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...
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.
[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:
[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)
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.
¹ Patrick Turnmire of Red Rock Acoustics.