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Now we are heading DOWN into the rabbit hole! What fun :) Acoustical engineering is outside of my realm of training, clearly different from good old fluid mechanics and such. Resonance distortion, different paths, pressure gradients ... it does make sense that there can be some interesting things going on in this cabinet.


Anyway, I get to sit back like the guys in the research foundations that provide funding and let other guys do the heavy intellectual lifting. Hopefully SD weather will cooperate with measurements this coming weekend.



Somewhat related: I picked up my 5 kg of washers from Grainger, PLENTY of added mass for the SLAPS I bought. The difference between models and reality was well nicely demonstrated last year in the sub build I first found over on data-bass but is also over here: http://data-bass.ipbhost.com/topic/746-passive-radiator-project-or-captivator-in-a-tiny-box/ The finding that 'small' parameters are not necessarily valid at modeling high power operation was fascinating to me, especially that the PRs perform BETTER than predicted at high excursions. Anyway, I have (at the moment) 10 JBLs and two leftover (and OLD!) HE-15 Stryke drivers. I was thinking I would build PR subs with the HE-15s and do nearfield with sealed JBLs. Then a couple of weeks ago RedFive started posting some crazy modeling of multiple driver subs, first the 9 ported JBL design, and now this Devastator.



Insanity is an interesting thing. I find myself as a silent observer and then end up kicking in $ to get this current project built. The worst of it is that our intrepid builder is obviously FAR off the deep-end as noted by his OTHER projects :eek: Chris, you had best make sure that your homeowners insurance isn't going to require an additional rider for sonic damage!


RedFive, thanks for the design work ... and congratulations for the MOST IMPORTANT project you have underway: your wife & tiny-but-growing child :cool:!
 

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Crap...Wright model won't work. Do you still have the measurements saved? If you do can you export the free-air impedance curve and the added mass curve as text or zma files. We can get them from that information with Bolserst's calculator spreadsheet.


Edit: Need the amount of mass used too!
Sorry I didn't save the added mass curve.

Sorry for being a complete noob here, but...

How does the 3rd harmonic distortion present? Is it an artifact of the driver’s distortion at the peak frequency, or is it dependent on the distortion at the 3rd harmonic? Does reducing the peak via eq reduce this effect, or is it inherent to the acoustic properties of the box?

I currently run Mini Devastators loaded with 18ds115’s in my theater. They have a huge peak in response. Model vs close-mic:

Subjectively, these are the cleanest subwoofers I’ve heard. Of course, the 18ds115 is a very low-distortion driver. And I am running with the peak significantly eq’d down.

Chris
So harmonics occur at multiples of the fundamental frequency, 2nd order occurs at 2x the fundamental, 3rd order at 3x and so on. So if your subwoofer is playing a 30hz tone the 3rd order harmonic will occur at 90hz. EQ does not reduce the level of the harmonics since they are generated off the fundamental frequency.

For this Quad JBL devastator design the peak in response looks like it occurs around 87Hz, the 3rd order harmonic that will land on that peak is then generated by a 29Hz signal. 29Hz looks to be about 8dB lower then the response at 87Hz which means that 3rd order harmonic will have an 8dB head start over the fundamental (nearly double the distortion %). The excursion maximum where 3rd order distortion will be most prevalent also occurs near that range so you end up with a double whammy.

Another example of where this can occur is on woofer or midrange drivers with a large cone breakup peak, any harmonics that land on that peak are amplified by it. So you would want to crossover at least 2-3x lower then the peak to prevent spikes in 2nd or 3rd harmonic distortion.

Now the effects on a driver with naturally low distortion occur just the same as a high distortion driver, but less distortion generated = less distortion to amplify.

A driver or enclosure design that can maintain a smooth flat frequency response will then be free of any of those harmonic distortion amplifying properties.

To put this another way say you have a driver in a subwoofer box designed to maintain a flat frequency response down below to below 20hz (lets say a ported enclosure). You also have a horn loaded enclosure which gives 10dB more output in the upper ranges but the same output (and excursion level) at 20hz. The ported enclosure will generally sound cleaner at low frequencies because the horn loaded enclosure amplifies the harmonics which occur at those lower frequencies. The ported design however ends up with much higher harmonic distortion in the upper frequencies since it requires much more power and excursion to produce the same output as the horn loaded design. So there are trade offs anyway you go.
 

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@Ricci

Chris @Chris Popovich took these measurements. Top is un-broken in driver, bottom is broken in with added dust cap.



Significant difference from @mtg90 ’s.

Chris P is going to try and find a broken-in driver and take a new measurement.

Chris

It makes me wonder if the manufacturing has actually changed along the way with these drivers. They change names, it is conceivable that they are doing some things differently in their construction.
 

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Discussion Starter #44 (Edited)
Sorry I didn't save the added mass curve.







So harmonics occur at multiples of the fundamental frequency, 2nd order occurs at 2x the fundamental, 3rd order at 3x and so on. So if your subwoofer is playing a 30hz tone the 3rd order harmonic will occur at 90hz. EQ does not reduce the level of the harmonics since they are generated off the fundamental frequency.



For this Quad JBL devastator design the peak in response looks like it occurs around 87Hz, the 3rd order harmonic that will land on that peak is then generated by a 29Hz signal. 29Hz looks to be about 8dB lower then the response at 87Hz which means that 3rd order harmonic will have an 8dB head start over the fundamental (nearly double the distortion %). The excursion maximum where 3rd order distortion will be most prevalent also occurs near that range so you end up with a double whammy.



Another example of where this can occur is on woofer or midrange drivers with a large cone breakup peak, any harmonics that land on that peak are amplified by it. So you would want to crossover at least 2-3x lower then the peak to prevent spikes in 2nd or 3rd harmonic distortion.



Now the effects on a driver with naturally low distortion occur just the same as a high distortion driver, but less distortion generated = less distortion to amplify.



A driver or enclosure design that can maintain a smooth flat frequency response will then be free of any of those harmonic distortion amplifying properties.



To put this another way say you have a driver in a subwoofer box designed to maintain a flat frequency response down below to below 20hz (lets say a ported enclosure). You also have a horn loaded enclosure which gives 10dB more output in the upper ranges but the same output (and excursion level) at 20hz. The ported enclosure will generally sound cleaner at low frequencies because the horn loaded enclosure amplifies the harmonics which occur at those lower frequencies. The ported design however ends up with much higher harmonic distortion in the upper frequencies since it requires much more power and excursion to produce the same output as the horn loaded design. So there are trade offs anyway you go.


Thanks Matt!

I was completely misunderstanding this concept apparently.
So, if I now understand correctly, in layman’s terms:

The horn loading in this design will amplify the distortion produced by the high excursion at ~30hz, where the third harmonic is at peak output.

Where as a ported box will be cleaner in the high excursion frequency range without the harmonics being amplified. Yet the higher frequencies will show more distortion without the horn gain for a given output level.

If I’m analyzing this correctly, the ported horn should still be cleaner than a quad sealed across the usable frequency spectrum at a given output level. Since the required input power/excursion should be lower.
At the expense of a much larger enclosure, of course.
EDIT: Nevermind, this last part is wrong because the high excursion range is outside of significant port gain.(hence high excursion)

Chris
 

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Discussion Starter #45
It makes me wonder if the manufacturing has actually changed along the way with these drivers. They change names, it is conceivable that they are doing some things differently in their construction.


Possibly, or just the same design with different lots/material suppliers. Or just build variations in these mass-produced drivers.

I believe @Chris Popovich has a ridiculous number of these drivers from different lots/times and said he gets relatively consistent measurements from them though. Maybe he’ll chime in.

Chris
 

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Thanks Matt!

I was completely misunderstanding this concept apparently.
So, if I now understand correctly, in layman’s terms:

The horn loading in this design will amplify the distortion produced by the high excursion at ~30hz, where the third harmonic is at peak output.

Where as a ported box will be cleaner in the high excursion frequency range without the harmonics being amplified. Yet the higher frequencies will show more distortion without the horn gain for a given output level.

If I’m analyzing this correctly, the ported horn should still be cleaner than a quad sealed across the usable frequency spectrum at a given output level. Since the required input power/excursion should be lower.
At the expense of a much larger enclosure, of course.
EDIT: Nevermind, this last part is wrong because the high excursion range is outside of significant port gain.(hence high excursion)

Chris
It does get hard to compare distortion profiles of different alignment types unless you build and measure them, but in general if output and/or excursion of one type is lower then another you will get less distortion from the driver in those frequencies.

In a design like this such a narrow response peak serves minimal benefit in terms of usable output since it will end up largely flattened with EQ. If the horn design could have been adjusted to have that peak flattened or rounded off (perhaps not feasible given size constraints and driver parameters) the 3rd harmonic distortion produced in 25-35hz range would be reduced 3-6dB.
 

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Possibly, or just the same design with different lots/material suppliers. Or just build variations in these mass-produced drivers.

I believe @Chris Popovich has a ridiculous number of these drivers from different lots/times and said he gets relatively consistent measurements from them though. Maybe he’ll chime in.

Chris

Yeah, I do realize manufacturing tolerances often are far too broad. Interested in some rather concerning info related to this, but in a different field?


A few decades ago when generic drugs were first being allowed into the US market, I was absolutely STUNNED when I learned the standard - that for a generic to "pass" then:



  • 70% of the pills sampled had to have ...
  • 70-130% of the potency of the branded drug :eek:
Think about that, 30% didn't even have to meet ANY sort of standard, and the remaining 70% only had to be in a tremendously broad range! You see, my friends, this is what happens with an ever expanding bureaucracy. It becomes far more cost-effective for pharmaceutical manufacturers to spend money lobbying for poor standards rather than for R&D and manufacturing optimization :rolleyes:. I was an engineer before going to med school (in the days when Fred & Barney were still babysitting Pebbles & Bam-Bam) and worked in process control. There is no excuse for such lax standards for human meds, but that's the state of it!




Sorry, I will now step down off of my soapbox and return to being a quiet old guy sitting in the corner. Carry on :cool:
 

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Here are the measured specs from Chris's impedance files with the semi inductance. Doesn't radically change things from the basic factory specs but it does change the models. I'm a little skeptical of that SD figure. Seems high.


If I get some time maybe I'll give a shot at modeling this cab with the drivers separated inside of Akabak just to have a look. Haven't had an excuse to use Akabak in a long time.


What's the internal height of the ports and the depth inside from the front panel to the driver baffle?

Sd=551.00
Bl=12.39
Cms=1.63E-04
Rms=7.86
Mmd=153.97
Le=2.75
Re=3.31
Leb=0.74
Le=5.97
Ke=0.29
Rss=507.10
Rms=4.16
Ams=1.69E-05
 

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Discussion Starter #49
Here are the measured specs from Chris's impedance files with the semi inductance. Doesn't radically change things from the basic factory specs but it does change the models. I'm a little skeptical of that SD figure. Seems high.


If I get some time maybe I'll give a shot at modeling this cab with the drivers separated inside of Akabak just to have a look. Haven't had an excuse to use Akabak in a long time.


What's the internal height of the ports and the depth inside from the front panel to the driver baffle?

Sd=551.00
Bl=12.39
Cms=1.63E-04
Rms=7.86
Mmd=153.97
Le=2.75
Re=3.31
Leb=0.74
Le=5.97
Ke=0.29
Rss=507.10
Rms=4.16
Ams=1.69E-05


Thanks Josh!

Ports are 2” high, 25.25” long

Horn is 7” high

Chris
 

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Here are the measured specs from Chris's impedance files with the semi inductance. Doesn't radically change things from the basic factory specs but it does change the models. I'm a little skeptical of that SD figure. Seems high.


If I get some time maybe I'll give a shot at modeling this cab with the drivers separated inside of Akabak just to have a look. Haven't had an excuse to use Akabak in a long time.


What's the internal height of the ports and the depth inside from the front panel to the driver baffle?

Sd=551.00
Bl=12.39
Cms=1.63E-04
Rms=7.86
Mmd=153.97
Le=2.75
Re=3.31
Leb=0.74
Le=5.97
Ke=0.29
Rss=507.10
Rms=4.16
Ams=1.69E-05
I used 1/2 the surround as per the previously posted measurements (in this thread) for consistency's sake. I use less for my personal use as I agree with you. If you'd like me to re-do it with ~1/3 the surround (IMO reasonable) or something different just say so and I'm happy to oblige. I have the DATSV3 and the test JBL about 4' from my desk.

Consistency wise, I've found most of the CS1214, GX1200, CX1200 to measure about the same. There are minor manufacturing variances which is to be expected, but in general they come in about the same, which is different than the JBL spec sheet and the other guys measurements.

FWIW I test the same way every time; driver has little foam blocks on the backside to let it breath, and is upfiring since that's the easiest way for me to be consistent each time in my particular setup. If you'd like me to do something differently just say and I'll give it a go.

Remember, I'm dealing with sealed boxes so most of the worries simply go away (and some headroom for that luxury). Running 40 or so sealed gets the job done though. ;)

Chris
 

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Discussion Starter #51
A few decades ago when generic drugs were first being allowed into the US market, I was absolutely STUNNED when I learned the standard - that for a generic to "pass" then:
  • 70% of the pills sampled had to have ...
  • 70-130% of the potency of the branded drug :eek:
Think about that, 30% didn't even have to meet ANY sort of standard, and the remaining 70% only had to be in a tremendously broad range!

Wow, those do seem to be very loose specifications.

From my very limited knowledge of medications, don’t many work based on continuous dosing and latent level in the system? I can understand how those could still be effective at least.
Anything single dose-controlled just sounds dangerous though.

In my road-material field, anything below 85% of design is remove and replace.

Chris
 

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I used 1/2 the surround as per the previously posted measurements (in this thread) for consistency's sake. I use less for my personal use as I agree with you. If you'd like me to re-do it with ~1/3 the surround (IMO reasonable) or something different just say so and I'm happy to oblige. I have the DATSV3 and the test JBL about 4' from my desk.

Consistency wise, I've found most of the CS1214, GX1200, CX1200 to measure about the same. There are minor manufacturing variances which is to be expected, but in general they come in about the same, which is different than the JBL spec sheet and the other guys measurements.

FWIW I test the same way every time; driver has little foam blocks on the backside to let it breath, and is upfiring since that's the easiest way for me to be consistent each time in my particular setup. If you'd like me to do something differently just say and I'll give it a go.

Remember, I'm dealing with sealed boxes so most of the worries simply go away (and some headroom for that luxury). Running 40 or so sealed gets the job done though. ;)

Chris
How squishy are those foam blocks? I've noticed if a driver is allowed to bounce or vibrate up and down at all during sweeps it throws off the measurements.
 

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@Ricci is the sharing of the QW resonator where the subs tend to get more picky or is it equally due to the bass reflex system being shared? I am more or less curious if there were multi QW resonators each with one sub if that would potentially make the situation better or more predictable.
 

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Discussion Starter #57
@Ricci is the sharing of the QW resonator where the subs tend to get more picky or is it equally due to the bass reflex system being shared? I am more or less curious if there were multi QW resonators each with one sub if that would potentially make the situation better or more predictable.


From my very limited understanding, the problem is completely due to the variable loading from different driver position along the path of the QW.
Two drivers side by side would likely work fine. As many as you want in individual IDENTICAL qw’s with a shared rear chamber should also be fine.
My understanding is completely from reading @Ricci’s design threads though, so I may be WAY off.

Chris
 

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Somewhat related: I picked up my 5 kg of washers from Grainger, PLENTY of added mass for the SLAPS I bought.
I used lead with my original model SLAPS (roughly 4-5" diameter and washer thick also) as I found that with to much weight extending off the bolt sagged the PR's due to having no traditional cone and spider design. Especially since we had to cut the rear foam holes larger ourselves due to turbulence.
Great units though and I punished mine for 8yrs before floods took them...I still have 2x12" packed away! lol
 

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Thanks to @Ricci, @Chris Popovich, and @mtg90 for the help.

Here’s the HR model updated with Chris P and Josh’s specs:


And compared to factory spec:
Both 1200w, no HPF. Black is factory.


Chris
The first and second pic doesn’t line up? Is the first one with HPF?
 
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