posted
Setup, The box is in the trunk of a civic facing forward. The center of the rear decks surface area has been cut out (sawzalls kick ass) and the port is firing up though the rectangular hole cut in the rear deck. The rear deck also has mdf over it, which mounts to the front of the box completely sealing off the cab from the trunk.
I noticed that if im moving around in the car with the seats out, there is a null in the center.
Hypothesis, The null is caused by the wave reflecting off the front of the car and causing cancellation.
Theory, Say you have a long cylinder (for simplicity) one end has a sub, the other end is a boundary. Follow the wave traveling down the cylinder. When it is ¼ wl (wavelength) from the boundary, the reflected wave will be 180 degrees out of phase with the incident wave. At the ¼ wl from the boundary It travels 1/4wl, reflects and travels ¼ wl back. That’s ½ a wavelength.
Measurements in my car, I took a meter and measured the distance from the speakers to the null at 40,50,60,70 and 80hz. I then calculated the ¼ wavelength at each frequency. If my hypothesis is correct, the distance from the speakers to the null + the ¼ wavelength should all be the same for each frequency. Since the boundary in the front of the car (windshield ect..) is oddly shaped I can’t really measure from the null to it.
With the exception of 40hz, where there was little cancellation anyway they are all approximately the same. The distance from the speakers to the front of the car the wave sees as a boundary is about 90”.
This sucks, look at how much spl I loose with respect to what my body feels, Like 10dB! Theoretically, there is nothing I can do other than get a different car. According to the data it has nothing to do with the placement of the speakers, It’s the actual distance from me to the windshield.
If this is hard to follow, draw it out on paper. That’s what I did before I took measurements. Once in the car taking measurements it was obvious, the higher the frequency the closer the null was to the front of the car.
BTW, My car hits harder if I scoot up. Way up!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
cool report... I know it sucks huh. That's why when I have the $$$ I am going to build an extreme SPL LIMO!
-------------------- (1) PPI AMP PCX-2200 (800 Watts @4ohms Bridged) PCX-480 for my highs (2) DD 3512a's in a 4.4cube box with 50sq. inches of port tuned to 33Hz Street 1-2 [TEAM DD] 147.7dB My Sound Domain page Posts: 1655 | From: Tucson, AZ | Registered: Jul 2001
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quote:Originally posted by Mr.Dank: Setup, I noticed that if im moving around in the car with the seats out, there is a null in the center.
This "NULL" is often referred to as an acoustical shadow. I too have noticed that "NULL" in my civic. We both basically have the same setup, cept that I have my port in-between my two 15"s (on the baffle). I found that I lost that "NULL" when I moved my box into my back seat..but am planning on moving it back. Not that you care . Interesting test results though.
-------------------- Andrew Kemendo Team Pro Creations FAA Pilot #2718765 Oderint dum metuant
posted
Thanks... Soundrew, Ya, I want to keep a back seat for a while... I would never waste a back seat for a box anyway, only for a wall. "build a wall" thats what the voices in my head say....
My old car had one.....and its hard to go back to the trunk. Im getting there though.
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
What body style of Civic are you refering to? I really hope its not a 96-98, if so its time to go car shopping.
-------------------- My Ride: '98 Civic LX My System: Panasonic 501U Diamond Audio M361i 3 way Pioneer 6X9s Hifonics Zeus BXi1500 2 RFP 1415 and not enough sound deadening material SPL:? Posts: 801 | From: Cornfield IL | Registered: Oct 2000
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that's what my friend told me summer before last...he was surprised as all hell when i called the next morning i said i got about 19 cubes to work with lol. he said he was just kidding...didn't think i'd get serious hehehe
-------------------- "Those who give up liberty for the sake of security deserve neither liberty nor security" - Benjamin Franklin
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