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-   -   Explain the push-push thing to me (http://www.diyaudio.com/forums/multi-way/10305-explain-push-push.html)

 kneadle 25th January 2003 02:28 AM

Explain the push-push thing to me

I searched the Forum for the answer, and I see "push-push" all over the place, but it's not quite explained.

Here's what I know: two drivers mechanically mounted at 180 degrees in order to cancel the Newtonian effects of their respective motions.

I also know that push-push means that you have to vary how you input your T/S parameters.

What else is there?

I'm tossing around the idea of building a type of TLb like is on the t-linespeakers site. How do you run Martin King's mathcad for that? It's obviously not twice as long a pipe.

Thanks,

Dave

 planet10 25th January 2003 07:26 AM

Re: Explain the push-push thing to me

Quote:
 Originally posted by kneadle I searched the Forum for the answer, and I see "push-push" all over the place, but it's not quite explained. Here's what I know: two drivers mechanically mounted at 180 degrees in order to cancel the Newtonian effects of their respective motions. I also know that push-push means that you have to vary how you input your T/S parameters. What else is there? I'm tossing around the idea of building a type of TLb like is on the t-linespeakers site. How do you run Martin King's mathcad for that? It's obviously not twice as long a pipe.
You have it pretty close... all the parameters remain the same* except Vas doubles. This means the enclosure size doubles. Since the resonances don't change the TL length remains the same but the cross-section doubles. You can model it as a single driver and then double the cross-section (ie as i did when i pinched Bob Brines 40-1354 ML-TQWT design for my Bipolar-pipes) or you can just model a single driver with 2x Vas.

*(if you can actually do a design where you can get the drivers magnet-to-magnet your Qt may go down -- it also becomes a shielded driver).

You want to rigidly couple the 2 drivers.

A couple more links with pics:

push-push woofer

Dipole concept that loads the base drivers push-push (the guy i did it for had 140 6" VIFAs in his basement)

as i post this Apple (mac.com) is having some problems so the pics aren't coming in, try again later.

dave

 halojoy 25th January 2003 07:45 AM

Push-Pull

Mostly used in subwoofers.
2 woofers, one in front and one in backside of box
While the front pulls out form box
the other pushes into the box.
This helps a lot as the work on each woofer
is much less.
At the same time the air pressure inside the box
is about the same, all the time.
also called "constant-pressure" loudspeaker

Here is a guy telling about why he used push-pull:
I used a push-pull mounting arrangement for the drivers. With the drivers mounted back to back the inertia from cone movment cancels resulting in a major reduction in cabinet vibration. To accomplish this I built a small box to fit onto the end of the PVC pipe with the drivers mounted on opposite sides. To take advantage of the cancellation effects the drivers need to rigidly connected to each other. I used solid maple crossbraces between the drivers to provide a simple but reasonably rigid connection. Even with the woofers really cooking the cabinets are dead still. I had always figured that cabinet vibrations were primarily excited by sound waves. This experience would suggest that the majority is actually from the cones inertia.

A variation I almost didn't try was mounting one of the woofers backwards (with the magnet facing out) and then wire the drivers out of phase. The theory is that woofer cone movement is not perfectly linear. In response to a symetrical sine wave the outward motion of the cone does not match the inward movement. By mounting the drivers face to back and wiring out of phase the non-linearities cancel out. I figured that with a high quality driver and limited excursion that the cancelation effects would not be audible. Boy was I ever wrong! The bass was notably cleaner and controlled with lots of slam. Subjectively there seemed to be a little less bass which is a clear indication of less distortion. After hearing the difference I would never consider any other topology than out-of-phase push-pull. It is more than a little odd looking but it fits with my non-conventional theme.

halo

 planet10 25th January 2003 08:03 AM

Re: Push-Pull

halo,

There is a lot of misleading and even out-right wrong info in this post:

Push-pull is a technique whereby both drivers load the box at the same time (ie both push on the air in the box in sync) but with one of the drivers physically reversed so that as one moves out from the basket, the other moves in. The idea is to cancel the non-linearities inherent in lessor drives wrt how they move in vrs how they move out. It actually only averages the differences but that can still make an improvement.

Quote:
 Originally posted by halojoy Mostly used in subwoofers. 2 woofers, one in front and one in backside of box While the front pulls out form box the other pushes into the box. This helps a lot as the work on each woofer is much less. At the same time the air pressure inside the box is about the same, all the time. also called "constant-pressure" loudspeaker
If you actually used two woofers like this in a box you would get almost no bass. It is literally a dipole on a small baffle, everything cancels.

Constant pressure usually refers to an isobarik box where the front driver loads into a small chamber. In the back of that chamber is another driver loaded into a box. The back driver can be seen to act as a pressure relief for the front driver and for a large part of its range the small chamber has constant pressure (in Latin iso-baric)

Quote:
 Here is a guy telling about why he used push-pull: [I]I used a push-pull mounting arrangement for the drivers. With the drivers mounted back to back the inertia from cone movment cancels resulting in a major reduction in cabinet vibration. To accomplish this I built a small box to fit onto the end of the PVC pipe with the drivers mounted on opposite sides. To take advantage of the cancellation effects the drivers need to rigidly connected to each other. I used solid maple crossbraces between the drivers to provide a simple but reasonably rigid connection. Even with the woofers really cooking the cabinets are dead still. I had always figured that cabinet vibrations were primarily excited by sound waves. This experience would suggest that the majority is actually from the cones inertia.
he is actually describing a push-push system

Quote:
 A variation I almost didn't try was mounting one of the woofers backwards (with the magnet facing out) and then wire the drivers out of phase. The theory is that woofer cone movement is not perfectly linear. In response to a symetrical sine wave the outward motion of the cone does not match the inward movement. By mounting the drivers face to back and wiring out of phase the non-linearities cancel out. I figured that with a high quality driver and limited excursion that the cancelation effects would not be audible. Boy was I ever wrong! The bass was notably cleaner and controlled with lots of slam. Subjectively there seemed to be a little less bass which is a clear indication of less distortion. After hearing the difference I would never consider any other topology than out-of-phase push-pull. It is more than a little odd looking but it fits with my non-conventional theme.
This is a push-push system with the drivers mounted push-pull. It is harder to get the drivers rigidly coupled, but you get the benefit of reduction (or averaging) of the drivers exursion non-liearities.

An example

dave

 halojoy 25th January 2003 08:45 AM

what a kind correction of my "use-less post"

whatever you say, professor dave

at least I tried to answer our fellow
which noone else had bothered to do

halo sets the spotlight for you - the other does the main work

So, if my post was useless - when it comes to examine final results
it is up to any of you other readers - to think a little about

Was my effort spent in vain?
What did I get out of it?
Mostly I don't even get a mention,
very seldom even a thank you.
Even get some scorn thrown at me at occations.

Does this discourage me to post?
No,
I never expect anything good from anybody.

I do not have any illusions about any person
not even "my friends".

/halo doesn't ignore a guy with a question - others might ;) :)

 chris haberland 25th January 2003 01:53 PM

This was a interesting read, until you get to the last reply.Should be removed.Chris

 kneadle 25th January 2003 02:28 PM

I thought it was some nice poetry

 kneadle 25th January 2003 02:32 PM

Why must the cross section double? That's the part I don't get. I'm not even sure what the PRECISE meaning of "cross section" is.

Thanks Dave,

Dave

 ryoshu 25th January 2003 05:04 PM

Here's my understanding, Dave please correct me if I'm wrong :)

When you build a TL you want the line to be the 1/4 wavelength of the driver Fs and you also want the line to be the approximate volume of what the driver would see in a sealed box. When you use a push-push configuration the doubling of drivers doubles the Vas, which in turn means you need to double the cross-section, or volume of the line. (I may very well be wrong on equating cross-section with volume)

Getting the right line length, with line volume (and the subsuquent effects of stuffing the line increasing the apparent line volume) are what make TLs difficult to build.

Again, these are my impressions from the materials I've read (good info on t-linespeakers.org, btw). If I'm missing something please let me know. I'm going to build a set of real TLs once I'm done with my current project.

 planet10 25th January 2003 05:13 PM

Re: what a kind correction of my "use-less post"

Quote:
 Originally posted by halojoy Was my effort spent in vain? What did I get out of it?
Your post was not in vain...
It essentially became
a set of questions
bringing further light
(did you get some light to fight the SAD?)
to the question

I usually don't mention push-pull
I like the coupling better
but merit it does have.

Of late
since enlightened
the posts of halojoy
have more audio content than ever before
even mrfeedback
a post did make
with not a comment snide

halojoy the light
(probably needs some light)

dave

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