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Posts: 74
Location: Nashville TN
Joined: April 2, 2006
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I first became interested in minimizing diffraction when modifying a pair of NSM 10S Ultra Definition Mini-Monitors for nearfield listening. Rounded cabinet edges are said to help with diffraction, but very few speakers incorporate this into their design, and if your speakers weren’t manufactured this way, it is a difficult modification to make, and beyond what the average tweaker is willing to get into. Therefore, you are left with the option of some sort of diffraction minimizing device around the tweeter. I first tried a pair of the “Felt Defraction Rings for Tweeters” available from Madisound.com. They had a slight effect of making the high end a little smoother, so I knew I was on the right track. But there are two problems with these simple rings. First of all, these rings are very thin and are not thick enough to fully absorb the reflections. Research has shown that that in order to be of maximum benefit, they should be made of real wool felt in the proper density and of substantial thickness. Secondly, in order to help eliminate the problem with diffraction at the cabinet edges, it is recommended that the diffraction material extend all the way out to the edges of the baffle. (This may not be practical, or even necessary with large speakers / baffles, but with small mini-monitors like the NSM’s, it doesn’t make sense to stop with the diffraction material before you get to the edge.) The Madisound Rings help with diffraction immediately surrounding the tweeter, but do nothing for the diffraction at the edges of the cabinet. After more web searching, I discovered that a diffraction pad in the proper density, thickness and dimensions was available through a company by the name of ‘Diffraction Be Gone’ and are marketed as ‘Touchstone Tweeter Surrounds’. I contacted Jim Goulding at diffractionbegone.com/and gave him the make, model, & dimensions of my speakers and shared with him my plans to optimize them for nearfield listening. He assured me that the elimination of diffraction provided by his tweeter surrounds would be most beneficial in this particular application. Since I was not going to be using the grills on the NSM’s, I was a little concerned about how the diffraction pads would look. (The natural felt color of the pads are a grayish color.) Jim offered to dye the pads a brown color which would blend in better with the walnut wood finish of the speakers. I accepted his gracious offer, and within about a week, I had the pads and was ready to give them an audition. My first reaction after hearing my speakers with the Touchstone Tweeter Surrounds in place was “Good Gosh… these things are killing the highs!” But how could they? The pads weren’t in front of the tweeter, so there’s no way they could have been blocking the highs. Then it dawned on me - the highs that were now missing were not the primary highs emanating directly from the tweeter dome; they were the highs that had been reflected off the baffle area around the tweeter. These pads were doing exactly what they were intended to do. With the Touchstone Tweeter Surrounds in place, the highs were much cleaner sounding, a result of subduing the reflections which were arriving at my ear slightly behind the direct sound and smearing the top end. The imaging of the NSM 10S (already acknowledged as being one of the best in this regard) was enhanced with the use of these diffraction pads. With the tweeter reflections removed, the time and phase alignment of the drivers is preserved, leaving the subtle cues that assist with the pinpoint imaging of individual instruments within the soundstage. Needless to say, I am very impressed with this mod. The stark difference and apparent loss of highs that I noticed when I first installed the pads was because after hours and hours of listening to these speakers without the pads, THAT had become my ‘reference’ sound. But now, after spending a considerable amount of time listening with the pads in place, I prefer the way the speakers sound with the Touchstone Tweeter Surrounds, which is more natural and relaxed. The sound with the pads installed has now become my ‘reference’ sound, and removing them causes just as stark a difference, but not for the better. Without the pads, the speakers are brighter, harsher, fatiguing. Some have questioned why they should pay what Jim is asking for his Touchstone Tweeter Surrounds when you can buy the felt and cut them out yourself. Go ahead - buy a sample piece of 3/8” felt and try to cut it, but I’m telling you now, even with the sharpest knife or scissors, this stuff is almost impossible to cut and get a nice clean edge; and trying to cut out a clean hole in the center for the tweeter - forget it! (I think Jim uses a modified die cutting press to cut the felt which results in a nice precision cut pad.) Considering the quality look of these pads, along with the improvements they made in the sound of my speakers, I consider the Touchstone Tweeter Surrounds to be a bargain. Below is a photo of my NSM 10S Ultra Definition Mini-Monitors with the Touchstone Tweeter Surrounds installed. A complete rundown of the modifications I have done to these speakers can be seen in my personal email to Erol Ricketts which has been posted on the homepage of the NSMT website at nsmt-loudspeakers.com/. Ken Harris
Nashville, TN
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