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Acoustic Suspension Design - How tight is tight?

nniemiec

nniemiec

Active Member
I read somewhere that acoustic suspension design principles call for an enclosed box, but not air-tight.
Is this true and if so- what's the dynamic at play with a box that's not sealed properly vs. one that is too tight? I assume it's all about pressure, but I'm not schooled enough to understand what the principles are. I'm curious.
 
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nniemiec said:
I read somewhere that acoustic suspension design principles call for an enclosed box, but not air-tight.
Is this true and if so- what's the dynamic at play with a box that's not sealed properly vs. one that is too tight? I assume it's all about pressure, but I'm not schooled enough to understand what the principles are. I'm curious.
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I believe an acoustic suspension design speaker that isn't air tight technically speaking is a non-tuned bass reflex speaker by default. :confused:
 
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An acoustic suspension speaker cabinet should not be completely air-tight. It it were, it would become a barometer. Every change in barometric pressure would cause the cone to move in or out. The practical amount of air-tightness ranges somewhere between the cabinet being a barometer and a non-tuned bass reflex cabinet.

Generally, an acoustic suspension cabinet is thought to be sufficiently air-tight to function properly without being unduly subject to barometric pressure when the woofer cone takes more than about 1/2 second to return to rest after the cone is pushed in and held in place for several seconds, then released quickly. There is no strong concensus on the 1/2 second minimum. Some prefer 1 second, other prefer 2 seconds return time.

There is math that determines the degree of air-tightness or air leakage. It is based on the contained air acting as a spring for the driver.
 
A competently built sealed enclosure should be airtight at all seams. Then there's mounting the drivers. Even though gaskets are used to seal the driver, there's still some leakage, from the driver. Be it foam surrounds, pole piece vent, or some leakage from the gaskets, the enclosure does breathe a bit. And that should be enough. Good enough sealing is what you should aim for, so that leakage is not excessive. If one aims for some leakage, imho, it will be too much.
 
Pio1980 said:
The barometric venting is usually via the spider and voice coil cavity, thru the cone dust cap.
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Sometimes, sometimes not. For example, KLH required that the dust cap be sealed the same as the surrounds. Other speakers use a porous dustcap, which allows controlled venting and cooling.
 
Ideally an acoustic suspension speaker is sealed tight. Mcintosh use to use a form of plumbers putty to seal the speaker frame edge to the enclosure. AR was just as careful. The enclosed air acts as a spring to control the woofer linearity. If the enclosure leaks the speaker Q is lowered failing to extend the output to the lowest frequencies and compromising other variables.
 
The speaker cabinet should be as tightly sealed as possible. The size of the enclosure (ie. the amount of air inside) will determine the "tightness" or "springiness" of the enclosure.
 
spark1 said:
RTally is correct. There must be some small amount of air exchange. See Edgar Vilchur's original patent documentation.
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OK, I had to look up the patent. It is US Patent Number 2,775,309.

While a completely gas-tight seal for the enclosure is theoretically desirable, in a practical construction it is apparent that the speaker cone would function in a manner analogous to an aneroid barometer in response to variations in atmospheric pressure. The voice-coil would accordingly be continuously biased from its predetermined optimum position in the magnetic field due to atmospheric changes. Accordingly, a small amount of air bleeding or air leakage is permitted to exist between the inner and outer portions of the speaker enclosure in accordance with this invention. The amount of leakage is sucient only to equalize long-tirne variations due to atmospheric conditions but is insufficient to affect the inherent elastic stiffness of the enclosed air cavity for voicecoil excursions occurring when reproducing sound. In other words, the enclosure is designed to provide an acoustic seal, that is, one which does not allow significant air leakage over a period corresponding to a half-cycle of the lowest frequency encountered.
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Col. 4, line 68, to Col. 5, line 11.

If anyone should know, it would be Villchur. He puts it in terms of an acoustic seal (emphasis above from the patent).
 
Iv'e had real world experience with this issue, Cabinet volume 10.2 cu ft internal, 2 15" CTS high compliance acoustic suspension woofers per cabinet (13mm x max) with an fs of 18hz, the thiel sheet calls for a 19.8 cu ft per woofer. These two woofers in a sealed cabinet were disappointing, the low end started to roll off at 36 hz and had a horrible resonance at 60 hz, every surface inside was covered in 3" of fiberglass insulation. Many years later I decided to address the problem. On a hunch after reading up on ported cabinets and tuning software, I decided to install 2 precision ports 3"x12" long flared on both ends, that tuned the cabinets to 17hz and allowed the woofers to move, greatly increasing excursion, volume and the response now rolled off at 24 hz (flat down to 26 hz) minus 6 db at 20hz and minus 9 db at 16 hz. I could not believe the difference. These drivers were not recommended for ported cabinets, granted they probably would have sounded better with just one driver per cabinet but now I'm completely content with them. I'm still amazed at the difference. Even at 100 watts at 20 hz there just under the 13mm max x. That said, the cabinet software shows a flat response down to 20 hz with a 20 cu ft cabinet, can't imagine moving those around, the 10.2's are almost 200 lbs each. So I got what I wanted with half the size.
 
"The amount of leakage is sufficient only to equalize long-time variations due to atmospheric conditions but is insufficient to affect the inherent elastic stiffness of the enclosed air cavity for voice coil excursions occurring when reproducing sound."

He is talking about a "barometric leak." The contention is that the pressure change in the cabinet due to temperature changes (PV=nrT) would cause the rest position of the woofer cone to change from some normal position (say at 70 degrees F). In real life I wonder how much the cone would be displaced going from say 70 degrees to 100 degrees. Barometric leaks, very tiny holes, are put in the chamber between the diaphragm and back plate of very sensitive condenser microphones so the diaphragm is always flat at rest. This looks like a very high resistance in a mechanical/electrical analog circuit. I think for patent protection Vilchur was trying to address every possible scenario in his analog circuit description to prevent copiers from purposely putting a tiny leak somewhere in their speaker systems to claim they are different. When he mentions the effect in a half cycle to cone motion he is saying the only way to circumvent the patent is to have a "substantial" leak so it is not acoustical suspension.
 
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So where is the "small amount" of leakage occurring in an AR speaker? The driver surrounds?
 
Drugolf said:
So where is the "small amount" of leakage occurring in an AR speaker? The driver surrounds?
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I don't think there is any "special" leaky spot in the cabinet or woofer. You cannot hermetically seal a normal cabinet. The surround will leak, etc. Vilchur is stating that if the woofer is "springy, ie undamped" for at least a half cycle of motion, it would be a good seal.
 
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As the patent application indicates, a barometric vent doesn't need much more than time lag to equalize interior pressure differential.
 
Remember the cone is paper and could be porous. Water has a larger atom than oxygen and nitrogen and water will drip through a paper cone, so will oxygen and nitrogen equalizing the pressure.
 
Thanks for information here. I am building my first serious AS system and am obtaining valuable information in using the Altec 874 Segovia woofers. As I understand you want to obtain an ideal "Q" of .707, right.
The 874 woofer has a huge center vented magnet structure and is very loosely suspended. I have not measure it yet but it looks to be an ideal woofer for AS application!
DC
 
Norman Labs were really good acoustic suspension speakers and they used porous dust caps. I promise you those cabs were air tight and if you pushed the woofer cones in, it took at least a half second for them to spring back.
 
beans said:
Norman Labs were really good acoustic suspension speakers and they used porous dust caps. I promise you those cabs were air tight and if you pushed the woofer cones in, it took at least a half second for them to spring back.
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I agree-Norman Labs made some very nice sounding AS designs! That is the sound I hope to emulate with my woofers! Their Model 10's still impress me.. I have refoams dozens of these and love their build.
DC
 
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