Resonant Frequency of a wall set up

[bzhei]

hi all,

been building music boxes for quite some time now and am deciding on doing a burp box for a 98 Honda Civic.

well,i've been searching the web but i get different infos on how to find the resonant frequency of the said car.

the current set up is a straight wall with 2 Kicker CVX 15s tuned to 33Hz off a single RF t20001bd @ 1ohm. it's a couple of 5.6cubes(including port) stacked on one another. i'm quite happy with it since i'm getting 149.8dB on the AC SPL190 and 148.5dB on the TL using music. been wanting to throw in a burp box to get to the 150s.

i'm wondering if what i'm doing is right, so any comments/tips will be very much appreciated.

what i did was the ff:

1.) didn't remove the wall enclosure

2.) left the CVX in place

3.) covered the port with a 3/4 MDF piece

4.) threw in a 10" sub (1cu. ft. sealed) on the passenger seat

5.) did a manual sweep and recorded the SPL. (peaked at 41Hz and dropped beyond this)

6.) removed the 10" sub and threw in a 12" sub (sealed 1.2 cu. ft. sealed)

7.) did a manual sweep again and recorded the SPL (peaked at 42Hz and dropped beyond this)

with the recorded peak. i'm planning to do a 38Hz box. but i needed to know if i did the right thing in determining the resonant frequency of the car. what i plan to do is just remove the two enclosures and replace them with the new box i'm doing. but i'm wondering why the peak frequency is low (if that is indeed the resonant frequency of the car).

what worries me is i've read in different forums that the sealed box trick won't work in walled systems in determining the resonant frequency of the car. and to tune walled enclosure in the 45-55Hz range for max SPL. distance between meter (windshield) and subwoofer is 4ft exactly. so that translates to what, 72-74Hz?

any help would be appreciated. thanks

[Boon]

Yeah walled cars tend to peak really high... well they have a resonance node at whatever half their resonant frequency is so it's possible you're seeing a node of 82-84hz resonance.

[bzhei]

Yeah walled cars tend to peak really high... well they have a resonance node at whatever half their resonant frequency is so it's possible you're seeing a node of 82-84hz resonance.

thanks man.

so should i tune it around 75-77Hz then or am i already good at 38Hz? i already ran a couple of experiments.

so here they are:

1.)connected one t20001bd on the CVX on top (still using the 33Hz box). disabled the bottom subwoofer. used a 42Hz tone and metered a 141.8dB (AC SPL190) i built only 1 box so i don't have to adjust two enclosures at the same time.

2.) built a 4 cubic feet box tuned to 38Hz and metered, shortened the port, metered again, shortened the port and so on..

3.) the highest i got was 145.2dB which when i entered the port length and new box volume (for the sub) on my Term-Pro got me a 39.5Hz tuning freq.

4.) i did try to use a 60Hz tone but it only measured 130ish.

my thoughts are if the 141.8dB single sub experiment equates to 149.8dB when two of them plays, it does put me in the 150s when my new enclosure hits 145.2dB right?

thanks for helping man, really appreciate it.

[Boon]

No point tuning that high, you can't burp over 80hz in competition anyway

I would tune to the low peak, more fun that way

Pics and more specs of your box? Also the rest of your equipment? I'm sure your setup has a 150 in it somewhere, especially if you just fart-box it.

[bzhei]

No point tuning that high, you can't burp over 80hz in competition anyway

I would tune to the low peak, more fun that way

Pics and more specs of your box? Also the rest of your equipment? I'm sure your setup has a 150 in it somewhere, especially if you just fart-box it.

ok man, i'll shoot some today and post it up here. i'll run some more experiments and hopefully you can lead me doing the right thing.

and yah, i thought it's more fun coz i can still be able to play music with a 39-40Hz box right?

[bzhei]

ok man, i'll shoot some today and post it up here. i'll run some more experiments and hopefully you can lead me doing the right thing.

and yah, i thought it's more fun coz i can still be able to play music with a 39-40Hz box right?

here's the enclosure i built.

side by side comparison of my 33Hz music box right and 38Hz SPL enclosure.

the 38Hz box is 4 cubes with 72.5 sq in of port.

i resined it and did a 45 on every corner.

placed it in the car. the wall was already finished and sealed up when i did the test.

i did manage to get a 152dB on the SPL190 which is what i have. i have to wait for a comp to have it metered on the TL. i got the 152 on a 44Hz tone, not 42 anymore.. one t20001bd on 1 ohm.

i'm kinda liking the current enclosure since it can also play music pretty well.

now on to my next inquiry..

i calculated the actual driver impedance using a TPI A296 clampmeter to measure current and a TPI 440 to measure voltage. my amp is seing a 5.17ohm load per woofer @ 44Hz. basing from the 2 ohm birthsheet rating of the t20001bd i got, my amp is just putting out 1184watts in this enclosure.

i still have 3 more t20001bd sitting in the garage and was thinking on strapping two of them to each subwoofer. did some calculations and the amps will give out around 2085watts when strapped. was thinking i could use this type of set up for a 3second burp. any insights?

i installed them and prewired them for another set of tests and experiment.

by the way, here's the signal path for the subwoofer section:

alpine 9881 headunit

audiocontrol Four.1i

audiocontrol 6xs

rockfordfosgate t20001bd

[audiofanaticz]

I had a buddy that walled a civic with 2 15 inch juggs, Want to say 2 or 4 Us Amp Md3d's

I know the thing sounded like a fog horn when he burped it. Around 72hz if I remember right. It was pretty high!!

edit for vid.

[Boon]

I'm going to say the box needs to be wider across the car and shallower. Looks solid though. Also try and get the woofer as far away from the port as possible. Subs in the middle of the car with port on the drivers side isn't really ideal for SPL.

[bzhei]

@ bangin caddy

nice system bro. it does sound like a fog horn.

I'm going to say the box needs to be wider across the car and shallower. Looks solid though. Also try and get the woofer as far away from the port as possible. Subs in the middle of the car with port on the drivers side isn't really ideal for SPL.

ayt man, i'm gonna try and rebuild the box. but in the mean time, i'll try to hook it up with the 4 t20001bd and get record some numbers.

but let's say with the box i have now, is there any placement that would be ideal for SPL? like turning the box upside down so the port would be on top and the woofer below and vise-versa?

was thinking on getting a DC Audio Level 4 w 5 coil in about a couple of weeks so might rebuild the box when they arrive.

[Boon]

Mmmm could you flip the whole thing around and do ports on top subs lower? It's not ideal for SPL (port drivers side subs passenger side is the widely accepted optimum arrangement) but it might gain you some...