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<blockquote data-quote="shredder1" data-source="post: 8879928" data-attributes="member: 658920">If you are looking for low end extension I&#039;d opt for a +- 1.5:1 ratio and understand any resonance below the drivers Fs with a EBP over 60 will affect power handling. I like 1.5-2.25:1 on most 4ths depending on driver characteristics once the sealed side is determined. Always watch delay (-25ms) for an accurate result.Your peak is not what Winisd says, Experience and a designers knowledge can make the response you see FAR more accurate once cabin gain is applied to your modeling depending on your vehicle.&nbsp; Coupes and trunks generally gain around 50-60hz, Suv&#039;s with more cabin space gain anywhere from 45hz-35hz so anything you design could have a half (almost) octave lower gain to match. I use resonance and ratio to smooth final response out if everything else looks good. Consider this... all frequencies reproduced with authority is far preferable to making something that underperforms within a narrow bandwidth. In my exp a well designed 4th for 2-12s will at least reproduce -25-70hz if ratioed correctly.I&#039;d focus on making a 1.5:1 4th sized for 45hz sealed and enough port to keep velocities under 75fps myself. Everything else should fall in line once built. A 30hz 4th? comon&#039; bro...</blockquote>

[QUOTE="shredder1, post: 8879928, member: 658920"] If you are looking for low end extension I'd opt for a +- 1.5:1 ratio and understand any resonance below the drivers Fs with a EBP over 60 will affect power handling. I like 1.5-2.25:1 on most 4ths depending on driver characteristics once the sealed side is determined. Always watch delay (-25ms) for an accurate result. Your peak is not what Winisd says, Experience and a designers knowledge can make the response you see FAR more accurate once cabin gain is applied to your modeling depending on your vehicle. Coupes and trunks generally gain around 50-60hz, Suv's with more cabin space gain anywhere from 45hz-35hz so anything you design could have a half (almost) octave lower gain to match. I use resonance and ratio to smooth final response out if everything else looks good. Consider this... all frequencies reproduced with authority is far preferable to making something that underperforms within a narrow bandwidth. In my exp a well designed 4th for 2-12s will at least reproduce -25-70hz if ratioed correctly. I'd focus on making a 1.5:1 4th sized for 45hz sealed and enough port to keep velocities under 75fps myself. Everything else should fall in line once built. A 30hz 4th? comon' bro... [/QUOTE]

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