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Acoustic Blanket on Cornwalls?

dc270

dc270

Now Off the Reservation
I ask another question about modifications for my 1980 Cornwalls. Has anyone experimented on the front baffle by installing acoustice foam/ "blanket", say 1/2 thick or less, around the horn openings? This would act as a difraction blanket like AR uses on the AR9's. The therory sounds valid in practice for cone/dome drivers, just not sure for horn lens openings.

Another modification of mention for my CW's. I cut a rectangular hole on the back baffle, 2.5" x 3.5", centered, where the original connection area used to be. I put a .75" thick wood plate inside overe the cut out area with about an 1" over. I then installed two HD binding posts of gold color. This akes me feel better than just about anything else! Nothing alarms me more than to think of a wire dropping loose during a loud session.....I can sleep now!

I have the toe boards un-screwed from the bottoms and the loose veneer being glued back into place. So,e how the previous owned got these wet and they partially split. Soon as they are sanded and re-painted back to a new look then I will be screwing on each retangle, four, 2" HD, black carpet rollers. A big speaker that is easy to move is a big plus for me and y wife who like to clean under everything!
DC
 
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Nothing wrong with your AR9 blanket idea. Did something similar on a pair of Quartets. Placed a piece of 5/8" square high density fiberglass (felt would do) across the inside of the grill frame horizontally along the top edge of the mid and tweeter horn to catch any stray radiation headed toward the ceiling. These two strips helped a lot resulted in better stage and image quality. Best regards Moray James.
 
Interesting idea there, DC.

I've been wondering about the use of felt with horns after reading this article by Lynn Olson. The quote below the link is from that article. I found it very interesting and I may try this since I do not have the Tractrix horn in my Fortes.

The Art of Speaker Design
http://www.nutshellhifi.com/library/speaker-design1.html

From a technical standpoint, horn-loaded drivers typically have very low THD, IM, and FM distortion, uneven frequency response, reflections in the time domain, and very sharp cutoff characteristics at both ends of the frequency range. From the viewpoint of mainstream high-end designers, horns are beset by serious problems with impulse response, diffraction, and smooth dispersion.

The root of these problems, especially with cheaper PA-style horns, is the acoustic reflection from the edge of the horn-mouth. When a sound wave moves across a sharp boundary, it diffracts and re-radiates in all directions, like a separate driver located at the point of the reflection. The reflected wave from the horn-mouth then bounces back into the throat, which typically has a hard phase plug or a driver with a stiff cone. After it strikes that, it reflects right back outward again ... this succession of reflections is called a series reflection, and it is far more audible than the small ripples in the frequency response might indicate.

Although the frequency response doesn’t really indicate the full impact of the reflections, they show up in the impulse response or 3D waterfall display. (This is most clearly seen if the horn driver is measured without a crossover.) Inexpensive PA horns that are too short suffer most severely from this problem, and have the grossest "horn coloration" as a result.

There are solutions for this problem that work pretty well. If you can afford to lose 1 or 2dB or efficiency, you can line the inside of the horn with 1/8" wool felt. 1 to 3 inches extending from the lip of the mouth going inward will do the trick. The further you go back towards the throat, the better the damping, but if you overdo it, the bass response of the horn will start to droop, along with the efficiency. Think of it being like tweaking the VTA on your cartridge and you’ll be heading in the right direction. Of course, if you have access to a MLSSA or similar FFT system, you can adjust the impulse response to taste, as well as compensate the crossover accordingly. (Note: if you’re modifying a commercial horn, don’t forget to remove the wire mesh bug screen in the throat. The wire mesh creates a very unpleasant gritty harshness at levels above 90dB, and is only required for severe outdoor environments.)

The best solution is to eliminate the mouth reflection entirely. This has already been done with the Tractrix horn profile, invented by P.G.A.H. Voight in the late Twenties!

The Tractrix still has a sharp edge at the horn mouth, but the horn wall has already curved through 90 degrees before the sound hits the boundary. The reflected sound then has the difficult task of curving back through that 90 degree curve before it can strike the phase plug. Therefore ... no standing wave, only one modest reflection, and very little of the "horn sound" if the compression driver is correctly designed.
Click to expand...

edit...Lynn Olson's article "A Tiny History of High Fidelity" was an intersting read too
http://www.nutshellhifi.com/library/tinyhistory1.html
 
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Have you looked at the article Lynn wrote on the Chorus where he time aligned the tweeter> Impressive results. I have no idea where it is so you will have to track it down but well worth the time and effort. Best regards Moray James.
 
is it safe to say that 'damping' is generally a good thing for mid and treb horns regardless of model?
Would duct tape work good as the damping mtrl?
 
Ha, duct tape is for keeping your tee-shirts from getting dirty around the neck. You know multiple layers might just do the job. How many who knows. Why don't you try and compare it to duct seal or dynamat. Best regards Moray James

PS: duct tape also makes a fine under garment protector keeping your most personal fabrics like new.
 
Actually headliner material as use in automobiles for sound proofing, etc works quite well. You can get it in colors, it is a bit over 1/8" thosck foam and can be glued into place easily.

I read the articles you all were so generous to enclose- THANK YOU! It seems that a thing to look into would be to incluse about an inch of foam, or what ever your damping material is, into the mid horn mouth. There is a small amount of loss but managable. I am planing to look into doing this on my front baffles.

This is proving to be a very facinating project with many considerations, ideas and thing to experiment with.

Unfortunately things move slow for me as I am meding from foot surgery in late December. However I do manage to accomplish a little everyday and use my time to also investigate ideas I am using. You people are proving to be of great value in getting those ideas gathered for my evaluation. I should have photos soon to entertain us with!

Thanks again
DC
 
Here are some photos I promised as the project nears completion.

The horn drivers are front mounted, 2" casters installed on all four corners, toe boards repaired & repainted, Binding posts installed, drivers are sound wrapped, ALK networks built & installed, cabinets refinished & Crites 125 tweeters installed.

I still wanted to intall a sound blanket around the horns, finish painting the baffles, internal bracings and trim the grill covers to fit the horn cut outs.

As for how they sound........most incredible!! These upgrades ARE the formula for top shelf Cornwall 1's, using original woofer and mid horn drivers. I probably spent about $200 per cabinet on upgrades and untold hours of time. If I had to sum up the sound it would be "Articulate Authority"!
DC
 

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Nice work DC. I'd love to hear them. Thanks for sharing. It's inspiring.

Some folks give the Heritage series a bad rap because of how everyone has to upgrade them to get the sound you really want but I see it a bit differently. They are a very nice speaker but at the prices they often go for on the used market (I have just $200 in my Fortes), they are a great platform for those who love to involve our hands in this hobby as well as out ears. I LOVE fixing up old speakers and upgrading (if it has proven to be worthwhile) my gear. It's fun but also very rewarding to turn a good speaker into a great speaker. I'm sure you are really enjoying the fruits of your effort. ;)

mark
 
So are the horns flush mounted, or surface mounted. Looks like they are recessed into the baffle. I just received some Cornwall 1's that have the Crites tweets. But the mids and tweets are rear mounted.
 
I moved the rwo horn drivers to the front, what you see there is the thickness lip of the drovers plus some sealing caulk under the lip to prevent leaks. I would advise moving to the front of the baffles- much cleaner sound benefit. Bad new i you'll have to modify your grill covers. The single largest improvement was the ALK networks.

markw300d- you are exactly correct! You can take darn near any vintage speaker and make worth while improvement to them. It just so happens the pay off from this set of CW 1's was bigger than expected! These are excellent plat form to build/ modify from.
DC
 
Flush mounting them would alleviate having to change the grill. Hmmm. Have to keep this one in mind. Thanks.
 
dc270 said:
A Dremel tool & some time -it is easy enough!
DC
Click to expand...

A router would be the easiest way to counter sink the drivers into the baffle. Use a rabbeting bit with a bearing on it set to the depth and width of the driver and you're golden. You can also put a different bearing on it to adjust the width of the cut, too. Or, use a straight bit and clamp on guides across the cabinet to get the desired cutout.
 
avguytx said:
A router would be the easiest way to counter sink the drivers into the baffle. Use a rabbeting bit with a bearing on it set to the depth and width of the driver and you're golden. You can also put a different bearing on it to adjust the width of the cut, too. Or, use a straight bit and clamp on guides across the cabinet to get the desired cutout.
Click to expand...

That would have been the best way to accomplish the task at hand but the router base was to larger and I could not make the cuts without hitting the side walls, etc. Lack of proper tooling stopped me here. As it is they turned out good- but better would have been to recess them as was pointed out. Good point!
DC
 
You can always drop a plate in which extends up above the outside lip of the speaker use that surface to route from. That's what I did to cut a larger rebate on a pair of Quartet cabinets with a large hand held router. That worked well. Best regards Moray James.
 
moray james said:
You can always drop a plate in which extends up above the outside lip of the speaker use that surface to route from. That's what I did to cut a larger rebate on a pair of Quartet cabinets with a large hand held router. That worked well. Best regards Moray James.
Click to expand...

That's a good idea! I am expecting the blanket material any day now. I plan to strip the baffles clean then lay the material out and trim around the openings. I have some good 3M spray adheasive and will glue it down with that.

The look I think will be intersting as should be the function. I will experiment with putting a 1" strip just inside the mid horn mouth to check those reflections traveling back into the horn.

Is there an outlet to cquire more original grill cloth material?

Thanks
DC
 
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