The 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 Vandersteen VR-5 speakers. It’s sort of a marriage of the two designs, taking the best of the aesthetics of both and incorporating them into one awesome speaker. The dimensions are 12-1/2″ at the bottom with a 7° slope on each side 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 at 14° to mimic the VR-5 and reduce the effects of edge diffraction. 1/4″ chamfers around all exterior edges finish off the cabinets 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. But it’s not just the looks, the dimensions, the angles, the top chamfer all serve a purpose, too. Read on to find out why. Or skip my mumbo jumbo ramblings and move on to the pictures at the bottom. Your choice.
How I came up with 14° sloped front baffle was by drafting up side views of the raw drivers in Delta CAD using the .pdfs from Parts Express’s website. Phase/time alignment is achieved by placing the acoustic centers of each speaker in the same vertical plane. It is generally believed that the acoustic center of a speaker sits roughly at or around the voice coil location. The exact acoustic center is much more difficult to ascertain, so we are simply going for a best approximation. 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 active (MiniDSP), I will have independent control over the time/alignment of the woofer and tweeter. But if someday I decide to re-purpose these speakers, I may design a passive crossover for them and in that case I’ll be all set with the speakers physically already 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 improves (or degrades) the sound.
Vance Dickason described how a sloped front baffle could be used to counter the effects of a “polar tilt”, an upward (or downward) shift of the main lobe at the crossover frequency which occurs when the woofer and tweeter both have even-ordered filter slopes (2nd, 4th, 6th order). He claimed this tilt was about 15° based on the general offset of an average woofer and tweeter combo. In a passive crossover if the desire is not to physically set back the tweeter from the woofer, as is with most traditional speakers (and most of the speakers I’ve built) then one could shift the phase 90° (electrical phase) by changing one of the driver’s crossover slopes to an odd-ordered slope while leaving the other even. You actually see this quite often in commercial and DIY speakers where the woofer has a 12 dB slope (2nd order) and the tweeter has a 18 dB slope (3rd order). Now whether or not that was done to bring the polar tilt back to 0° or if it was just to protect the tweeter from low frequencies by increasing the filter cutoff rate is really up to the designer, but the benefit is mutual. And how much does a 15° polar tilt really affect the way a speaker sounds? Well, it can do a lot actually, especially if you’re shooting for a specific sound stage and response at a single given point in room, such as a listening position, seated, between the speakers at ear level. But the effect is about the same as moving from a seated position to a standing one. Most speakers should “sound good” no matter where you sit or stand, but if you’re searching for perfect phase and frequency response, with accurate soundstage directly in front of the speakers, then it’s important to at least recognize how the physical (and electrical) timing of the drivers affecting the projection angle of the main lobe.
The slope of the sides also has some merit to it – non parallel walls. Sound traveling across the width of the interior of the speaker will be forced to reflect downward and thus with each reflection the response decays, or rolls off, much quicker than if the walls were parallel. This does not mean that you get away without damping and proper insulation however. It just reduces the effect to some (very) small degree and makes the use of proper damping and insulation just that much more effective. Obviously again there are thousands of speakers aren’t built this way and they still sounds fantastic. The reason for the 7 degree angle was mainly aesthetics. I drew the speakers with various different angles from straight up and down to the max angle the woofer and tweeter would allow. Then I picked something in the middle that to me just looked the best. Then when I measured it and it came up to be 7° I thought perfect! It’s exactly half the slope of the front baffle so they should look good, like they match, like the speaker has some deliberate intent behind its design. Obviously the more exaggerated the slope, the more benefit is gained, though arguably the benefit is negligible in such a small space, but proper interior damping is not negligible and also is not visible whereas the exterior dimensions of the enclosure are visible and can make or break the aesthetics, in my opinion. As before, this is the first speaker I’ve built that didn’t have plane Jane parallel walls so I am curious to see how they turn out.
Another dimensional design consideration is the 3″, 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 Watt doesn’t have it, so it is even necessary? The answer is both yes and no. Aesthetically, it completes the speaker, in my mind, it’s that final detail that just makes the speakers, 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 almost 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. Baffle edge diffraction occurs anywhere there is a discontinuity on a surface where sound is present. Wilson’s Watts combat this problem with a type of acoustic foam attached directly to the front baffle around the tweeter and woofers. This way the sound waves traveling across the front of the baffle are attenuated before they even get to the edge, therefore reducing the audible effects of edge diffraction by the listener. While in principle this works, it also looks ugly. Just my opinion though. 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, voice coil, materials, glues, 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 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 the mic is still picking up that you just can’t let go. But that’s not to say that some speaker designs 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 with their 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 design. So a designer must balance form with function. The complexity of the shape of the DM-4 Cabinets doesn’t lend itself to an easy edge diffraction model and Jeff’s tool doesn’t exactly allow the sloped sides or a single 3″, 14°chamfer across the top to be modeled. So I don’t have a good estimate going in how much these cosmetic changes will really alter or improve the sound. But I certainly intend on taking a whole bunch of measurements to find out. 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.
So for these speakers there are four things that help to reduce the effects of edge diffraction:
The first being flush-mounted drivers. It goes without saying that just about everybody in the audio industry understands and believes the benefits of flush-mounted drivers. Measured response plots can easily 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 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. The second design feature that is helping to reduce edge effects is the top 3″ chamfer which looks like a 118° corner (instead of 90°). This provides a smoother transition from the front baffle to the top baffle. This results in less disruption in the waves traveling over it. The third thing is the angled sides of the cabinet. The distance from the center of the tweeter to any single point along the entire edge of the cabinet is not the same. The 7° slope provides an ever-changing edge that prevents the stack-up of diffraction at different frequencies (or at different times for the same frequency). Though to be fair, basically all rectangular speakers have this advantage, unless the tweeter is mounted in an enclosure all by itself with a round front baffle. The last design feature that helps reduce edge diffraction is the 0.25″, 45° chamfer along every edge of the enclosure. This creates a similar effect to diffusing the edge and thus reducing the disruption of sound waves as they travel over/around it. Granted this contour is small, only 0.35″ across its surface, it is just enough to help smooth the frequency response of the most critical frequencies from the tweeter. Edge effects are a peculiar thing because at low frequencies, the wavelengths are so long that the edge does little to disrupt the wave since it represents such a small fraction of its total wavelength. But baffle step losses do occur for this very reason where the driver transitions from radiating from 2pi space to 4pi space. Edge diffraction effects sort of start to take over just above the 2pi frequency point but then decay again as the tweeter frequency increases so much that it begins to operate as a single point source radiating directly into the air without ever reflecting across the front baffle to the edges. So there’s this really critical area in all speaker designs that is roughly from only a few hundred Hz to upwards of 10 kHz where you must be concerned about both the baffle step losses and the edge diffraction effects of a speaker cabinet. That’s actually a lot of bandwidth if you think about it! You don’t want to screw this part up. Anyway….
The only other thing I didn’t go into was the bracing technique I used. I’ll save that for another post, but even the bracing has some intentional design behind it and isn’t random. The main thing I considered was doing enough bracing to push the fundamental panel resonance out of range of the woofer (>2 kHz) as much as was practical and to mimic the lower cabinet design. This box will have so much insulation (carpet pad, cotton fill, polyfil) so it shouldn’t make a whole lot of difference but I did have some concept going in to stiffen the sides/top/bottom panels as much as possible. 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, if you made it this far, I’m impressed. Now you may view pictures. Which is all you really wanted to see anyway. Check them out below. And check out the video montage as well. Enjoy!