Post by: Shane Presley on July 09, 2009, 10:52:26 PM
Post by: Shane Presley on July 09, 2009, 10:56:51 PM
Post by: Shane Presley on July 09, 2009, 10:57:49 PM
Post by: Shane Presley on July 09, 2009, 10:58:46 PM
Post by: Tom Manchester on July 17, 2009, 04:05:55 PM
Post by: drewgandy on July 17, 2009, 09:51:06 PM
| Tom Manchester wrote on Fri, 17 July 2009 15:05 |
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? |
3 thoughts:
1. Some people wouldn't know that it was a "bass reflex" without some round holes.
2. They originally had just the slot but then realized it was tuned too low so they added the holes.
3. The round port is a "test" port for checking that your woofer box is working. While playing some bass you put your hand in front of the port and if you feel air chugging then it's good.
drew
Post by: The Guy on July 20, 2009, 05:10:51 PM
| drewgandy wrote on Fri, 17 July 2009 21:51 | ||
3 thoughts: 2. They originally had just the slot but then realized it was tuned too low so they added the holes. drew |
I think you're onto something here. The SB1000 original model did not have the supplemental round ports, and the new SB1002 is also lacking these ports.
-J
Post by: Tom Manchester on July 20, 2009, 08:39:07 PM
Post by: drewgandy on July 21, 2009, 01:08:58 AM
| Tom Manchester wrote on Mon, 20 July 2009 19:39 |
Perhaps it had to do with the switch from older RCF L18-p300's to the newer 18-sound drivers that they added the ports. |
Ok, now Shane has to put the drivers back in and measure the resonant frequency then plug the holes and do it again. My first thought from Tom's post was that the tuning would change pretty dramatically with the additional holes; so much that just a woofer change wouldn't explain it. But maybe the shift isn't so much.
drew
Post by: Jon Waller on July 22, 2009, 10:23:06 AM
| 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.
Post by: Phillip_Graham on July 22, 2009, 12:13:02 PM
| Jon Waller wrote on Wed, 22 July 2009 10:23 | ||
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...
Post by: Jon Waller on July 22, 2009, 03:05:58 PM
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.
Post by: Tim Charpentier on July 23, 2009, 03:13:06 AM
Post by: Phillip_Graham 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.
Post by: Jon Geissinger on August 16, 2009, 06:56:38 PM
Post by: Evan Kirkendall 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
Post by: drewgandy 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
Post by: Jeff Robinson on October 17, 2009, 10:40:46 AM
| drewgandy wrote on Mon, 17 August 2009 18:56 | ||
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
Post by: Jeff Robinson 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
Post by: Art Welter on October 17, 2009, 12:40:46 PM
| Jeff Robinson wrote on Sat, 17 October 2009 09:13 | ||
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.
Post by: Jeff Robinson on October 17, 2009, 01:30:47 PM
| Art Welter wrote on Sat, 17 October 2009 11:40 | ||||
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. |
I stand corrected. Never having run a system with variable tuning combining an unrestricted large port with undersized porting, I was giving opinion rather than speaking from experience. Even my 12" 1 cu ft 90 Hz cabinets have a pair of ducted 3" ports, specifically to avoid high velocity conditions in the ports (I have heard chuffing, I don't like it and will not allow my designs to make it). I still hold the opinion that the large slot will prevent backpressure from forcing pulses of air in and out of the ducted port. Have you heard this specific model cabinet in a system? I may have to crack a book and learn how to do reynold's number calculations (Phillip Graham's math skills are impressive, I never went beyond dif. eq. myself) if I want to answer this conundrum.
Thank you,
Jeff Robinson
Post by: Art Welter on October 19, 2009, 06:01:04 PM
I have heard SB1000 port chuffing, but I don’t know if they had the additional round ports.
There are several interesting things going on in the SB1000 that make for more variables than a simple model.
The rectangular duct is coupled to an expanding opening, so it’s tuning is not the same as it would be on a flat surface.
The round ducts are roughly 18” in front of the rectangular duct, if they do “choke” at high power they would also change the phase a bit too, since they would lead the other port by a bit.
Your previous statement from Sat, 17 October 2009 08:40 “Actually, the enclosure tuning shifts up in frequency (when the small round port 'chokes' at high power levels)”
Is incorrect. More port area makes for a higher FB (tuning).
Drew Gandy was also mistaken
“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”.
Electro Voice called covering one of the ports for a lower tuning the “step down” mode for the TL cabinets. Of course, the smaller port area then made for more “chuffing”, but that is a different can of worms.
My guess on the whole mess is the large “venturi port” tuned a bit lower than EAW wanted, and an easier fix than re-tooling or re-building existing cabinet shells was to simply add a couple of ducts to raise the tuning, giving more of the 60-80 crap everyone seems to want.
And if you wanted to make the cabinet the same as the others, you could simply cover the round ducts.
Everybody’s happy.
And if Phil’s theory is correct,
“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....a variable box tuning that dynamically shifts depending on the velocity of air in the smaller ports.”
It would do no harm in the process, as the “chuffing” would not be noticed with all the raging HF that would generally accompany the sub being pushed to the point where the air is blown out of the smaller ports.
Art Welter
Post by: RYAN LOUDMUSIC JENKINS on October 21, 2009, 06:09:26 PM
My TCS 2800 subs ate built to be VERY similar to the SB1000s. They do however have a different porting to them. There is no big open port in the back of the chamber. Rather there is just two small hole ports on each side.
If everyone is correct that the small ports in the SB1000 affectively become nulled at high output, It makes me wonder if the TCS box would be similar only at high outputs, act more like a sealed box? Hmmmm?
Post by: Phillip_Graham on October 22, 2009, 08:11:34 AM
| RYAN LOUDMUSIC JENKINS wrote on Wed, 21 October 2009 18:09 |
This is a really interesting subject that got me to thinking....Not always a good thing. My TCS 2800 subs ate built to be VERY similar to the SB1000s. They do however have a different porting to them. There is no big open port in the back of the chamber. Rather there is just two small hole ports on each side. If everyone is correct that the small ports in the SB1000 affectively become nulled at high output, It makes me wonder if the TCS box would be similar only at high outputs, act more like a sealed box? Hmmmm? |
The SB1000 is not acting like a sealed box at high power, but it is changing the effective port mass. The small and large ports are in parallel at low power, and when the small ports are shunted out, only the (larger) effective mass of the central vent remains.
Your TCS box is very likely behaving like a sealed box at high power levels, which may, or may not, be entirely bad.
This is VERY crude rule of thumb, but if the port area of a vented box is not at least ~30% of the Sd of the driver, there is a very good chance for substantial port compression issues.
Post by: drewgandy on October 23, 2009, 12:10:03 AM
| Art Welter wrote on Mon, 19 October 2009 17:01 |
. Drew Gandy was also mistaken “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”. Art Welter |
?
I think I was asking a question so I'm not sure how I was mistaken. Elaborate?
Post by: Art Welter on October 23, 2009, 03:19:37 PM
| drewgandy wrote on Thu, 22 October 2009 22:10 | ||
? I think I was asking a question so I'm not sure how I was mistaken. Elaborate? |
Drew,
Sorry, my mistake about you being mistaken.
Less port area (from a shunted port) would make for a downward tuning.
Art Welter
Post by: drewgandy on October 24, 2009, 12:23:17 PM
| Art Welter wrote on Fri, 23 October 2009 14:19 |
Drew, Sorry, my mistake about you being mistaken. Less port area (from a shunted port) would make for a downward tuning. Art Welter |
That's fine, I think there were a few different things going on in the posts. My question is about whether we want a downward shift or upward shift in response to either heating or excursion. Any thoughts?
drew
Post by: Art Welter on October 24, 2009, 03:30:48 PM
| drewgandy wrote on Sat, 24 October 2009 10:23 | ||
That's fine, I think there were a few different things going on in the posts. My question is about whether we want a downward shift or upward shift in response to either heating or excursion. Any thoughts? drew |
In general, a downward shift in tuning at higher power would be preferable, as more excursion is required to reproduce lower frequencies at the same SPL.
Post by: drewgandy on November 03, 2009, 02:01:46 AM
| Art Welter wrote on Sat, 24 October 2009 14:30 |
In general, a downward shift in tuning at higher power would be preferable, as more excursion is required to reproduce lower frequencies at the same SPL. |
But doesn't that open a larger excursion peak above resonance?
drew
Post by: Art Welter on November 04, 2009, 03:13:06 PM
| drewgandy wrote on Tue, 03 November 2009 00:01 | ||
But doesn't that open a larger excursion peak above resonance? drew |
That is true, so the question becomes what stresses the speaker more, LF or the upper range?
Frankly, a shifting tuning with a fixed volume does not seem a good idea to me.