when designing a 2 woofer enclosure...

i am in the process or designing a dual ht sub.....
i was wondering what i should do about the volume of air inside?
say i was installing woofers that would norminaly go in a 2cu. ft. enclosure when used in seprate enclosures......when they are mounted in a dual configuration should i have 4 cu. ft. of air inside the enclosure?

thanks,
Slice:)
 
Re: Isobarik

Ap said:
Another example is the Isobarik or Compopund enclosure where drivers are mounted face to face or back to back with only one driver visible. In this case the box volume is half that required by a single woofer.
Check out www.diysubwoofers.org for more info.

A clarification. The above is a method of using 2 woofers in a box & halving the volume instead of doubling it. It is not an example of push-push.

dave
 
that is exactly how i was planning on doin it....
a push push configuration as it is called.
basickly you just mount the woofers back to back on each parrolell wall.... right?
is anything else requaried for a push push configuration woofers?
note: i say at the:Push-Push Sonotube Woofer says i have to do more?
like bolt them together and add a spacer.....what is the perposs of that?

thanks,
Slice
 
AndrewJ said:
The advantage of bolting them together via a spacer is to cancel the mechanical reaction force generated by the drives. This produces a dramatic reduction in the cabinet vibration, and is well worth doing whenever you have multiple drivers

Andrew says it pretty good. Instead of the cone movement causing the fixed part of the driver and the box to move, the other driver resists this motion. The greater the coupling between the drivers, the larger the cancellation.

Now someone is going to say "the mass of the cone moving the box, ha ha". Well talk to the guys with the walking subs, and keep in mind that what sets a great hifi apart from just a good one, is what they do down 40 dB.

Even just mounted on opposite sides you will get some benefit, but the tighter you can mechanically couple the drivers, the greater the benefit. Wedging something between the magnets is step 1, aluminum, brass, or some other stiff non-magnetic material is better than PVC or wood, but use whatever works for you. Running ready-rod as bolts ties the fronts of the drivers together (and also stress-loads the box walls, pushing panel resonant frequencies up). If you use the ready rod it needs to be damped as well, even some shrink wrap would probably do the trick.

dave
 
how do i do this?
how do i bolt the drivers together?
do all drivers have such holes?
and where can i get an the marerials needed for this?
what type of bolt should i use?
how do i install it?

thanks.....
i would also like some sugestions for drivers.....
i like poly cones not paper.......
i am looking to drive both subs with a 350w @ 4ohm apex senior
plate amp.....

the subs prefrable should be 175w RMS each......

thanks,
slice
 
slicemaster101 said:
how do i do this?
how do i bolt the drivers together?
do all drivers have such holes?
and where can i get an the marerials needed for this?
what type of bolt should i use?
how do i install it?

You have to be creative when building it. All drivers (well almost all) drivers have holes for the mounting bolts. Just run the fatest piece of ready rod (hardware store) thru these holes so they stick out (cur them such that they stick out just enuff to be tidy. You want to put a nut and a washer on the inside at either end & then a washer and a nut to hold the driver in.

i would also like some sugestions for drivers.....
[/B]

Steve at ApexJr is bundling the amp you are looking at with the Stryle 12" woofer -- just get 2. I have a pr of these on the way to replace my single Shiva. Really nice driveres.

dave
 
Stryke SAE 1024 on the Stryke site, but the pictures are better on the ApexJr site. Even then the woofer still looks better than in the pictures -- it is one stunning piece of kit -- especially considering the $100 price.

And you are overly concerned with the power rating. It is way more than the amp puts out, but you are more likely to blow a speaker with too little power than too much.

The cone material isn't listed, but i know it is not paper, and Nick uses a version of poly for his cheaper drivers (and paper for the more expensive ones).

Ready rod is a piece of steel rod that is threaded for its entire length.

dave
 
thanks, now i know what ready rod is.....
i have just never heard it called that.....
you might think i am overly conserned with power rating but i just want drivers the i know would respond well to the anount of power i am feading them..... and you said it your self that it is easer to blow a speaker that is underpowered rather then over.

well what do you sugest for a spacer....
i would like something that is fairly inexpencive but good.....
would pvc or abs work?
what do you sugest?

thanks,
 

Bill F.

Member
2001-11-15 5:25 pm
SW MI
Stop the presses!

Nobody's yet mentioned the benefits inherent in push-pull!

Fact: Push-pull alignments are very beneficial in that they largely cancel even-order distortion caused by driver motor and suspension nonlinearities. Take a peek at the M&K MX line for example. Siegfied Linkwitz also discusses this benefit in the dipole woofer section of his Phoenix project.

I submit that in a cabinet of decent stiffness, an opposed push-pull alignment, especially with cast-basket woofers, will yield excellent force cancelling. I mean, how compressible is MDF along its axis? Not very:). Why throw away a chance to cut your harmonic distortion?

Bill
 
note: i kinda wanted a push push configuration to cut down on vidration of the enclosure.....

tell me what you think about these drivers from JBL

they are the JBL GT1241 and the power rating in w rms is 18-300w rms and i dont remember what the peak was....

these babies go down to 18hz!

tell me what ya think....
 

Bill F.

Member
2001-11-15 5:25 pm
SW MI
Hey Slice,

Just so we understand each other, an opposed push-pull will not create any more cabinet vibration than an opposed push-push with backplates joined. Equal Newtonian forces will be acting on each other (cancelling) the same with either alignment.

The difference is that the joined push-pushers act on each other through their backplates while the push-pullers act on each other through their baskets and the bracing/cabinet walls. Assuming decent bracing members between the driver basket rims, this force is transmitted as compression/tension along the axis of the walls/bracing, *not* as flexure across the axis which is seen as external vibration.

I agree with Dave that what a sub is doing 40dB down is what separates audiophile from audio-pile. Just beware: at decent excursion, driver harmonics may be only 20dB, 10dB, or even less under the surface! And they don't care to what lengths you went to join your driver backplates. The only way to fight them is to turn them against each other.

Now, let me add that if you want to ignore this and go ahead with the other fine recommendations on this thread, I'll leave you alone. Hey, I'll even back you up! You'll still end up with a fine sub.

Bill
 
Bill F. said:
Stop the presses!

Nobody's yet mentioned the benefits inherent in push-pull!

Fact: Push-pull alignments are very beneficial in that they largely cancel even-order distortion caused by driver motor and suspension nonlinearities. Take a peek at the M&K MX line for example. Siegfied Linkwitz also discusses this benefit in the dipole woofer section of his Phoenix project.

I submit that in a cabinet of decent stiffness, an opposed push-pull alignment, especially with cast-basket woofers, will yield excellent force cancelling. I mean, how compressible is MDF along its axis? Not very:). Why throw away a chance to cut your harmonic distortion?

Indeed push-pull has the benefits you describe. No an MDF cabinet is not stiff enuff to yield excellent force cancelling -- good force cancelling (if appropriatly braced), but not excellent. You can hear the difference between a pr of woofers in a box and the same pair tightly coupled. When it comes to a choice, i select woofers with good linearity (like the Lambda* built Stryke units) and push-push. Now best might well be 4 woofers in an isobaric push-push push-pull configuration (4 woofers in a box with the same net volume as 1 woofer).

An example of a push-pull woofer

* i talked with Nick about Push-Pull with his drivers and he said that they were linear enuff that they gained nothing in terms of distortion from push-pull (at least his upper range ones anyway).

dave
 

Bill F.

Member
2001-11-15 5:25 pm
SW MI
Dave,

I don't doubt the benefits of backplate coupling, especially when it allows you to flexibly decouple the woofer baskets from the enclosure. Short of emperical testing/comparison, I won't be able to prove or disprove my inclination that good between-driver bracing can be as quiet as backplate coupling. So I'll give you the benefit of the doubt and trust your ears over my hunches.

*However* your caveat about woofer linearity is a giant variable. You reference the Lambda drivers, which are some of the very best in the linearity department. I'm suspicious that almost anything less would benefit from push-pull alignment, especially if called upon to perform significant excursions. Along the continuum of declining driver quality, there is some crossover point at which your coupled push-push idea gives way to push-pull, in terms of what yields the greatest benefit. Where that point lies is a hard question.

Wouldn't it be nice to have both? :)

I just had a eureka moment--a vision of the best of both worlds:

What if you performed a dustcapectomy on one woofer, and then solidly coupled the exposed pole piece to the backplate of the second woofer by way of a large hardwood dowel, or some other extrememly stiff cylinder? Viola! A firmly coupled push-puller!

You could bolt the cylinder to the pole piece with a lag bolt through the vent. Actually, you could decapitate both woofers and bolt from both ends. Then you could stick a phase plug on the outward-facing woofer. Who needs duscaps anyway?

Bill
 
Bill,

Your points are all VERY valid, and point out some the things you have to pay attention too when building these contraptions.

Your suggestion to couple the push-pull woofers is something good to think about -- it gets rid of all sorts of ills -- dust-caps, pole-piece vents, cancels distortion and cancels force.

This is still a good way to do it too (and no surgery required):

push-pull-push-push.gif


dave