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Impedence curves and resonant frequency

 

Silver Member
Username: Zacdavis~

Post Number: 160
Registered: Sep-04
well i guess i've got my car peaked. i'm using 4 amps that run at .5, so i ordered 4 dual 1 ohm subs. those impedances match well for street beatin, however for competition....not so much.
so let me explain why.
every box has a resonant frequency, as does every vehicle. now, as those resonant frequencies are reached impendance curves result. in short...even though a .5ohm sub is connected to the amp, the amp may "see" an ohm load twice as high because of the impedance curve. so in reality my amps are seeing a near 1ohm load even though the sub is .5. i have found the exact resonant frequeny of my car and complimented it with a matching box tuning. its maxed out. now i must buy dual .5ohm subs, this will show the amp a .25ohm load at rest, but at full burp the amp will "see" .5 :-)
 

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 4681
Registered: Dec-03
usually at or near the Fs of the sub, the impedance should drop drastically.
the impedance curve would rise as you rise in frequency toward the low-pass filter point.
not to mention, the sympathetic frequency of most vehicles is around 65Hz, and the Fs of most subs is closer to the 30-21Hz range.
All you should really need to do is re-dune your enclosures.
If you're going for SPL, I'd tune the enclosures to the symp. freq. of the vehicle cabin anyway.
get the most SPL from combining peak efficiency of the cabin and the enclosure together.
don't forget also factors like if this is a sealed enclosure, it'll be a flat response curve to about 9Hz since you have cabin gain to compensate for roll-off below the transfer point.
quite a few factors to take into account.

Granted I was never much for SPL events though.
If I was, I'd just use a 6th or 8th order enclosure and swamp the subs with power.
tune the box to the resonant frequency of the cabin and see how it works out.
Personally, I'm much happier with a SQ system, that's loud enough to satisfy just about anyone *in* the car without actually killing someone.
 

Silver Member
Username: Zacdavis~

Post Number: 163
Registered: Sep-04
unfortunatly we arent talling about the subs free air resonance, spl is much more involved than just the subs physical characteristics.as you know, all cars tune differently. my explorer tuned to 43 hz, the car i compete with tunes at 63. this frequency should be used in determining the size and tuning of the enclosure.
however, just moving the box into different positions in the vehicle can change the loading of the subs, this is just one way impedance curves and spikes can be noticed.
the only real point to my first post was that when i burp my subs the amps see 1ohm, even though the subs are .5
i can see that you've got a handle on how box tuning and cabin resonant frequencies should parallel one another, and most all other variables that come into account when trying to achieve maximum spl.
i appreciate your input on re-tuning my box, but my box is already tuned to the exact cabin resonant frequency of my car. under extreme sound pressure levels the cars frequency can change. at 160db (termlab), my car tunes too 63:-) i have no choice but to buy lower impedance woofers for this application. wish me luck at finals glass. btw, nice to hear from ya again, the last time i posted when you were around was last winter. i forgot my password so i had to change my screenname from zacdavis to zacdavis~, go figure:-) remember motoman22, hydro, n all those guys? well anyways nice too hear from ya again.
 

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 4717
Registered: Dec-03
sounds good.
glad to see ya back again.
hope finals go well.
speaking of physics, yeah at 160dB or higher, I'm sure the change in pressure in the vehicle's cabin can even alter the Fs of the vehicle among other things. Considering hight above sea level plays a big factor, and the pressures reached in the car cabins car be right up there as well, but short of finding a way to read barometric pressure in the cabin when burping the system, and calculating those effects, it'd be hard to compensate accurately for that sort of thing.

here's a thought.
contact the mfgr of your subs, and see if you can get them to custom wind some voice coils for your subs.
specify the exact impedance you're after and explain why.
Hell they may even sponsor the job if they like the idea.
worth a shot anyway. I know some companies like RE will custom wind coils if you contact them. Their subs are usually dual 2 ohm, but they offer dual 4 ohm on request.
 

Silver Member
Username: Zacdavis~

Post Number: 165
Registered: Sep-04
you've hit the nail right on the head, i experienced an audio control mic get 2db tighter within 8hours because of a storm that rolled in, humidity. funny how much weather can compromise an accurate reading, thats just one reason the termlab is so much better, it has proven to be somewhat resiliant in comparison.(to the a/c)
oh ya, i've already ordered dual .5 subs, they'll be within ten thousanths of an ohm :-)
gotta love DD
i heard you've built a mopar, sounds pretty radical
(I spelled resiliant wrong didn't I?)
 

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 4761
Registered: Dec-03
you're fine.
I'm working on one now, yeah. an old '66 charger.
www.wickedcases.com/charger/

what better way to get a good solid basis for a sound system, than good old detroit steel.. buried in spray-liner hehe should give a pretty solid piece of groundwork for the audio system. don't have to worry about plastic vibrating or thin sheet metal heh
Just need the system loud enough to drown out that 493 stroker.
 

Silver Member
Username: Insearchofbass

Post Number: 883
Registered: Jun-04
i have to say ive been doing my homework over the past 2 years on spl stuff and reading this thread has added to my knowledge base thanks zac and glass.
 

Silver Member
Username: Fishy

Tamarac, FL USA

Post Number: 576
Registered: Sep-04
It confused me.

:-)

-Fishy
 

Silver Member
Username: Insearchofbass

Post Number: 884
Registered: Jun-04
reread it fishy i know you can understand it....i know i know more stuff to remember but put it all together and your more prepared to get the most from your audio system
 

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 4790
Registered: Dec-03
may help to have a notepad handy so you can graph what he's talking about to visualize it.
 

Bronze Member
Username: Slap_johnson

Arizona

Post Number: 77
Registered: Aug-04
zac, you hit 160 on the new termlab :o, thats extremely loud, seans right, this thread did really increase my knowledge about spl, knowing very little about it and only designing 1 spl box that took forever to design/make/install for one of my teachers lol, it did help a lot that he had a degree in engineering and knowing quite a bit about physics, it was an enourmous 6th order bandpass with a DD9912!!! running off a crossfire vr2000d in a civic si hatch i think? how this man makes this much money, i dont know, but anyways, the box was tuned to 57 hz i believe (resonant frequency of cabin), and did very well hitting 157 on audiocontrol, very proud the day we finished that enclosure. on a side note, i think it is abosolutly hilarious that a teacher has the loudest system on campus and probably top 3 in the city, sounds absolutly terrible though and cant sit in the thing for more than like 5 sec. oh and best of luck to you at finals
 

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 4845
Registered: Dec-03
that sounds like a 6th order for sure hahaha

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