![]() ![]() But for this vented sub box, we want zero boost, in fact the ideal roll-off is where the Q=0.7 which can be achieved by changing R9 and R11 at a ratio of 1:2. ![]() Depending on the values selected, you can actually create a bass boost circuit, which can be used to model bass boost circuits of common plate amps to see how they will sum with your enclosure, which is what I did with my Sonorous 10S subwoofer. ![]() The input stage is unity gain, and provides a good buffer stage to the source, the LPF stage is set to about 90 Hz, but can be adjusted as shown with three resistors and last stage is the subsonic stage which is set by only two resistors. I show that circuit here, which is made up of an input buffer stage, which can be set just about any gain with no EQ, there’s an 18 dB/octave low-pass filter which can be set to any frequency desired, and then there’s the 12 dB/octave high-pass filter (the subsonic filter) which also can be set to any frequency and any Q desired. I built a model of the exact crossover that makes up the EV XEQ-2 and ran an AC simulation which lets you see the exact response of the filter. So how do we determine where to set the corner frequency of this subsonic filter? Well, that’s where MultismLive comes into play. In this case I am using a fairly simple 12 dB/octave active filter that requires only one op-amp to implement. Steeper slopes can help, but also make the filter more complex. The biggest problem with subsonic filters is by design, they limit excursion, and in doing so if not set to the correct frequency, will limit the output at and above tuning, which we do not want. A subsonic filter should be implemented to keep the driver under control below tuning to prevent possible driver damage, lower distortion, and keep the amplifier happy (i.e., possibly going into protect mode during playback and interrupting your movie/music). most if not all amplifiers are not even flat 20 Hz, meaning below 20 Hz the amp is starting to roll-off anyway, thus limiting the actual excursion at the driver and lastly 3: eh, it’s just modeled data, who really cares? All valid, but flawed points on their own. there is little source material that is actually going to push the sub to those levels below 20 Hz and 2. So even though this sub is already tuned pretty low, if you didn’t want to implement a subsonic filter, you could argue three points: 1. Passive radiator designs are bit better and sealed designs are even better yet, but they too can benefit from a properly designed subsonic filter to accompany them. Regardless of the driver and enclosure, this is a pretty common behavior that will occur with any vented design. This looks pretty good in the simulation, has a decent f3, a nice and flat in-band response, without any peaking or drooping, but if you look at the excursion below 19 Hz, you can see that Xmax is widely exceeded, as is the assumed Xmech. In this case I’ve got a Dayton Audio RSS315HF-4 in a 95-liter enclosure tuned to 22 Hz. First we’ll start with a simple simulation of the driver and enclosure. So with that let’s start out with this simple example, which is based on a sub I built several years ago, so we can see how it all plays out. frd files which can have any shape, slope, Q, response, etc. Unibox 4.08 on the other hand will let you import. Plus it’s not obvious how to go from the simulation to a real-world active filter, unless you’re building a system with DSP. You can create simple filters using the built-in filter tool, but is limited to certain prescribed filter types. I took a stab at using WinISD to do this, but it doesn’t provide the option to import. Unibox is a bit dated for speaker enclosure design and runs in Excel, which not everyone uses, but is my favorite box simulation software of choice. The two main tools for achieving this are NI’s MultismLive and Unibox 4.08. I’ll do a quick re-cap of my previous post on this topic, but the main idea behind this post is to show how to design subsonic filters and then how to model them into a subwoofer design before you even buy or build anything. And more recently since the availability of online, web-based simulation tools, such as National Instruments MultismLive, taking the guesswork out of active filter design is a thing of the past. While my specific example incorporates this super-old (and no longer available) EV XEQ-2 crossover, the methods employed herein are practical for anyone looking into subsonic filter design, how it works, and the best way to incorporate it into you next sub project without sucking the life out of your bass. It’s been a while, but I recently got around to actually doing what I said I was going to do all those years ago. So this is going to be a Part II to my original post from 14 years ago where I first discussed the topic of proper subsonic filter design in conjunction with subwoofer enclosure modeling.
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