Ummm, they all have o's in them?
Ummm, they all have o's in them?
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Ported boxes, shop vac's, and voice coil cooling
This was a tangent in notnyt's thread that I think might be of more general interest.
It went like this:
For some reason that caused some thoughts to reach full clarity after years of almost-there.
SeekingNirvana's suggestion isn't all that far-fetched.
Years ago I was showing off a new ported sub to a friend and fellow mechanical engineer.
I was playing a 35 Hz sine wave at high level and told him to feel the air near the port.
He looked around the box for other openings and asked in wonderment "Where's the air coming from?!"
I told him it was just an illusion because it was AC flow.
That's true but it's highly asymmetric, with a much more focused and higher velocity airstream on the outflow half of the cycle.
This is because on the outlow, the air volume can only come from the confines of the port, but on inflow there's nothing to prevent it coming from all directions; more area for same flowrate means lower velocity.
Because air pressure on a surface increases with the square of velocity (one factor of two because there are twice as many air molecules hitting the surface per unit time, and another because each one is hitting twice as hard), the net effect will be to push objects away from the port.
Another example of this which I've made reference to before is shop vac's; you have to put your hand within a few inches of the inlet port to feel the airflow, but you can feel the air from the exhaust port many feet away.
Then there's voice coil cooling.
Some years ago I was talking with the dislustrious Carlos Beltran about the cooling design his Acoupower high end sub drivers, which had 6" VC's and could take a couple of kW *continuously*.
I asked him how cooling could be effective when the voice coil area can't breathe in both directions - wouldn't it just draw in the hot air it just squirted out?
The answer was no; same as above, most of the air that's squirted out is ejected far away, and cool air from all directions nearby is drawn in.
It went like this:
Quote:
Originally Posted by notnyt /t/1516234/8x-18-lms-5400-ultras-sealed-to-27-cube-15hz-ported-build/50_50#post_24383162
One left, then a bunch of cleanup... dust and wood chips all over![]()
For some reason that caused some thoughts to reach full clarity after years of almost-there.
SeekingNirvana's suggestion isn't all that far-fetched.
Years ago I was showing off a new ported sub to a friend and fellow mechanical engineer.
I was playing a 35 Hz sine wave at high level and told him to feel the air near the port.
He looked around the box for other openings and asked in wonderment "Where's the air coming from?!"
I told him it was just an illusion because it was AC flow.
That's true but it's highly asymmetric, with a much more focused and higher velocity airstream on the outflow half of the cycle.
This is because on the outlow, the air volume can only come from the confines of the port, but on inflow there's nothing to prevent it coming from all directions; more area for same flowrate means lower velocity.
Because air pressure on a surface increases with the square of velocity (one factor of two because there are twice as many air molecules hitting the surface per unit time, and another because each one is hitting twice as hard), the net effect will be to push objects away from the port.
Another example of this which I've made reference to before is shop vac's; you have to put your hand within a few inches of the inlet port to feel the airflow, but you can feel the air from the exhaust port many feet away.
Then there's voice coil cooling.
Some years ago I was talking with the dislustrious Carlos Beltran about the cooling design his Acoupower high end sub drivers, which had 6" VC's and could take a couple of kW *continuously*.
I asked him how cooling could be effective when the voice coil area can't breathe in both directions - wouldn't it just draw in the hot air it just squirted out?
The answer was no; same as above, most of the air that's squirted out is ejected far away, and cool air from all directions nearby is drawn in.
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a...ha!
i've wondered about that in a few different contexts myself, particularly air flow around voice coils.
thanks noah.
i've wondered about that in a few different contexts myself, particularly air flow around voice coils.
thanks noah.
you're welcome, and props to you for your prolific output of insightful and helpful posts
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I'd have thought there would be more interest in this; maybe the thread title isn't catchy enough.
ok, changed it to make it a bit more intriguing and sound less like homework
I'd have thought there would be more interest in this; maybe the thread title isn't catchy enough.
ok, changed it to make it a bit more intriguing and sound less like homework
They can get 'quite' hot inside with continuous high signal levels. I had a QSC PLXII 3602 at clipping levels into a 15" TC3000, ( titanium cone driver with aluminum former butted directly up to to the back of the cone, connected with a thermally conductive epoxy ) for several hours with music. I decided to check the temperature of the driver before shutting it off for the night, I just about burned my hand on the cone. The cone was alarmingly hot, as well as the surround, the frame and even the MDF box was warm to the touch on the exterior. The box had a 2" layer of Roxul rockwool on 3 out of 6 sides.
I set the sealed box on it's back, put a wet towel on the cone with an icepack on it, then removed the driver from the box. I didn't take any measurements but the air inside the box was probably 140F or hotter. After detatching the top assembly from the motor I had to carry the motor with oven mitts to put it outside on the concrete steps to cool down, it was the middle of winter.
IIRC, the motor on the TC3000 is around 42 pounds of steel and ferrite, there was a lot of thermal mass to soak up the heat from the coil. Even then there was a bit of distortion inside the former where the lead in wires take a turn. There is a fairly large air gap in the motor, and the coil did not rub.
I did take a picture of the coil former on that top assembly when it was out.
They can get 'quite' hot inside with continuous high signal levels. I had a QSC PLXII 3602 at clipping levels into a 15" TC3000, ( titanium cone driver with aluminum former butted directly up to to the back of the cone, connected with a thermally conductive epoxy ) for several hours with music. I decided to check the temperature of the driver before shutting it off for the night, I just about burned my hand on the cone. The cone was alarmingly hot, as well as the surround, the frame and even the MDF box was warm to the touch on the exterior. The box had a 2" layer of Roxul rockwool on 3 out of 6 sides.
I set the sealed box on it's back, put a wet towel on the cone with an icepack on it, then removed the driver from the box. I didn't take any measurements but the air inside the box was probably 140F or hotter. After detatching the top assembly from the motor I had to carry the motor with oven mitts to put it outside on the concrete steps to cool down, it was the middle of winter.
IIRC, the motor on the TC3000 is around 42 pounds of steel and ferrite, there was a lot of thermal mass to soak up the heat from the coil. Even then there was a bit of distortion inside the former where the lead in wires take a turn. There is a fairly large air gap in the motor, and the coil did not rub.
I did take a picture of the coil former on that top assembly when it was out.
I've run 2 600Wrms 12" subs for ~6yrs and never worried about them even in summer, till now
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