figured I would post this here as well
The reason you would want to know the “cabin peak” or resonant freq of your car could either be; you want to build a spl car and would like to know where to start your tuning at, or you are designing a box the correct way around your cabin geometry and acoustics.
The easiest way to find out your resonant freq of your cabin is to use a flat responding box, flat responding in ½ space, place it in your cargo area where your sub will be facing the same way your sub will face. Then place meter sealed legal on dash. Then you will want to run single tones, not a sweep, until you find the loudest.
Now if you’re wanting to know how your cabin responds to make a box around the acoustics it’s done differently, you will need same equipment as above but you will need to do a lot more work. Set it up same way as before, sealed legal on dash. Or if meter is the least of your worries as it should be put the meter in the listening position where ever your head is. You will need to take measurements inside of the car and outside as well, your outside measurements should be in a wide open area, no walls nearby, with the meter on a stand to middle of cone 3ft in front of cone
use this graph to record the score in each category
You only have to record up to where your subs low pass is, remember the low pass is not a dead cut off but an attenuation by a set number of db’s most are 12db slope on mono amplifiers. So knowing that if you go past what your sub woofer will be crossed at you will end up building a big peak into your enclosure to only cross it out later. What you want to do is find the difference between the inside and outside scores. After you have all the info you want, it’s time to model your enclosure, a good free program is winisd. To model your enclosure you will need to know 3 things,
the t/s parameters of the subwoofer(s) broken in t/s is best but pre broken in is better than nothing.
The space available
And the goal you have in mind
After you enter the subwoofer data in and select the enclosure you want to design you will end up with a graph that shows the predicted response of your subwoofer and that box in ½ space. What you need to do to find out how it will act in your car is add the amount of difference in the 2 scores you got before to the graph. This will get you much much closer to getting an enclosure that works WITH your cabin and not fighting it. The last thing to remember is to make sure your subwoofers response will join the mids response so there are no dead octaves or lost notes in your music.
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