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Old 29th August 2011, 09:51 PM   #1
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Default resonant frequency

the other day i put together a system to find the resonant frequency of various vehicals. it's a 1.1cf box with a 10" punch hx2 rfd2110 and a ~200w punch 2-ch amp. after displacement the box is 1cf, the sub only has 1 working 2 ohm coil and the amp stays at full pass. i have a 12v power supply and an oscilloscope at my house so i set the amp so it would'nt clip using a 60hz sine wave. i also checked the output voltage with my fluke 112 rms multimeter. i got ~18vac @60hz no clipping, i'll use that for a refrence later. i made it so i can clip the amp's power wire to the cars battery and i use my phone for the input. the bad part is the 1/2 blown subwoofer, it's a 2-ch amp so i can only use
1-ch of the amp with the single 2 ohm coil. i took it outside, set my term-lab mic about a foot of so in front of the sub and measured every frequency from 25hz to 65hz and averaged around 115db. i have 4 vehicals so i did test in evry one of them and got various results. oh yea according to an enclosure program the sub & box should have a Q of .76.

testing proceedure:
first i point the sub different ways and run a 20hz-120hz sweep and see what i get the the highest on. once i get that i will point the sub in the direction that gave me the highest numbers and will do sine waves in 1hz increments around my highest output frequency from the sweep.

example- if i peaked at 55hz on the sweep i would meter 1hz at a time between 50hz and 60hz and record all my results on the chart.

should i indivualy meter a wider variety of frequencies?

in my 2000 nissan altima i noticed it had 2 peak frequencies in my sweep.
@ 30hz = 118.13db
@ 49hz =118.11db


but i have a few questions.
would it be better if my setup was louder than 115db overall?
a friend of mine has a kicker comp 10 for sale but when i plug the specs into th software it says i will Q .5 with my 1.1cf box. should i wait for another sub that will have a Q of .7?
is there any way to improve my testing method?
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Old 29th August 2011, 10:20 PM   #2
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Join Date: Nov 2003
Location: Louisiana
If you want to find the vehicle's transfer function, take the speaker outside as far from any buildings/walls or any other boundary as possible. Run pink noise, get an average reading and save it.

Install the speaker in the vehicle, play pink noise, get another average reading and save it. Then display the difference between the two stored readings. That will show you what the vehicle does to the frequency response of the woofer.
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Old 29th August 2011, 10:30 PM   #3
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how do i get an average reading and save it?

can i do this with the term-lab software?
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Old 29th August 2011, 10:32 PM   #4
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I would expect that you could. It's a basic function for spectrum analyzers. Go to the termlab forum. They would be able to tell you and may have better suggestions.
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Old 29th August 2011, 10:37 PM   #5
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i don't have access to that function.
i paid over $1000 dollars for the meter and software package but i didn't include the Broadband RTA Upgrade. it was another $300!!!!!
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Old 29th August 2011, 11:40 PM   #6
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There may be free (or cheap) RTA software on the net.

Try posting a question requesting suggestions for the best free (or cheap) RTA software on the software forum. Explain what you're trying to do.
Software Tools - diyAudio
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Old 30th August 2011, 01:44 AM   #7
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is it common for some frequencies to play lower in the vehical that they did on the outside test?
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Old 30th August 2011, 02:28 AM   #8
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It's possible due to cancellation (I'm assuming that you used precisely the same signal level in and out of the vehicle).
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Old 30th August 2011, 02:35 AM   #9
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as best as i could.
i'm using the batteries (3-kinetic drycells) in my car for a power supply with the car off. so i would imagine that it has some voltage drop after a # of test. i will address that later. as far as everything else it it's all the same.
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Old 30th August 2011, 02:38 AM   #10
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If you want to match the levels, measure the AC voltage going to the speaker with a test tone and then switch to pink noise (or whatever you're using for the test signal).
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