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.