What Thiele/Small numbers do I need?

[Tom Loizeaux]

I want to upgrade a pair of 4-12 PA columns. These are sealed units. The original worked the speakers in an "acoustic suspension" mode.
What T/S numbers do I need to look at, and what should they be, to get 12" drivers that perform well in the low end in these coulmns. The columns are about 16" x 10" x 53" inside (4.9 cu. ft.), minus a little bracing and 4 small-framed 12" drivers. Since there are no partitions, I suspect that the internal volume should be divided by four.
What Qts numbers, and what values of other perameters, should I look for drivers for low end performace in this cabinet?

Thanks,

Tom

[frank23]

I don't know much about what Qts you'd need, but you could try some figures from different JBL drivers in a box calc program and see what low freq extension that would give.

It wouldn't be too much work to define the box [volume divided by 4 as you say] and put in T/S parameters for several 12". And you could try what they turn out with a reflex port added to the same volume.

I like the excel based Unibox300 myself for quick modelling. It also provides an ïdeal cabinet" option and you could choose the driver that equals 1/4th of your volume and combine 4 of those

greetings, frank

[duaneage]

You could also cheat a bit and stuff the cabinets with fill for about a 15% increase in acoustic volume. With 1.22 cu feet, not including the drivers themsoves and bracing, your going to be hard pressed to find 12 inch drivers that are loose enough to work properly in that volume. 128H might do it but they are precious and you would not have much low end in a sealed combination since the EBP designates them as vented box drivers.

Some car speakers may work in that arrangment. If you can get specs from JL audio ( they at least publish the TS numbers) or from Kicker you may find a driver that works in 1 cu ft without too much rolloff. The 4 driver arrangement will end up giving you about 10 hz lower cuttoff due to mutual coupling.

I would also consider an open back cabinet with MI speakers. Depends what your planning to use them for.

[Tom Loizeaux]

So, would a speaker with a Qts of around .15 work better in this cabinet (with 3 other similar drivers) better then drivers with, say, a Qts of .25 and higher?
Any help please?
Thanks,

Tom

[frank23]

hi tom,

I put the 2203H and 2204H values in the box simulation program and calculated both for:
- closed box 50liters [so 200liters with 4 drivers]
- bas reflex box of 50 liters with port optimized for driver

the 2203H and 2204H values are in the screenshots for the closed box. There you can see their Qts are 0,14 and 0,35 respectively as can be found in the T/S paramaters list from JBL

you can check the frequency responses for yourself

frank

[jpb_dk]

Hi there
There are several parameters to keep in mind. I will try to explain it simple. Some of them are the Q's, but they are not the only one to care about. It really depend on what You wanna end up with. I would like to help you if i can. SO one of the Q's that are not mentioned are the end-resoult. Lets call it Qtot-acoustic. (the Acoustic Q value measured in a box, open or closed, with damping material or not, but the end-resoult when you play music)

The electrical Qtotal is a amplifire-problem. Buy a Krell or Threshold amp if everything goes wrong. Personally i love the setup of a Threshold S300 for the bass unit and a Threshold S/3 for midrange and top.

When You define the "goal" you do a step-one calculation, and after you fine-tune it. The thing is really that You CAN get around a shitty kabinet-design and end up with a wonderfull bass reproduction you have never imagined. Just have to know something about the "parameters".. Not to forget the worst of it all. The filter. If you draw a "generator-diagram" of it all (bass unit, cabinet influence, maby a port, filter-components and impedanse-corrections) You will see that the filter-components are a real mess in that calculation. No wonder everyone talke so much about the filter. French "chef's" says its all in the soup or the sauce", anyway..

If this subject is interesting (generator-diagrams) of filter, cabinet influence ect.. bla bla, i will find a bottle of whisky and some good cigars and find some of my reports from speaker-designs/tests i have done in the past in my professional workinghours and try to make a step-one introduction. Sorry but i really cant remeber it.

So back to your question "What Qts numbers, and what values of other perameters you should look for". I would say somehow the same parameters as the original bass-unit unless you wanna redesign/build the box and the filter. You can do alot with the damping material, but not all.

NB: if someone have experience running a JBL4410 with active filter i like to know. I use my 4410 in my home-studio to play backing-tracks when Aerosmith are on tour and can not come around and play with me *LOOL*
They shall have a sub to give that punch to make the drums realistic. Maby it will be better to sell them and buy another spaker designed for that from the beginning. Something has to be done, as that filter is shit (talking about its all in the soup) I will apriciate to hear

[duaneage]

let's keep it siimple.

Qts below .35 indicates vented boxes and the need for a tuned port
Qts .35-.40 can usually go either way

Above .40 and out to .50 are definately sealed box drivers and even infinate baffle open back enclosures. The drivers' internal dampining is enough that the cabinet does not contribute to final Q.

That being said, I think you would get better results with

A. a larger box that is ported with drivers havinga Qts of around .20 - .25

B. A 3 driver system that is sealed with .35 Qts drivers and a fair amount of stuffing.

Vas numbers and FS numbers are interrelated in that the Qts determines the lift in the speakers resonant frequency and the Vas determines how much air it takes behind the driver to do that. But the Qts is a good starting point, lower Fs numbers will yield lower bass extensions.
A decent book on the theory and design of systems is the Loudspeaker Design Cookbook. It has been published for about 27 years and is as good a guide as any for coming up with a system.

Most of this forum is dedicated to reproducing JBL designs and tunings with emphasis on tweaking system. When you decide to make a move PM me and maybe I can help you out. I have done numerous designs with off the wall and off the shelf drivers from different companies. I also am not far from you , maybe we could collaborate on a design.

Can't let all those west coast guys have all the fun.

[Tom Loizeaux]

...Above .40 and out to .50 are definately sealed box drivers and even infinate baffle open back enclosures. The drivers' internal dampining is enough that the cabinet does not contribute to final Q...

Thanks to all of you who replied.
The above quote has me confused. How can a Qts of .4 or .5 be correct of a speaker in both air suspension and infinate baffle? Infinate baffle means no back pressure or support as the cone moves, where Air Suspension means a great amount of back pressure or support as the cone moves.
Also, I feel I may have inverted my understanding of Qts. It seems that a soft suspension has a high Qts number (.4 and up) where a stiff suspensuion has a low Qts number (.4 and down) Is this correct? If so, a typical driver for a small sealed cabinet should have a Qts in the neighborhood of .5 to .6, or higher. Right?

In any case, I'm still looking for the numbers for Qts, Fs and any other relivant figure that will lead me to a 12" speaker that will deliver reasonably good low end in a 1.2 cu.ft. sealed box.

Tom

[duaneage]

Thanks to all of you who replied.
The above quote has me confused. How can a Qts of .4 or .5 be correct of a speaker in both air suspension and infinate baffle? Infinate baffle means no back pressure or support as the cone moves, where Air Suspension means a great amount of back pressure or support as the cone moves.
Also, I feel I may have inverted my understanding of Qts. It seems that a soft suspension has a high Qts number (.4 and up) where a stiff suspensuion has a low Qts number (.4 and down) Is this correct? If so, a typical driver for a small sealed cabinet should have a Qts in the neighborhood of .5 to .6, or higher. Right?

In any case, I'm still looking for the numbers for Qts, Fs and any other relivant figure that will lead me to a 12" speaker that will deliver reasonably good low end in a 1.2 cu.ft. sealed box.

Tom

Sorry for not really keeping it simple. Actually .4 - .5 are high Qts values but not that high.

Ex. If you mount a driver with a Qts of .55 ( some musical instrument drivers and most small car speakers are even higher) on a large open baffle you will get a system Q of .55 . Some people claim that .5 is ideal, at least as far as transient response it is pretty close to perfect. Drums would sound very natural but unless the driver is huge you would not get a lot of output necessarily.

Qts is also known as a quality rating and refers to the drivers resonance amplitude at , well, resonance. Low Qts speakers are charactorized by VERY large impedance peaks caused by loose suspensions, large powerful magnets, big voice coils, and large amounts of cone travel. Low resonant frequencies, like 20 HZ, Think 128H and 2214H drivers.

High Qts drivers have stiff edge surrounds, strong spiders, less maximum cone travel, smaller magnets (sometimes not) and pretty small impedance peaks compared to their low Qts brothers. Think Eminence MI 15 inch woofers. They also have much higher resonant frequencies, sometimes as high as 60 hz

Drop a low Qts speaker intoa sealed box and the volume needs to be smallish to provide cone support, after all the driver is pretty loose. This in turn lifts up the resonant frequency as the cone sees air support contributing to the drivers suspension ( ala acoustic suspension). Q rise too, since this controls the cone and resuces that impedance peak, AFter all, if the cone moves less because of the small box holding it , how can that magnet generate a large amount of imedance. You end up with a big woofer in a small box without that much bass as a result.

theteh same size as the example above and you get a different story. Big high bass peak caused by the box siffing up the cne even more. You take a driver that resonates naturally at 50 hz and enclose it, pretty soon your maybe looking at a 75 hz peak and the .55 Qts is now .9 or higher.

What does this have to do with your requirements? Well for starters I think that 1.2 cuft witha 12 inch driver is a bit too optimistic. That would be a square box that is 12 3/4 on each side! Push the dimension out to 1.75 and then you have options like vented low Qts drivers or medium Q (.38) drivers with a decent Fs. You really need air to get bass, I used to have a chart somewhere that laid it out.

Without substantial room gain, like in the trunk of my Escort, your not going to get decent performace from such a small volume without compromising bass resonse, transient response, power handling, or all three.

3 woofers in 4.9 gives you 1.64 cu ft a piece.

[Tom Loizeaux]

...High Qts drivers have stiff edge surrounds, strong spiders, less maximum cone travel, smaller magnets (sometimes not) and pretty small impedance peaks compared to their low Qts brothers...
...What does this have to do with your requirements? Well for starters I think that 1.2 cuft with a 12 inch driver is a bit too optimistic. That would be a square box that is 12 3/4 on each side! Push the dimension out to 1.75 and then you have options like vented low Qts drivers or medium Q (.38) drivers with a decent Fs...

I think I'm still confused. You're saying that a low Qts means a softer surround and spyder, right? That's what I originally thought.

As far as changing the size of the cabinet, well that's not an option. These are vintage finished units and I simply want to upgrade the drivers since the originals are 30 years old and were a low end driver to begin with. The bass response (all I need is down to 50Hz) is actually OK now, it's the high end that really needs help. I've added a twin bullet pack to the top of each column, and it makes a HUGE improvement! I also changed the top 12" to a decent quality co-ax that uses a 1" compression driver, small intrigal horn and has a fairly soft surround.
Now I want to "beef up" the other drivers with speakers that will work in this cabinet and handle more power.

...so what Qts numbers, and other numbers, should I look for in selecting these other 12" drivers?

Tom

[duaneage]

OK, fair enough. Now that we have an idea of what your going to do with it let's look at the possibilities.

if you mix differnent woofers into the same cabinet and drive them from the same amplifier your going to have some strange impedance loads. Some amplifiers won't like the different phase angles the drivers are going to generate. Speakers are also motors and generate voltage as they move. Current lags voltage as the speaker moves out, voltage lags current as it moves in. The problem is when different drivers in the same cabinet interact with each other your going to need a degree in calculus to determine what will happen. So there are two choices (three actually)

1. Select drivers with close values that will behave the same from 50 Hx to around 250 Hz. This may or may not be easy. It depends on what the coazial driver has for TS values. You might find woofers that offer similar Qts and Fs. Those are the two values that matter in this case and they would need to be tested to be sure.

2. Drive the coaxial woofer with it's own amp and the others from a second source. Choose FS numbers that match and try to tame the resonance peak as much as possible. Alternatively you could isolate the top driver from the others with a baffle inside (the third solution) in effect creating two distinct systems with only a small loss in volume (maybe 150 cu inches) due to the baffle.

I recommend using the third solution although I can't recommend drivers off teh top of my head. That depends on your budget and overall availability. Most of the super JBL pro stuff is going to need large cabinets and the low QTS 12 inch drivers from JBL are expensive and not suitable for that application.

If you want to delve more into these things PM me and we can chat more on the phone sometime.