Cab Dampening for Cornwall II

[Kriton]

After going to Hope in July, and talking with the engineering staff, I

decided to make my last (hopefully) modification to the CWII's and I

lined the cabs with some foam dampening material from Parts Express

(eggcrate stuff).

For those of you who didn't read the CW III thread, Mr. Delgado ended

the controversy about the dampening material in the CW; the CWIII has

the same foam inside it like the RF-7 - when I pointed out to him that

the CWII did not come with any kind of foam, he appeared to be shocked,

saying "it needed it", putting that controversy to rest for me at any

rate.

I guess the next question that I have is, do I cover up the crossover

with foam? For those of you who don't know, the crossover on the

CWII is attached with screws to the back of the speaker just above the

woofer magnet, but below the 2x4 front to back brace...will it matter

if I place the foam over the crossover - either heat, sound wave

reflection, anything else? Wouldn't be difficult to cut the foam

around it, but I would like the foam all around if possible.

What do you guys think?

K

[colterphoto1]

The foam or other dampening materials are used to dampen internal resonances or 'standing waves' in the cabinets. I have been 'corrected' in the past on this next point, but I believe that generally accepted speaker construction call for the following- The materials are USUALLY placed on one of each of two parallel surfaces in order to dampen resonances. In MOST cases this is the top, one side, and back of the cabinet. In NO case should the material interfere with the transmission of air through any port cavity.

I would think that in the case of your CWII, you could either move the crossover to the non-dampened side of the cabinet or cut out the foam around the crossover. Heat is not an issue. Likewise, there is enough diffraction occuring from the front panel that dampening the entire back wall might not be necessary. You may cover the crossover if you wish. You may also have to carve a hole in the material adjacent to the Mid driver, as it nearly contacts the back panel.

On SOME Cornwalls, material is also placed on top of the port 'shelf', NEVER on the bottom of the cabinet, that's port area, and SOMETIMES the entire interior, including motor board is covered. My above rules are the GENERAL rules.

Go ahead, flame away, make my day. I have no idea what I'm talking about once again.....

Michael

[Kriton]

Mike,

I talked to Roy quite a bit about this, and he told me that the inside

of the CWIII is completely covered with foam, all except the motor

board. I was not sure whether this included the top of the shelf,

but I did not put any foam on the top of the shelf, because the top of

the cab was covered, so I figured it might not be necessary.

The foam sheets fit like a glove by the by, and required minimal easy

cutting...I have the crossover covered at this point, and I just wasn't

sure about the foam directly on the crossover - I did cut out the foam

for the 2x4 brace AND the transformer (transducer) what-ever-the-hell

the standing gizmo is...using a heavy stapler to attach...going easy

and will be reversible...

Going to put it back together tonight, will let you know what I find...

Bruce

[johnyholiday]

some feel that an inductor of a crossover ,close to a magnetic

field,effects the inductors own, magnetic building ,an collapsing

fields,if this holds true for transformers i have no idea,can the

crossover network go microphonic, only the experts know,most coils can,such

as a cheap guitar pickups ,that are not wax potted~wax potting -a method

saturating a pickup in wax to hold the coil and any mechanical

parts absolutely rigid to prevent undesirable microphonic feedback~ or an

ignition coil, thats not

encased properly, an in oil ,hope for the best prepare for the worst

[Kriton]

Huh? WTH does that have to do with the question? Man you are just not right.

K

[Hardhead]

By the way, does anyone have an idea of the frequencies of the standing waves we're talking about? Considering the size of the Cornwall box, I can't imagine the frequencies would be very low. Does anyone know how to calculate the frequencies?

Once we have an idea of which standing waves we're concerned with, we may have a better idea of how to deal with them.

[colterphoto1]

By the way, does anyone have an idea of the frequencies of the standing waves we're talking about? Considering the size of the Cornwall box, I can't imagine the frequencies would be very low. Does anyone know how to calculate the frequencies?

Once we have an idea of which standing waves we're concerned with, we may have a better idea of how to deal with them.

Consult DrWho for that one. But standing waves are upper mid tones that usually add a 'tubby' or 'boxy' sound. Just knock on the side of the box for an approximation.

M

[johnyholiday]

Huh? WTH does that have to do with the question? Man you are just not right.

K

An opinion of a counter slow stochastics

oscillating indicator the more divergence with "Man you are just not

right" the better, poor Roy a discussion with a basic carbon unit about

foam

[Audiokid]

DON'T DO IT! Your veneer will warp if you 'dampen' it (my mother used to 'dampen' the clothes with water before ironing them). Now if you're talking 'DAMPING', that's a different animal. (Just one of my pet peeves).

Dave

[Kriton]

Rethought my response - edited to eliminate overly vicious rebuff.

Mea Culpa.

K

[BS Button]

Back to the issue at hand. I actually replaced one panel in my Cornwalls with foam that was too thick. It choked the airflow to the ports and caused a noticeable lack of bass. Al Klappenberger talks about the woofer enclosures in the Klipsch Heritage line on his website. Now, maybe he's just referring to the Folded horn woofers here, but he cautioned to leave the bass enclosures alone and I'd have to agree, based upon my experience. The foam you chose:i.e. peak and base dimensions on the convolute should be less than an inch, combined, and especially on the back panel. That air has got to travel somewhat freely to the ports. Non-vented enclosures are probably fair game for more insulation, but not vented. Also, consider the width and length on the back panel to make sure it is no more than what came in the Cornwall 1. I changed back from a thicker, larger pc of foam to a densified polyester fiber pad of 3/4" thickness, and reduced the width and length to the factory size....suddenly bass response was back to normal. And, good greif, don't worry about covering the crossovers. My .02.

Note: This modification was only performed on the back panel. The other parts of my Cornwalls are the stock tissue paper.

BS

[Kriton]

Thanks BS for adding to the discussion, I guess I got carried away a

little higher, just hate when people set out to make other folks look

stupid.

I agree with you on blocking the ports...the egg crate stuff I am using

from Parts Express is not high end foam by any means as this was more

of a test... but the foam that Klipsch is using on the CWIII is the

same heavy tempurpedic-like egg crate that is used in the RF-7's...it

appeared to be as thick as the stuff used in the RF-7 and it clearly

covered the inside of the cabs, which are the same apparent size.

The only surface not treated was the motorboard, that I was able to

discern.

As I said I did not get a chance to see in the test cab to notice if

the foam was placed on top of the shelf, and that is a question I would

like to answer. Due to the fact that this is a test job, I did

not (thankfully) use adhesive to apply the foam, just a wall stapler

with 1/2"staples that can be easily removed. The foam is only

down to maybe an inch above the port shelf and no lower, so as not to

restrict the ports.

The foam is easily an inch at the highest tip of the egg crate, and I

did have a problem with stuffing the mid-horn back in the case. I

did not remove any foam behind it however (as Mike suggested), and I

was going to ask if it was necessary to do that...the horn is snugly

held and the magnet is directly in contact with the foam. Is this

a problem, does the mid really need need to be self suspending?

What are the pros and cons?

In your opinion, what should the effect be with placing foam in this

cabinet, what should I be hearing, if it is working well? I have

not had a chance to do any critical listening at this point, but I can

say that the sound I am hearing so far is more ...detailed (?)...on a

blind test my wife thought that the modified speaker sounded *louder*

on first blush...but I don't think that is it, I think it was just

clearer, if that is possible.

Any opinions on what the foam should do from a hypothetical point of view?

K

[wstrickland1]

DON'T DO IT! Your veneer will warp if you 'dampen' it (my mother used to 'dampen' the clothes with water before ironing them). Now if you're talking 'DAMPING', that's a different animal. (Just one of my pet peeves).

Dave

I wonder if he's been making a fool of himself since 1969?

[colterphoto1]

The stuff that Todd gave me with the decorator Heresies, looks EXACTLY like what they were stuffing in cabinets in HOPE.

Very softly convoluted, not 'eggcrate', fairly soft foam approx 1- 1 1/2 inch thick.

Michael

[BS Button]

First of all, the discussion about eggcrate and tempurpedic like foam is just jibberish. It all comes down to the density and air-flow or porosity of the (open cell) foam. You can acheive the very same results inside the cabs with flat slab urethane or polyester foam. You're just trying to knock all the activity down a bit. I've always liked Colter's explanantion of just using one sheet on oposing sides, makes perfectly good sense, to me. If somone was to use a visco-elastic memory foam as you suggest, the density and measured airflow far outweigh the importance of the elasticity.

What's the goal of lining the cabs with foam? What are you trying to acheive in a vented design?vs. that of a sealed enclosure?

BS

[Kriton]

[dbflash]

I used Deflex Panels in both my Cornwalls and Heresy's.

10 panels was enough to do both pairs.

Madisound.com

Danny

[dragonfyr]

First of all, the discussion about eggcrate and tempurpedic like foam is just jibberish. It all comes down to the density and air-flow or perosity of the (open cell) foam. You can achieve the very same results inside the cabs with flat slab urethane or polyester foam. You're just trying to knock all the activity down a bit. I've always liked Colter's explanation of just using one sheet on opposing sides, makes perfectly good sense, to me. If someone was to use a visco-elastic memory foam as you suggest, the density and measured airflow far outweigh the importance of the elasticity.

What's the goal of lining the cabs with foam? What are you trying to achieve in a vented design?vs. that of a sealed enclosure?

BS

Some good points and some good questions.

A few thoughts and considerations...

Is this proposed dampening to reduce standing waves and cabinet resonance effecting woofer response, or to minimize radiated mid frequency vent noise, or...?

Or perhaps you are considering this just because others have done so and been pleased with the results (which is a valid reason as well! It just makes defining the solution alittle more difficult! As the specific problem to be solved still has to be identified...)

Allot of the sheet style foams are as reflective as they are absorptive...as their acoustical impedance varies with frequency. They will simply move the problem surround if they are not used 'surgically' - matching need to the response.

Lossy fiberfill is more effective for LF standing waves while sheet material such as sonix would be more appropriate for mf/hf vent noise suppression...

Defining the problem and the goal certainly help in defining the best solution.

[DrWho]

The foam is easily an inch at the highest tip of the egg crate, and I

did have a problem with stuffing the mid-horn back in the case. I

did not remove any foam behind it however (as Mike suggested), and I

was going to ask if it was necessary to do that...the horn is snugly

held and the magnet is directly in contact with the foam. Is this

a problem, does the mid really need need to be self suspending?

What are the pros and cons?

In your opinion, what should the effect be with placing foam in this

cabinet, what should I be hearing, if it is working well? I have

not had a chance to do any critical listening at this point, but I can

say that the sound I am hearing so far is more ...detailed (?)...on a

blind test my wife thought that the modified speaker sounded *louder*

on first blush...but I don't think that is it, I think it was just

clearer, if that is possible.

Any opinions on what the foam should do from a hypothetical point of view?

Well any standing waves inside the cabinet will probably be somewhere around these frequencies (in Hz):

395.1

569.8

966.6

One thing to note, the 3rd harmonic of 395.1 (1185.3Hz) and the 2nd

harmonic of 569.8 (1139.6Hz) are very close together, not to mention

also very close to 966.6 (within one note). Does anybody know what the

crossover frequency and slope is on the cornwall? I would guess

something like 12db/octave at 500Hz, which puts the woofer only 12dB

down at 1kHz (where it seems any standing waves should be centered).

Having the woofer very close to the middle of the speaker is really

going to amplify the second harmonics (probably why klipsch moved the

woofer up in the newer cornwall 3).

In order for absorbtion to be effective at a particular frequency, it

needs to be thicker than a quarter wavelength. 40Hz has a wavelength of

28.25 feet, thus requiring absorbtion at least 7 feet thick! The reason

I bring this up is because the dampening material you put inside the

cabinet is going to have very little if any effect on low frequencies

because they simply aren't going to see the material (but rather the

hard face of the cabinet behind it). I really don't see how you are

going to affect the tuning as long as you don't block the port, or do

anything crazy. The addition of loose polyfill inside the cabinet does

increase the apparent size of the cabinet to the driver, but this would be a

good thing because the cornwall is already undersized and has a slight

peak around 80Hz because of it.

I think the biggest change you noticed is due to the rear panel being

firm against the back of the squaker. At first I was thinking that

vibrations from the motorboard and the compression driver itself would

get amplified through the rear panel (which may very well be the case),

but then I was thinking that you will also be further dampening the

rear panel. When I was conducting a chorus II versus cornwall

comparison session at my place, I noticed that the rear panel of the

cornwall was vibrating quite a bit (playing at around 80dB). Try

uncoupling the squaker from the rear panel (remove the foam in the way)

and see if the sound doesn't go back to what it was previously. If this

is the case then I would try installing some cross-bracing of your own

to dampen the rear panel (instead of relying on the squaker to do it).

Or perhaps the extra clarity you notice is just that the squaker is

better braced inside the cabinet (so energy from the motor is being

used to move air instead of the whole driver/horn combo).

I just wanted to comment that most of this is speculation (except the

stuff about the absorbtion). I bet someone at klipsch would be able to

tell us the exact numbers and the source of the problem if we really

wanted to know. One thing I've thought a lot about and I figure now is

as good a time as any...how easy would it be to remove the front baffle

and put on a new one? Klipsch has already arrived at the conclusion

that the woofer needs to be moved up (and I know someone has that

specific number), so why don't we just follow suit and revamp our

cornwalls? There are a lot of cornwall versus lascala opinions floating

around and I wonder how they might change when comparing against a cornwall

without standing wave problems.