What would happen if you covered a speaker with 2" of fiberglass to reduce diffraction?

[cavedriver]

I was looking at a picture of a Snell Aii with the grill off and I am reminded that Peter Snell, Kevin Voecks, and John Dunlavy all experimented with limiting diffraction (I presume) from the speaker front baffles by using felt or fabric damping to varying degrees. Of all the designs people have seen, is anyone covering the majority of a speaker's surfaces with sound absorbent such as an inch or two of fiberglass that can achieve near 100% absorption of higher frequencies?

I imagine this would do some very strange things to a speaker's radiation pattern, besides sapping a lot of power. Looking at the Olson paper I can't even match the resulting "shape" to any of his experimental shapes when it comes to edge diffraction (if you were to literally cover the entire surface of the speaker and not just part of the face as shown below). Picture of Dunlavy SC-IV from the web for reference:

 

[RayDunzl]

There is no problem that can't be solved by creating another.

 

[cavedriver]

There is no problem that can't be solved by creating another.

yes of course it occurred to me that you could simply set a tweeter in free space, but at some point for bass drivers you need a cabinet if you want a closed baffle, so do please go with me here a little

 

[RayDunzl]

I'm really not the one to ask.

I have too many other problems to address (per those who don't like this design)

16 years ago, just after moving in.

 

[DVDdoug]

I would expect very little difference once you are a few inches away from the tweeter.

And a properly designed waveguide, properly integrated with the baffle, is probably a better solution.

Covering the sides and back (if that's what you're suggesting) wouldn't be much different from putting a block of fiberglass in the room, or a fluffy dog walking in.

 

[thewas]

Such solution can actually sometimes really smoothen the on- and off-axis response like in this example

https://www.facebook.com/klangundton/posts/pfbid028funM9ZbwUisGUwepDEvZnwzG959sou7wZBEFySFYyymmqViA9E5YuDvet19pCz1l

but you need to do measurements to make sure you don't make things worse.

 

[Jim Taylor]

And a properly designed waveguide, properly integrated with the baffle, is probably a better solution.

Andrew Lipinski seems to agree on both points. Stereophile review of the L-707:

Lipinski Sound L-707 loudspeaker | Stereophile.com

Street buzz is a force to reckon with. When an audiophile whispers to me that a piece of new equipment sounds unusually good, I'm interested. When two manufacturers of other equipment independently tell me "You've got to listen to this speaker," I get excited. I first heard about Lipinski...

www.stereophile.com

 

[kemmler3D]

It would depend on how far the absorptive material extends away from the front of the baffle. If the tweeter was way down in a well of fiberglass, you'd basically just end up narrowing the dispersion accordingly. This might help or hurt depending on how the speaker was behaving before, but for a modern well designed speaker I think it'd be more of a problem than a solution.

If it was really thick it would also work for the mid and woofer, to an extent, but the effectiveness would tend to decrease with frequency, so you'd end up with a pretty dark power response without taking corrective measures.

 

[dwkdnvr]

John Bau also did this with the Spica TC-50 (and other variants) which was pretty famous back in the day for holographic imaging. It's a pretty brute-force method and I'd tend to agree that a properly designed waveguide and edge treatment mostly render it obsolete, but I'd also say that it was very successful in the designs that used it.

 

[Francis Vaughan]

In defence of John Dunlavy, he wasn't just messing about with no idea what he was doing. He did have a proper anechoic chamber and took significant pains to research and measure what he did. But this was the 80s. There were limits to what he could achieve and address. Famously he was an antenna design engineer, invented the log-periodic antenna, so not exactly dumb. He retired, came to Australia to settle down, and got bored, so started a loudspeaker company. (He did RF consulting on the side as well.) Ironically the business was very successful, he un-retired, and later went back to the US, where DAL was founded, and he ran it until his passing. He clearly didn't take well to retirement.
His whole idea was to apply antenna design ideas to loudspeakers. I remember hearing this before I encountered any of his designs, and was really puzzled as to what this might mean. The answer was pretty simple. He was laser focussed on on-axis response and phase preservation at the listening position. Just like an antenna. There was scant attention paid to off axis response, or indeed anything beyond the experience at the perfect listening position. The results were little short of revelatory at the time. For many the Duntech Sovereign was the endgame loudspeaker of the age. But not everyone is prepared to put up with the downsides of this design regime.
I remember John telling me that the felt was specially sourced from South Africa, as that was the only source he had found that had the right mix of characteristics to work properly. There is a clue here. The exact felt configuration was chosen to meet a specific goal, and there was a lot of trial and error and measuring to get there. Just blindly slapping felt around isn't going to work.
In the modern world things like off-axis response and directivity are valued, and we have the tools to get them right. It is a lot less likely that slapping absorbing panels of felt or fibreglass on a cabinet is going to do anything useful. Minimally you would want to be clear about what the problem you are trying to solve is, with measurements, and then apply what we now understand about managing sound. You would almost certainly find that a solution involving adsorbing bits was way down the list of useful solutions, if it was there at all.

 

[ssashton]

Fiberglass or rock wool etc are actually somewhat reflective, due to the high flow resistivity, which is one reason anechoic chambers use wedges.

Any application of absorber around d the driver could help at some frequencies but make others worse, so you'd need to measure and tweak.

Waveguide and smooth rounded baffle is superior.

 

[DanielT]

Stig Carlsson experimented with felt placed around the tweeter and sometimes on the baffle BUT he did a lot of measurements. Unfortunately there are no measurements with and without felt reported but I suspect he came to the conclusion that they were better with felt.

Some of his models:

Some measurements on a model. Measures well:

(I'll add a little "cheating" to that measurement, the same speaker box + felt around the drivers/baffle but a upgrade kit with SB Satori MW16P-8 and SB Satori TW29R-4 plus a new crossover)

CarlssonKult

History of the Carlsson loudpeakers - CarlssonPlanet

Carlsson speakers have been around for fifty years. We will cover the speaker models decade for decade. The fifties The sixties The seventies The eighties The nineties The new millennium The fifties In 1953 Stig Carlsson designed the peculiar Coalscuttle, a cannon look-alike bass reflex...

carlssonplanet.com

I don't know when and under what conditions it makes an audible difference but any curious DIY's can test that themselves. Measurements and listening with and without felt.

 

[eddantes]

I measured something like this. Here's my thread: https://www.audiosciencereview.com/...ing-observation-real-or-misinterpreted.59809/ While it might have some positives in one aspect; like anything in audio - there are side effects that may render the exercise moot.

 

[Francis Vaughan]

There is an interesting idea that has burbled around in the background for a while.
What if a waveguide was made from something that exhibits some absorption? It isn’t really clear what the answer might be, but the answer might not always be bad. Modelling it would be challenging, the usual BEM assumptions probably break badly. But there is the nagging feeling that maybe the idea could prove useful.
It opens up the whole box of possible meta material ideas as contributors to a design. But at a significant leap in complexity and general grief.
Perfect for hobbyists.

 

[Duke]

What if a waveguide was made from something that exhibits some absorption?

Open-cell foam waveguides in the Acoustic Research MGC-1, designed by Ken Kantor:

 

[ssashton]

There is an interesting idea that has burbled around in the background for a while.
What if a waveguide was made from something that exhibits some absorption? It isn’t really clear what the answer might be, but the answer might not always be bad. Modelling it would be challenging, the usual BEM assumptions probably break badly. But there is the nagging feeling that maybe the idea could prove useful.
It opens up the whole box of possible meta material ideas as contributors to a design. But at a significant leap in complexity and general grief.
Perfect for hobbyists.

Like the early Genelec coaxial, that had foam over the full midrange driver which acted also as the surround.

 

[dylanfane825]

I was looking at a picture of a Snell Aii with the grill off and I am reminded that Peter Snell, Kevin Voecks, and John Dunlavy all experimented with limiting diffraction (I presume) from the speaker front baffles by using felt or fabric damping to varying degrees. Of all the designs people have seen, is anyone covering the majority of a speaker's surfaces with sound absorbent such as an inch or two of fiberglass that can achieve near 100% absorption of higher frequencies?

I imagine this would do some very strange things to a speaker's radiation pattern, besides sapping a lot of power. Looking at the Olson paper I can't even match the resulting "shape" to any of his experimental shapes when it comes to edge diffraction (if you were to literally cover the entire surface of the speaker and not just part of the face as shown below). Picture of Dunlavy SC-IV from the web for reference:
View attachment 481497

Yeah, you’re right - fully covering a speaker in sound-absorbing material like fiberglass would get weird fast. A little felt or foam around the drivers can help reduce edge diffraction, but if you cover most of the cabinet, you’d basically kill off a lot of the reflected high frequencies and mess up the dispersion pattern. The sound would get dull and directional, and you’d lose some efficiency since the energy’s being absorbed instead of radiated.

Guys like Snell, Voecks, and Dunlavy only used felt in specific spots to control reflections right around the drivers - not to absorb the sound, but to smooth the transitions at the edges. If you wrapped the whole thing in thick fiberglass, it wouldn’t behave like a normal loudspeaker anymore; it’d be more like a nearfield absorber with a speaker buried inside it.

 

[Elkerton]

There are a lot of measurements on the effects of varying amounts of felt in this link:

Diffraction Doesn't Have to be a Problem

 

[DanielT]

In Qln One speakers there was a tweeter with felt around it. I don't know if it added anything positive or not:


I am now replacing the tweeters to Dayton Audio ND25FW-4 with WG plus SB Acoustics SB15NBAC30-4 woofer. A fun little project, which nevertheless for some inexplicable reason is taking a long time. Nothing directly wrong with the original Seas tweeter, mostly because it is just a fun project.

 

[Y~BNA]

Look at professional near field monitor builders. Like Genelec. Do THEY fool around w/magic ideas like that? No? You can bet it's for a reason.

Speaker building is all about even power response AKA equal polar response AKA constant directivity. Whatever you think of, should bring you a step closer to THAT goal. Every aspect of Genelec's monitors is aimed at that: Mid is the tweeter wave guide (coaxial); Three ways function as point source by woofer projecting AROUND the mid/hi baffle; etc.

Given a nice flat power response on axis, everything else is moot until you achieve some acceptable level of constant directivity. How absorbing a small portion of an already quickly diminishing frequency range will help that, that would be the answer to your question.

My two cents... Cheers!