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Passive Radiators

Discussion in 'Amps and Cabs [BG]' started by Saephan215, Apr 29, 2010.

  1. Saephan215


    Apr 29, 2010
    I've been looking at the specs for the Walkabout Scout 1x15 and was wondering, what is a Passive Radiator? How does it affect the sound quality?

    Also, what is a "premium horn attenuator & instant reset horn protection"? Is the horn that small thing that resembles a speaker located on the top-left corner.

    Thanks in advanced.
  2. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    A passive radiator is like a speaker without a magnet or coil, its another way of tuning the enclosure, like a port. Not common because they are much more fiddly than a hole of a certain length.

    The horn is a tweeter for the top and will look like another small speaker. In this case you can turn it down and if you and overload it by clipping your amp or something, you can reset it.
  3. The big advantage to a passive radiator is getting a large vent area in a small box.

    The square area of the vent is a function of the speaker cone area and displacement. The vent area has to be of sufficient diameter to avoid high velocity air noise ('chuffing').

    To get a sufficiently large vent area, the vent has to be ducted (extended). This gets tricky very fast in a small cabinet.

    As noted above, the passive radiator is a cone without magnetic motor assembly. Significant mass is added to the back of the cone to tune the resonance of the PR in its particular cabinet.

    This mass can be quite substantial, and induces an unopposed vibration into the the cabinet. Think of a BMW horizontal motorcycle engine, but with only 1 cylinder.

    Adding a 2nd PR to the cabinet, on the opposing wall, does the same thing as the 2nd cylinder in the beamer: vibration cancellation.

    PRs are not used commercially because two puts the price point far above the rest of the crowd, and makes the cab unsaleable.
    Dual PRs are a very good small box solution, but only for the DIY builder.

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