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!!!

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

TroyD wrote:

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?


And here's a excerpt of a review 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, 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

madmax wrote:

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

Larry

Larry Chanin

Flash21 wrote:

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

Larry

nadams

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

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

Flash21 wrote:

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 **** 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

Larry Chanin wrote:

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

George Grand wrote:

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 **** 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 "****" 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

Flash21 wrote:

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.

1. A psychoacoustic effect whereby our brains blend the delayed reflected sound and the direct sound via Precedence Effect, thereby smearing the sonic image.
2. 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

Flash21 wrote:

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

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...