Passive Radiator

[Simon Barden]

From my Speaker Design Cookbook, passive radiators need to be used with a driver with a high cone mass, which can be achieved by adding mass to the speaker cone, if it doesn't have sufficient mass already. Passive radiators will extend the low frequency response (at the expense of losing high end from it), but high mass speakers are less efficient, so you lose maybe 3dB - 6dB of overall volume. So to get the same output level, you'll need at least double the amplifier power. You don't get something for nothing, so you're trading more low end for less output.

I'd guess that commercially available bass speakers tend not to be high mass (Qms is the parameter to look for) and this should be in the 7 to 10 region for a passive radiator to work properly. Looking through the Eminence bass speakers, there are a few with a Qms in this region, though not many, and most are in the 4 to 5 region. The high Qms speakers by nature have lower efficiencies than the lower Qms speakers, and the passive radiator would drop this efficiency even more, so they would need to be paired with much larger amplifiers to achieve similar outputs.

You need to remove a lot of material from the speaker enclosure to install a passive radiator, which then makes the enclosure weaker, so it needs more strengthening. Which then puts cost and complexity up. Porting a bass enclosure is a lot easier, and cheaper.

You get a few passive radiator designs in hi-fi and monitor speakers, where stage volume levels aren't required and mid an/or HF drivers can add in the higher frequencies the passive radiator reduces. You lose some of the drawbacks of ported design like port noise at high volumes, but you do gain a few others like increased group delay (frequencies being produced over a period of time rather than all at the same moment).

I'm just reporting stuff from the book, and have no real understanding of these passive radiator systems, so can't currently enlighten you any further.

BTW, the current Bugera BT108 has a vented back, rather than fully open one, so I'd imagine that it is acting as a basic port. The port size probably needs to be quite large to get the right response, and it looks larger because it's a small enclosure.



If you took that back off, then you'd get much less low end from the unit and you'd probably quickly destroy the speaker if playing at loud volumes as that bit of wood will still help reduce speaker excursion..

Remember that it's a product built to a very low budget, so you'll always get compromises with something like that.