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Infinite Baffle Subwoofer

The goal of any subwoofer is to deliver the lowest octaves of music with the least amount of distortion possible at sound pressure levels that match those of the rest of the system.

Since my Martin Logan rig can play very clean at SPL's greater than 105dB, I need low frequency reproduction that can keep up and pressurize the 3K cubic foot home theater.

While I had two Velodyne ULD-18's in my home theater before, they could not move enough air to keep up. Which brings me to the main point of all great subs: they have to displace air in significant quantities.
Most commercial subs have relatively small volumetric displacements, and I'll use the rather large ULD-18 here for comparison, which displaces a total of 2.15 liters (or 131 ci). In contrast, my IB sub displaces 15.24 liters (or 870 cubic inches), which 6.5 times as much one Velodyne.

The other characteristic of Infinite Baffle subs is that they have very low distortion figures due to operating many drivers in their cleanest ranges and not forcing them to compress air in a box. This IB is the perfect math for the ultra-low distortion Martin Logan electrostats.

I innovated a design for in-room manifolds (called an 'outie') with this particular design, as can be read in the design and install discussion I wrote at the time.

The system specifications are:

  • System Frequency Response
    18 - 120 Hz +/-2dB
  • Crossover Frequency
    80 Hz at 24 dB per octave
  • In-Room manifold
    Dual layers of16 ply Baltic-Birch plywood
    Glued and screwed together. Glued and screwed to floor
  • Drivers
    Stryke AV15 Mk-II
  • Sensitivity
    92dB/1 watt/meter
  • Amplifier
    Crown XLS602 - 600x2 into 4 ohms = 1,200 watts total to IB
  • Impedance
    2 drivers series wired on one VC to yield 4 ohms , with each pair wired to one amp channel
  • Weight (drivers + Box)
    140 lbs (not that it matters, as it's bolted and glued to the floor now)

Full View of IB