Pio1980
AK Member
A controlled bit of 'seepage' is necessary to allow for barometric changes. Otherwise, the neutral position of the woofer and it's voice coil would shift with barometric changes.
AR-3a: What is the importance of having an airtight seal on woofers?
- Thread starter mhjmhjones
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- acoustic research ar-3a
onplane
What! No Wake???
No acoustic suspension system can be 100% air tight. If they were they'd become huge barometers. That is the "rest position" would be a function of elevation above sea level.
As N8Nagel said the seal has to be good enough to prevent air escaping at 20Hz. Air escaping slowing to compensate for pressure differentials (weather related or elevation related) is not only OK; it is necessary!
Indeed, there is an article over is CSP where they talk about how AR technicians tested for "leaks" or "birdies" by means of a large low frequency signal and a stethoscope.
Funny enough, but "leaks" in ported or vented systems are just as devasting to low frequency response. Yes, they are designed to leak, but the only leak is through the tuned port. Further, finding "leaks" in vented systems is much harder for the average consumer.
Regards,
Jerry
As N8Nagel said the seal has to be good enough to prevent air escaping at 20Hz. Air escaping slowing to compensate for pressure differentials (weather related or elevation related) is not only OK; it is necessary!
Indeed, there is an article over is CSP where they talk about how AR technicians tested for "leaks" or "birdies" by means of a large low frequency signal and a stethoscope.
Funny enough, but "leaks" in ported or vented systems are just as devasting to low frequency response. Yes, they are designed to leak, but the only leak is through the tuned port. Further, finding "leaks" in vented systems is much harder for the average consumer.
Regards,
Jerry
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sKiZo
Hates received: 92644 43.20°N 85.50°W
I imagine any non-vented speaker benefits from having the seal as tight as possible. I also expect a certain amount of seepage is built into the driver materials to allow compensation for atmospheric pressure changes. Those should be slow and gradual enough that it shouldn't be a concern during use.
Any thoughts on resealing the cabinets themselves? Those tend to get a bit loose over time due to shrinkage of the panels. I've also found that a lot of speakers develop leaks around the input panels. Black silicone is your friend here.
Any thoughts on resealing the cabinets themselves? Those tend to get a bit loose over time due to shrinkage of the panels. I've also found that a lot of speakers develop leaks around the input panels. Black silicone is your friend here.
Here's what my rehabbed speakers currently sound like:
I used the pre-existing mortite and filled it in around the woofer thinking it was more of a vibration dampener than an air seal. Most of the original mortite stayed in place on the woofers when I took them out. I'm happy with the bass sound and I wouldn't want it much more bassy.
I used the pre-existing mortite and filled it in around the woofer thinking it was more of a vibration dampener than an air seal. Most of the original mortite stayed in place on the woofers when I took them out. I'm happy with the bass sound and I wouldn't want it much more bassy.
Lesterbest
Super Member
How can anyone compare an AR 4x to
For your information mortite drys out with time (40 yrs). It's maliable like clay when new.. That may have been the onset of your issue. Also I question how anyone comparing an AR 4x to an AR 3a expects to be taken seriously. This thread is the blind leading the blind
GD70
Lunatic Member
Two different woofers to illustrate a point, that was the reason for mentioning the 4x woofer. My 3's dust caps are sealed, the 4's are not. I know they are two completely different speakers.
onplane
What! No Wake???
Are the dust caps on the 4x's originals?
When you perform the pressure test on the 4x's, do the woofers return slowly?
Jerry
Apolek
Well-Known Member
I
If you're happy, that's all that matters. 95%, 96%, 99.999%... doesn't matter; all that matters is that you like the way they sound. Enjoy!
If you're happy, that's all that matters. 95%, 96%, 99.999%... doesn't matter; all that matters is that you like the way they sound. Enjoy!
centsless
Super Member
The only thing to add is you don't use PLUMBERS PUTTY!!...ever.... to seal your drivers. It hardens and cracks in a short time. Remove that old mortite and get some duct seal from a hardware store. It is cheap and works excellent for sealing drivers and stays flexible for years...many years. I roll it on a table with my hands to get a 1/2" thick rope and simply apply it around the perimeter. Some use the foam gaskets also that AR used later in the 70's.
Lesterbest
Super Member
Mortite is fairly inexpensive. The advantage is it gives a good seal but allows drivers to be changed without cabinet damage. In that era, we replaced all defective drivers with new drivers. We banged out 44 AR 5s in one day that AR donated to MOMA. In a Soft Machine concert, something oscillated. We replaced every driver & every crossover.
GD70
Lunatic Member
Hey Jerry!
Yes, they are original. When I gently push in the woofer cones, they return fairly quickly, but not as quickly as the 4301b's which are ported.
I did apply a very thin coat of the sealer on the surrounds and did notice an improvement in the bass output.
Glenn
onplane
What! No Wake???
Glenn, a significant amount of air can escape through the dust cap. I know, because I had a refoam with the correct foam, but wrong cap.
I sealed the cap and all was well again.
Try this, lay the 4's down, put a tissue over the dust cap and push in quickly. If the tissue moves at all, I'd seal those caps. Acoustic suspension depends upon the "trapped" air to provide the restoring force. This very linear restoring force is what allows AS speakers to have that exceptionally low distortion.
Regards,
Jerry
twiiii
Lunatic Member
AR- 3 like KLH 5 Like Mac ML1 require 100 percent sealed cabinet. We applied 50 watts sine wave generated with an oscillator signal from 20 to 100 Hz , listening for buzzes, whistles, rubbing and other forms of distortions. Sometimes the dried out surround could be a big offender. Loose dust caps and warped rubbing voice coils and bent bobbins were a real problem. Some times to much power applied to the speakers damaging the voice coil physically as well as potentially damaging the voice coil electrically were revealed.
Lesterbest
Super Member
Once you sweep a bad AR 3, 3a & 5 MR @ 750 Hz, it will honk @ you. That's the frequency where most MR issues are. There's no substitute for sweeping. Problems manifest themselves quickly. Every audio club should have a signal generator.
Well, the good news is that when I push in on the woofer cones, it returns slowly. Much slower than when I pushed the cones in while they were out of the cabinet.
...and that is good enough. The woofer returns slowly as the air seeps back into the cabinet to equalize the pressure. It just needs to return more slowly than when it is not installed in the cabinet.
AR did factor in some "leakage". The AR-4x woofer, with its untreated cloth screen dust cap and lightly treated cloth surround, is the best example of this. A bit of leakage around the shafts of the original AR pots also takes place, so glopping lots of sealant on a 4x woofer surround to make the woofer return more slowly is counterproductive. Some manufacturers later began to design otherwise acoustic suspension speakers around specifically controlled leakage. This is known as an "aperiodic" design, the most popular example being the Dynaco A25..
Roy
GD70
Lunatic Member
Thanks Roy for helping clear this up. I didn't realize the pots allowed leakage as well.
Glenn
Hi Glenn,
The pot leakage is tiny, but in the 4x it adds to a greater amount than other AR models of the era.
Tom Tyson, owner of the original AR engineering files, has suggested that the AR-3 and early 3a cloth surround woofers' dust caps were not completely sealed to allow some leakage. I do know that when I've replaced or sealed these dust caps, it has slowed some of these woofers to the point of just creeping back when pushed in...so I'm no longer doing this if the cabinet seems sealed well enough. Later 12 inch woofers had completely sealed dust caps, but allow some leakage through the foam surround.
Roy
KingBubba
"Too Much Stuff"
I am surprised to hear that plumber rope dries and cracks quickly. I have plumbing at home and speakers with the rope used in it and both items are still as soft and sticky as the day it was installed. Is it possible that we are not talking about the same item? I am curious because I have never heard anyone say this before. Maybe I should start a new thread with this question.
easy enough to get something that works better, use "3M Strip Caulk" available at auto body supply or a better parts store - probably ver yclose to original material
Where an actual gasket was used I've used Harbor Freight solid black toolbox drawer liner cut to shape, much sturdier than the crap foam originally used.
Where an actual gasket was used I've used Harbor Freight solid black toolbox drawer liner cut to shape, much sturdier than the crap foam originally used.
