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Bass Cannon
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Cannon 01
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Bass Cannon - Concept
Behind the ELF Waves
I would love for someone to write something up here in more detail.

In the meantime, lets just discuss the principle. The basic problem of speaker design is that the wave coming off the back of the driver is 180 degrees out of phase with that coming off the front. So in essence a driver with no enclosure makes no sound, since the two signals cancel each other out.

Many speaker designs focus on simply destroying the wave coming off the back of the driver. You cant just mount it in a small box though, because

  • If there is too much back pressure generated by the enclosure, the movement of the cone will be restricted and not as much energy will escape the front. So the volume of a speaker box would ideally be infinite, so that no backpressure is generated, and no funky waves bounce back at weird phase angles and impinging on the cone while it is trying to do its job.
  • Unless you build a box like a tank, it will be hard to prevent some of the sound from the back of the driver making its way out of the box. This is why people end up putting multiple of layers of MDF and loaded vinyl mats in their enclosures, and generally building them to not resonate, etc.
  • Some speakers seek to transform a portion of the backside energy and pipe it back out at a more suitable phase angle. The wavecannon is fits in this category. The idea is to make a resonant pipe with a driver placed in the middle somewhere, so there is some length of pipe attached to the front and some length attached to the back. The lengths of the pipe are adjusted such that when the signals are exiting the two ends of the device, they are in phase. It turns out that the driver should be placed about 1/3 of the way down the pipe, so one section is twice as long as the other.

    The nice thing about this design is that you are using 100% of the driver energy, which is more than can be said for most speaker designs. It is also relatively easy to build. The downside is that it has resonant modes, and so the frequency response will not be flat. Fortunately the human ear is not very sensitive to non-flat frequency responses at the low end. So while the cannon isn't great for a general purpose speaker, it makes an efficient, powerful, and easy to build subwoofer.

Design partially original and partially ripped off from other websites
by Holly Gates