Felt around tweeters

[Flash21]

One part of replacing my tweeters (see other thread) that I didn't mention was removing the felt I had glued to the upper baffle and tweeter faceplate on my Monitor 10Bs years ago, based on a article I read claiming reduced reflections and improved imaging. I never kept the article, so I was wondering if any of you guys have it? I would like to read it again...

[dorokusai]

AR called it the Acoustic Blanket I think....George, Russ? Dahlquist used velvet on the front baffle of the DQM series.

It's not a crazy idea, and it just depends on if you subscribe to that theory.

[ND13]

I've never felt up or around my tweeters. You're a sicko!!! :D :D :D

[TroyD]

If I remember correctly, Stereophile did an article on the same premise about a year ago.

YMMV, however, I believe in the Acoustic Blanket.

BDT

[hoosier21]

If I remember correctly, Stereophile did an article on the same premise about a year ago.

YMMV, however, I believe in the Acoustic Blanket.

BDT

Makes since to me, worry about treating the room, the first reflections and all, so why not dampen the reflections right at the tweeter

[madmax]

I messed around with the rings you wrap around the tweeters. Drove me nuts for several weeks! I could make any particular piece of music sound better but others would suffer. Crazy!
madmax

[Larry Chanin]

Hi Steve,

Here's a link that may interest you.

Legitimate Tweak, or Worthless Gimmick? (http://www.polkaudio.com/forums/showthread.php?t=27696&highlight=tweak)


And here's a excerpt of a review (http://www.stereophile.com/standloudspeakers/1203acarian/index1.html) of a reasonably priced bookshelf speaker in which the reviewer describes his experiences with and without the felt diffraction ring.

When the original Li'l Rascal was introduced in 1999, its tweeter was fitted with 1"-wide felt rings around the circumference of its dome, to modify re-radiation of high frequencies from the cabinet edges. According to Marchisotto, the ring purifies and smooths the speaker's high-frequency response. The first production run of the Mk.II version, however, was sold without the rings. The idea was that a ringless speaker would produce a livelier high-frequency presentation, and thus be a better match for inexpensive electronics with less revealing high-frequency response.

Marchisotto ultimately changed his mind, and the Li'l Rascal Mk.II now comes with diffraction rings. As it turned out, my review samples were of the early, ringless variety; JA felt that I should listen to them—and that he should remeasure them—with rings installed. Acarian Systems mailed me a pair of rings, and I fitted them to my review samples.

The diffraction rings tamed some of the vocal sibilants I'd noted on the Madeline Peyroux recording: the highs sounded smoother and more refined, and Mark Ribot's dobro was a touch less metallic. The difference was less noticeable on Mighty Sam McClain's Give It Up to Love—with the rings, the reproduction of vocals and cymbals was almost indistinguishable from the sound without the rings—but the Fender Stratocaster's high-frequency attacks were tamed a bit. (The latter difference was akin to turning down the Twin Reverb amplifier's treble knob one number—more subtle, in guitar terms, than flipping off the Bright switch, or swapping a silver-panel, 1970s-vintage amp for a mellow pre-CBS version from the early 1960s.) The more highly modulated passages of Janis Ian's "Walking on Sacred Ground" (LP, Breaking Silence, Analogue Productions CAPP027) were less forward and more natural with the rings fitted, but there was very little difference on the low-level passages.

Overall, the effect of adding the tweeter diffraction rings was subtle, and more noticeable with some recordings than with others. The effect was not as great as switching from the Creek to the Audio Research amp, for example. On balance, however, the rings resulted in a more balanced, musical, and involving presentation overall. Acarian Systems has promised to provide diffraction rings free of charge to owners of early-production Li'l Rascal Mk.IIs. I recommend that they be taken up on the offer.—Robert J. Reina

In this section of the review entitled Measurements Postscript (http://stereophile.com/standloudspeakers/1203acarian/index4.html), you'll see the author provides actual before and after measurements. In this case there was a small, measurable improvement in the frequency response and in the speaker's lateral radiation pattern.

No doubt the effectiveness of this approach is speaker specific and depends on how well the speaker already deals with diffraction without the diffraction ring.

Larry

[Flash21]

Thanks for the ideas, I have found the Stereophile article which references the old AR Acoustic Blanket...I'll have to look into that a little more. Wish I could remember the specific article I saw, but it was years ago and I don't have a clue if it was even media (vs. internet-type blather).

I remember an obscure old speaker brand called Avid that was big-time into the reduction cabinet of reflection/refractions. The drivers were carefully mounted flush to the front baffle, with smoothly rounded transitional elements to avoid any edges that might "catch" sound waves. The grill frames were recessed into a groove around the outer edge of the cabinet so they couldn't reflect sound, and the outer edges of the frames were rounded too.

But for all that they didn't use any absorbent material on the baffle itself...

[BobMcG]

I messed around with the rings you wrap around the tweeters. Drove me nuts for several weeks! I could make any particular piece of music sound better but others would suffer. Crazy!
madmax

Seems to Me: Probably no different than the felt blankets. Might be a plus sometimes and a minus other times. Maybe if it was a actual plus all the time, all speaker manufactures would be incorprating the theory into their designs. I mean why not. They'd be selling a superior product with little extra investment.

[Flash21]

The rings seem a little different to me, unless they are 100% absorbent they have to be focusing the sound somewhat, like a horn. The felt I put on the baffle would not redirect the sound at all, just prevent reflection off the baffle.

[dorokusai]

I've used those rings as well and hated them. Thankfully they aren't expensive.

[Larry Chanin]

Hi Steve,

Here's an other scientific approach to the issue. It shows that all "felt" is not created equal. In this example one type of felt ring hurts frequency response whereas a thicker felt block improves the response. Needless to say the in either case the placement of the felt in relation to the tweeter assembly is very important.

Felt Rings verses Felt Blocks for Baffle Diffraction Reduction (http://www.speakerdesign.net/felt/felt_ring_vs_blocks.html)

Larry

[Larry Chanin]

Thanks for the ideas, I have found the Stereophile article which references the old AR Acoustic Blanket...I'll have to look into that a little more. Wish I could remember the specific article I saw, but it was years ago and I don't have a clue if it was even media (vs. internet-type blather).

I remember an obscure old speaker brand called Avid that was big-time into the reduction cabinet of reflection/refractions. The drivers were carefully mounted flush to the front baffle, with smoothly rounded transitional elements to avoid any edges that might "catch" sound waves. The grill frames were recessed into a groove around the outer edge of the cabinet so they couldn't reflect sound, and the outer edges of the frames were rounded too.

But for all that they didn't use any absorbent material on the baffle itself...

Hi Steve,

Is there an Internet link to the Stereophile article on the AR Acoustic Blanket?

Here's a DIY article on using the felt type blocks referenced in the David L. Ralph article.

Making felt front baffle diffraction pads (http://home.comcast.net/~audio-worx/page90DIY-FeltPads.html)

Larry

[nadams]

Also note that Polk did this themselves on the SL2500 tweeters... as seen here:

http://www.king-nerd.com/dood/images/temp/tweet/imag0001.jpg

[Flash21]

Here is the Stereophile article... http://stereophile.com/reference/704cutting/index.html
it is about cabinet diffraction, and mentions the Acoustic Blanket on page 2

A quote:
"In AR's listening tests, fitting this blanket resulted in three distinct areas of subjective improvement: better tonal accuracy, more stable stereo imaging, and a narrowing of the width of virtual sound sources—all much as you might anticipate from removing the frequency-domain ripples and time-domain smearing that short-period reflections introduce."

[Flash21]

The felt I used was just a very thin layer across the entire upper baffle...it had very little thickness to speak of, but presumably provided a less reflective surface for the baffle. Just made sense it would reduce image smearing...how much? I dunno...

My concern with those thick rings is that they are acting as a partial reflector, depending on absorptive properties (natural wool, synthetic wool, etc.). On the other hand, they are effectively shadowing the rest of the baffle from the tweeter, which would reduce baffle reflections quite a bit...

[Larry Chanin]

The felt I used was just a very thin layer across the entire upper baffle...it had very little thickness to speak of, but presumably provided a less reflective surface for the baffle. Just made sense it would reduce image smearing...how much? I dunno...

My concern with those thick rings is that they are acting as a partial reflector, depending on absorptive properties (natural wool, synthetic wool, etc.). On the other hand, they are effectively shadowing the rest of the baffle from the tweeter, which would reduce baffle reflections quite a bit...

Hi Steve,

The article by David Ralph would suggest just the opposite. The 1/2" "real" felt performed measurably better than the the 1/8" synthetic felt. This would suggest that the difference, at least in this case, lies more in the absorptive properties of the diffusion device more than the thickness.

Larry

[dorokusai]

Adding felt to the front baffle, of the driver array, is a long time Dahlquist DQ-10 tweak.

[George Grand]

The Acoustic Research Acoustic Blanket, helping people get better sound since 1979. I posted these findings YEARS ago on this forum, but as usual WTF do I know? Don't buy special anything. Go to Home Depot or Lowes and get the thickest shit you can find. The AR stuff is about 3/4" thick.

On the AR-9 the entire front baffle area is blanketed. The dome tweet, dome upper mid, and 8" cone lower mid are all encompassed. It works like nobody's business.

[Flash21]

Hi Steve,

The article by David Ralph would suggest just the opposite. The 1/2" "real" felt performed measurably better than the the 1/8" synthetic felt. This would suggest that the difference, at least in this case, lies more in the absorptive properties of the diffusion device more than the thickness.

Larry
Oh, of that I have no doubt...I guess what I was trying to say is that the very thin self-adhered felt I used didn't really have a side surface to reflect off of. It was just a way to reduce the reflectivity of the baffle itself.

Because when you introduce a ring of 1/2" material of ANY kind around the tweeter, you must start thinking about the reflective/absorptive properties of that material. To be sure, different materials will reflect differently, in differing amounts and at different points in the spectrum - but nothing absorbs 100% of the energy. What is not absorbed must be reflected. So where does the reflected energy go? In some cases, a ring configuration could be like cupping your hands around your mouth...trading dispersion for projection of those frequencies.

In other words, with the ring tweak you are changing the frequency response of your speaker, whereas felt on the baffle is only intended to improve imagining by reducing unwanted reflections, without materially affecting frequency response.

Having said all that, you may very well like the sound with the rings, which is fine...there is no absolute right/wrong answer here...

[Flash21]

The Acoustic Research Acoustic Blanket, helping people get better sound since 1979. I posted these findings YEARS ago on this forum, but as usual WTF do I know? Don't buy special anything. Go to Home Depot or Lowes and get the thickest shit you can find. The AR stuff is about 3/4" thick.

On the AR-9 the entire front baffle area is blanketed. The dome tweet, dome upper mid, and 8" cone lower mid are all encompassed. It works like nobody's business.
Hi George - I am not claiming to be breaking new ground here, just was trying to track down old info I remembered vaguely from years ago...maybe it was yours!

A couple of questions:

What type of "shit" are we talking about here? Felt of some kind?

How is the edge where the "felt" (or whatever) meets the driver handled? is the felt compressed down to avoid creating a hole around the driver?

Does thin self-adhered felt accomplish anything? (I'm guessing not, if they are using 3/4" stuff...

[Larry Chanin]

Oh, of that I have no doubt...I guess what I was trying to say is that the very thin self-adhered felt I used didn't really have a side surface to reflect off of. It was just a way to reduce the reflectivity of the baffle itself.

Because when you introduce a ring of 1/2" material of ANY kind around the tweeter, you must start thinking about the reflective/absorptive properties of that material. To be sure, different materials will reflect differently, in differing amounts and at different points in the spectrum - but nothing absorbs 100% of the energy. What is not absorbed must be reflected. So where does the reflected energy go? In some cases, a ring configuration could be like cupping your hands around your mouth...trading dispersion for projection of those frequencies.

In other words, with the ring tweak you are changing the frequency response of your speaker, whereas felt on the baffle is only intended to improve imagining by reducing unwanted reflections, without materially affecting frequency response.

Having said all that, you may very well like the sound with the rings, which is fine...there is no absolute right/wrong answer here...

Hi Steve,

I agree that we have the potential to both improve and hurt imaging with the use of diffraction devices. However, I would like to address a few of your concerns.

As we know diffraction is a form of Early Reflection. Early Reflection hurts imaging two ways.


A psychoacoustic effect whereby our brains blend the delayed reflected sound and the direct sound via Precedence Effect, thereby smearing the sonic image.
Comb Filtering whereby the reflected sounds and the direct sound constructively and destructively interfer thereby causing nulls and peaks in the frequency response, and causing lobing in a speaker's dispersion pattern.


In either case if the reflected sound is 9-10 dB less than the direct sound, our hearing won't be able to perceive the negative effects. Therefore, we don't need perfect 100% absorption, just enough for the range of frequencies of interest.

For the higher frequencies the more dense a given absorber is, the more acoustically reflective and less effective as an absorber it will become. The thicker an absorber is the more effective it will absorb lower frequencies.

Therefore a thick, porous absorber may prove to be more effective as a diffusion device than a thinner, denser absorber, even if they are made of the same material. This is due to the fact it has higher absorption over a wider range of frequencies.

With regard to your concern about focusing the sound with a diffraction ring, one way in which to determine how directive the radiation pattern of a speaker is would be to measure its lateral response. In the Stereophile article by Robert Reina they measured the lateral response both without and with the diffraction ring. The results showed "the overall dispersion with the tweeter ring is smoother overall, wider above 7kHz, and no longer has the off-axis "horns" at 6.5kHz." So the elimination of the reflections from all directions around the tweeter did not result in focusing the sound, just the contrary.

Larry

[Flash21]

Makes sense, mostly...I wonder if the improved frequency response in the test was due to favorable absorption of frequency peaks by the material, or if the ring was preventing reflections off of the surrounding baffle...or both.

[Larry Chanin]

Makes sense, mostly...I wonder if the improved frequency response in the test was due to favorable absorption of frequency peaks by the material, or if the ring was preventing reflections off of the surrounding baffle...or both.

Hi Steve,

As I mentioned the frequency response of a speaker is affected by the degree of comb filtering occuring at the listening position. When it occurs, it creates nulls and dips in the speakers frequency response. Comb filtering is reduced if the reflected sound is less than the direct sound, as would occur by adding an absorber to the baffle, or it can be eliminated if the sound reflected off the baffle was redirected to a location other than the listeners, as in a specular reflection.

So yes, in theory, it could be a combination of fortuitous redirected reflections and absorption.

However, if you'll notice the measurements made by David Ralph showed an improvement in frequency response for the "real" felt, and he didn't use a ring configuration. This in conjunction with Robert Reina's measurements with a ring would suggest that its more a function of favorable absorption, the factor common to both experiments.

Your prior comment may be contributing to some confusion on the subject:

In other words, with the ring tweak you are changing the frequency response of your speaker, whereas felt on the baffle is only intended to improve imagining by reducing unwanted reflections, without materially affecting frequency response.
In both cases we are improving the speaker's frequency response by reducing the amplitude of unwanted reflections, because that's what reduces comb filtering, and it is the reduction of comb filtering that improves the frequency response. You can't reduce unwanted reflections without improving frequency response.

Larry

[George Grand]

It is a combination wool/felt type of affair on the original AR blanket. It does not taper, it is full thickness throughout. The "cut-out" around the tweet, upper mid, and lower mid are all different. The tweeter cut-out is a circle, just big enough for you to remove the entire tweeter assembly without disturbing the blanket. You can see the mounting screws. The dome upper mid-range has a rectangular cut-out, considerably larger than the assembly itself. The 8" cone lower mid (the same 8" driver that is used as a woofer in most if not all 8" woofer based AR systems of that time period) has a circular cut-out, again just large enough to accomodate the removal of the entire woofer without disturbing the blanket.

I have heard/read about a "plug" for the upper mid-range area that seems to decrease that rectangle, but mine did not come with it. A little more on the rectangle, the horizontal edges of the cut-out are flush with the top and bottom of the driver. There is about a 1 1/2" gap between the side edges of the driver and the edge of the blanket.

Maybe with a little help from Stephie I can shoot an image and present it here tomorrow.

[George Grand]

From the AR-9 owner manual:

"On a conventional loudspeaker enclosure, sound waves from the highrange and midrange units move across the front panel and are reflected from obstructions such as mounting screws, moldings, decorative parts, and even from the cavities formed by other drive units. When the sound waves reach the enclosure edges, they are reflected again. The direct and reflected sound waves interfere with each other in a way that varies with frequency, blurring the stereo image and making frequency response uneven.

These effects are suppressed in the AR-9 by an Acoustic Blanket, a layer of absorbent material on the front panel surrounding the lower-midrange, upper-midrange and highrange drivers. The Blanket keeps sound from spreading to, and being reflected by the enclosure edges and the cavity formed by the 8-inch lower-midrange driver. The absence of the usual interference effects caused by such reflections, when coupled with the vertical drive-unit placement described in the next sub-section, gives the AR-9 exceptional imaging, stereo localization and very smooth frequency response."


That last sentence? They ain't lyin'. They were being modest about it too.

[scottyf]

Another speaker from the 90's that used felt was the Dunlavy V's. The felt was in layers around the tweeter and midranges. I heard a pair at Galen Carol's house and they sounded great! (of course, they were set up perfectly) At the time, very expensive and very tall!

[TroyD]

For pics of the acoustic blanket, click here (http://www.polkaudio.com/forums/showthread.php?t=34997)

[ttrentt]

I must also add, most of JBL's famous speakers also used foam around the tweets, including the famed LE25 tweeters.

[F1nut]

JBL made a famous speaker!?!

[Flash21]

Can't quite tell from the pics, is the tweeter flush with the Blanket, or does the Blanket stand proud of the tweeter (tweeter flush with the cabinet)? Just thinking about adapting this kind of thing to my 10Bs, where the tweeter is flush with the cabinet...

[Larry Chanin]

Hi All,

I started experimenting his afternoon with these cheap neoprene diffraction rings (http://www.audio-ideas.com/tweaks.html) on my SDA-CRS+ which I'm using as a center channel speaker.

I've already upgraded the tweeters to the silk dome replacements. Here's a graph showing the before and after frequency response.

This seems to work in theory. I'm considering the more serious thick wool felt approach.

Larry

[ttrentt]

JBL made a famous speaker!?!

Haha. Sorry, I am a big JBL fan - I know they seem to be frowned upon in this Polk community. Considering some of the prices JBL products pull on eBay, I would say they made some famous speakers. I am still looking for a set of 4311s :)

[Vr3MxStyler2k3]

I am seriously thinking about trying these...

Larry did you notice a difference in sound, beyond the measurements?

[Larry Chanin]

I am seriously thinking about trying these...

Larry did you notice a difference in sound, beyond the measurements?

Hi Sid,

I haven't gotten that far yet. I've got five more tweeters in my setup and I want to measure what's going on before I get distracted by critical listening.

I don't have a trained ear and I don't want the placebo effect to unduely influence me. Frankly, I'm guessing that I won't be able to discern a difference with these rather basic rings, especially since its not feasible for me to do a blind A/B test where I can instantaneously switch between with and without the diffusions rings. Even if it turns out that I can't hear a difference, at least the measurements will allay any fears that I might be hurting my setup.

The measurements suggest that this is not entirely one of those worthless tweaks and it may be worth the additional effort to experiment with an AR Acoustic Blanket type approach. As you know the vintage speakers have sharp edges and may be more susceptible to diffusion effects than the modern Polks that have carefully designed grills. (http://www.polkaudio.com/education/tech_article.php?id=24) So applying thick absorption to the baffles of vintage speakers might give us more bang for the buck than if they were applied to a new Polk speaker.

Larry

[Vr3MxStyler2k3]

I actually just took some foam tape I had laying around (1/8" thick, 1/2" wide) -- and put it around my tweeters...I cant really say I hear a difference, possibly less sibilance in some songs, not sure - still listening.

[Larry Chanin]

I actually just took some foam tape I had laying around (1/8" thick, 1/2" wide) -- and put it around my tweeters...I cant really say I hear a difference, possibly less sibilance in some songs, not sure - still listening.

Hi,

I'll be traveling for the next several days and won't be able to get to this for a while. I was thinking that a crude way to do an A/B test would be to install the diffraction devices around the tweeters on say just your left main speaker and leave them off your right. Then listening to a recording centered on the soundfield, change the balance from all the way left to all the way right and back. Then see if you can hear a difference between the main speakers.

Larry

[George Grand]

Famous speakers and good speakers are two different things. Blose are pretty famous too wouldn't you say?

The AR Blanket extends 3/4" past the tip of the domes. The tips are not at all flush with the outer edge of the blanket.

I told you it worked Dr. Chanin.

[Larry Chanin]

Famous speakers and good speakers are two different things. Blose are pretty famous too wouldn't you say?

Hi George,

Perhaps infamous would be the more accurate term. ;)


I told you it worked Dr. Chanin.

I'd say the guys to convince would be Max, and to a lesser degree Mark. :D

Seriously, I think measurements might still be important to make sure the absorber is being placed correctly.

Larry

[ttrentt]

Famous speakers and good speakers are two different things. Blose are pretty famous too wouldn't you say?


haha. You got me there. Will always be a huge fan of older JBL though. The big cast frame woofers were always beautiful to me :)

[scottnbnj]

...I was thinking that a crude way to do an A/B test would be to install the diffraction devices around the tweeters on say just your left main speaker and leave them off your right. Then listening to a recording centered on the soundfield, change the balance from all the way left to all the way right and back. Then see if you can hear a difference between the main speakers.

maybe another step might be to listen in mono. if you have a solid, stable image to start with (before adding felt to one side) and the changes hold true to your graph, this should be revealing.

)

[Larry Chanin]

...I was thinking that a crude way to do an A/B test would be to install the diffraction devices around the tweeters on say just your left main speaker and leave them off your right. Then listening to a recording centered on the soundfield, change the balance from all the way left to all the way right and back. Then see if you can hear a difference between the main speakers.

maybe another step might be to listen in mono. if you have a solid, stable image to start with (before adding felt to one side) and the changes hold true to your graph, this should be revealing.

)

Hi Scott,

I'm afraid I don't follow. Can you elaborate?

The reason I suggested the method above is because we have very poor memories when it comes to compairing sounds. If more than a few minutes go by it is extremely difficult to make correct critical comparisons. That's why the switching in A/B test must be done almost instantaneously. Twirling the balance dial is almost instantaneous and it might be fast enough to let us make a meaningful comparison. Of course without being a blind A/B test this method is not scientific and is still subject to placebo effect.

By the way, I just attempted an other measurement using one of my main speakers SDA-1C's. There was almost no measurable difference. So small that they should be inaudible to human hearing, less than 1/4 dB. Refer to a closeup of the frequency responses.

Larry

[scottnbnj]

nt, sorry i'm gonna try again

)

[scottnbnj]

sorry larry, i'm not a very technical kind of guy, but, i played with pink noise and real time analyzers a long time ago and have been trying to spend more time listening than thinking about listening for quite a while, so, i'm not really schooled or fluid in all this stuff,.. and elaborating on anything often makes my head hurt, i hope it doesn't make yours hurt too.

anyway. i agree with what you said about a/b'ing, and i wasn't at all trying to say don't do that or it won't work. looping a short revealing clip of music might make it a bit easier. listening to both channels in mono just lets you shift your attention more quickly, focus more acutely and have a real time comparison or control reference for some things.

i think we agree that measurements like your graphs could mean alot of different things. for where i was going earlier, if you're familiar with the image in mono with a variety of tunes and the mains showed the large difference (as in graph one) with felt added to one side, i would think, with that magnitude of change, it would reveal distinct image shifts or instability. the types of tunes that it showed up on and how might make the differences easier to hear and make sense of than test tones and a/b'ing alone.

i mean, in a case where there's a db or two of gain or cancellation on one side, the image is shifting, and it's audible if you pick tunes that reveal it.

i guess you measured with a continuous tone. you've thought about this alot so you know, but for others, common spl measurements from continuous tones don't differentiate between direct, reflected or any other sort of sound, distortion or cancellation, much less how much later it arrives than the direct sound from the drivers. so, if the gain was from reflections that travelled a much longer path than driver > baffle > listening position or resonances that, say, built up in the room or enclosure surrounding the speak, it would take a longer sustained note for real world (music) gain to approach what the test tones showed.

so, say the mains' mono image is normally centered, solid and stable over a wide frequency range. but, when you add thick felt to one speak, the image in narrow frequency ranges stuck to or shifted or fluttered mostly towards the untreated speak on only long sustained notes or sounds. my first guess would be that the dbs of difference were later reflections or resonances that the felt somehow lessened the impact of on the treated speaker side of the room rather than merely diffraction off of some part of the baffle bouncing straight to the listening position. that sort of gain would be much easier and quicker to identify and isolate with the right tunes and a pair of speaks in mono than with spl measurements and/or a/b'ing.

another quick example might be if the gain effects clarity. it's not uncommon for a strong center image to draw towards the problem speaker side of the room too. listening in mono would just quickly isolate the problem further. treat the image to where it is stable and well centered, clarity is regained, and peace is restored relatively painlessly.

that the difference didn't hold true on the mains, as you said, obviously means the difference will be much more subtle, like soundstage layering and/or shades of blackness (more what i'd expect from controlling just baffle diffraction). being that you're using sda's might make things more complicated too. nonetheless, listening in mono still might help you tell a little more about what's going on and how much difference you could expect to hear with both speaks treated. like lots of other tools, the more familiar you are with it, the deeper it lets you hear and read into what's happening.

sorry i went on, i hope it made some sense.

)

[Larry Chanin]

i guess you measured with a continuous tone. you've thought about this alot so you know, but for others, common spl measurements from continuous tones don't differentiate between direct, reflected or any other sort of sound, distortion or cancellation, much less how much later it arrives than the direct sound from the drivers. so, if the gain was from reflections that travelled a much longer path than driver > baffle > listening position or resonances that, say, built up in the room or enclosure surrounding the speak, it would take a longer sustained note for real world (music) gain to approach what the test tones showed.

Hi Scott,

Sorry for the delayed reply, as I said I was traveling.

Actually I was using a short MLS (Maximum Length Sequence) test tone. This is a sort of white noise.

As I mentioned earlier, we are dealing with two phenomena associated with Early Reflections:


A psychoacoustic effect whereby our brains blend the delayed reflected sound and the direct sound via Precedence Effect, thereby smearing the sonic image.
Comb Filtering whereby the reflected sounds and the direct sound constructively and destructively interfer thereby causing nulls and peaks in the frequency response, and causing lobing in a speaker's dispersion pattern.


The first effect has absolutely no bearing on measurement method. It is a function of how our brains work when interpreting sounds spaced short intervals apart.

The second, comb filtering, is a transient effect that occurs when the first reflections interact with the direct sound. A resonance is a steady-state effect caused by the formation of standing waves. This requires sustained reflections and takes some time to build after the sound is applied, and some time to dissipate after the sound ceases.

The changes in frequency response that I, and the other folks were measuring, is not likely to have anything to do with resonance effects. In order to have a measurable effect on resonances (the modal response of a room) requires the addition of a significant amount of aborption, i.e. a significant amount of the surfaces of the room must be covered with absorbers. These felt strips and rings were so small and thin as to have no measurable effect on the formation of standing waves. In fact, the addition of a person into the room had many times the absorption of the diffusion devices and therefore would have had more effect on resonance than the diffusion devices.

What the small diffusion devices were doing was reducing the amount and level of the first reflection reaching the listeners. By removing the effect of these early reflections the amount of comb filtering was reduced and the frequency response at the listening position was improved.

Larry

[scottnbnj]

Actually I was using a short MLS (Maximum Length Sequence) test tone. This is a sort of white noise.

very cool. mlssa was still too new when i was hacking around with this stuff for me to get my hands on it. but, i keep stumbling across software on the net...

i went to the acoustisoft website. do you have any thoughts about the add-ons and how etf compares to other similar programs?

)

[Larry Chanin]

very cool. mlssa was still too new when i was hacking around with this stuff for me to get my hands on it. but, i keep stumbling across software on the net...

i went to the acoustisoft website. do you have any thoughts about the add-ons and how etf compares to other similar programs?

)

Hi Scott,

No, I'm afraid that ETF is more than a handfull for me and I haven't purchased any other comparable products.

I have purchased their R+D add-on. It's really a work in progress. It's supposed to help you identify the resonances in your room and set up your equalizer to deal with them. Its a rather time consumming process that requires taking up to 32 measurements throughout the room. It takes a pretty decent PC processor. With my 1 GHz cpu it takes quite a long time to calculate results even when its set for "fast compute".

Larry

[Larry Chanin]

Hi,

For those folks considering using felt treatments, here's a response I received from David Ralph to some questions I had.


I just ran across your excellent article on felt diffusors. It was very helpful in getting an appreciation for the pros and cons of this approach.

In your article you state:

"My recommendation is to not have any sort of treatment closer then the edge of the driver mounting plate, or at least not much closer."

I was wondering whether you might amplify on this statement. I know that some speaker manufacturers avoid the mounting plate with blankets of felt, but I alway thought this had more to do with maintaining access to the drivers for possible maintenance. Did you do any measurements of mounting closer to the drivers that showed poor performance? Do you have any other tips for placement?

[Dave]
Access to the units is good to maintain, but felt can have 2-sided tape added if removal/replacement is needed. Placing a 1/2" thick piece of felt too close to the tweeter dome can create more problems than it helps. This is because felt does reflect to some degree. It also can actually create something of a "chamber", in that the dome can be "boxed in" if felt on several sides is very close. But I do place it a bit over the faceplate. My latest system uses two smaller, 1/4" thick strips that overlap even more. The thinner felt has less of a reflection component due to less surface area on the side.


Do you have any opinions regarding a more aggressive approach such as using blankets of thick wool felt that covers large amounts of the baffle?

[Dave]
Note that John Dunlavy (of Duntech and Dunlavy Audio) used very thick felt blankets on the entire front of his very large designs. This has more to do with effectiveness in the midrange, as what's important is to have a significant distance through which the sound passes. The lower the frequency, the more is needed to be effective.

Regards,

Dave

[Larry Chanin]

Hi,

Here's an other response I received from Thomas over at Audio-worx who wrote the DIY article.


In your article you state:
"In this example a Masonite template was made to elevate the tweeter, to avoid destructive interactions with the grill cloth frame."

And you reduce the thickness of the felt.

I was wondering whether in an application where the speaker grill was not going to be mounted, whether you think the tweeter would have to be elevated or the felt thickness reduced?

You also mention that the cutout around the tweeter should be square rather than round. I was wondering if you could elaborate on the reasoning for this?

I also read the excellent article by David Ralph that you referenced. Have you done any similar measurements of the affects of this treatment on frequency response?

Do you have any additional tips on felt placement?

Thanks very much.

Larry Chanin


Hi,

Elevating the tweeter was specific to that design. I was using a premade enclosure with a factory grill frame made from 3/4" material. As a result I wanted to get the tweeter as high as possible to avoid diffraction interference with the grill frame.

Jon and I have been using felt on all our designs for more than a decade. There are probably some measurements around, but they were done long before we started posting on the net. I linked to Dave's testing because it's representative of what happens when one uses the felt.

If you look at the designs from Avalon acoustics, their entire grill frames are completely lined with wool felt with cutouts sized to the drivers.

One shouldn't use round cutouts close to a tweeter. Doing that would impact specific frequencies depending on the diameter of the hole.

Buy some felt and experiement. Use masking tape to place pieces around the baffle and listen to the effect. I'm sure you'll like what you hear.

Regards
Thomas

[Flash21]

Good info Larry, addresses some of the questions I had...thanks for the legwork.

[Larry Chanin]

Good info Larry, addresses some of the questions I had...thanks for the legwork.

Hi Steve,

You're welcome.

At some point I think I'll experiment with large amounts of the thick wool felt.

Right now my measurements show with the exception of one borderline case, for the most part the cheap neoprene rings yielded results that should be impossible to hear. That is, most of the changes in measured dB was below even the most acute human hearing. Where there were a few dB difference which might be heard by someone with a good ear, I found that the rings improved some frequencies and hurt others.

I ended up leaving one ring on my lower center channel and leaving the rest off of all my other speakers.

Larry

[Larry Chanin]

Hi,

For those interested in this topic I found two more revelant articles:

Understanding Cabinet Edge Diffraction by Andy Unruh (http://www.speakerdesign.net/understand.html)

Felt Effects on Baffle Diffraction by David Ralph (http://www.speakerdesign.net/felt_amelioration/feltssdescr.html#MT_Module)

Larry

[nathanso]

Nice trivia, Doro! I've got a pair of black velvet fronted Dahlquist DQM-3's I've had for 20-years, and a set of acoustic blanketed AR90 towers that I've had for a month or so. The AR's blanket is far thicker than any velvet. NHT also used foam bits (that always remind me of bike helmet pads) stuck to the fronts of their speakers in the area of the tweeters.

[Larry Chanin]

Hi All,

I bought a 1'x6'x1/2" chuck of the F13 wool felt. It is the less expensive stuff at $33.55 with shipping.

Attached is a photo of my SDA-CRS+ center channel with the baffle covered with the felt.

Also attached is the before and after frequency response.

Larry

[scottnbnj]

when you have everything put together, do you normally have something covering the framing around the speaks?

)

[Larry Chanin]

when you have everything put together, do you normally have something covering the framing around the speaks?

)

Hi Scott,

Do you mean like this?

http://mysite.verizon.net/res8ycu4/sitebuildercontent/sitebuilderpictures/shadowd.jpg

Its an acoustically transparent shadowbox.


You can see more of my setup by clicking on the link in my signature.

Larry

[dorokusai]

Nice trivia, Doro! I've got a pair of black velvet fronted Dahlquist DQM-3's I've had for 20-years, and a set of acoustic blanketed AR90 towers that I've had for a month or so. The AR's blanket is far thicker than any velvet. NHT also used foam bits (that always remind me of bike helmet pads) stuck to the fronts of their speakers in the area of the tweeters.

Thanks Nathan :D

Larry - Great job with the extra information, it's been a nice read. I've always believed in the benefits of diffraction treatments but it's a tricky operation. I've flattened out the freq response in some older speakers with wool/felt, but it also degraded the midrange quite noticeably in almost all of my non-scientific tests. It's also so speaker dependent that while fun, can be a real PITA to tweak over time. I swept alot but the figures, while sweet on paper, didn't move me audibly. That foam ring stuff is bonafide crap....it's much more worth the time and effort to do exactly what you are doing now.

I plan on using it on my DQ10's when I get off my ass to restore them as it's already a known improvement for that particular speaker.

It's was fun to play around with, but eventually the ADD kicked in and I forgot where I was.

[scottnbnj]

Do you mean like this?

sorry larry, i didn't mean 'covering' visually (those grille covers look fickin a sweet btw). i meant acoustic treatments on the wood framing members extending out beyond the face of the baffle.

)

[Larry Chanin]

Larry - Great job with the extra information, it's been a nice read. I've always believed in the benefits of diffraction treatments but it's a tricky operation. I've flattened out the freq response in some older speakers with wool/felt, but it also degraded the midrange quite noticeably in almost all of my non-scientific tests. It's also so speaker dependent that while fun, can be a real PITA to tweak over time. I swept alot but the figures, while sweet on paper, didn't move me audibly. That foam ring stuff is bonafide crap....it's much more worth the time and effort to do exactly what you are doing now.


Hi Mark,

Thanks.

Assumming that the felt still introduces diffraction off the edges of the felt cutout, the article by Andy Unruh helped me understand Thomas' comment about why its better to cut square or rectangular holes in the felt. If the ring was round the distance from the tweeter to the ring would be the same so all the delays between the reflections and the direct sound would be the same. This would result in the biggest peaks and nulls in the frequency response. By using square or rectangular holes the delay distances vary, therefore the diffusion is spread out and the resulting peaks and nulls in the response are lessened.

Unfortunately, getting a better feel for the theory still doesn't help me figure out the optimum dimensions of those cutouts. :(

In my last measurement it would appear the felt degraded the midrange. It introduced a maximum difference in frequency response of about 3 dB, which would be audible, but just barely to a trained listener.

Most of the differences in the frequency response from the neoprene oval ring resulted in changes which never would be audible, and any listening tests would be futile.

Larry

[Larry Chanin]

sorry larry, i didn't mean 'covering' visually (those grille covers look fickin a sweet btw). i meant acoustic treatments on the wood framing members extending out beyond the face of the baffle.

)

Hi Scott,

Thanks.

No I haven't tried to treat the frame, but it did occur to me that they might be the source of diffraction.

To try to see if there was a measurable effect I pulled the center channel speaker out further. I figured that this should yield less reflections off the frame. Surprisingly the frequency response got worse.

Larry

[scottnbnj]

I figured that this should yield less reflections off the frame. Surprisingly the frequency response got worse.

go with the force larry. your instincts were right.

)

[BobMcG]

After reading all the good input on this subject it looks like (at least to me) that if a speaker designer has figured a felt overlay into the design of the speaker it's a definite go. If not, it's more of a hit or miss thing.

[Flash21]

I suspect you're right...

[Larry Chanin]

After reading all the good input on this subject it looks like (at least to me) that if a speaker designer has figured a felt overlay into the design of the speaker it's a definite go. If not, it's more of a hit or miss thing.

Hi Bob,

The way I figure it is that if the speaker designer didn't already take measures to address diffusion off of sharp cabinet edges via rounding the edges, specially designed grillwork, or felt treament, etc, then there may be room for improvement. As I mentioned, some of the vintage speakers seem to be better candidates for this simply because they don't seem to have the attention to this issue that the newer designs seem to have.

Since the improvements are rather subtle, this would suggest that any experimentation would require a means of accurately measuring the results of the treatment to make sure it really is an improvement. Doing this entirely by ear may result in improvements in only a selected range of frequencies and may actually hurt the frequency response in an other range.

Unfortunately even with measurements the placement of the felt in relation to the drivers remains a big unknown factor. That is, we can't predict the effect changing the placement of the felt will have on our measurements. Therefore, we are left with a lot of time consuming trial and error. Definately a hit or miss undertaking if you will. :(

Larry