Unbaffled – A Tale of Three

In this installment of Unbaffled, I test three different dipole low-frequency configurations.

Up first is a 4 x 15″ driver configuration.  The woofers are value priced models from Goldwood.  You can see something on the cones – that is silicon that I added to them in order to lower the resonance frequency from 30 Hz down to 24 Hz.  (As a side note, I would recommend using a black silicon instead of clear.  The clear silicon doesn’t have a very attractive appearance.)

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I also built an H-dipole recommended by Linkwitz.  This was really easy to build, OK to move, and much stiffer than the 81″ tall panel version above.

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Finally, six of the 12″ carver hex woofers from the original Carver Amazing loudspeaker.  Notice that half of the drivers are reversed – and connected electrically out-of-phase – so that some of the even order non-linearities would cancel out.  The drivers are connected 3 in series / 2 groups in parallel.  The panel is about 15″ wide at the bottom as I was not able to find the 24″ wide board that I used to make the 4 x 15″ dipoles.

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Here are some measurements adjusted to be equal in output.  All measurements are smoothed by 1/24th octave filtering to help reduce the noise from the wind.

  • Green = H-dipole response measured with the ground plane method at 2 meters.
  • Blue = the 4×15″ dipole panel with the microphone at 1 meter, 41″ off the ground (which is the center of the 4 drivers).  The level is reduced by -3 dB relative to the H-dipole.
  • Red = the 6×12″ dipole panel with the microphone at 1 meter, 41″ off the ground.

I was surprised at how close the H-dipole configuration behaved compared to the panel.  I was expecting the H-dipole to show more peaks and dips and otherwise be a poorer performer.  Not so!  In fact, I like the fact that the H-dipole forms a naturally well braced cabinet.  The two panel designs both bend like a reed in the wind and flop around at low frequencies (4 Hz is especially violent).

Comparison_NoEQNext up are some distortion plots.  An important caveat is in order here – the wind and automotive traffic were rather problematic during testing.  The plots are in the same order as introduced above.  The H-dipole plot was the least windy and shows the best results.

Dipole_4x15_distortionH_dipole_distortionDipole_6x12_distortionAnother note is in order… the 6 x 12″ dipole panel was damaged because it fell over and cracked.  Moreover, I had to swap out two of the drivers as they now buzzed.  (I had just enough spares.)  You can see that the panel isn’t quite straight at the top.  The  whole thing fell apart when I finally removed the drivers for the next prototype.

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Unbaffled – NOS Carver Drivers

I have been purchasing used Carver amazing drivers for some time.  Today I put my collection of original hex cone drivers to the test in a full six driver dipole configuration.  The drivers are connected in two sets of three wired in series, the two sets then wired in parallel.  Each driver has a DCR of 3.5 ohms, so the net impedance is a reasonable 5.3 ohms.

2016-03-05 6x12 2mGPTest configuration is a ground plane measurement at 2 meters.  I also did a similar measurement with the 4×15″ driver dipole panel I built with Goldwood drivers.

Compare_0dBWI know what you are thinking, these are the same speaker measured twice.  Nope!  I have added some silicon to the cones of the 15″ drivers to decrease their resonance to 24 Hz / raise their Qts to about 3.5.  They are very close in specs to the original Carver drivers at 24 Hz / Qts = 3.0.  Note the baffles are even different – the baffle is 24″ wide for the 4×15 and 15″ wide for the 6×12 – so maybe that compensates a little for the small difference in Qts.

2016-03-05 4x15 close micClose_mic_15For reference, consider a close mic measurement of one of the 15″ drivers (blue) compared to the array 2 meter ground plane measurement (green).  You can see that the dipole bass cancelation is -15 dB at 24 Hz.  You can also see multiple dips at 35, 57, 121, and 141 Hz.  I had the speakers facing into the garage, so I am wondering if these dips were due to the finite baffle, or a garage Helmholtz resonance.

So another test I tried was to put the 6×12″ on its side and compare to the standing array.

2016-03-05 6x12 on side

A little bit of gain, but otherwise the same response.