Speaker Surround Seal

[tysontom]

> i think speaker quality should be judged on

>how well the speaker is doing what its trying to do, not just

>range and acuracy.

The definition of "accuracy" in loudspeaker reproduction is the ability to faithfully replicate the input wafeform signal -- neither adding to nor subtracting from the original. No speaker can do this with 100% accuracy. A lot is involved in this, but a good example of excellent accuracy is shown in this image of the Acoustic Research AR-3 (predecessor to the AR-3a) reproducing the sound of a Steinway piano. A. is the oscilloscope waveform of the signal from the recording microphone (a calibrated Neumann condenser); B. is the oscilloscope waveform of the output of the AR-3 playing the same passage and C. is the electrical input to the input terminals of the AR-3. Peak power being fed to this AR-3 was between 130-200 watts, approximately, in order to play a piano at realistic levels.

http://www.classicspeakerpages.net/dc/user_files/1246.jpg

AR-3 Accuracy

>surround is

>indented toward the back of the speaker instead of appearing

>as a bubble, was that an attempt and dampening ?

There is no real difference in the performance of a woofer if the surround is convex or concave. Concave surrounds are easier for a manufacturer to handle and ship, in that the raw driver can be placed face-down. Convex surrounds will flatten if placed face-down on a table.

--Tom Tyson

[DaveD]

Tom,

As usual, your elucidation on the acoustic seal of speaker surrounds being different than an air tight seal is a revelation.

I took my KLH 17 pair to a local speaker builder who sells his own boutique line of speakers on the internet, Jay Waxler. I asked him to check the capacitors, and they are all still within spec so I let them be. They are the yellow covered Mexican polystyrene ones.

Jay told me that the woofer surrounds were leaking and that this reduced bass performance. Yes, I had just read about this on the forum and it is true that I remembered the 17's being stronger in the bass years ago. Jay said that it would be a mistake to use something that stiffens the surround too much to change the compliance.

I reaearched this sealing process as best I could at the time. I got some free samples of three different silicones from 3M, one of them a pretty thin liquid. I tried thinning regular silicone caulk with something that was recommended, I can't remember what. I also got the product sold by Orange County Speaker which is a latex product with water solvent, XL40. I put a thin coat of each one of these on a piece of old tee shirt material. I took a soda straw and used it to try to blow through the treated areas after they had dried. As I remember, all of the silicones sealed the cloth very tightly, and so did the XL40. But, the cloth was much stiffer to various degrees with all of the silicones and almost as limp as untreated cloth with the XL40 applied. It was the clear winner.

I asked Jay to apply one pretty thin coat of the XL40. It turns out that he decided to apply two coats, both very thin according to him. Well, I trusted his judgment, and thought at the time that an air tight seal was what was needed. Now, I have learned that an air tight seal is not desirable.

Boy, the bass output has certainly improved at both the upper range and at 40Hz. A hollowness to vocals that I attributed to the tweeters turned out to be a weak lower midrange and upper bass from the woofers in the crossover region. The speakers are now much more musically satisfying and right sounding.

I have a very strong 40Hz on the Stereophile Test CD warble tones, and the upper bass tones are also strong. The ones that are weakest are the 63Hz and 50Hz tones. This could be due to the fairly poor setup and environment which they are in-- a large concrete basement room.

When I press gently on the woofer cones, they do now have much more resistance than they did before being sealed. When I pull my hand away, the cones snap back part of the way, and then more slowly move for the rest of the way forward until they stop. One of them is a little more slow returning than the other one. The slower one takes about a second to stop moving forward, to recover from the amount of pressure that I am comfortable applying. I am cautious about this, and don't try to press in too hard.

I just got a pair of KLH 23's, and the surrounds on them are much darker from the original black sealer than the ones on the 17's were before they were re-sealed. They are also much more evenly coated. When I press on them they have more resistance than the unsealed 17 woofers did, but less resistance than the 17 woofers do now with the sealer. They also snap back faster than the 17 woofers do. But, of course, this is a completely different speaker and this may be the reason for the difference. I have not tried to assess the bass performance of these 23's here, but they sounded bass heavy sitting on the floor and being powered by an old Pioneer receiver at the home of the fellow who sold them to me for $59.00. On the floor near the wall they should sound that way, I think.

How can one tell if too much sealer was applied to cloth surrounds, enough to create the adiabatic barometer effect that you described in your post? Or, too much creating a negative effect on the woofer compliance. And, if there is too much, could adding some evenly spaced pinholes remediate the mistake with regard to the barometer effect? In thinking about this, I wonder if it is is possible for a heavily sealed cloth surround to be more airtight than a butyl rubber surround. It seems the answer is that it can't be.

More importantly for my selfish reasons, do my descriptions make you think that my 17 woofers have too much sealer on them? I plan to listen to the 23 pair for bass satisfaction before thinking about applying any sealer to their woofer surrounds.

[Guest palomar]

Dave,

It's very difficult to seal the system so tightly that it would act like a barometer. The cone itself, and possibly the dust cap, (although I don't know if KLH used sealed dustcaps), as well as any number of other seal points almost always have very small leakage which is acoustically invisible but keep the speaker from being really air tight.

If you did manage to attack every possible leak point, and truly make your system hermetically sealed, your cones would soon get blown out of your speaker or sucked in, depending on what the barometric pressure was when you sealed them, and what the barometric pressure changed to.)

As long as you didn't put so much sealer on the cones that their flexibility significantly changed (and it sounds like you experimented quite a bit to avoid that), then I wouldn't worry about over-sealing the speakers.

And even if the flexibility changed a little, I doubt it would have much effect on the system performance given that the compliance of the air in the cabinet is far greater than that of the suspension.

Gary

[DaveD]

Thanks, Gary. We have some wicked atmospheric pressure changes here in Tornado Alley, and the speakers are still in place. I will rest easy with regard to them being sealed too tightly, but maybe not with regard to the weather.

[Guest denmarkdrivers]

I'm getting conflicting reports about "Air tightness"

I suggested foam surrounds let a minute amount of air through wich may help its rage.

I was told it does not let any air through, i always though it let a very small amount through.

If the surround doesnt let any through, would that equate to the speaker needing a port ?

..... craig

[Guest palomar]

Craig,

I don't know whether foam is truly airtight or not, but there are usually enough very minor losses that it shouldn't matter. The real test would be to gently push the cone in perhaps a quarter inch or so - enough to move it in noticely without pressing it in too hard.

If the cabinet were truly air-tight, then the cone would be difficult to push in, and it would spring right back when you let it go. I've never seen a speaker sealed this tightly, and if it were, as I mentioned earlier, it wouldn't last long.

The normal situation is to have the cone move in moderate resistance, and then when you let it go, have it come back within a second or so. (Even if it took 10 seconds, it would still be fine. In order to be a problem it would have to come back so slowly that the weather conditions would change faster. Perhaps they do in Tornado Alley!)

If the cone pushes in very easily, and springs right back, then the leak is too large - either surrounds have to be resealed, or there is significant leakage elsewhere.

Gary