Anti diffraction pads on SDA's

dorokusai

The cheap sticky pads are not worth the time in any way shape or form, and really don't count as a plausible method here.

If you really want to experiment, get some wool pads or a thick felt media as you mentioned. There are multiple manufacturers that have used these materials succesfully and historically. Spica, Green Mountain, Rogers, ProAc and even KEF have used this method with great success. Others have used a segmented baffle, rubber or plastic with a dimpled or serrated groove structure.

It's a cheap enough tweak to play with but you need more than ears to really get anybody to give a crap.

drumminman

Heard back from Jim at Diffraction Be Gone. I sent him a link to the 1985 Stereo Review review of the SDA SRS by way of explaining the concept of SDA. Because of the unique design of these speakers, he's thinking that the anti diffraction pads may deteriorate the sound.

Priced 95% wool felt sheets, 12" square at McMaster-Carr: $35 each.

From what I've read Doro is right about attempting to use cheap synthetic felt. It doesn't work well, if at all. So now I have to decide if I want to spend $70 + shipping on an experiment. :question:

As Clint Eastwood said, "Well, punk, do ya feel lucky?"

geoff727

drumminman wrote: »

Because of the unique design of these speakers, he's thinking that the anti diffraction pads may deteriorate the sound.

How so?

dorokusai

I'm with F1Nut in his earlier assessment but it's something you'll have to try on your own....as well you should. As far as it causing a deterioration of the sound, I agree since the SDA effect is somewhat dependent on the front baffle, and the reflection/cancellation of the waves produced begin at that point. It wasn't designed with thick chuncks of wool pasted to the baffle.

drumminman

geoff727 wrote: »

How so?

Jim's (Diffraction Be Gone) honest assessment is that the mids are likely blowing spreading waveforms from the tweeters into the soundfield. If the tweeter waveforms aren't reaching the front baffle edges then the anti diffraction pads won't help and may hurt.

VR3

Thats interesting... hmmm

geoff727

Hmmmm, I'm going to politely voice some skepticism on that. First of all, that's not how sound waves propagate. One sound wave doesn't actively confine another like that, unless it's completely overpowering (like a jet engine vs. a tweeter). Sound waves tend to pass through one another. Plus, even if this were true, what about the SDA-1C, or 1B, or 2A, or 2B, or CRS? The tweeters are mounted above the mid-bass array.

If a sound wave propagating from the tweeter(s) is not high enough in frequency to retain highly directional characteristics, then it's going to meet the sharp edge of the enclosure and be re-radiated as an out-of-phase signal; that's diffraction. It does one thing to stereo imaging....degrades it.

I'm not saying that anti-diffraction felt (or whatever material) will for sure improve the sound that any given listener perceives from their speakers; this is a highly subjective thing. But, the fact is, diffraction on sharp-edged anclosures exists. And i will bet dollars to donuts that anti-diffraction treatment could not hurt in any way. Just like was said above...try it out!:)

I am, however, skeptical of a ring that surrounds the tweeter. At wavelengths where the tweeter is not yet becoming highly directional, I'm dubious about the absorptive effect of such a small area of treatment. That's why I used several inches on Fongolio's -1C's.

Anyway, for an interesting study on the listener subjectivity of diffraction effects, get into Dickason's LDC, Chapter 6.

G~

heiney9

geoff727 wrote: »

Hmmmm, I'm going to politely voice some skepticism on that. First of all, that's not how sound waves propagate. One sound wave doesn't actively confine another like that, unless it's completely overpowering (like a jet engine vs. a tweeter). Sound waves tend to pass through one another. Plus, even if this were true, what about the SDA-1C, or 1B, or 2A, or 2B, or CRS? The tweeters are mounted above the mid-bass array.

G~

What is your take on the D'Apollito array?


A loudspeaker configuration developed by and named for Joe D'Appolito, in which a high frequency driver, or tweeter, is positioned between two midrange or low frequency drivers that each cover the same frequency range. Depending on the exact implementation the speakers can be positioned with a vertical and/or horizontal orientation. In either case the two midrange drivers serve a couple of purposes: they combine to create a larger effective woofer or midrange driver size, and they also serve to control the dispersion of the tweeter. The tweeter's output is somewhat corralled or contained by the sound coming from the midrange drivers in a similar way to how two parallel surfaces control dispersion. There are some variations on the design where two same sized woofer/midrange drivers may cover slightly different frequency ranges, however those aren't considered true D'Appolito designs. (Input from an inSync Reader:) The D'Appolito design specifies a third order crossover network. The tweeter is coordinated with the woofer so that at the selected crossover frequency, the drivers all have similar horizontal dispersion. (This is not easily accomplished because many drivers behave badly at the extremes of their range.) The advantage of doing it all correctly is one of the most seamless blending of drivers possible. The result is an absence of any sudden change in directivity with frequency. This may not mean much for monitors where there is a limited listening area, but in a typical room where a large percentage of the sound is reflected by the room, the effect is dramatic.

It seems to go against your statement.

H9

geoff727

Interesting highlighted section, H9. Where did that come from, by the way? What I said is true, but it may be simplistic. Maybe way too simplistic, because this is going to start to get really complex when relative phase is considered between the two sources (I think that's where the highlighted section above is going). My initial thoughts were of SPL plots I've seen of cross-firing sources, but again, that may not be going nearly deep enough into it. And then it really becomes deep when considering distances between sources, frequencies covered, the effect of the crossover network, etc. Again, simplistically, that's why radiation patterns (which are vertically symmetrical) change in shape in the MTM array when different driver spacing and xover networks are used. And in the case of the SRS, SRS2, 2.3, etc. a variation will certainly apply, even though the speakers are not true side-by-side MTM's. Even so, it begs the question...just how much sound from the tweeters does reach the edge of the cabinet, considering the distance between the acoustic centers of the tweeters and the mid-bass drivers, the operating frequencies, etc. It would certainly be interesting to measure, although i don't know if it would do any of us any real good.

I know this is going off topic, so I should probably just stop. But, this has peaked my interest here a bit. Time to get out the books....again!

G~

heiney9

geoff727 wrote: »

Interesting highlighted section, H9. Where did that come from, by the way? What i said is true, but it may be simplistic. Maybe too simplistic, because this is going to start to get really complex when relative phase is considered between the two sources. And then it really becomes deep when considering distances betwen sources, frequencies covered, the effect of the crossover network, etc. Again, simplistically, that's why radiation patterns (which are vertically symmetrical) change in shape in the MTM array when different driver spacing and xover networks are used. And in the case of the SRS, SRS2, 2.3, etc. a variation will certainly apply, even though the speakers are not true side-by-side MTM's.

I know this is going off topic, so I should probably just stop. But, this has peaked my interest here a bit. Time to get out the books....again!

G~

With respect to sda's that have the tweeter's arranged vertically, they all use a progressive point source, so the typical radiation patterns are quite different and really don't have a lot to do with D'Appolito or standard radiating patterns.

"Polk's Progressive Point Source technology to maintain Constant Vertical Directivity of mid and high frequencies, which prevents undesirable beaming. As frequencies increase, the tweeter array adjusts its radiation area and eventually becomes an ideal point source at the very highest frequencies, eliminating frequency interactions and reflections between multiple drivers." - From Polk Sales Literature

The tweeters each play at a different level of output.

My point is that SDA's are a little different and I would suspect the typical radiation patterns attributed to standard forward facing multiple driver speakers can't be applied apples to apples.

Joe08867

I agree H9. If you use geoff's school of thought then MTM designs would sound just like any other box speaker design. Of course the midrange changes the characteristics of the tweeter as a woofer does to a midrange. Music waves are like water waves. Tweeters make small tight waves, mids make mid size waves and woofer and subs make large waves.

I have used felt around a couple of speakers, it will help when the tweeters smear or have a wider dispersion pattern than the mid or woofer they are working with. I also cover the area around the tweeter so that only the dome shows through. Having it just on the baffle face and not on the tweeter body is not an effecient use of the product IMHO.

heiney9

geoff727 wrote: »

Interesting highlighted section, H9. Where did that come from, by the way? G~

Sweetwater web page, in the glossary section. But search D'Appolitto on the web and you'll get all kinds of hits. I chose that definition for it's simplicity and directness. There are several other links that go into a lot more detail.

H9

geoff727

Above edited.

Joe08867

I could see the idea of anti diffraction pads in the case of SDA2's or CRS's, since the have a single tweeter above the mids and a large surface surrounding them. But not in any of the other SDA's including the SDA1's as the tweeters are set at different frequencies to self cancel diffraction issues as H9 stated above. And not in the SRS2 with a single tweeter as the mids surround the tweeter also canceling any diffraction.

Joe08867

Geoff, I do agree with you about the tiny felt ring around the tweeters. I don't think this was used for diffraction as much as control of the material being used for the dome. In one case Mylar in the other a Tri-lam but both have limitations and take mechanical devices to eliminate certain issues and limitations associated with there designs. Not unusual as certain companies used felt over there tweeters to help control there dispersion and keep them confined to a certain area of frequencies.

Sorry I do not know all the lingo but I would bet you get my drift?

Fongolio

Joe08867 wrote: »

I could see the idea of anti diffraction pads in the case of SDA2's or CRS's, since the have a single tweeter above the mids and a large surface surrounding them. But not in any of the other SDA's including the SDA1's as the tweeters are set at different frequencies to self cancel diffraction issues as H9 stated above. And not in the SRS2 with a single tweeter as the mids surround the tweeter also canceling any diffraction.

So.....does this mean the felt on my 1C's is not really effective since the tweeters cancel diffraction issues, or is the felt actually degrading the output from the tweeters? I'm still not convinced that the felt is uneffective. Perhaps some experimetation is in order. Geoff, will I hurt the finish in any way by peeling the felt off? I don't want to do damage. The blown drivers earlier this year was far too stressful, and I don't want to feel that way again.:sad:

heiney9

Fongolio wrote: »

So.....does this mean the felt on my 1C's is not really effective since the tweeters cancel diffraction issues, or is the felt actually degrading the output from the tweeters? I'm still not convinced that the felt is uneffective. Perhaps some experimetation is in order. Geoff, will I hurt the finish in any way by peeling the felt off? I don't want to do damage. The blown drivers earlier this year was far too stressful, and I don't want to feel that way again.:sad:

Can't say. I take it you've never heard your 1C's sans the felt? Experimentation is definitely in order. I have 1C's and see no reason to try this. I couldn't be happier with the output, but then I have them set-up as per Polk. There will be some variation simply based on how we each have them set up in our listening environment. So is would suspect some may feel they have an improvement and some may not.

H9

Face

Surrounding a tweeter with Mids will not cancel diffraction, if anything they can cause it.

Kelvin, if you're happy with the way you're speakers sound, keep them on. The felt pads do change the sound, but for better or worse is up to you. Many OEM's have been doing this for years including Tannoy, Wilson, AR, etc...

geoff727

Kelvin, 3 PM's sent.
G~