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The speed of sound at sea level = 1,116.43701 feet per second, but that can change with temperature and altitude, http://science.howstuffworks.com/question182.htm , I've always used 1132 ft per second as my feet per second for sound, so measure the total distance in your vehicle that you will have for sound waves to form and multiply that by four, because full sound waves can be heard in the quarter wave distance, which means if the full wave distance is 56 ft you only need a quarter of that distance to hear it or 14 ft, so a measurement of 14 ft is the quarter wave length and you multiply that by four.

 

14 ft x 4 = 56 ft / 1132 =  20.21 Hz, 14 ft of distance is about what you need for 20 Hz to form.

 

So if you have a hatchback car and the longest distance from the windshield to the back glass is 8 1/2 ft or 8.5 x 4 =34 ft , 34 ft would be the length of the full wave, but you only need a quarter of that distance to hear it, the 8.5 ft you measured, take the full wave length distance and divide it by the speed of sound (1132) and you get 33.29 Hz

 

8.5 ft in car measurement x 4 = 34 ft / by speed of sound 1132 = 33.29 hz

 

 

You know when you open your windows in your vehicle the low bass gets louder, more distance more low bass, less distance less low bass, Example: regular pickup truck cab about 6.5 ft left to right.

 

6.5 ft x 4 = 26 ft / 1132 = 43.53 hz, not so low but if you put your subs in the bed with a hard cover and port into the cab low bass.

 

 

I believe it's 56" / 12" = 4.66" x 4 = 18.64 ft / 1132 = 60.72 Hz