0.707 is the correct Qt for a sealed box woofer if you want a Butterworth (flat) response. The Qt for a ported box woofer is 0.38.
Acoustic Suspension Design - How tight is tight?
- Thread starter nniemiec
- Start date
dc270
Now Off the Reservation
It will be a sealed enclosure, as was the original Segovia 874 design. I think the terminology I am looking for is an ideal Qtc of .707.
As quoted from the Home Theater Forum:
Qtc - value for the damping provided for a driver in a sealed enclosure. Denotes the enclosures ability to control the driver response at resonance. Qtc = 0.707 is the optimum value for sealed enclosures, providing flattest response and highest SPL
DC
As quoted from the Home Theater Forum:
Qtc - value for the damping provided for a driver in a sealed enclosure. Denotes the enclosures ability to control the driver response at resonance. Qtc = 0.707 is the optimum value for sealed enclosures, providing flattest response and highest SPL
DC
mitchalee
Active Member
I think we need to get back to basics. What is the function of the trapped air? How tight does the cabinet need to be to perform this function?
Well, it seems to me the old phrase "air spring" says it all. The air needs to compress and rarify enough to damp the woofer cone's motion. So the cabinet does not need to "perfectly air tight" to perform its function. It just needs to be tight enough so that as a practical matter the sound is not effected by any slow air leaks.
Well, then, how tight is that? Well, put that computer or slide rule away and think about it.
A practical test would be to drive the woofer with no cabinet and then gradually increase the air-tightness till no further sonic benefit is gained. Oh, but wait a minute, if there were no cabinet the back wave would be 180 out of phase, so it would damp the woofer! So the test need to begin with an infinite baffle of some sort. Then we can gradually various amounts of air with various degrees of firmness for the woofer to use as it "air spring" and by simple inductiuon we could eventually find the point at which additional tightening would give no further sonic improvement.
Basically we are not trying to make the cabinet absolutely air tight or even find some sweet spot for its tightness. What we are really doing as a practical matter is finding the point of diminishing returns at which we are beginning to waste time and money for further no sonic gain.
My guess is that we reach diminishing returns fairly easily and quickly. Again. think about it: If the air leaks in the cabinet do not significantly change the air pressure the cone feels, then it is sonically meaningless. Now the cone is generally moving fast. Even deep bass notes with an ordinary acoustic suspension KLH, AR, Microtech, Teledyne, EPI, et al will be moving back and forth at 40 to 120 times a second. So unless the inside air can re-equalize with the outside air significantly faster than that, then I do not see a problem. One would only be shaving a bit of the resistance off of ones "air spring".
Now, don't get me wrong. I see no problem with an obsessively tight box that grips the air inside like a passionate love such as The Long Haired Lady of Grim Death. I mean, it's not a problem! But the speaker will not be sounding better after any air leaks are no longer allowing the inside and outside air to equalize in, maybe, somewhere between a tenth and a half a second. Just a ball park guess. It could be more like 5 seconds for all I know. But it's not an hour. Or even a minute. And probably not a split second.
Well, it seems to me the old phrase "air spring" says it all. The air needs to compress and rarify enough to damp the woofer cone's motion. So the cabinet does not need to "perfectly air tight" to perform its function. It just needs to be tight enough so that as a practical matter the sound is not effected by any slow air leaks.
Well, then, how tight is that? Well, put that computer or slide rule away and think about it.
A practical test would be to drive the woofer with no cabinet and then gradually increase the air-tightness till no further sonic benefit is gained. Oh, but wait a minute, if there were no cabinet the back wave would be 180 out of phase, so it would damp the woofer! So the test need to begin with an infinite baffle of some sort. Then we can gradually various amounts of air with various degrees of firmness for the woofer to use as it "air spring" and by simple inductiuon we could eventually find the point at which additional tightening would give no further sonic improvement.
Basically we are not trying to make the cabinet absolutely air tight or even find some sweet spot for its tightness. What we are really doing as a practical matter is finding the point of diminishing returns at which we are beginning to waste time and money for further no sonic gain.
My guess is that we reach diminishing returns fairly easily and quickly. Again. think about it: If the air leaks in the cabinet do not significantly change the air pressure the cone feels, then it is sonically meaningless. Now the cone is generally moving fast. Even deep bass notes with an ordinary acoustic suspension KLH, AR, Microtech, Teledyne, EPI, et al will be moving back and forth at 40 to 120 times a second. So unless the inside air can re-equalize with the outside air significantly faster than that, then I do not see a problem. One would only be shaving a bit of the resistance off of ones "air spring".
Now, don't get me wrong. I see no problem with an obsessively tight box that grips the air inside like a passionate love such as The Long Haired Lady of Grim Death. I mean, it's not a problem! But the speaker will not be sounding better after any air leaks are no longer allowing the inside and outside air to equalize in, maybe, somewhere between a tenth and a half a second. Just a ball park guess. It could be more like 5 seconds for all I know. But it's not an hour. Or even a minute. And probably not a split second.
The barometer statement earlier is correct, you don't want that.
There's an interview with either Vilchur or Allison where they talked about a problem in shipping where in normal shipping
and handling the sides of the cardboard boxes got compressed by stacking or lifting and perhaps repeatedly bouncing around.
Because of the large area of the cardboard this caused huge excursions of the woofer inside the box when they had the best
seal possible for the woofer since the pressure on both sides of the woofer could not equalize. Woofers were being damaged
in shipping. They said that a small leak was the solution so that the box/woofer did not respond to the shipping box compression.
I'd think that a 1/16" to 1/8" hole would do it but it might chuff and I've never seen one in an AR speaker, perhaps they somehow
allowed the paper cones or spider/dust cap to have a very minor leak, not sure.
There's an interview with either Vilchur or Allison where they talked about a problem in shipping where in normal shipping
and handling the sides of the cardboard boxes got compressed by stacking or lifting and perhaps repeatedly bouncing around.
Because of the large area of the cardboard this caused huge excursions of the woofer inside the box when they had the best
seal possible for the woofer since the pressure on both sides of the woofer could not equalize. Woofers were being damaged
in shipping. They said that a small leak was the solution so that the box/woofer did not respond to the shipping box compression.
I'd think that a 1/16" to 1/8" hole would do it but it might chuff and I've never seen one in an AR speaker, perhaps they somehow
allowed the paper cones or spider/dust cap to have a very minor leak, not sure.