Box resonance?

[KennyG]

Does max impendance cause box resonance...or, is it the other way around? Does box resonace cause max impendance. I would assume that the heavier the cabinet, the lower the Fb? (box resonance)
Can anyone tell me how this "vent" works...it is at partsexpress's web site, under "cabinet port tubes", page 3, the Scan Speak vent at the bottom?
It lowers Fb by what, allowing internal cabinet pressures...or lack of, to vent???

[Lexman]

I don't have the answers, but I am hoping that someone can address this issue, back to the top.

I mean come on, this guy is from Earth, we need to help him if we can.

Lex

[Bing Fung]

I don't know the answer Lex or surely I would have helped...




Bing

[ThomasW]

Oops sorry , missed this while I was on vacation

The "box" resonance is simply a function of it's construction. Some people believe that thick, massively dense, boxes are necessary for good sound, not necessarily true. What's important is that the resonance of the box, is outside the frequency range that the box is operating. So if you're building a sub, and you can create a stiff enough box so that it "rings" like a bell when you tap it that's perfect (this is for a wooden box, not a "tube" sub). The resonance of the cardboard in a tube sub, is already higher than the frequencies being reproduced. That's why no bracing is necessary for the tube. Also the cylindrical shape equalizes the pressure across the entire surface.

The "aperoidic" vents have been around for ever. How does it work? The question should be, does it work? It's in essence a very small damped port( the damping makes it function as if it were physically larger). I've never tried them, they just seem too wierd. I sure that if they were of great benefit that everyone would be using them, and believe me they're not. The number of them used depend on the size of the woofer being used. So for a big woofer you'd end up swiss cheesing the cabinet with multiple holes. Probably not a good idea

A port functions just like when you blow over the top of a pop bottle. (That's the easy answer )

The reality is that the driver, the box, the port all work together and form an circuit (just like an electrical circuit). The port functions as a fixed resistance in the circuit. So when you "tune" a port you are actually varying the resistance by changing the size of the port. This in turn, changes the tuning of the circuit. For the long drawnout explanation see the Vance Dickason's book "The Loudspeaker Design Cookbook"




theAudioWorx
Klone-Audio

[KennyG]

Thanks Thomas, my copy of the Cookbook just came in off backorder yesterday, and I have been pouring over it! The aperoidic vent is mentioned in there, and they say it does impact the cadinet in a way that makes it act larger, you don't feel it would be of benefit in trying to get the cabinet size down to something reasonable? (think twin 15's in a 4X Vas enclosure)
I'm really trying not to give up on the ultimate sub in a cabinet idea!

[ThomasW]

Kenny

All ports help make the box smaller.

The "vario-vents" are really not intended for subwoofer use




theAudioWorx
Klone-Audio

[KennyG]

O.K. let me attack the issue of cabinet size in a different way.
What about a Isobarik, Infinite baffle, (is there such a thing?) the cookbook says a good Isobarik sub's cabinet is half the Vb of a standard configuration. If a Isobarik sub was built with 2X's the Vb of the twin 15's Vas would the perforamce be close to a 4X's Vb standard IB?

[ThomasW]

Kenny

All speakers have even and odd order harmonics. In an isobaric design you bolt two drivers together facing each other, then bolt them into a a cabinet with the magnet from one of the drivers facing out. The even and odd order harmonics cancel each other out. This results in marginally better sound, and a smaller box.

By definition isobarics are all infinite baffle(the sealed box type of IB). The advantages are as stated above. Beyond that I doubt that "true IB" performance can be obtained with any sealed box even one built with the 4XVas/driver.

The 4XVas is just a "guess" used to try and approximate a really large room. But in reality is still a closed box and will have the inherent problems associated with any "box"

Remember that a isobaric doesn't function like 2 drivers. It functions like one really good driver in a small box.

Also you seem to keep forgetting that a regular sealed box (nonisobaric)using 2-15" needs EIGHT times Vas, not 4. For a standard sealed box to try and function like a IB you need ATLEAST 4XVas per speaker. So for 2-15"s in an isobaric configuration the box size must be atleast 4XVas

The only way to answer your question is to build a test box. All the speculation and computer modeling isn't a substitute for a test box.




theAudioWorx
Klone-Audio

[Lexman]

I thought an isobar mount just means 1 driver is inverted. For example, MK uses 1 outward firing driver, and one mounted isobar. If you take the cover off an MK MX125, you see the magnet of the isobar driver down low. But the drivers are not coupled together. Is that not a true isobar in that situation?

Thanks,

Lex

[KennyG]

Yeah I realize that to use a pair of 15's my Vb will be 4X the Vas of both drivers, which is somewhere around 20 cu.ft. ouch that's a back breaker!
Also Lex your drivers are mounted opposing each other, but are also wired out of phase, which actually makes them work in-phase, and I'm guessing that the physical shape of the cones being 180 degress out, changes their radiating patterns and stress characteristics causing the cancellation of certain harmonic distortions???

[ThomasW]

Sorry my answer was confusing. :?

There are several types of isobaric design.

1)two drivers in a box, each driver in a separate enclosure. The enclosures are "piggy-backed"' Advantage smaller box, magnet doesn't stick out

2) two drivers mounted in a (push-pull)configuration that has the drivers facing into each other. Advantages are second and third order harmonics are cancelled. This means better quality sound and a smaller box.

3)hybrid, this is the M&K design should have some of the harmonic cancellation of the push-pull, along with the smaller box.

The thing to remember is that with any isobaric design the benefits you're getting are better quality performance (lower distortion)and a smaller box. What you don't get is the output increase from using 2 drivers, like you get with 2 drivers facing out, side by side in a large box.

So if you want the output level from dual 15"s, and you want the benefits of an isobaric design, then you need 4-15"s mounted as 2pr/isobaric cabinet. This way each cabinet can be built as 2XVas




theAudioWorx
Klone-Audio

[KennyG]

Oh crud, the plot thickens. (or sickens?)
As I think about it perhaps the increased output is not what I need...the sub section of my mains features 6 12's.
What I'm really after is a sub to use with the center and surround channels, an enhancement of the main's sub section. So an Isobarik 15, built as big as is feasible and set towards the back of the room may be all I really need, or want.

[ThomasW]

Kenny

There's really not much significant LFE in the effects channels.

A ported sub using 2 Tempests will go lower than your Legacy 12"s.

Also using a rear sub can greatly increase the potential for room nulls.




theAudioWorx
Klone-Audio

[KennyG]

True, but at this time I've set my cross-over point at 50hz for the center and surrounds, and would eventually like it to be closer to 75hz. The 75hz setting really cleans up voices.
For me it's always been about quality over quantity, and the more I read, the more I beginning to form the opinion that a Isobarik sub using a push-pull design will produce the cleanest sound of all the cabinet designs...but having never built any, and having only owned velodyne's (ULD15 & ULD18, which were servo feedback) it's hard to know what to expect from any one design. Although the lower freqs of the vent design are enticing, I've never heard a vented sub that I thought matched either of the velodynes.
Like I said, the one thing I do value is quality, and if I lose a few db that's o.k...my pre/pro has two sub outputs, I can always build another!!!

[ThomasW]

If quality is the goal, then build the push-pull isobaric with a sealed box sized to a Qtc of 0.5. This is critically damped, it will have the lowest distortion and tightest bass possible.




theAudioWorx
Klone-Audio