New Sub – Reciprocity Test

A technique recommended for finding your optimum subwoofer position is to place the subwoofer in your primary listening position and go around the room and listen to various candidate subwoofer locations.  Today I did such a test.  Instead of listening, I used a microphone, of course.

Reciprocity Test

The three curves above are smoothed with a 1/12 octave filter.  The green curve is the raw sub measurement.  It is obtained by placing the microphone very close to the driver cone so as to swamp out the influences of the room.  The red curve is the left corner of the listening room, and the blue curve is the right corner (where the subwoofer was located in all previous tests).  The left corner is next to a couch, while the right corner is open to the room.

Which to pick?  A subwoofer in each location would be ideal.  The peaks / dips would partially cancel.  In the end, I selected the left corner.  It is smoother above 100 Hz where the subwoofer needs to integrate with the main speakers.

How well did the reciprocity hold up?  I moved the subwoofer from my primary listening position and into the corner.  (Pause, catch breath, this thing is HEAVY.)  The big dip at 70 Hz has filled in nicely, while the response above 150 Hz is rougher.  Acceptable.

Reciprocity Test After

New Sub – In the Game Room

New Sub to the Right

Sub to the right, hiding underneath the table.  Crown XLS1500 sitting on top.

Now comes the moment of truth.  What does my technological terror sound like?  Well, listening was short lived.  The Crown XLS Xi1002 kicked the bucket some time during Tron.

BnC Corner

I did manage to do some testing before the amp died.  The green curve represents the average of 6 measurements about the game room couch at ear level.  The dark blue curve is the close mic’ed response shifted to match the general characteristics of the average measurement.  The red is the difference of these two.

The obvious is the broad peak F1 centered at 40 Hz.  This is a very pronounced room resonance with a long decay corresponding to the room dimension of 14′.  Very unpleasant.  (The room also has a dimension of 17′ on the long size, but it opens to a stair well and a bathroom, so no mode observed at 33 Hz…)  There is a smaller resonance H2 at 80 Hz, and a possible dip H3 at 120 Hz.  The next trend – and this is a good thing – is the gain of +9 dB / octave below 50 Hz.  This partly counteracts the 12 dB/octave roll-off of the sub below 67 Hz.  Over-all this is a very extended in-room bass response.

I purchased a new amplifier – a Crown XLS1500 on sale at Guitar Center.   Ran the AutoEQ feature of my Onkyo receiver.  The AutoEQ algorithm plays a series of noise bursts, equalizing between bursts.  You could hear it extend the final half-octave as it iterated and optimized its filters.  I’ll have to take measurements later.

Next I played a couple of flicks – Tron, Operation Swordfish, House of Flying Daggers, and Pearl Harbor.  The scene in HoFD with the drums is just amazing.  Tight.  The aircraft roars in PH are clear, no excessive rumble.  Quite a bit of cone motion during the various explosions.  The gun fire is somewhat muted in the sound track, so doesn’t have quite the impact it could.

New Sub – The B&C 18″

Next up is the B&C.  Start with the frequency response, measured with the mic close, with pink noise, and averaging 1000 times.  The Velodyne and PVR responses are shown for reference.  Can you tell which is the PVR and which is the B&C?  I double checked my notes, so not sure why these look the same, other than I must have over-wrote the PVR data :-(

BNC Response

Now for the good stuff, the distortion measurements.  These were difficult due to the high output and the driver.

  • 35 Hz, 17″, 2.83 vRMS –> 2.7%
  • 35 Hz, 39″, 5.9 vRMS –> 4.2%
  • 35 Hz, 79″, 12.9 vRMS –> 8.1%
  • 35 Hz, 79″, 22.5 vRMS –> 11.7% at an estimated 3.6 mm peak displacement
  • 49 Hz, 39″, 2.83 vRMS –> 1.1%
  • 49 Hz, 79″, 5.92 vRMS –> 1.5%
  • 49 Hz, 79″ + reduced mic gain by -6 dB, 12.9 vRMS –> 2.93% at an estimated 1.9 mm peak displacement
  • 67 Hz, 79″ –> 0.56%
  • 67 Hz, 79″ + reduced mic gain by -6 dB, 5.95 vRMS –> 1.3% at an estimated 0.95 mm of displacement

Why the additional measurement at 49 Hz?  I have a theory as to why the distortion measurements are much higher at 35 Hz than 49 Hz.  First, the displacement isn’t higher vs. the drive voltage for the two frequencies.  This is because below the box tuned resonance of 67 Hz displacement flattens out.  Not the same for output.  Consider this picture of distortion vs. transfer function:

BnC 35 Hz dist

Notice the gain at 60 Hz is almost 12 dB (4x) higher than at 35 Hz.  That is, when taking a measurement at 35 Hz, the second harmonic is getting amplified !!!  The same thing does not happen at 67 Hz.

BnC 67 Hz dist

New Sub – Measuring the PVR 18″

Next up for testing is the PVR Audio 18SW2000 in a 3 cu.ft. (net) enclosure.

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First up is the frequency response measurement.  Performed with the mic close-up, using pink noise averaged 1000 times.  Red curve is the Velodyne for reference.

PVR Response4

The Velodyne subs have a built-in amplifier.  To match up with the PVR I carefully recorded the gain settings on the microphone.  I dialed in the PVR at +6 dB higher than the Velodyne to account for the fact I have two of the Velodynes and I am looking for a net improvement over the pair.

THD test results – not an improvement !

  • 35 Hz, 17″, 2.83 vRMS = 1.6 watts into 5 ohms –> 4.5%
  • 35 Hz, 39″, 5.92 vRMS = 7.0 watts into 5 ohms –> 7.4%
  • 35 Hz, 79″, 12.93 vRMS = 33.4 watts into 5 ohms –> 16.6%
  • 67 Hz, 17″ –> 2.0%
  • 67 Hz, 39″ –> 2.0%
  • 67 Hz, 79″ –> 3.9%

New Sub – Establishing a Baseline

My current home theatre setup uses a pair of Velodyne MiniVee 10″ sealed subwoofers.  So the first step is to take some measurements of the Velodynes as a performance baseline, that way I know I am getting an upgrade.

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Took one of the two subs out of the system for testing.  Testing starts with a close mic frequency response test.  Method was pink noise, averaged 1000 times.  A lot of averaging was required to counter the sounds of cars passing out on the road.

Velodyne Response

Next I wanted to test the distortion at a couple of frequencies.  I picked 35 Hz and 67 Hz as two frequencies easily within the capabilities of the sub.  My current microphone pre-amp doesn’t have a low-enough gain setting for testing at large signal levels, so I had to carefully manage the signal level.  The first measurement is at a distance of 17″ with the microphone on the ground, making it a ground plane measurement.  The signal level is increased +6 dB, and the distance doubled to compensate.  Then repeated again.  The results:

  • 35 Hz, 17″ –> 3.5% THD
  • 35 Hz, 39.4″ –> 8.1% THD
  • 35 Hz, 78.8″ –> 15% THD
  • 67 Hz, 39.4″ –> 1.0% THD
  • 67 Hz, 17″ –> 1.0% THD
  • 67 Hz, 78.8″ –> 1.0% THD

The SPL guesstimate is 90-91 dB @ 2 meter ground plane for the 78.8″ test cases (equivalent to 90-91 dB at 1 meter in half-space).  Obviously very nice results at 67 Hz, the Velodyne isn’t sweating at all.  The 35 Hz is a challenge.


New Sub – B&C 18TBW100

My instincts are telling me that the PVR Audio 18SW2000 is not going to work out for my project goals.  Not enough Xmax.  Not even close.  Also, the Vas is larger than advertised, the appropriate box for a Qtc of 0.707 is more like 3.0 cu.ft. and the fc will then be around 67 Hz.

So I started looking for an alternative 18″ driver.  After reading various reviews, the B&C drivers looked like a good candidate.  Their 18TBX100 came out a few years ago.  Reviews consistently said “this is the 18″ pro sub driver to beat.”  Then B&C came out with the 18TBW100 to one-up their own accomplishment.  The spec sheet is impressive.  Mechanical limit is over +/- an inch without damage!  B&C also measures what they call Xvar, the point at which one or both of the suspension or magnetic circuit have diminished to 50% of the small signal values.  The magnetic circuit also has shorting rings so that high power levels don’t contribute as much distortion.  This is a feature I see in all the top tier sub woofer drivers – pro and home theatre.  The cost is double the PVR Audio 18SW2000… and I’m guessing its twice the driver.  Just what I need!

BnC 18TBW100 specs