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Author Topic: Push pull double 18  (Read 23538 times)

Craig Hauber

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Re: Push pull double 18
« Reply #50 on: March 24, 2018, 08:45:26 pm »


What did EV do with their MTL-4 design to address this?


I ended up storing them on their backs.

When I discovered the issue it hadn't progressed too far so I simply rotated the drivers and let gravity correct the issue for a bit before swapping them to the side drivers.
We handled them with forklifts so it didn't matter which way they were setting on a pallet.  They were essentially a cube so it didn't affect truck-pack or warehouse racking. 

The mid-high box  was the main reason for the forklift (anyone who has used those can understand why) and luckily the 10's in those were all mounted vertical.  any cone sag at all in those would be noticeable quickly due to the phase plug -which I doubt that there would be any in such a small driver that still used the same spider as it's larger 18" sibling. 
I actually had bigger issues with recone-errors in those where a slightly off-center dustcap could reduce excursion significantly due to collision with the phase plug.  (You wouldn't believe how many reconers I've encountered that just eye-balled the placement of those!)

Back on-topic, I also found humidity has quite a bit to do with cone sag too. 

Storage doesn't seem to affect sag as much as heavy use.  With the low-Xmax drivers in the EV subs you always found Xlim quickly then backed-off to just a bit before it and held it there all night :) 
I think the constant straining of the driver's suspension really aggravated the sag issue.  Lightly used installed versions didn't seem to have as bad a problem.

Nightclubs with fog machines are another situation.  I have one client with manifold type boxes mounted sideways where the driver's spiders are saturated in condensed fog fluid -probably due to the constant drawing of air through them while the driver is pumping.  Just awful sag issues -to the point where the coils are almost resting on the back-plate.  Many times the glue holding them to the basket lets-go  I have to change them out often and this is with modern very well built heavy-duty B&C, Radian and RCF drivers. 
The DJ's pound on the system, it's in a humid area (an ocean shoreline) and the excessive fog use just make it a woofer torture chamber. 
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Craig Hauber
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Jeremy Young

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Re: Push pull double 18
« Reply #51 on: March 26, 2018, 07:24:26 pm »

Thanks Craig, that answers my question and more, thank you for sharing your experiences.  I sure am learning a lot, which is why I keep coming back to these forums.
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Brown Bear Sound
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Mark Wilkinson

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Re: Push pull double 18
« Reply #52 on: March 27, 2018, 01:28:25 pm »

Craig, I'd like to thank you too.  Very real world info.

Between your post and more digging, I've decided the only way I'll build horizontal, is with some rotation scheme in mind,
and/or possibly going on with the Marc S coil-holding idea.

I've begun making distortion tests on the push-pull I posted, along with testing it swapped to push-push (clamshell like).
I've found I need to get a little more experience with REW's sine sweeps, or better yet it's stepped-sine tests, before I have confidence to trust (or post) what I'm measuring. 
Listening to sweeps and pink, the two are a little different, but I don't know how to describe that yet, or what it means for music.

Clamshell would make rotation very easy, just swap the drivers every now and then.....I'm almost hoping I can't hear or measure an improved difference against push-pull  lol 
Reduced cabinet vibration, and sounding like my single 18 vented diys,  is all I've really been after...
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Mark Wilkinson

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Re: Push pull double 18
« Reply #53 on: March 28, 2018, 08:46:38 pm »

Got a nice day of testing in......

Since the whole idea of push-pull is 2nd order harmonic distortion reduction,
I decided to test push-pull inverted, vs push-push (clamshell) using the same box.
I mean, why not just flip the driver in the plenum around and make a direct comparison, right ?!

REW has a stepped-sine harmonic distortion routine that is really cool. 
It spends a couple of seconds at each sine step to determine distortion.
You set how many sine steps per octave the neighborhood can stand  ;)   I used 12.

First plot is push-pull.
Second is push-push.
Both were at 2 meters, so add 6dB to SPL.

I don't see enough of a clear case for push-pull.................?? 
Plus, the suspension and coil vent noise below 35Hz, was much louder with the pull driver when exposed in the plenum, than when the driver rests in the cabinet as a push driver like its brother.

So, I'm going with push-push.   
This gives the vibration reduction (which is dramatic vs a regular dual 18), and allows for a very easy rotation of drivers to get over the horizontal sag stuff.

Last plot is a transfer for the push-push.  Dark green is raw, blue is processed.
My SPL Leq method of finding sensitivity gives 101.3 dB 2.83v at 1m, with the processing in place.
THD hit 10% right at 130dB 1m.

Happy camper!!

« Last Edit: March 28, 2018, 08:52:22 pm by Mark Wilkinson »
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Art Welter

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Re: Push pull double 18
« Reply #54 on: March 29, 2018, 07:22:32 pm »


My SPL Leq method of finding sensitivity gives 101.3 dB 2.83v at 1m, with the processing in place.
THD hit 10% right at 130dB 1m.

Happy camper!!
Mark,

Very low distortion- the BMS 18N862 sure is clean!
I see it is available 4 or 8 ohms.
What is your nominal cabinet impedance?
What was the voltage of the test signal?
Did you happen to measure excursion during any of the tests?

Your test results here show the fundamental both at 119.6 dB, on the DIY site, they read 120.9 for push-pull, 121.2 push-push at 2 meters.
Only a 1.6 dB difference, but distortion going from around 3% to 10% with what could be a 4.4 dB change or a 2.8 dB change  (or something different) makes the detail a bit more "interesting" since

Your cabinet response has a 10 dB peak around 150 Hz, what is the "SPL Leq method of finding sensitivity"?

Art
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Ivan Beaver

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Re: Push pull double 18
« Reply #55 on: March 29, 2018, 08:53:50 pm »


Last plot is a transfer for the push-push.  Dark green is raw, blue is processed.
My SPL Leq method of finding sensitivity gives 101.3 dB 2.83v at 1m, with the processing in place.
THD hit 10% right at 130dB 1m.

Happy camper!!
What sort of processing were you using?

It appears to have a boost at the lower freq.  So how was the 2.83V determined?

Is it at the boosted freq? or some other freq?

Any boost at lower levels to flatten freq response, will have to be deducted at higher calculated SPLs, because there will be more power at those freq (assuming flat impedance)
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

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Mark Wilkinson

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Re: Push pull double 18
« Reply #56 on: March 29, 2018, 08:55:45 pm »

Mark,

Very low distortion- the BMS 18N862 sure is clean!
I see it is available 4 or 8 ohms.
What is your nominal cabinet impedance?
What was the voltage of the test signal?
Did you happen to measure excursion during any of the tests?

Your test results here show the fundamental both at 119.6 dB, on the DIY site, they read 120.9 for push-pull, 121.2 push-push at 2 meters.
Only a 1.6 dB difference, but distortion going from around 3% to 10% with what could be a 4.4 dB change or a 2.8 dB change  (or something different) makes the detail a bit more "interesting" since

Your cabinet response has a 10 dB peak around 150 Hz, what is the "SPL Leq method of finding sensitivity"?

Art

Hi Art,

I'm using two 8 ohm.  An impedance curve is attached, however it's in-room next to other subs....

The distortion sine-stepped measurements were probably averaging right around 25v rms. 
I didn't grab voltage averages, because I had 100ms of cool down waiting in between each sine step.

Differences between SPL on distortion graphs here and on DIY, are simply due to freq of cursor when I captured the graphs.
I put graphs up here in distortion %, as it's the more common language.
And on DIY in dB relative, as that's more common there. Didn't try to match, or even know, where cursor freq was.

Hey, if I reran distortion using final push-push processing, it shouldn't make a damn what freq i choose ! ;)

The "SPL Leq method of finding sensitivity" is very simply.....put processing in place (as per blue trace posted)....simultaneously meter SPL LEQ and average RMS voltage. 
Do the math and bring it to 2.83v at 1m.

An aside.....I really, .....really, want to hear what's wrong, or more fair, than that method for a sub sensitivity spec......
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Mark Wilkinson

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Re: Push pull double 18
« Reply #57 on: March 29, 2018, 09:00:33 pm »

What sort of processing were you using?

It appears to have a boost at the lower freq.  So how was the 2.83V determined?

Is it at the boosted freq? or some other freq?

Any boost at lower levels to flatten freq response, will have to be deducted at higher calculated SPLs, because there will be more power at those freq (assuming flat impedance)

Hi Ivan,
see reply to Art, it should answer your question as to how 2.83v was determined.
Which you will see is not at any any freq, but at an average of all, of the processed response.

Processing is a custom FIR setup.



« Last Edit: March 29, 2018, 09:04:36 pm by Mark Wilkinson »
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Mark Wilkinson

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Re: Push pull double 18
« Reply #58 on: March 29, 2018, 09:02:08 pm »

delete modify attempt that misposted
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Ivan Beaver

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Re: Push pull double 18
« Reply #59 on: March 29, 2018, 09:37:24 pm »


An aside.....I really, .....really, want to hear what's wrong, or more fair, than that method for a sub sensitivity spec......
I feel that loudspeakers should be measured (at least as far as sensitivity and response) with NO processing of any type.

Just the measurement system into the amp, and to the speaker.

It gives a much better indication of the natural raw capability of the loudspeaker.

Anybody can add eq to bump up a part of the response to tame the peaks as needed.

Assuming the generator is flat, and is using a swept sine wave, you can easily measure the applied voltage.  2.83V is suggested.

I never measure at 1m, it is to close to get numbers that translate to other distances.  It is better to get several meters away.  I prefer to increase the drive voltage at the same dB as the distance takes away.  10m=20dB is a easy one to remember and the voltage increase is simply 10x more.

Another advantage to this method is that any power compression will be present, which gives an idea of real world numbers in usage.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
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ProSoundWeb Community

Re: Push pull double 18
« Reply #59 on: March 29, 2018, 09:37:24 pm »


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