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Author Topic: PICTURES inside an SB1000  (Read 18711 times)

Phillip_Graham

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Re: PICTURES inside an SB1000
« Reply #10 on: July 22, 2009, 12:13:02 pm »

Jon Waller wrote on Wed, 22 July 2009 10:23

Tom Manchester wrote on Fri, 17 July 2009 16:05

Neat. I've always wondered what the smaller tube ports in addition to the large center slot port accomplished. Why not one or the other? Anyone know?



Perhaps it has to do with driver cooling.  The round port tubes look like they end right above where the magnets on the drivers would be, and they help heat escape that would build up in the front corners of the box, above the woofers.  This could improve driver reliability and reduce power compression losses.



I doubt it has anything to do with driver cooling.

If I was going to add overly small ports like these to a box like this, it would be for one purpose, and that would be to dynamically shift the tuning frequency of the box.

If you build an equivalent circuit model for the box alignment, with both port systems in parallel, this design would have a certain small-signal tuning frequency.

At high output levels the resistive component of the port impedance rockets up due to turbulent flow conditions (which you can estimate from reynold's number calculations).  This effectively shunts these small round ports out of system, leaving the tuning set by the large slot port.

So my hunch is variable box tuning that dynamically shifts depending on the velocity of air in the smaller ports.

Clever, really...
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Jon Waller

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Re: PICTURES inside an SB1000
« Reply #11 on: July 22, 2009, 03:05:58 pm »

Phillip, I think you are right.  Clever of them and you...

But for this to work, the turbulent flow conditions in the small ports could give rise to 'chuffing' noises (a Richard Small term, I believe) at high power levels.  Perhaps this drawback, combined with newer, larger xmax drivers, are why they were not included on the new SB1002 model.
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Tim Charpentier

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Re: PICTURES inside an SB1000
« Reply #12 on: July 23, 2009, 03:13:06 am »

I have a buddy who built 8 of these in May/June...He said they were relatively straightforward to build...Unfortunately, the proper porting was an issue for him...He wasn't sure which version of the sub had the "best" port configuration...I've seen the boxes (came out great), but didn't take a close look at the ports...
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Phillip_Graham

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Re: PICTURES inside an SB1000
« Reply #13 on: July 23, 2009, 09:06:04 pm »

Jon Waller wrote on Wed, 22 July 2009 15:05

Phillip, I think you are right.  Clever of them and you...

But for this to work, the turbulent flow conditions in the small ports could give rise to 'chuffing' noises (a Richard Small term, I believe) at high power levels.  Perhaps this drawback, combined with newer, larger xmax drivers, are why they were not included on the new SB1002 model.


Jon,

I've read a couple of papers on these turbulence effects, and the change in the resistive component of the acoustic impedance of an undersized port can be extreme, like more than 2 orders of magnitude!

If the change in these small ports was similarly extreme (likely), they would nearly shunt themselves out of the practical acoustic system w.r.t. the large port.

Clearly, though, air has to move through them to cause the rise in acoustic impedance.  If the effect is pronounced very near the inside end of the tube, perhaps there would be no "chuffing" at the outlet of the port tube.  I truly don't know.

Not having experience with this particular SB1000 variant first hand, I don't know if this is audible or not.  I know that if I was the manufacturer I would do a series of nearfield acoustic measurements of the cone, large port, and small port, at different power levels and signals, and I would try to correlate this to the impedance spectra taken under the same stimuli.

Academic musings worth probably less than 2 cents.
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Jon Geissinger

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Re: PICTURES inside an SB1000
« Reply #14 on: August 16, 2009, 06:56:38 pm »

I hope those little bitty wires are for some LED or sensor or something and they are NOT the speaker wires!?!?

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Evan Kirkendall

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Re: PICTURES inside an SB1000
« Reply #15 on: August 16, 2009, 07:58:55 pm »

Jon Geissinger wrote on Sun, 16 August 2009 18:56

I hope those little bitty wires are for some LED or sensor or something and they are NOT the speaker wires!?!?





Why? That's a pretty common cabinet wire size...



Evan
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drewgandy

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Re: PICTURES inside an SB1000
« Reply #16 on: August 17, 2009, 07:56:42 pm »

Phillip Graham wrote on Thu, 23 July 2009 20:06



Jon,

I've read a couple of papers on these turbulence effects, and the change in the resistive component of the acoustic impedance of an undersized port can be extreme, like more than 2 orders of magnitude!

If the change in these small ports was similarly extreme (likely), they would nearly shunt themselves out of the practical acoustic system w.r.t. the large port.
...
Academic musings worth probably less than 2 cents.


I had a post put together a couple weeks ago and thought it posted but somehow it did not. So this is a little late to the discussion...

I kind of gave up on vented things many years ago on account of the vent velocity issues so I haven't given this much thought in quite awhile.  Mostly I'm wondering if a downward movement of the tuning is actually what you want when the vc heats up and/or the woofer is really moving.  Also, do you consider the vc temp to be the biggest characteristic begging dynamic manipulation?

drew
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Jeff Robinson

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Re: PICTURES inside an SB1000
« Reply #17 on: October 17, 2009, 10:40:46 am »

drewgandy wrote on Mon, 17 August 2009 18:56

Phillip Graham wrote on Thu, 23 July 2009 20:06



Jon,

I've read a couple of papers on these turbulence effects, and the change in the resistive component of the acoustic impedance of an undersized port can be extreme, like more than 2 orders of magnitude!

If the change in these small ports was similarly extreme (likely), they would nearly shunt themselves out of the practical acoustic system w.r.t. the large port.
...
Academic musings worth probably less than 2 cents.


I had a post put together a couple weeks ago and thought it posted but somehow it did not. So this is a little late to the discussion...

I kind of gave up on vented things many years ago on account of the vent velocity issues so I haven't given this much thought in quite awhile.  Mostly I'm wondering if a downward movement of the tuning is actually what you want when the vc heats up and/or the woofer is really moving.  Also, do you consider the vc temp to be the biggest characteristic begging dynamic manipulation?

drew


Actually, the enclosure tuning shifts up in frequency (when the small round port 'chokes' at high power levels). Consider that for a given box tuning frequency the larger the port, the longer the duct. Conversely, a small but long port will have a lower tuning frequency (for a given enclosure size). The amount of energy you can push through the porting system is proportional to the radiating area.

Robert Bullock's published info is a really good way to use Thiel/Small specs to look at the whole range of possible tunings for any given loudspeaker (math is involved).

Jeff Robinson
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Jeff Robinson

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Re: PICTURES inside an SB1000
« Reply #18 on: October 17, 2009, 11:13:02 am »

Jon Waller wrote on Wed, 22 July 2009 14:05

Phillip, I think you are right.  Clever of them and you...

But for this to work, the turbulent flow conditions in the small ports could give rise to 'chuffing' noises (a Richard Small term, I believe) at high power levels.  Perhaps this drawback, combined with newer, larger xmax drivers, are why they were not included on the new SB1002 model.


With the parallel large port you are not pressurizing the back chamber, which would cause air to pump in and out of the port (what I have always regarded as "chuffing"). The turbulence restricts the small port's acoustic output to much less than the rest of the system (cone and slot port). To hear any chuffing you'd have to risk permanent tinnitus by sticking your ear right on top of the port when the input power level is set to "stun".

Jeff Robinson
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Hail to thee, blithe Spirit!
Bird thou never wert
That from Heaven, or near it
Pourest thy full heart
In profuse strains of unpremeditated art
...
Such harmonious madness
From my lips would flow
The world should listen then, as I am listening now!

Art Welter

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Re: PICTURES inside an SB1000
« Reply #19 on: October 17, 2009, 12:40:46 pm »

Jeff Robinson wrote on Sat, 17 October 2009 09:13

Jon Waller wrote on Wed, 22 July 2009 14:05

Phillip, I think you are right.  Clever of them and you...

But for this to work, the turbulent flow conditions in the small ports could give rise to 'chuffing' noises (a Richard Small term, I believe) at high power levels.  Perhaps this drawback, combined with newer, larger xmax drivers, are why they were not included on the new SB1002 model.


With the parallel large port you are not pressurizing the back chamber, which would cause air to pump in and out of the port (what I have always regarded as "chuffing"). The turbulence restricts the small port's acoustic output to much less than the rest of the system (cone and slot port). To hear any chuffing you'd have to risk permanent tinnitus by sticking your ear right on top of the port when the input power level is set to "stun".

Jeff Robinson



When a sub is run without the upper speaker, port chuffing is easily heard without sticking your head in the cabinet, because the chuffing is high frequency, octaves above the fundamental tones.
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