BASSBOSS Questions

Hello David. What would be the actual cutoff freq of a subwoofer horn? In a blog post you wrote that 30 Hz is not feasible since requiring 2.86 m length and 3x3 m perimeter. Given a sub cross over at 80 Hz then a horn cutoff of say at most 60 Hz still means 1.43 m length (which is doable) and 1.5 x 1.5 m perimeter (still large!). In the VS21 the horn was combined with the low freq part but it looks like it is a way smaller horn, hence with a presumably much higher cutoff. So what would such horn help whenever crossing over at 80 Hz? Where is the contradiction? 🙂

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David Lee

The term “cutoff frequency” is a bit misleading. The output doesn’t cut off, it rolls off. It just loses efficiency and effectiveness below a given frequency, and that frequency is influenced by the three factors of length, radiating area and rate-of-expansion, aka flare rate. If all 3 of those factors are matched at a single frequency, that horn should work very well down to that frequency. If you’re not trying to use the horn to deliver low frequencies, it can be much smaller. If the horn is very effective at 120Hz, it will still be providing some sensitivity advantage at 60Hz, and quite a bit at 80Hz. In the case of the VS21, the desired output sensitivity of the horn is relative to the output sensitivity available from the vented section. If the horn were more effective, it would dominate the output of the vented section, making the result skew away from flat response.

To give you a sense of the relative scale required to deliver the relative ranges, in our SubMergence systems we use 4 x 18” woofers for the lowest frequencies and 1 x 18” horn-loaded woofer for the upper-bass impact. That horn is very efficient above 50Hz but it’s as big as a ZV28. It takes the deep-bass output of two ZV28s to keep up with it. It also requires offset compensation to align the direct radiating drivers with the horn, so multiple amplifiers and DSP channels are needed. The VS21 takes the concept of combining the vented and the horn-loaded outputs into one box, but unlike many other boxes that have put those sorts of elements together in the past (and present) the VS21 doesn’t allow the horn-loaded section to dominate the character of the box.

In the VS21, a longer horn would introduce phase offset issues. A larger radiating area would make the cabinet bigger and, along with a faster flare-rate, would shift the response higher up the frequency spectrum. As it is, the horn section in the VS21 provides more sensitivity than needed, The low-pass filter is used to flatten the response, with the advantage being a lot of amplifier headroom in the upper-bass frequencies.

The horn in the VS21 is a compromised horn, but it does everything it needs to do to provide upper-bass impact to complement the vented section. The horn’s output sensitivity serves to compensate for the inertia from the mass of the 21” cone. Without it, the power required to provide enough acceleration to deliver impact would be much higher. With it, the cone can move a shorter distance to produce the required SPL, thus reducing the demand for power and the stresses on the cone and suspension of the driver.

~DL

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