Home Forums DIY Speakers and Subwoofers Looking for a linear subwoofer that can still take some power? Reply To: RE: Looking for a linear subwoofer that can still take some power?

  • mistersota

    June 12, 2021 at 8:10 pm

    Making enclosures for cars is a completely different ball game than in a home theater thanks to cabin gain. This is a 10-12db/octave rise starting at the frequency that is 2x the length of the cabin (most are around 60hz). So in order to design a proper enclosure for a car, you MUST add a linkwitz filter when looking at the response. If you don’t, you will get a huge peak in response around your port tuning. Generally, due to this cabin gain, you will need to make the enclosure smaller and tuned higher, which are good things. You also need to simulate the voice coil inductance. This is because subwoofers tend to have a high inductance which can dramatically change the response. Some don’t, but almost all with high excursion and power handling do. Here is an example with the MX15 you mentioned, and I own.

     First, here is the difference in response by simply simulating the inductance. the box size and tuning is exactly the same, but the one with the hump is with inductance simulated. Don’t worry, it’s actually not a problem in the end.

     Now let’s look at the linkwitz transform. To add a 12db/octave rise at the frequency you want to put a filter with these values. f0 is the starting frequency that depends on your vehicle. I’ll use 60hz because it’s fairly standard.

     Here is what that filter looks like when applied to the mx15. As you can see there is a 20dB peak at the tuning frequency due to that cabin gain, which is not what we want to see.

     To combat this you have to tune higher and make the box smaller. This graph shows the difference between the recommended box that would work well for something like a home theater and what would work well in a car. The one with the peak is a 155 liter box and the flattened one is 64 liters.

     I’m sure you noticed that most car audio subs seem to have a high fs compared to home theater subs, this is because the manufacturers know that the cabin gain will fill in the low end. Play around with the filter on other subs and you’ll find that you’ll find more subs that fit your application. You’ll find that there are lots of subs that will play lower than you expect, and the higher tuning and smaller box will help with the excursion.

    I’ve been into car audio for a while and would say you shouldn’t worry about finding an “SQ” sub too much. The benefit of low distortion really isn’t that important with subs because once you get playing loud enough that the distortion might be audible, things are shaking and rattling anyway. Flat frequency response is much more important which comes from a properly designed enclosure more than anything.

    Some of this stuff can be complicated, so if you have any questions let me know.