box tuning question

[unhealthy]

if i have an 18 cubic foot box(after all displacement) with subs up and the port is 47" wide x 6" high across the entire bottom of the box aimin backwards how deep would the port need to be to be tuned at 28 - 29 hz?

thanks for any help, let me know if you dont understand what im asking.

[Thewes]

if i have an 18 cubic foot box(after all displacement) with subs up and the port is 47" wide x 6" high across the entire bottom of the box aimin backwards how deep would the port need to be to be tuned at 28 - 29 hz?

thanks for any help, let me know if you dont understand what im asking.

According to bassbox pro 44.67 inches but that seems kind of steep.