18" IB3 Impedance Curve Questions

[onebadmonte 235]

This is my setup: A US Acoustics USB-2150 rated for 450 watts x 1 @ 4 ohms powering an 18" Fi Audio IB3 at 4 ohms. I have a seemingly solid install now, but my score has stayed the same, 132 and change on both the termlab and my spl labs lcd bass meter. Check out my build HERE. I decided to test the output of one of my sub amps. I tested with tones from 25hz to 65hz. I checked the AC voltage output of the amp. Simultaneously I checked the output amperage to the sub at the positive wire to the sub. With these numbers I calculated the wattage by multiplying the volts times the amps. I also calculated the impedance by dividing the voltage by amperage.

This is what I got

25Hz 53.0Volts 3.4Amps 180.2Watts 15.58Ohms

30Hz 48.7Volts 4.0Amps 194.8Watts 12.17Ohms

35Hz 47.3Volts 4.8Amps 227.0Watts 9.85Ohms

40Hz 46.6Volts 5.2Amps 242.3Watts 8.96Ohms

45Hz 46.7Volts 5.1Amps 238.1Watts 9.15Ohms

50Hz 47.2Volts 5.1Amps 240.7Watts 9.25Ohms

55Hz 47.9Volts 5.0Amps 239.5Watts 9.58Ohms

60Hz 48.5Volts 5.0Amps 242.5Watts 9.70Ohms

65Hz 49.1Volts 4.9Amps 240.5Watts 10.02Ohms

My subs are 4 ohm. Looking at the impedance curve I'd be inclined to say that these are 8 ohms subs. Is this normal for the impedance curve of a 4 ohm sub to be so high? I disconnected the sub and it checked in at 4.2 ohms. Are there other factors contributing to this? Do you think the seat back and small volume of air in front of the sub affecting the impedance curve much? What else should I be looking at?

[Fi Car Audio 64]

Seat back and small volume are definitely changing the curves. You are somewhat running a reverse aperiodic chambered enclosure. This will not model as a true IB due to the front loading of the sub. Remove the seat and things will change.

DCR should be 3.8-4.0 Ohms on a meter than reads 0 when crossed out. We are slightly higher on the DCR with these as they are normally in true IB applications in home audio 99% of the time vs car. If there is no way of working around the seat loading... we might need to look at a recone with a custom coil that knocks the DCR down lower and gets things a little closer to 4Ohms at the mins of the curve.

Thanks,

Scott

[REVpuggie 23]

HMMMM , I'd say that that impedance curve is spot on to what I'd expect from a Woofer with an Re of 4ohm on sinewaves and impedance rise taken into account .......

[onebadmonte 235]

I've metered the system with the seats in and with the seats out. The measurements were with in tenths if a db. Unless there is anything else I can do it looks as if the fat lady has sung. No offense Scott, I'm sure you a good looking guy. Do you think the 2 ohm sub would have been a better option?

[REVpuggie 23]

don't think impendence is really an issue here .

try metering it with the trunk open (and doors shut offcourse) ... how big is that trunk ,10 or 11 cuFt ..... way to small to house two of those beasts and you are in fact smothering them

.... maybe trunkspace was stated somewhere in your other topic but did read it entirely ... .if it were 11cuFt Qtc would be somewhere around 0,93 ... you need to get it down to 0,7 (36 cuFt) at least ... 0,5 would be my goal , ther is no way you're going to achieve that in a trunk for 2 18inch IB woofers ... my suggestion would be make a hole (or two) what is short and big enough not to act as a port to the outside .

test with trunk open and see how that goes ... might also be an idea to listen to your system and how it sounds ... if and when that works you could make the ventilation holes that are often behind the bumper bigger and if there are flaps remove them for starters .

[Fi Car Audio 64]

Send me an email to scott(at)ficaraudio.com and we get some things squared away.

Thanks,

Scott

[onebadmonte 235]

e-mail sent