A passive radiator is basically a loudspeaker cone without the voice coil and magnet; it can be used much like a vent and tuned to a certain frequency by adding extra mass to the cone. The benefit is obviously that there’s no need to construct long vents or that the subwoofer suffers from vent noise, and lastly the woofer is protected from large excursion by the passive radiator. Again we start off with a 30-liter enclosure and add the Peerless XLS10 passive radiator which comes with a mass of 400-grams. This passive radiator is essentially a XLS10 woofer without voice coil and magnet. The passive radiator can be modeled in much the same way as a vent, as it works in similar fashion, which results in the graphs for spl, and excursion for both the woofer and the passive radiator below.
As can be seen from the graphs the power needed on the amplifier side is much lower, just 120-watts and the excursion for the woofer is far below the maximum. Unfortunately the passive radiator is past its maximum excursion of 22-mm. The only way to counter this is to tune it to a lower frequency by adding more weight to the passive radiator. This is simply done by removing the screw on the back of the cone and adding metal rings to increase the weight. By adding 200-grams, net total weight is 600-grams, the passive radiator excursion is reduced by 40% as per the below noted graph.
As an added advantage the low frequency response is more linear and extended to the left of the graph, also note that the required amplifier power has not increased. What is clear though after this exercise is that by using a passive radiator we’ve been able to model a subwoofer using Peerless’ XLS10 woofer and the XLS10 passive radiator that meets all of the design requirements we’ve set forth. The response of the subwoofer can easily be corrected to offer a flat response by adding a simple filter at the input of the amplifier. In a practical situation moving the subwoofer closer to a wall, or in a corner, has a similar effect, due to the natural low frequency boost, so no equalization is necessary to get a linear frequency response. Now that we’ve modeled the subwoofer correctly it is time to start thinking about construction, as although we specified the enclosure as a 30-liter box, a little more effort is required to design a good enclosure for the subwoofer.
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