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[Jeff Robinson]

Art Welter wrote on Sat, 17 October 2009 11:40

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.

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

[Art Welter]

Jeff,

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

[RYAN LOUDMUSIC JENKINS]

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?

[Phillip_Graham]

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.

[drewgandy]

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?

[Art Welter]

drewgandy wrote on Thu, 22 October 2009 22:10

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?

Drew,

Sorry, my mistake about you being mistaken.

Less port area (from a shunted port) would make for a downward tuning.

Art Welter

[drewgandy]

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

[Art Welter]

drewgandy wrote on Sat, 24 October 2009 10:23

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

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.

[drewgandy]

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

[Art Welter]

drewgandy wrote on Tue, 03 November 2009 00:01

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

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.