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Discussion Starter · #1 · (Edited)

Foreword:

I was provided a Rythmik F12 Subwoofer consisting of GR Research's SW-12-04 Subwoofer Servo Driver and Rythmik's AE370PEQ Amplifier. These two came loaded in the Rythmik provided enclosure and sold as the FG12. Dale has provided testing of the loaded enclosure; you can find it here. In order to not be redundant and use the resources I have to provide the site with a large data set, I will not be providing data on the FG12. I will, however, be providing data on the subwoofer and amplifier components.

Given the subwoofer is designed with the use of the AE370PEQ servo feedback system in mind, I have provided data of both the raw driver and with the AE370PEQ's servo circuit connected. This will give you all a good idea of how the driver performs on its own should you want to purchase the two components at separate times and add the amplifier later. I suggest this because, frankly, the driver itself performs very well in my tests and I find it is a high value woofer that will suffice most DIY'rs needs until they are able to complement the performance with the servo-featured Rythmik amplifier.

Now, let's get started...

Product(s):

GR Research combo which consists of the SW-12-04 Subwoofer and Rythmik A370PEQ Amplifier. The sample sent to me was preloaded in a Rythmik enclosure which make up the product known as the F12 Subwoofer.






SW-12-04 Subwoofer Driver Test Results:

Small Signal Analysis:

The following are the Thiele-Small parameters I measured using the Klippel LPM module.

Code:
[U]Electrical Parameters[/U]			
Re	3.52	Ohm	electrical voice coil resistance at DC
fs	20.8	Hz	driver resonance frequency 
			
[U]Mechanical Parameters[/U]			
(using test encl.)			
Mms	135.212	g	mechanical mass of driver diaphragm assembly including air load and voice coil
Mmd (Sd)	123.809	g	mechanical mass of voice coil and diaphragm without air load  
Rms	4.287	kg/s	mechanical resistance of  total-driver losses
Cms	0.432	mm/N	mechanical compliance of driver suspension
Kms	2.32	N/mm	mechanical stiffness of driver suspension
Bl	12.365	N/A	force factor (Bl product)
			
[U]Loss factors[/U]			
Qtp	0.407		total Q-factor considering all losses
Qms	4.128		mechanical Q-factor of driver in free air considering Rms only
Qes	0.408		electrical Q-factor of driver in free air considering Re only
Qts	0.371		total Q-factor considering Re and Rms only
			
[U]Other Parameters	[/U]		
Vas	133.2333	l	equivalent air volume of suspension 
Lm	86.73	dB	characteristic sound pressure level (SPL at 1m for 1W @ Re)
Lnom	87.28	dB	nominal sensitivity (SPL at 1m for 1W @ Zn)
			
Sd	466.98	cm²	diaphragm area





As you can see above, the Fs is low at 20.8 Hz. Qts and Vas ultimately net you to a Qtc (in-box Qt) of 0.70 in an enclosure size of 1 cubic foot (not accounting for displacements of driver and bracing). That's pretty respectable on its own. Decreasing enclosure size allows a bit more output in the 80hz region, however, that results in a loss of lower end output. This is where the amplifier's servo circuit will help; you can smooth out the impedance bump caused by an undersized enclosure (often noted as box resonance) and extend the low end. This information will be covered further in the amplifier discussion section. The rest of the parameters above can help you determine an enclosure size, but I encourage you to consider the implications of this when factoring in the accompanying Rythmik amplifier as it permits adjustments that will alter and potentially make moot of some enclosure design choices.


Large Signal Analysis:

The following is an inside peek in to the inner workings of a speaker driver. The Klippel LSI parameter is used to provide Linear Xmax: maximal excursion within some distortion threshold. In this case, 20 % total harmonic distortion which is becoming the standard for acceptable subwoofer distortion thresholds¹. The data below provide engineers the ability to better understand how their product is performing in order to make adjustments to the design in order to achieve maximal performance or high ratio. It is worth mentioning that no driver is perfect and while some curves may look great, I've found the real engineers are making compromises where it makes sense.

The linear excursion is typically comprised of three main aspects: motor force over excursion, suspension over excursion, and inductance over excursion. All of these parameters play in to the previously mentioned 20% THD threshold used to determine the linear Xmax. Each of these components have a role and result in their own component-based linear Xmax value. The least of which is used to determine the driver's linear performance for specification purposes.

The motor force, below, is given over excursion (driver voice coil in through coil out). What is shown is the force of the motor is nearly constant throughout its mechanical range, with a slight tilt to the curve illustrating an outward shift relative to the coil at full excursion.




Below are the suspension measurements. In this case, there is also a slight forward offset and asymmetry to the driver.




Lastly, we consider the effect of inductance on the driver's performance. Below are the results of both the inductance due to excursion (Le(x)) and the inductance variation due to current through the coil (Le(i)). As the driver moves, the coil does, too. When current flows through the voice coil, there is a resistance to that flow of current known as inductance. As a driver moves forward and backward, the current in the coil changes. As the coil is closer to the magnet the magnetic field is higher than when the coil is 'out' and away from the magnet. So, you can imagine as the coil moves inward toward the magnet the inductance (or resistance to current) increases and as the coil moves out the inductance decreases. This results in a non-linear inductance performance. This is not desired. So, many manufacturers will put a shorting ring on the motor assembly (placement varies from design to design) in an attempt to cancel the magnetic flux as the coil enters the motor area where inductance is typically high. You can find good info and probably a much better explanation than I've given via Google if you wish.

Below is the result of the inductance as the driver moves through it's excursion.




Here is the result of the inductance over current:



What you see in the Le(x) graph is a forward offset of the shorting ring by about 7mm. The Le(i) is pretty well flat with only about 0.10mH difference between highest and lowest measured point.

So, Bl(x), Kms(x), and Le(x) have been provided. The following values associated with each parameter comprise the components' limiting factor in excursion:
Code:
[u]Displacement Limits[/u]	 	 	
X Bl @ Bl min=70%	>17.2	mm	Displacement limit due to force factor variation
X C @ C min=50%	>17.2	mm	Displacement limit due to compliance variation
X L @ Z max=10 %	14.2	mm	Displacement limit due to inductance variation
X d @ d2=10%	        47.4	mm	Displacement limit due to IM distortion (Doppler)
As mentioned previously, each component plays a role in determining the driver's linear excursion. In this case, Le(x) being 7mm offset results in it being the limiting factor, resulting in the driver's linear Xmax being 14.2mm. Had the shorting ring been placed further in to the resting position of the driver's coil in order to center up the symmetry, the performance would have likely increased to >17mm and further testing would have been necessary to determine the linear excursion capability.

Where ">" is given, this simply means the data was not resolved. I usually push drivers until all parameters are resolved but upon further testing, I found little use in seeking resolution with Bl and Cms as the forward offset of the Le(x) measurement always resulted in having to stress the driver beyond my comfortable limits in order to resolve numbers for the sake of resolving them. In other words, I saw no need given the limiting factor had been determined and subsequent (more brutal) testing proved of little use.

Subwoofer Driver Conclusion:
On its own, this driver performs very well. The low Fs of 22.8Hz, reasonable enclosure size requirements, high linearity and very respectable 14.2mm linear excursion help make this a great value at it's current MSRP of $179.



Rythmik A370PEQ






¹ Patrick Turnmire of Red Rock Acoustics.
 

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Discussion Starter · #3 ·
lol.

Well, after I went to both Brian and Dannie's site, I realized that some of what I plan to discuss has been covered there pretty well (ie: the parametric EQ comparisons and the phase adjustments). So, I'm kind of wondering if it's worth the trouble. I may still do it, though, to provide a different way of viewing the features. Frankly, when I was on Brian's site I started to glaze over in his discussion of the phase delay... I felt it was maybe a bit too in depth; should've kept it top level given his target audience. Or broken it down like I do sometimes with the "Get to the point" section and then the "Nerd-out, dude!" section. ;)
 

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I think that what you are planning is very important for potential customers to have from as you offer a 3rd party/non-biased point of view.
 

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Not yet but I have been slammed since Monday at work and my Mac MB crashed. I pick it up today. I also have only had them since Saturday.
 

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Discussion Starter · #9 ·
think I'm having a change of plans here...

I'm considering evaluating this as a set without the box. at least for now. but, I'll sit on this a bit and think some more.

for now, here's a teaser picture of the measured FR of the driver only.

PS: I'm going to need a crash course in image sizing/managing here. ;)
 

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Ha, I can help with the imaging sizing. I usually cut them to 800X600 and then depending on usage will insert from 'My Photos' for a scaled down that will launch in a new browser or insert image like you have done in your post.
 

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Do you have any image editing software?

The forum software caps the images at 800 pixels, but that can be enlarged by a click on the bar above the image. We like using the 800 width to try to keep most of the graphs we post at a comparative size.

I generally edit/crop my images to the 800 pixel width with Paint Shop Pro and then upload them to our image gallery where it will create thumbnails you and use to link to a larger image... or you can use the full size image. There are several options you can choose with the gallery.
 

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Discussion Starter · #13 ·
I can size the images before I export them. I just wasn't sure what would be best. Thanks for the advice.


Regarding my testing, I've contacted Klippel about using the CEA2010 standard and received this reply:
Thank you for bringing this up. We have had discussions about this some ago, which didn’t lead to any result. But your inquiry brings this subject up to our minds again.



At the moment it is not possible, to do such a measurement with our measurement system.

But we are discussing a way to include this into our TRF or DIS module.



I will let you know, as soon as we have a solution for this.



Kind Regards,

Daniel

So, for now, I may let Dale handle this aspect of testing.

I have also determined over the past few days' testing that I need a microphone that can take higher SPL levels than mine can withstand. I prefer nearfield measurements which means the mic is exposed to higher output since it's physically closer to the source. I've been shopping but it looks like I'm spending $500-600 for what I need. Once I get that, though, I'll be good to go and my data will be almost bulletproof. In fact, nearfield measurements are better than farfield (groundplane, in the case of subwoofer testing) with sealed subs. With ported, the ground plane is used to capture the combined response of the port and sub. In the nearfield, this can be compensated by doing multiple measurements and combining the response. The good thing about nearfield is that it excludes all other forms of noise (room noise, environmental noise, reflection, etc) because the mic is so physically close to the driver and therefore swamps out anything else in relation to the original sound.

Make sense? Here's a good article:
http://www.audioholics.com/education/loudspeaker-basics/subwoofer-measurements

Another good one:
http://www.synaudcon.com/site/author/pat-brown/measure-a-sub-place-a-sub/


Bottom line: There are tradeoffs. A lot of people seem to be using the groundplane method and there's not really anything wrong with it. I just feel, for my own use, nearfield is best. Unfortunately, I have to work around the limitations of mic SPL capability or buy a new mic that can take higher output levels.
 

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So a Behringer ECM8000 won't work?

I know it will withstand 140db in a car, so I can't imagine it not being suited for anything in HT.
 

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Discussion Starter · #15 ·
It may do 140dB but the problem is the distortion value associated with that. There are no mfg specs but some DIYr's have found the ecm8000 to have high distortion values as SPL is increased. Here's an example:


At 118dB, THD is 1%. For what we're doing here, we need a mic with very low THD at high SPL levels. Otherwise, the accuracy of the data is called in to question.



side note: Dale, would you mind lending me your calibrator if I were to order a new mic for this?
 

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I would check with Herb at Cross-Spetrum Labs and see what he has that may be lower distortion and less cost before I spent $500 on a mic. If it ends up being what you need, then so be it, but you may can save a little. I also wonder about the IBF Acoustik mics we have (Dale and myself). They are considerably less.
 

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Discussion Starter · #17 ·
I'll have to look in to the possibility of using XLR mics for this. I'm not sure if the Klippel machine provides 48v phantom power. Right now I'm using an external power box for ICP voltage supply to a BNC mic.
 

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Discussion Starter · #18 · (Edited)
The EMM-8 you guys use doesn't state THD @ SPL. I can only find this:
peak acoustic input > 120 dB SPL


Sonnie, do you guys have a good rapport with ISEM? Or a good contact? They have ICP mics... I may ask them if they have a product that fits my needs. If you assume a max output of 120-130dB at 2m and I'm measuring in the nearfield at a few inches, the SPL in the nearfield is ~158dB. If I back off to about 8 inches or so (this is a swag; empirical data would drive the max distance I could be) it would lower to 150dB at the mic.

The alternative is to measure farfield but, frankly, the cons outweigh pros in my case. Measuring nearfield is easier and allows me to measure whenver rather than waiting for good weather or breaking out the gear late at night while the neighbors are sleeping.
 

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Discussion Starter · #20 ·
The EMM-8 is RCA and includes a mic amp if you buy the kit.

IBF-Akustik MP-1r-KIT

Wolfgang Frank is very good about answering emails: [email protected] ... if you wanted to ask him about the THD.
Right. But they state a peak of 120dB. Most of the peak levels they define in their specs are at 3% THD. This makes me think that the mic you and Dale use succumb to this spec as well. This may be something you guys want to look in to.

I'll shoot them an email about my needs and let you guys know what I hear back. :)
 
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