posted
ok, i finally got off my butt and did some measurements (with the doors open...but with em closed the impedance goes up so at this point i'm not concerned).
Correct me if i'm wrong but when measuring for impedance (taking V and I measurements) it doesn't really matter at what volume right? if i'm wrong then back to the vehicle i go...
anyway...i took readings at 5Hz increments from 20Hz to 60Hz and every Hz from 27-33Hz cuz my box is supposed to be tuned to 30Hz.
However, upon calculating (using Z=V/I) i came up with a maximum impedance at about 40Hz. my other computer with all my saved info on this stuff is down right now otherwise i prob wouldn't be asking this stuff.
a ported box has a maximum impedance at the tuning freq right? or was it a min at tuning and a rise to either side of that freq?
anyway...the lowest impedance that i calculated was a measly 8.56 ohms! can this be right or does it have to do with the volume level i was using (very low, 3 or 4 out of 33 due to neighbors)?
i have each of my subs (2 ohm DVC SHockerII 15's) in their own 6 cubic foot chamber after displacement, s'posedly tuned to 30Hz with 30sq. inches of port area.
at 30Hz i measured 29.6mA and 306mV so that should be 10.34 ohms right? oh, the VC's are wired in series and the subs in parallel if that makes a difference.
help! thanks
if these are right i'm gonna wire my subs down to a .5 ohm load and check the impedance for that. these subs are way loud for the power that they're receiving if i'm right...hehehe
------------------ Team "Under Pressure"--now in Minnesota for the summer with the stroke of a cone, i can make you moan wall w/two shocker15's and a cadence a7hc, soon to be 2 class D's
[This message has been edited by alaska (edited 07-07-2001).]
posted
hmmm....maybe not...forgot about the DCR. Chris M. reminded me when i was reading a thread about running crossfires at lower than 1 ohm.....still, any help plz
------------------ Team "Under Pressure"--now in Minnesota for the summer with the stroke of a cone, i can make you moan wall w/two shocker15's and a cadence a7hc, soon to be 2 class D's
posted
For a ported box, the lowest impedance is the tuning freq. the impedance will peak on both sides of the tuning freq also. As far as the min impedance being higher than you expected, What did you use to read it? I would do some tests to see if the measurements where accurate. Try a know resistor value and measure the current, and try measuring the voltage across a resistor in series with the subs to determine the current. Maybe your meter has a problem measuring current accurately if the load is inductive.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1247 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
it's a Fluke multimeter...i know the voltage measurements are right...but it's only good up to 300mA i think.
------------------ Team "Under Pressure"--now in Minnesota for the summer with the stroke of a cone, i can make you moan wall w/two shocker15's and a cadence a7hc, soon to be 2 class D's
posted
if you wanna measure the impedance of the driver try this.. http://www.installer.com/tech/sit.html I set this up tonight and with some playing I got it very close to accurate (closer than most other methods anyways) it gave me some interesting results anyways.. right coil left coil 20hz=3.69 20hz=3.58 25hz=6.07 25hz=5.89 30hz=12.12 30hz=11.85 35hz=3.78 35hz=3.72 40hz=3.51 40hz=3.41 45hz=6.21 45hz=6.19 50hz=7.15 50hz=7.02 55hz=4.93 55hz=4.81 60hz=4.45 60hz=4.3 65hz=3.89 65hz=3.82 70hz=3.19 70hz=3.12 75hz=2.92 75hz=2.86 80hz=2.85 80hz=2.79
------------------ Digital Designs 9515b Memphis 1000D Pioneer source MB Quart fronts ADS power plate (soon Adcom 4404) RW Audio processors LD4.12MBR Member of Team Hertz Canada
Posts: 735 | From: Calgary AB | Registered: Oct 1999
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posted
yeah, i've read that....just haven't tried it.
just have to make sure that the resistor can handle the wattage being supplied, even at low power levels
------------------ Team "Under Pressure"--now in Minnesota for the summer with the stroke of a cone, i can make you moan wall w/two shocker15's and a cadence a7hc, soon to be 2 class D's
quote:Originally posted by Atomic Fusion: Ehh dank, you got that bass ackwards. It's highest impedance at tuning not lowest :P
Adam
Hmm... not the last time I checked
The subs will reach their minimum impedance (never below dcr) at the effective tuning freq of the box in the vehicle. It will have a impedance spike on either side of this.
------------------ Nathan Munson Eastern Audio
Worlds Loudest Certified 1 woofer DB Dragster -- 170.8 in Vegas
posted
Here is a response graph for the Scan Speak 25W/8565 woofer. The rough lines are (obviously) frequency response and the smooth line that rises as it moves into the 1 khz+ range is impedance. Notice the large peak just shy of 30 Hz. That is the fs of the driver, and that's the spike I'm talking about. Same thing happens at tuning frequency. http://www.scan-speak.com/scs_prod/woofers/freq_25w_8565-01.gif
Adam
------------------ System: Undecided, as usual... (__I__) HAIL CARTMAN!
posted
Nope, In a sealed enclosure the peak IS determined by the Fs of the driver, you are correct, but in a ported enclosure, their are 2 very distinct impedance peaks on either side of the tuning freq, with a dip AT the tuning freq.
------------------ Nathan Munson Eastern Audio
Worlds Loudest Certified 1 woofer DB Dragster -- 170.8 in Vegas
posted
Adam, that graph is probably for free air. When you put it in a ported box you will get two resonance’s. One above and one below tuning. Basically, the speaker is nothing more than a mass and spring. This mass and spring is usually modeled as an electrical equivalent circuit for analysis. Once that is done you can treat it as a circuit and everything is just math from that point on. Those peaks in the impedance curve are when the equivalent inductance and capacitance of the system start exchanging energy with each other. This is called resonance. You can put very little energy into the system and the mass will move like crazy. Trust me, I’m an engineer. The impedance is minimum at the tuning frequency.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1247 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Ah okay I found an impedance plot of a woofer in a ported box. It's tuned to 22 Hz and there is a peak at about 13 Hz and 38 Hz. So you're correct. My deepest apologizes Dank
Learn something new every day.
Adam
------------------ System: Undecided, as usual... (__I__) HAIL CARTMAN!
posted
I thought it would be minimal at tuning frequency because the damping on the speaker is greatest. The little motion by the speaker creates less counter-EMF, which would result in a lower impedance. I guess thats a explaination, not sure if its right...sounds good to me tho
------------------ www.lowfreqaudio.com Team S.O.S "We're still Badass" 1 C-V Stroker 18d4 2 Cfa-1000ds 156.9 outlaw
posted
from the measurements I took, ea1 is right,the HIGHEST impedance was at the freq the port seems to do most of the work.. this was not the port tuning freq but about 10hz higher, by ear this is the loudest freq, even though the impedance is highest. but the box is tuned to 40hz, (pretty much the lowest impedance on my scale) so as he said there is a spike below 30hz for me which corresponds to the fs of the driver, and 50hz which is where it seemed loudest.. at port tuning 40hz the impedance is lowest (in that range it does drop at 70+ though) things would probably change at high power though, because I took the same set of measurements with my hatch open and the numbers were VERY different, so an increase or decrease in presure in the box/car would probably change the impedance of the driver at some freqencies, this may be why some ppl have a hard time getting louder after they break the 160-170 range, impedance may rise (or decrease) as pressure increases. something to think about anyways.
------------------ Digital Designs 9515b Memphis 1000D Pioneer source MB Quart fronts ADS power plate (soon Adcom 4404) RW Audio processors LD4.12MBR Member of Team Hertz Canada
Posts: 735 | From: Calgary AB | Registered: Oct 1999
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quote:Originally posted by ea1: Nope, In a sealed enclosure the peak IS determined by the Fs of the driver, you are correct, but in a ported enclosure, their are 2 very distinct impedance peaks on either side of the tuning freq, with a dip AT the tuning freq.
I've got to agree with Nathan on this one. His comments are 100% inline with all of the impedance plots that I have ever done over the years. I started doing impedance plots back in 1993 when I started working with AP setups and found that the information I gathered on them was also very useful in vented and sealed applications as well. Impedance plots for sealed and ported enclosurs inherently different.
------------------ Chris Dilbeck Ampman Audio Sales/Technical Support http://www.ampmanaudio.com
quote:Originally posted by RWAudio: I took the same set of measurements with my hatch open and the numbers were VERY different, so an increase or decrease in presure in the box/car would probably change the impedance of the driver at some freqencies
The Accoustic loading on the subwoofer system will change the impedance plot. With the interior of the vehicle closed up the loading on the subwoofer system will be greater and it will definitely change the impedance plot. When I used to do a lot of AP setups in cars I soon realized that the AP membrane tuned outside the vehicle never performed as well as one that was tuned in the vehicle with the doors,trunk,window closed. Of course this was for SQ, but the same principles apply in relation to impedance plots.
------------------ Chris Dilbeck Ampman Audio Sales/Technical Support http://www.ampmanaudio.com
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