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What does the excursion of this design look like in the 20-30hz range?

The response of this design looks very similar to the Quad GRS 10" 6th order series tuned bandpass sub I built a little while back. One of the main issues with that design was that the peak in the response (~105-110hz on mine) caused an amplification of the driver 3rd and 4th order harmonics in the 25-35hz range which just happened to coincide with the excursion maximum where such distortion was most prevalent.

The JBL's are likely cleaner but I'd watch for the same kind of rise in distortion in the 20-30hz range.

Edit: looked at my measurements again and it was just the 3rd order harmonics that got boosted.
I've been playing around with dual GRS 12SW4's in BP4 in HR. I tried the BP6's, but the graphs were ugly.
 

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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 see you made it over from diyaudio.com!
 

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Discussion Starter #64
Sadly I had some other stuff to take care of tonight, so just a few teaser pics:

This looks promising:


They’re here! A day early too!


Test cart assembly:


I’m off work tomorrow, so this is priority one. I’ll get the test cart working and break the drivers in a bit free-air.

Bad news, it is currently snowing, so ground plane is probably a no-go. Hopefully close-mic will tell us enough. Along with some demo material of course.

Chris
 

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^^^ The AWSOME factor is off the charts on this one !!!!

We are close to getting answers on this project, so cool ;)
 
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It’s like Christmas, but for everyone who isn’t even getting one of these “presents”.

:D
 

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Josh always pops in from time to time with very helpful info.
I know! I follow his builds too! Josh Ricci is a subwoofer designing monster! I've been registered to the data-bass forum for some years too. I might have posted once or twice.
 

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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.

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.
1. Why does it matter what happens at 87hz when most of us would have a 12db low pass filter at 80hz for home theater?

2. I thought the original Devastator was designed for more mid bass (60-80hz) due to BP6's flat responses in a room adds too much sub bass (20-40hz)?

I designed my negative flare tapped horn for flat response. I originally had it firing into the corner. The SPL was great! When I turned the enclosure 180*, more mid bass kicked in and sounds better to my ears.

Pic 1 = 2013 firing into corner.
Pic 2 = 2019 firing away from corner.
 

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1. Why does it matter what happens at 87hz when most of us would have a 12db low pass filter at 80hz for home theater?
You aren't sending 87Hz to the sub if it's harmonic distortion. You are sending 29Hz. The sub itself creates the harmonic output. Your crossover is in the signal chain. The 87Hz distortion is created during output.

Can't EQ/filter anything that isn't there to begin with.
 

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Discussion Starter #71 (Edited)
1. Why does it matter what happens at 87hz when most of us would have a 12db low pass filter at 80hz for home theater?


Not applicable here, as @Heide264 explained, but...

You also have to remember that crossovers are not hard cutoffs, but slopes. With a standard 12db BW at 80hz, you’re only down about 4db at 87hz.
This is why rule-of-thumb is to keep the first port resonance one octave above crossover frequency. Many of us push that down to 130hz or so for an 80hz crossover on the low-tuned designs, but not ideal.

Chris
 

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Cool! This is a little bit like watching a child open my Christmas gift, it is fun being part of this (even if only in the simplest way, a few keystrokes to buy drivers). Chris, your investment of time, material, and expertise in this experiment is great. It is strange being on this side of things, I have always been a 'doer' rather than an observer.


I think that my contribution to the JBL 12" story may be a 4 driver / 2 PR sub, not just randomly thrown together, but with thought-out parameters of cabinet size & PR mass. Just got WinISD running under Wine, still trying to figure out what driver specs to use.


Eagerly awaiting some numbers (on the Dev4x12). BTW, it is sunny but cold down here in AL ... definitely no snow :D
 

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Ok...Sorry for not following up here but I've been busy.


Finally had a chance to fire up Akabak and run this cab real quick. No inductance modeling on this. It's like I suspected. Driver excursion behavior looks nasty unless all of the drivers are wired in parallel. There is still some uneven behavior with them in parallel but it looks much better. Anything involving series driver wiring appears to cause things to go awry at least according to Akabak.
Wish I had better news.


Teal is the HR type sim with the drivers sim'd at the midpoint between the physical locations.
Green and black curves are the FR with the driver pairs sim'd at their separate physical locations. Black is with parallel drivers and green with series/parallel









Cone excursion with the HR type sim with the driver placement averaged.









This is with all drivers wired in parallel and modeled with the driver pairs separated locations. Not too bad.









Here is with the drivers wired in series / parallel. Ugh oh...All drivers in series looks the same.









That's all I have time to post now. Gotta run.
 

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Interesting graphs, wondering about how different wiring patterns might work, such as vertical vs horizontal pairs run as two separate channels. Then, if one were running two Devs (8 drivers) what happens with 2 lower & 2 upper drivers wired in parallel, then run in series with the second Dev. That would give all the drivers seeing the same relative position in the horn the same circuit. Wiring would be a little bit of fun, but honestly it wouldn't be that big of a deal, especially if they were located on the same wall.


Hmmm ... might need to get playing on a tuned PR box!


A funny observation from the pics: 2 pairs of drivers ordered at the same time ( I think) and yet 2 different "models"!
 

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Discussion Starter #77 (Edited)
Finally ready to fire this thing up.

And I check in here to see that Josh has posted the models. Huge thanks @Ricci!

It looks to me like the response curve is generally unaffected in the operating range. Diaphragm displacement isn’t pretty, but could be worse. Too bad these are 4 ohm drivers, and I don’t have a 1ohm stable amplifier.

Two questions Josh.
First, how much power does it take to hit 15mm excursion series/parallel?
EDIT: With a 19hz 2nd order BW HPF.

Second, could I sidestep this issue by running two sets of parallel driver pairs on identical amplifier channels, instead of running one bridged channel?

EDIT: Third question, does it matter which drivers are series, and which are parallel? I assume not if all-series looks similar. I have them wired series with horn path, parallel across horn path.

Close mic response should be posted shortly!

Thanks,
Chris
 

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Discussion Starter #79
This sub has some potential. I’m nervous due to Josh’s excursion sim, but know the BOSS guys have proven these drivers to be quite durable.

So I cranked it up a bit. It has the “devastator sound”, and is more usable without EQ than my mini’s are.
I was trying to listen for driver distress, but didn’t hear anything.
It is fully capable of rattling all the sheetmetal in my shop: the main door, ductwork, and ceiling were all buzzing severely.
The air velocity in the horn opening is impressive. I don’t remember that kind of velocity from the mini’s. Due to the higher compression ratio possibly?

It has way more output than the “control sub”, a PA460 tuned to 27hz in 3.5cf.

I’ll get some ground plane measurements tomorrow if the wind cooperates.
Hopefully, setting the quad just outside my shop door will work. I don’t have the ability to get if to the middle of a field.

I’ll definitely have to get it inside for a side-by-side with a mini devastator.

Chris
 
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