SPEAKER ACCESORY REVIEW
Diffraction Be Gone
List Price - $49.95 / pair
Jim Goulding of Diffraction Be Gone offers a simple brute force method of controlling loudspeaker diffraction for the audio consumer. Diffractionbegone’s Touchstone Tweeter Surrounds are high quality wool 3/8” - deep surrounds with a hole for the tweeter. The idea is to control diffraction through absorption. Given the exact model of speaker, they custom cut the tweeter pad to fit the speaker dimensions. The pads are cut to cover the entire front face of the tweeter area in an effort to most effectively control diffraction. A special request must be made if one wants to use these in combination with their speaker grills.
Last year at a New England audio DIY get-together, an acquaintance applied a wool felt treatment to a set of speakers that I had built and brought along with me. These speakers had little built-in diffraction control, so the effect was quite noticeable. I remember the image popping into focus while the soundstage took on greater depth. I applied my own treatment a few days later. Going the DIY route and attempting to cut wool felt in a professional manner proved daunting and I wished for a professional product. Wish granted. When offered the chance to review the tweeter pads from diffractionbegone, I was struck by the name and could only think of the guy from the OxyClean commercials screaming at me. After shrugging that image off, with earnest curiosity I quickly accepted the task of determining the effectivenessof the product.
Diffraction is essentially a reflection of the initial sound wave off the speaker front bezel that can interfere with the reproduction of the initial waveform. A simple visual example would be a wave coming through a hole in a wall dividing two bodies of water. The opening in the divider can be considered the tweeter. As the waves come through the opening, they radiate out 180 degrees on the horizontal plane in the water. The waves that radiate out to the sides reflect off of the wall. One can consider the faceplate of the speaker as the wall. As the waves come out, some of them bounce off the wall with phase differences causing ripples. These ripples, when it comes to loudspeakers, can be considered a series of nulls and phase reversals of the original waveform. An audible effect of this is the sound changing rather dramatically as one tilts their head around a little while listening. Some speakers do this more than others. These ripples of sound are also arriving ever so slightly later than the original waveform resulting in what some call time smear. To the listener, it sounds exactly as described; a smearing of the image and the instruments and voices within it.
Many audio pundits would argue whether the effects of time on the sound are audible, using all sorts of math analogies and likely taking things to a personal level. Personally, I believe that if some of the original waveform is arriving before or after the rest, the original waveform is not preserved. This is something I would try to avoid if possible. That being said, the waveform from the tweeter in the speakers I used most extensively in this review arrives before the woofer, so the time is already affected somewhat. Back to time smear. The manufacturer claims that by absorbing the reflections off the face of the cabinet, reflected sound that would otherwise be mixing with the original waveform after travelling a slightly larger distance, the smear from these reflections is effectively removed, or at least diminished. After using these pads for a few months now, I've become a believer.
Diffraction has another facet called "edge diffraction". When the sound waves emanating from the front of the speaker reach a sharp edge or corner, bad things happen. The waveform is distorted or scattered in odd ways beyond the edge of the speaker. This effect is not really dependent on frequency so much as it depends on the angle of the edge. The diffracted waveform beyond the edge of the speaker now has the opportunity to bounce off something else in the room and arrive at a later time with the phase shifted. It is the combination of the direct waveform and the diffracted waveform, likely out of , that affects the frequency response. Essentially, a speaker with a square, flat side edge can throw a waveform in a very specific direction, making room interaction and speaker placement more difficult.
Some speaker companies control edge diffraction only by using a large roundover on the baffle. I had some Revel F30s which did this and were pretty easy to place in the room if one ignores the rear bass port. Folks will argue over the benefits of a rounded edge on the sound. In my own experiments, I found that a 3/4 or larger rounder reduces the effects of the sound changing as you move your head around. Others control diffraction by controlling the waveform via a waveguide. The venerable Andrew Jones did this for KEF and now with Pioneer/TAD to great effect. The TAD speakers were quite nice at this year's Rocky Mountain Audio Fest.
Others go the completely opposite route by elminating much of the cabinet and putting the drivers nearly out in the open with little faceplate to deal with. One example is Vandersteen. Green Mountain Audio takes this one step further in it's Calypso speaker by carefully shaping the separate cabinet for each driver such that the waveform behaves quite nicely based on lots of fancy science. Regardless of the science, they sounded good and that's what really matters. Both of these companies happen to use wool felt treatments based on both theory and extended listening.
Another form of absorption was done by Celestion with the SL series back in the late 80's. They used a series of vertical deep ribs along the face of the speaker, both trapping some of the reflected sound as well as diffusing it. Thiel's method of diffraction control is a combination of rounding the cabinets as well as using super flush grills that seem to have no audible effects due to a rather small edge surface. The felt treatments help absorb the sound that would otherwise be diffracted.
In my experience, using grills with a large square edge make it real easy to play "pin the tail on the tweeter" as the speakers seem to call attention to themselves instead of disappearing. Not much fun for those of us who find keeping the grills on necessary.
Applicable speakers? Only a few of my own speakers would actually be able to use the felt tweeter pads from Diffraction Be Gone. Many of our readers will run into the same issue as some speakers just won't benefit or be able to use this tweak. The following speakers hanging around my home need not apply.
Maggies - There is absolutely no way that the felt pads would be applicable here although I wouldn't be surprised if Jim Goulding came up with a felt tweeter strip for the side with the ribbon tweeter as a follow on product.
KEF C95 - This is an old set of speakers sitting in my basement but I couldn't use these either. The tweeter is concentric with the woofer using the woofer as a waveguide. One certainly cannot affix a felt pad to the woofer and the waveguide negates the need for the felt pad in the first place.
Insignia - Yes, I actually fell into the trap and bought a set of these out of curiosity. They have the same issue as the KEFs. They are fine for the garage however.
Radio Shack LX-5 with the dipole Linaeum tweeter. The tweeter sits on top of the speaker essentially out in the open so the felt tweeter pad is of no use there either.
For most of the auditioning of this product, I used a set of DIY speakers designed using the ICD functionality in the WooferTesterPro. These are my main speakers in my main listening room. I do not miss the Revel F30s they replaced. Most DIY speakers I have heard have been either frighteningly bad or just OK so if I were reading a review and they used DIY speakers, I would be skeptical too.
I had a friend try this with his Devore Gibbons for a couple weeks just to get a production speaker into the mix.
This is the speaker before the treatment is applied. Note that it already has several forms of diffraction control built in. I did not have a traditional square box speaker to use. The front baffle has a ¾” round over, the tweeter is offset to help avoid any peaks from reflections off the face, and the sides are curved to help even more. Would diffractionbegone help even with a fair attempt at diffraction control?
Note that the panel behind the speaker is merely an acoustic room treatment.
Here is the speaker with the felt treatment applied. Later I found that I could incorporate the felt into the grill itself and still have the sonic benefits but keep the grills handy to help protect against prying hands. As one can see, it’s not pretty. Black would have been much nicer but it’s really the sound that matters.
ARRIVAL AND SETUP
The pads arrived in a bubble wrap envelop with Velcro tabs and a nice note explaining the concept. The pads were cut explicitly to what I specified as these are not production speakers. Too bad I forgot to mention the offset tweeter. It was my fault but he offered to send another set with the correct dimensions. With a 30 day money back guarantee, the company is kept on it’s toes keeping customers happy.
I quickly applied the pads to the speakers and sat down to an already warmed up system.
I immediately noticed an increased sense of “you are there” after applying the treatment while simultaneously losing a bit of the immediacy in the sound. The tonal balance shifted to a tiny bit more mellow which was later shown as slight smoothing of the response as well as a small peak around 6Khz getting tamed.
Center image focus was smacked right into place to the point where I had to remove the pads and go back and forth. With Nora Jones, “Feels Like Home” on CD, she was right there singing for you. The sweet spot got larger such that two folks sitting side by side could both listen in the sweet spot. Too bad my wife is not a Nora Jones fan. Her response was, “that sounds really good. Can I go now?”
On a whim, I threw in Led Zeppelin “Houses of the Holy” on CD. I was surprised to hear a difference here too. Sure, the imaging improved but the sense of space was heightened giving the recording a bit more of a garage band feel. I kept it in the CD changer for nearly a week and kept going back to it. The voice was more palpable and the guitars better delineated. Again, I had to go back and forth because these tweeter pads were really making a difference. One example is that it was easier to delineate the separate simultaneous guitar riffs in Over the Hills and Far Away.
At the Rock Mountain Audio Fest, I was lucky enough to catch the Fry Street Quartet more than once. I really enjoyed their performances and felt compelled to pick up a recording of them. This album… errr… I mean CD did not have the same songs they played in Denver but their sound was still there. This release ranges from strikingly engaging and dynamic to a softer side and every bit of it was even better with the tweeter pads applied.
Note: A full review of this recording is upcoming with interviews of the artists - publisher
This release from John Coltrane is always soulful and sultry but became even more so with when listening with the tweeter pads applied. I found myself not really analyzing the music as just enjoying it and forgetting about this whole review thing; the ultimate signal that something has improved the system. I had to go back later and explicitly compare to find that the sense of space in the recording was more obvious and the delineation of the horn textures was clearly better while reducing any listening fatigue even further. I also noticed that the dynamic transitions from loud to soft conjured a bit more of an emotional response.
I lent the Diffraction Be Gone's to a friend who has Devore Gibbons and all Bryston Separates. He tried them but never even really sat down to listen because they did not fit under the super flush Devore grills. Given how beautiful his home is, I only gave him a smidgen of a hard time about that.
These things are not pretty and may cause your significant other to wince. They look a bit like somebody punched a clean hole through an old gray sponge. A simple solution was to embed the pad into the grill after some careful trimming. A little hot glue tacked it in place after more listening just to be sure. It was only then that I realized that I would end up buying them. I contacted Jim Goulding about that and I need to note right here that Jim Goulding basically said that I could keep them. I felt a little strange about it but that’s how it went down. Either way, there was no way I was going to let these go after listening with them after finding a way to incorporate them into the grills.
I took some measurements in my basement lab to see if there were any measureable differences. I used a WooferTesterPro with 512 data points from 500Hz to 20Khz and an impulse test signal in an effort to minimize the acoustics of the lab itself. This is just a reminder of how much the room can affect the sound. Even with the impulse response, it will measure different depending on whether the lab is dirty and cluttered or clean indicating that it should be better treated acoustically.
Here is the measured response without the felt pad. The db response is in pink and the phase response in blue.
This is the measured response with the felt pad applied. Notice the little peak around 5Khz is tamed and that phase response, in blue, is smoother
Here is an overlay of with and without the felt pad applied for comparison
Again, the response is a bit smoother and the measured phase is noticeably smoother.
It seems that the most obvious effect was off axis. Here is an overlay of with and without the pad at 30 degrees off axis. Without the pad is pink db and bright blue phase while with the pad is yellow db and deep blue phase. Some peakiness around 6Khz is tamed and the phase response is better too.
• Bryston B100 SST integrated amplifier
• Nakamichi MB2-S as transport
• Panasonic DVD S97 as transport
• Pacific Valve modified DAC-AM
• Belden 1694A digital coaxial cable.
• DIY monitors designed using the ICD section of the WooferTesterPro and set of sand filled speaker stands
• BlueJeansCable LC1 interconnects in 2 foot lengths
• Kimber 4TC speaker cable
• Audioquest NRG-2 and Volex 17604 power cords
• Furman PST8-Digital surge suppressor/filter
• Hospital grade PS audio outlet
• Many room treatments
You won’t find us waxing poetic about this speaker tweak but it works swimmingly in the right application. They improve imaging, soundstage, detail, and even reduce listener fatigue. At only $49 per pair with a 30-day money back guarantee, it is certainly worth trying. Sonically, these things have Mojo with a subtle but obvious improvement you will notice right away. The caveat is that not all speakers will benefit or even accept these felt pads. However, if you own a speaker with a flat front such as those pictured in this review, the DiffractionBeGone tweak might prove very beneficial.
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