Gaskets? Would replacing these have any discernible effect on the sound quality?

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sxp Posted: Fri, Aug 14 2015 12:24 AM

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Hi, I have a pair of re-foamed CX 100s, but the gaskets around the rear panels and around the Front top and bottom drivers are really quite tired and flat looking. Would replacing the gaskets have any discernible effect on the sound quality? If so are the gaskets available from anywhere, or would it be worth using a good-quality polyurethane or neoprene foam tape or the like?

I'd really appreciate some advice

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Jeff replied on Fri, Aug 14 2015 1:22 AM

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The CX series are "acoustic suspension" speakers, that is they are designed as a closed, sealed box. They operate by giving the woofer a certain springiness of air sealed in the cabinet to work against and the woofer specifications are chosen, as is the size of the cabinet, to work together to create a certain low frequency response. Different size boxes, or leaky boxes, will not give the exact response designed. So, personally if they were mine I'd change the seals/gaskets. One way to see if they are very leaky is to gently push in on the woofer, putting the force as evenly as you can by pressing on the woofer near the edge of the center cap gently using two or three fingers, making sure you don't push it at an angle where you feel the voice coil scraping. Push it in reasonably far, then see how fast it pops back when you release it. If it's still sealed tight, it should spring back fairly slowly, if it's very leaky, it'll pop back faster. How fast is fast and how slow is slow is hard to describe unless you've done it before of course! But you should be able to tell.

Might consider changing the crossover cap while you're in there if you have to open them up and change the gaskets if you have the skills, they're fairly old speakers and sometimes the capacitors can deteriorate and change value with time.

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Jeff replied on Fri, Aug 14 2015 1:25 AM

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Of course, if they weren't refoamed properly you can also get air leaks around the foam surrounds that will act the same way, any leaks show up in this test. I had one I refoamed that was leaky that I had to pull and go around the edge until I found the part that wasn't glued down properly, resealed it and all was fine then.

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Geoff Martin replied on Fri, Aug 14 2015 11:53 AM

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Hi,

In addition to what Jeff said:

In a closed-cabinet loudspeaker, the leaks themselves can be audible due to air moving through the hole(s).

Play some music that has low-end-only and listen for "fluttering" or "fffffff" noises around the gaskets.

Some examples of stuff-to-play

"Angel" by Massive Attack (album: Mezzanine) (just the first couple of seconds)

"The Game Has Changed" by Daft Punk (Tron Legacy soundtrack)

"Alien Bashment" by Rudimental (Home)

Another option is to listen during the kick drum hits on a cleanly recorded and sparsely-mixed tune like

"I bid you goodnight" by Aaron Neville (Warm Your Heart)

Cheers

-g

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sxp replied on Fri, Aug 14 2015 12:09 PM

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Hi Jeff, thank you, based on your advice they definitely need new gaskets all-round as there are clearly visible gaps, especially at the back of the cabinets. The re-foaming looks very clean, centralised and completely adhered, although I will definitely do the tests on the drivers you suggest once the cabinets are airtight, as this may give a false reading until this is done?

I did some googling to see if there were any gasket kits available for both my CX 100s and CX 50s, but couldn't find anything specific; do you know of any back and front gasket kits that can be purchased?

Thank you for the additional advice about the crossover capacitors, what's the best way to tell if they do need replacing? 

If they do need replacing, I have no experience with this kind of thing, although I'm fairly handy with the soldering iron. I did some googling specifically for the CX 100s and CX 50s that I own, but couldn't see any specific kits; are there any available that you could recommend please?

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sxp replied on Fri, Aug 14 2015 12:16 PM

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Hi Geoff.. some really great suggestions of music there to test with, but I've not noticed any fluttering or fffff noises; but the gaps in the gaskets at the back of the units are very visible, so may not produce the desired the telling fffff noises??

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sxp replied on Fri, Aug 14 2015 12:16 PM

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Hi Geoff.. some really great suggestions of music there to test with, but I've not noticed any fluttering or fffff noises; but the gaps in the gaskets at the back of the units are very visible, so may not produce the desired the telling fffff noises??

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Geoff Martin replied on Fri, Aug 14 2015 2:28 PM

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Hi,

If there's a visible gap into the inside of the enclosure, then it could be that the hole is just too big to cause enough turbulence to be audible.

If you blow through a straw, you'll hear a ffffff sound. If you blow through a 3" diameter pipe, you won't. The reason is that the air velocity and the shape of the edges around the gap combine to create turbulence. The lower the air velocity (because of a larger hole or pipe), the lower the turbulence, and the less noise will be produced.

So, if your gaps are huge - take an extreme case, you lose the entire cabinet, and all you're left with is a woofer floating in space - then everything Jeff said will be true, but there will certainly be no funny noises that sound like a fffffff.....

It could also be that there is a gap/hole somewhere else (like through the surrounds) which works in parallel with the back gasket leak, making a bigger hole and therefore a lower air velocity...

Cheers

- geoff

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Jeff replied on Fri, Aug 14 2015 3:06 PM

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As for the capacitors, obviously if they're leaking or appear damaged that's one sign, but it usually requires you to measure them which you probably don't have the equipment to do unless you're a audio nerd/retired engineer like me! I can measure capacitance with my multimeter, and also have a little device that measures ESR (equivalent series resistance) of the cap. Replacements are pretty cheap and don't require mad soldering skills to change out so if there's any doubt I usually change them even if I don't measure them.

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sxp replied on Sun, Aug 16 2015 4:12 PM

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Thanks guys, that's really helpful.. Does anyone know of any back and front gasket kits that can be purchased?

Also, are there any capacitor crossover kits or the like that I can purchase?

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Jeff replied on Sun, Aug 16 2015 6:00 PM

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I don't know of any but I haven't really looked for any. I have enough scrap rubber and foam material around I keep for just this purpose, and cut my own gaskets using the originals as a template. My wife always complains that I save packing material from every box that comes into the house but it pays off!