If its recomended (by manufacturer) to have a sub box with 2.5 - 3 internal volume and mine has less (2.4), will my subs sound like crap? How far can i be off from what they recomend before i can hear the difference? Thanks
they could. alot of that depends on the driver and the enclosure type. some speakers will not nessesarily sound like crap, but may just not have as much decibel output. if you get anything in too small of an enclosure it will sound crappy yes. most speakers will be able to be in an enclosure that is 10%-15% smaller or larger before they are out of the recommended spec. that doest mean they wont work. it simply means they were designed to sound a certain way in a certain sized enclosure. sealed enclosures are not near as exacting. they can be out of the recommended size either larger or smaller and its not gonna be as big of a deal as a ported enclosure. ported enclosures need to be much more close to the recommended size and changing that will result in much poorer quality not to mention the fact that if its not tuned right with the correct port volume, speaker damage will occur.
You can put some Polyfill in the enclosure and it will sound like the enclosure is slightly larger than it actually is.
flashover95, the Q is likely over 1.0 unless you use the larger airspace. This is a result of an overly sharp and generous peak in the output, probably over +6db. What you hear is too much volume at around, most likely, ~50 Hz. When you then try to compensate for the peak using tone controls (etc.), the low end of the frequency spectrum is further minimized. The result is less-than-ideal response.
Manufacturers try to recommend the least amount of airspace requirements for their subs due to the limited amount of free space in vehicles. They will often give you options of sizes while warning that going smaller than the largest will result in less low end. The sound, though, will not necessarily be described as "crappy"...it depends on the gear and quality of install, all of that. It just might not be ideal.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.