winkychevelle 10+ year member
CarAudio.com Veteran
Is it normal to peak lower in your vehicle then the tuning of your box? If I build another box tuned to 30hz. would it peak @ 27hz.?
That varies vehicle to vehicle. Your low frequency peak could be the combination of the lowish tuned box and the cabin gain lining up at that frequency.
Only way to find the cabin gain is to subtract the frequency response of the same enclosure outside of the vehicle.
Usually you should do this first before designing the box so you can manipulate the box to work with or around the cabin gain.
If you have a low end peak in you cabin gain you could potentially tun you box higher than normal to have a more flat response higher into the frequency range.
A high end gain might require you to tune ultra low to get low bass.
Every vehicle is different and every time you add significant deadener or make a big space consumption change you should do a new test.
Adding a 10ft³ elcosure in the back of a Tahoe is going to change your cabin gain similarly, sealing off the trunk changes cabin gain, minor deadening doesn't change much but going from no deadener to fully treated is enough of a difference to warrant a new test.
You don't have to dive deep into the weeds I only recheck when I make massive changes in my system.