The Subwoofer DIY Page - Projects
Proof of Concept #6 - Dayton PA310 MLTL
27 October 2018
So, having upgraded my POC3 build with an Eminence Kappalite 3012LF, I now had a spare Dayton PA310 driver on hand. I also had on hand a full sheet of 0.75 MDF that was a few years old and slowly swelling due to moisture. And it being Christmas vacation time, I had a bit of free time on my hand. So, enter "Proof of Concept #6".
For a vented subwoofer system one of the factors that influences the upper frequency limit is the "organ-pipe" resonance of the vent. For best results, that should be located at least one octave above the intended passband for the subwoofer. This can be a bit of a problem, as to tune a box low with a suitably-sized vent, the vent has to be long, which in turn reduces the first resonance frequency of the vent, which in turn reduces the subwoofer's upper frequency limit. However, my "Boom Unit" Mass-Loaded Transmission Line (MLTL) design basically proved that it was possible to extend the upper frequency limit a bit by positioning the driver in the box such that the first first vent resonance was significantly reduced. I wanted to see if this idea could be extended for pro audio use. I also wanted to try out an alignment with a higher Fb for the PA310 driver, as low Fb alignments like the POC3 suffered reduced power handling on the passband due to the PA310's limited excursion capability (5mm Xmax).
The image below shows what I eventually came up with, using an updated version of the Excel workbook that I used to design the "Boom Unit". Target Fb is just below 50 Hz, and a bit less stuffing is being included in the sim, as too much stuffing will kill the response at the low end. The result is a fairly small (2.3 cu.ft. net) box that has dimensions that allow it to be easily carried by one hand (if a handle is mounted on top). Response starts dropping off just above 50 Hz, but this might be a good match for my car's system that has EQ points at 50 Hz, 30 Hz and 20 Hz. I can boost at 50 Hz and cut at 30 Hz and 20 Hz to both improve the box's power handling and output.
And some build pictures...
After construction (and before doing any work on the finish), I put the build through a series of tests. First of all, I took a measurement of its impedance curve - this is usually the FIRST step I do with any completed build, as the impedance curve can tell you a lot about how successful the build was, i.e. was the target Fb acheived, or are there any leaks in the box, or are there any problems with panel flex. The build passed the test, with Fb checking in about 0.5 Hz higher than predicted, likely because I used a little less stuffing than planned.
Next I tested its performance of POC6 against the POC3 loaded with the Eminence driver. As expected, POC3 had more output at 50 Hz and below, but then POC3 is a much larger box, around 4 cu.ft. net. Suprisingly, POC6 was a match for POC3 from 60 Hz onwards, and actually had a better (read: smoother) frequency response from 100 Hz to 200 Hz. The offset position of the driver didn't totally null out the first port resonance around 200 Hz, but it did kill enough of it to ensure that there was no huge dip at that frequency. In hindsight I should have left the baffle removable to allow me to experiment with different positions of the driver on the baffle.
Next, I did some linearity testing, basically by checking the response of POC6 at different input levels. The results were quite promising, with only 1.5~2dB of compression showing up at 50 Hz at the highest input level I felt comfortable driving the system with. At the highest level, distortion within the passband was still below 10%, but climbed quite drastically below Fb. The system could be driven higher with a steep filter below Fb. but not much higher. Of interest is that this level of compression is still significantly higher than what was observed with the POC3 tapped-horn design, which only shows about 0.3dB of compression before the distortion in the passband hits 10% - another win for tapped-horn alignments. The POC6 MLTL is however smaller and therefore more portable, and it should be possible to run it a little higher in both frequency and SPL than POC3.