Foam inserts pt.2.....

[Russnohio]

I received some foam inserts from JPG a couple weeks ago and I have them mounted , obviously. They fit perfectly, nice & snug and I really do like how they look! They actually look better in person rather than the pics if that makes any sense. I think the camera flash makes them give off a "glint" which they don't otherwise have. While grey..they are a very dark grey. As for the difference in sound? Well I'm really going to have to do some more extended listening. My ears are telling me that there IS...but I'm having some difficulty in being able to describe it....I want to say smoother...less "harsh",...at LOUDER listening levels. The problem I'm having is the fact that they (the horns)sounded great to begin with. I'll try some A & B test next weekend in mono...going from one speaker with and one without. Too bad I don't have two pairs!

[Richard C.]

Russ, Your aftermarket foam inserts are built different than "my" aftermarket inserts, which I bought off eBay about 2 years ago. My side pieces taper from the thickness of the top & bottom pieces down to the horn flare - a nice gradual taper. I wonder which one is closer to the original, because mine were either crumbling or gooey.

That flash makes the horn look shiney, and both pair of my 811's are very flat.

Richard C.

<font color="#FFFFFF" size="1">[ January 06, 2008 10:21 PM: Message edited by: Richard C. ]</font>

[Altec19]

Here's a sales brochure with the original design:

Link to Altec Model 19 Brochure

[jpg]

Nice looking 19's Russ! Here's a couple of auction images I found over a year ago, they have a narrower taper like the one shown in the brochure previously mentioned, these might even be Richard's...
<img src=http://myweb.cableone.net/jpg/M19Surrounds/8d_1.jpg>
<img src=http://myweb.cableone.net/jpg/M19Surrounds/a5_1.jpg>

I asked my builder to make foam surrounds with the same foam & taper as those in the picture. He argued that since the horns have a 90 degree flare at exit that further tapering wouldn't have much if any beneficial affect, and that 45 degree cuts would be much cleaner, have a stronger glue bond, would be easier for him to make, and cheaper for me to buy. He also suggested using this style of foam, which is a much stiffer dense cell foam, like that used in flight cases. I agreed & had him build a few pairs.

I do hear a difference between no surrounds vs surrounds. I believe that adding the surrounds removes an early reflection due to the upper cabinet walls, which helps make the audio more intelligible.

[Old Guy]

What kills the 90 degree argument is the cutoff is not absolute- a 90 degree horn produces appreciable energy outside that angle.

Second guessing engineers without good reason is generally counterproductive.
Honestly- how much different one taper makes would have to be determined by experiment.

In the absence of a good reason. I'd go as close to original as possible.

Don't get me wrong improvements can be made if you know what you are doing. I'm not against that- just against making judgements like 90 degree horns don't have enrgy past 90 degrees.

Many db down can be quite audible-

[Richard C.]

Those look exactly like mine; they fit tight and look great!

Mine are made from a pretty dense foam, and I would say that a careful taper cut could be made with a bandsaw. I have a 20" bandsaw at work and I have used it to make several nice squared off foam blocks to aid in packing fragile items. A variable pitch metal cutting blade will do fine, as long as it isn't overrun, where it starts tearing / pulling.

Richard C.

[bfish]

There's a little how and why here.

In a nutshell;

"Transitions in the shape of a loudspeaker (e.g., edges, slots) behave themselves as acoustic sources. Arrivals from these features always follow the primary wave in time. Additionally, they are often reversed in polarity."

"The shape of a horn, by itself, does not entirely govern the horn's directivity. Diffraction from the mouth and from intermediate transitions can play as great a role in both response and directivity as the interior shape of the horn."

What isn't mentioned is the type or density of foam, and there's a lot of variation acoustically along those lines.

[Russnohio]

Hi guys! Interesting topic. There is not much discussion it seems regarding acoustical foam as applied to Altec horns. JPG actually sent these inserts to me for test purposes only, and that I'd return them later. After unpacking them I fully realized they were not 100% exactly identical to the ones Altec produced, but that was ok with me and in fact....I liked them so much I asked Jim to sell them to me. I feel I must give Jim a bit of praise as a nice guy for trusting a fellow member with his property because again, these inserts were on loan from him only and originally I had no intentions of actually buying them. When I popped them into place I wanted them! As to the foam material itself, I like that it is a much stiffer foam than the original in that they will not sag over time the way softer foam can. I'm still trying to determine any big improvement in sound but the thing is my living room is not well suited in using model 19's! I must have them placed so that each speaker sits next to a wall and that fact alone is going to cause HF reflection in itself. I'm beginnig to feel a proper test regarding HF reflection could only be done in a much larger room. My living room is 22'X14' with speakers on the short wall. I wonder if Altecs published model 19 preformance/expectations....the horn in particular....is confirmed from test results obtained in an anechoic chamber only?


Russ

[bfish]

Originally posted by jpg:
What I imply from bfish's message, is that in a perfect world, if you continued the flair of the horn all the way out to the edge of the cabinet would give you minimal transition. To go a step further, perhaps you could continue a radiused curve onto the upper & lower pieces of foam. Close?

<font size="2" face="Verdana, Helvetica, sans-serif">Only if rigid/reflective materials are considered. My take on Altec's use of foam around horns was intended to be absorbent rather than reflective. You're on the right track though.

Now, expand that understanding to include how a source's natural directivity varies with frequency/wavelength. Below ~200Hz, the expanding wave will wrap completely around a cabinet unless you add large wings, or build-in the wall IB style. As F rises, WLs shorten, first approaching the source size, allowing control of directivity, then becoming smaller than the source where they beam. Also consider that crossovers aren't band-stop devices, so sounds above/below the center frequency are still passed, but at a reduced, defined rate.

With that in mind, you can see there's only a limited BW for any given horn/driver/XO where edge diffraction comes into play.

Now consider the acoustic properties of the materials surrounding a horn/driver. Rigid material represents a boundary, and abrupt angular changes in it will diffract and become secondary sources. Soft material is much less predictable, so the tested spec of the given material should be considered. Materials will either reflect, absorb, or transmit specific frequencies. With foam products, the type (open cell/closed cell) and the density determine the acoustic properties.

Back to M19 foam. The obvious assumption for the use of foam is to reduce diffraction caused by the multiple cabinet edges/corners. A material that is absorbent from ~600 thru 2400Hz should effectively render the mentioned edges/corners invisible. Conversely, a material that is reflective in that band might make things worse.

At one time, I located an extensive chart relating the tested properties of various foams, and the differences in properties between them was surprising (to me). If I can find it again, I'll post it.

[jpg]

What I imply from bfish's message, is that in a perfect world, if you continued the flair of the horn all the way out to the edge of the cabinet would give you minimal transition. To go a step further, perhaps you could continue a radiused curve onto the upper & lower pieces of foam. Close?

To describe the foam used: This foam is very rigid & stiff. It doesn't bend or flex when you tap on it. It would take some force to dent.

The original foam in my 846U's is very soft, and has lots of give when you push on it. The side pieces are nothing more than a flat 45 degree wedge of foam with no taper.

<img src=http://myweb.cableone.net/jpg/846Ufoam.jpg>

I agree that an experiment is the only way to tell a difference. I'll hound my builder to make a tapered pair. BTW, he's very slooooow.... If and when he comes through, I'll report back with results. In the meanwhile, if anyone wants to compare these surrounds to their tapered pair or make some audio measurements, send me a PM.

Richard, have you tried making a thin cut at 20 to 30 degrees with your bandsaw? Rob mentioned that once he gets past 45 degrees that things start to fray & get ragged. Maybe that depends on the type of foam and condition of the blade. Also, do your surrounds cover the horn mounting screws & flange like mine, or do they start outside the horn flange like the 846U? Can you post a photo of the surrounds in place? Anyone with original surrounds out there?