So the PVR Audio driver showed up today. Chester was interested in it of course, had to scent mark daddy’s new toy.
The first test is for basic Thiele-Small parameters. I tried the test with the driver sitting on the carpet, but alas the magnet system is vented (for improved power dissipation) as shown below. Blocking the vent causes the parameters to change due to the air flow restriction.
The fix was easy… went to Home Depot and picked up some shelf brackets. Mounted four of them and created essentially a set of stilts to raise the driver up.
Parameters were measured with a small signal test. To measure Vas a set of 16 nickels (80 g) were placed on the driver. Results (compared to spec sheet):
- fs = 33.0 Hz (vs.38.4)
- Qts = 0.35 (vs. 0.36)
- Vas = 246 liters (vs. 173)
- Sensitivity = 97.6 dB (vs. 97)
Next, I tried to measure distortion. The configuration is with the stilts above. I wanted to look for the 10% level, since this is considered the maximum output level. At a test tone of 50 Hz, the distortion reached this 10% distortion level with a 9.08 vRMS –> 10.3 watts into 5 ohms. I used a displacement simulator and came up with only 1.76 mm peak one way. That is only 25% of the rated Xmax! Hmmm. Could this be the room effect (the room has resonances at 34, 67, 102, etc. Hz) or operating in free air?
The later is a quick fix, I already had a 2 cu.ft. test box handy from my college days. I made an adapter plate for an 18″ driver and repeated the 50 Hz measurement. Distortion dropped to 3.34% at an input of 9.1 vRMS and an estimated displacement of 1.25 mm. As I increased the volume my microphone pre-amp saturated and wouldn’t go any lower in volume. So I switched to a 30 Hz tone. The 10% THD level was reached at around 12.35 vRMS with 1.70 mm of estimated displacement. Again, hmmm, not what I was expecting.
I wanna new sub, one that will make the earth quake, and my heart jump. My current home theatre setup uses a pair of Velodyne MiniVee 10″ sealed subwoofers. They aren’t too bad, but I never felt they did an adequate job of the last half-octave in the 20-80 Hz LFE range.
So how to proceed? I have always liked sealed subwoofers. That requires moving a lot of air, so why not an 18″ driver, the big boy? I also thought the approach of Bagend to use a professional driver with electronic equalization was an interesting approach. The sensitivity of pro 18″ woofers is in the 95-99 dB range compared to the 84-92 dB range for home theatre products. The box size is kept reasonable by tuning to relative high frequency. Electronic equalization is used to make up the difference. In the case of Bagend, they equalize down to 8 Hz !!! That’s really low, and from reviews I have read requires at least two 18″ drivers for a home theatre setup. My goal is more modest, say 16 Hz would be nice, 20 Hz more realistic, with one 18″ sub.
So I started to poke around Parts Express and see what they had in the pro 18″ department. The target was a box size of 2 cubic feet and a sealed Qtc of 0.707. That put the cut-off frequency of 75-85 Hz for the candidate drivers suitable for a sealed enclosure (many pro drivers are designed for vented or horn enclosure designs). A sealed enclosure rolls off at -12 dB for each octave, so the amount of equalization required would be expected to be +12 dB at 40 Hz and +24 dB at 20 Hz for a 64 Hz target cut-off. That’s a lot! Assume a 97 dB efficiency, then to operate at 100 dB would require about 2 watts above 80 Hz, and around 512 watts at 20 Hz by these numbers! Assuming of course that the driver has enough Xmax. That is around 8 mm of driver excursion for an 18″ driver.
One of the candidate drivers was on sale, the PVR Audio 18SW2000. It would achieve a 82 Hz box tuning in 2 cu ft (actually a little less). Xmax looks good. Power handling more than enough. The frame is cast, not stamped. And did I mention it is on sale?