will have independent control over the time/alignment of the woofer and tweeter anyway. But someday I may decide to re-purpose these speakers without the active DSP so I will have to design a passive crossover for them and in that case I’ll be all set with the drivers physically already time aligned. I have to admit, my experience in this area is mostly textbook since I have not actually ever built a speaker with the drivers’ acoustic centers vertically aligned in this fashion. Like most speakers, I just allow the vertical offset to play out however it plays out on a normal, flat, vertical baffle. So this will be fun for me to see just how much it changes the way the speakers the sound.

Another cool dimensional design consideration is the 14° chamfer across the top/front baffle edge. So what purpose does this little design detail serve? It is just for looks? Did I just do it to copy the VR-5? The Wilson Watt doesn’t have it, so it is even necessary? The answer is both yes and no. Aesthetically, it completes the speaker, it’s that final detail that just makes the speakers great and sets them apart from the rest. It gives it that hi-fi, high-end look to it by simply cutting that corner at the top edge and smoothing its transition from the front to the top so that it, well, just looks cool. Sonically it should also reduce the effects of baffle edge diffraction around this surface. Baffle edge diffraction occurs anywhere there is a discontinuity on a surface where sound waves are present. I believe the Wilson Watt combats this problem with a type of acoustic foam attached directly to the front baffle around the tweeter and woofer. This way the sound waves traveling across the front of the baffle are attenuated before they even get to the edge, therefore potentially reducing the audible effects of edge diffraction by the listener. While in principle this should work, it also looks ugly. It’s probably one of the main reasons most speakers do not have some sort of acoustic foam on the front baffle surrounding the drivers, it can take a great-looking speaker and make it look terrible. Plus, I think it complicates the design in an area that does not need to be made complex. The construct of the tweeter, the voice coil, dome materials, doping methods, glues, the quality of construction, crossover type, frequency, etc. all play a much larger roll in whether or not a speaker sounds good. Once you start focusing on just baffle edge diffraction, even with a $50 tweeter, you’re really looking only to fine-tune what should already be a great-sounding driver. It’s taming that last little ripple in the response that your microphone is still picking up in the measurement that you just can’t let go. But that’s not to say that some speakers aren’t designed with edge diffraction as their sole feature with the entire focus being around proper driver/cabinet dispersion. Any speaker enclosure with huge rounded edges or extra ordinarily wide front baffles are taking into consideration these edge effects. Take almost the entire line-up of Thiel speakers for example or Sonus Faber. These guys believe whole heartily in the ill effects of edge diffraction and have designed in oversized front baffles and large-edge roundovers.

This effect can be modeled and there are tools out there for doing so. One of them is Diffraction and Boundary Simulator by our favorite Excel guru Jeff Bagby. Not only does the edge cause unwanted disruptions in sound but the location of the tweeter/woofer on the front baffle plays heavily into how bad these disruptions really are. Offsetting the tweeter from the center changes the distance from the radiating source such that the edge effects occur at different times with respect to each other. This can help reduce the overall effect of edge diffraction making the speaker’s FR response smoother. Unfortunately, the best-simulated design for diffraction tends to also be the worst-looking. So a designer must balance form with function in this sense. The complexity of the shape of the DM-4 cabinet doesn’t lend itself to an easy edge diffraction model and Jeff’s tool doesn’t exactly allow the sloped sides or a single 14°chamfer across the top to be modeled. But after researching it a bit more I came across another boundary simulation tool from the guys over at FRD Consortium called Baffle Diffraction Simulator. This tool will let me create the exact shape of the enclosure and then model its response. I’ve been playing around with it as well and it’s been pretty interesting the results. You can see a couple different screenshots below. I can say that the 1/4″ chamfer on all the edges, as small as it is, does provide as much as ±0.5 dB improvement in frequency flatness above 2 kHz. And the tapered sides and sloped front baffle provide a measurable difference as well when compared to a flat rectangular box with no chamfered edges. So moving on…

Another critical element to reducing edge diffraction effects is to flush-mount the drivers. Measured response plots can reveal the effects of edge diffraction for a tweeter mounted on the surface of a baffle (which might be only 5mm thick) compared to being flush mounted. Beside flush mounted drivers look soooooo much better. Finally a win-win for diffraction and aesthetics! I always like to flush mount my tweeters and depending on the design, will also do the woofers. Though the benefit of flush-mounting the woofers which have a lower crossover point is minimized due to the longer wavelengths. It still looks better to me!

So I’ve written plenty for this one post. The only thing I didn’t go into was the bracing technique I used. I’ll save that for another post, but the bracing definitely has some intentional design behind it and isn’t random. Recently I’ve adopted this design technique that starts with the bracing and enclosing a box around it, instead of building a box and adding bracing to it. It makes building the speakers really fun and is actually quite easy. The main consideration for doing the bracing this way is to brace each panel sufficiently such that the fundamental panel resonance is pushed out of range of the woofer (>2 kHz) as much as is practical. Also it mimics the lower cabinet bracing design. This box will be insulated using the same technique as the lower cabinets (with the carpet pad, cotton fill and polyfil) and should be extremely damped and extremely dead overall. The front and rear baffles are 1.75″ thick and the sides top and bottom are 1″ thick. Tapping/rapping on the cabinets with my knuckles provides a nice, dead, thump in response. So they sound very solid, very dead. Anyway, here’s the pictures, you may draw your own conclusions of the bracing design. Is it enough to do the trick?

Alright, enough talking, here’s all the pictures. Check them out below. And check out the video montage as well. Enjoy!

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