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Hollis613 
Member - Posts: 15
Member spacespace
Joined: January 19, 2004
Posted: August 23, 2004 at 12:25 PM / IP Logged  

Hi guys just a quick question.

I built a sub box for a freind and it is 2.85cuft. She has 2  12" Alpind type-r. I have one 4 inch port. How long should the port be. Whats a good frequency? 40 ? 30?

Also 2.85 is the size before the subs are put in. I'm not sure how much room they take up.

SnomanF150 
Silver - Posts: 269
Silver spacespace
Joined: September 19, 2002
Location: United States
Posted: August 23, 2004 at 4:53 PM / IP Logged  
grab you a nanogram for port lenghts (you can google search for them), or go to calculators on the left of your screen and look for the "port lenght calculator"
I usually port around 28 - 32 Hz.. Find the resonance freq of the car with a tone generator and port to that if you really want to knock the paint off..
the12volt 
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Administrator spaceThis member has made a donation to the12volt.com. Click here for more info.spaceThis member has been recognized as an authority in Electrical Theory. Click here for more info.spaceThis member has been recognized as an authority in Mobile Audio and Video. Click here for more info.spaceThis member has been recognized as an authority in Mobile Security and Convenience. Click here for more info.spacespace
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Posted: August 23, 2004 at 4:58 PM / IP Logged  
You can find a Nomogram here, it's the first file on the page: https://www.the12volt.com/downloads/downloads.asp?catid=47
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Hollis613 
Member - Posts: 15
Member spacespace
Joined: January 19, 2004
Posted: August 25, 2004 at 2:34 PM / IP Logged  

How would I go about finding the resonce frequency of a car?

She really unhappy with these type-r subs so every little bit helps

gibo 
Member - Posts: 3
Member spacespace
Joined: August 26, 2004
Location: United States
Posted: August 28, 2004 at 12:04 AM / IP Logged  
Hollis613 wrote:

How would I go about finding the resonce frequency of a car?

She really unhappy with these type-r subs so every little bit helps

 
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
 
So you can use this formula, ? ft in car x 4 = ? / 1132 =  ? 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.
 
What the magic distance that the SPL Guys use ?
 
I believe it's 56" / 12" = 4.66" x 4 = 18.64 ft / 1132 = 60.72 Hz

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