Did i calculate this right?

Bronze Member

Username: Infinity_addict

Santa Rosa, California
USA

Post Number: 72
Registered: Jul-05

Posted on Thursday, August 11, 2005 - 03:03 am:

k, so im lookin at the manny for my head unit. sez it puts out 22w rms at 4ohms. so using ohm's law i calculate voltage (voltage = square root of watts x ohms). so i get v=9.38...

where im going with this: i blew my 3.5" speaker. so i replace it with a bookshelf speaker from an old stereo. this speaker is 6ohm. once again using ohms law i calculate for watts this speaker will be recieving. watts=volts squared divided by ohms. i get 14.666666.... watts.

did i do all the math right? will the speaker be given 14.6 watts rms?

Gold Member

Username: Glasswolf

Wisteria, Lane
USA

Post Number: 9095
Registered: Dec-03

Posted on Thursday, August 11, 2005 - 11:45 am:

er yah, but does it raelly matter?
it'll just be quieter than a 4 ohm speaker would, but you already knew that :P

Gold Member

Username: Glasswolf

Wisteria, Lane
USA

Post Number: 9097
Registered: Dec-03

Posted on Thursday, August 11, 2005 - 11:48 am:

oh and your math didn't account for efficiency of the head unit's amplifier, or the curve of that efficiency at various loads, etc.. but it's close enough.
you know the amp is 22 watts @ 4 ohms.
you know that'd be 11 watts at 8 ohms, give or take.
6 ohms is half way between that, so all you had to do is add 11 and 5.5 t get about the same result at 6 ohms. much less work :-)

Bronze Member

Username: Infinity_addict

Santa Rosa, California
USA

Post Number: 74
Registered: Jul-05

Posted on Thursday, August 11, 2005 - 02:06 pm:

yah, but isn't it true that halving the load doesnt give exactly 2x the power? this is true cuz theres a square in the formulas, yah? if i had some response curves for the HU, i think i might've used em ;-)

yah, i know it's quieter, but i'm happy the crackling of the old speaker is gone!! maybe someday i'll get some real components! ha!

Silver Member

Username: Touche6784

USA

Post Number: 589
Registered: Nov-04

Posted on Thursday, August 11, 2005 - 02:50 pm:

W=V*I
V=I*R I=V/R
W=I^2*R=V^2/R
halfing load doubles amperage assuming voltage remains the same. so formula becomes:

W = (2I)^2 * R/2
= 4I^2 * R/2
= 2(I^2 * R/2)

that had nothing to do with the square. what you talk about has to do with limits in amplifier tech, not physics.

Bronze Member

Username: Ghowards

Clark, SD
USA

Post Number: 28
Registered: Jul-05

Posted on Thursday, August 11, 2005 - 03:02 pm:

hey does anybody know of any brands that make just 7'' component speakers. I have a 2003 eclipse and it has 7'' front speakers and 6x9 rear but all I can find is 5x7's. Anybody know of any please let me know. preferably if they have 2 ohm speakers. Thanks.

scuba steve

Silver Member

Username: Scubasteve

Annapolis, MD

Post Number: 382
Registered: May-05

Posted on Thursday, August 11, 2005 - 03:13 pm:

i'd say just go for 6.5s and make or buy a mounting bracket for them to fit your car. Anyway, your car should have stock 6.75s, not 7s.

Bronze Member

Username: Ghowards

Clark, SD
USA

Post Number: 29
Registered: Jul-05

Posted on Thursday, August 11, 2005 - 03:18 pm:

ill prolly have to buy a mounting bracket. is that something I can just get about anywhere. no the 2000-2003 eclipse GTS has factory 7'' front and 6 x 9 rear. the 95-99 had 6.5

Bronze Member

Username: Ghowards

Clark, SD
USA

Post Number: 30
Registered: Jul-05

Posted on Thursday, August 11, 2005 - 03:27 pm:

ok steve the more I look it looks like it could be 6.75's. The chart I found said 7'' but ill prolly trust your word over that. Is there anyway to check it for positive.

scuba steve

Silver Member

Username: Scubasteve

Annapolis, MD

Post Number: 386
Registered: May-05

Posted on Thursday, August 11, 2005 - 04:43 pm:

I got 6 3/4 by looking it up on a site. Scosche has a site, so does crutchfield i think. The way to know if speakers will fit without modification is to measure the diameter of the hole in you car and compare it to the listed cutout diameters for different speakers.

Gold Member

Username: Glasswolf

Wisteria, Lane
USA

Post Number: 9113
Registered: Dec-03

Posted on Thursday, August 11, 2005 - 04:53 pm:

yeah how close to doubling the power an amplifier gets by halving the load depends a lot on the amplifier's power supply design.. regulated or not etc.. some are better at coming close to twice the power tahn others.

Silver Member

Username: Touche6784

USA

Post Number: 591
Registered: Nov-04

Posted on Friday, August 12, 2005 - 11:42 am:

infinity, i just looked at your calculation again, and you may have gotten the right wattage but you did it completely wrong. the voltage is supposed to be 14.4V, not what you got. you were supposed to calculate for amperage, voltage does not change persay, same goes with home audio or most electronics for that matter. and you plan to get a certification for this kind of stuff? wouldnt want you near my car.

Bronze Member

Username: Infinity_addict

Santa Rosa, California
USA

Post Number: 76
Registered: Jul-05

Posted on Friday, August 12, 2005 - 12:40 pm:

instead of insulting somebody who is new to this, maybe show me how to calculate it correctly?

btw, you essentially just told glasswolf you think he's an idiot too because he agreed with my calcs...

Silver Member

Username: Touche6784

USA

Post Number: 593
Registered: Nov-04

Posted on Friday, August 12, 2005 - 02:01 pm:

and? all people make mistakes or slips here and there. if you have an interest in something such as electronics i would hope that you have some knowledge in E&M. you used the right formulas and got the right answer but there was some extra thought that needed to be made to do it "correctly". if you are serious about becoming a pro installer i would think that you have bought books, take classes, learning something about electronics outside of an online forum and i would think that you are learning things there. if you just got into it and dont know a thing, well then my mistake. if you cant take a correction and some criticism then i think you should grow up because you go out to work and you screw up something really simple, you will get your @$$ handed to you.

Bronze Member

Username: Infinity_addict

Santa Rosa, California
USA

Post Number: 78
Registered: Jul-05

Posted on Friday, August 12, 2005 - 02:05 pm:

i thought i took it very well...

you still haven't shown me how to do it "correctly"...

Silver Member

Username: Touche6784

USA

Post Number: 594
Registered: Nov-04

Posted on Friday, August 12, 2005 - 02:17 pm:

now you want to be spoon fed? i gave you enough to figure it out. you just have to remember that voltage is a constant. re-read my posts and maybe you will figure it out.

Aaron Hobbs

Silver Member

Username: Hellbender

Los Angeles, California
USA

Post Number: 163
Registered: Apr-05

Posted on Friday, August 12, 2005 - 02:32 pm:

Christopher:
There's a big difference between positive criticism and insulting someone. You're comment was unnecessary If your true intentions were to simply correct his mistake and teach him the right way to do it.

Silver Member

Username: Touche6784

USA

Post Number: 596
Registered: Nov-04

Posted on Friday, August 12, 2005 - 02:43 pm:

well then my appologies. everyone have smiles on their faces now?

Bronze Member

Username: Infinity_addict

Santa Rosa, California
USA

Post Number: 79
Registered: Jul-05

Posted on Friday, August 12, 2005 - 02:47 pm:

thank you!

i don't think he knows anyways... he's posted more than enough times to just tell me and be done with it. i think my outcome was accurate enough.

Aaron Hobbs

Silver Member

Username: Hellbender

Winnetka, California
USA

Post Number: 164
Registered: Apr-05

Posted on Friday, August 12, 2005 - 02:51 pm:

Why cant we all just get along? lol.....

Silver Member

Username: Touche6784

USA

Post Number: 597
Registered: Nov-04

Posted on Friday, August 12, 2005 - 03:06 pm:

its always easier to call someone else stupid when you cant figure it out yourself. your calculation was correct and i think i might have to appologize for making that criticism. i forgot that often in physics there is more than one way to get the answers. my way was using current as my main number rather than voltage. your way is faster.

Gold Member

Username: Glasswolf

Wisteria, Lane
USA

Post Number: 9145
Registered: Dec-03

Posted on Friday, August 12, 2005 - 06:18 pm:

calculate for current as your variable, and voltage as a constant (12 volts since the power supply in the head unit is usually regulated anyway) is what he wanted you to spot, I believe.

anyway my way is still easier.. just pick the wattage half way between 4 and 8 ohms, since ya already know the power would almost double at half the load.. you can shave a couple watts off to account for the efficiency curve and you've got an answer you can do in 10 seconds in your head :-)

anyway, I can be an idiot at times, too *shrug*

Bronze Member

Username: Infinity_addict

Santa Rosa, California
USA

Post Number: 81
Registered: Jul-05

Posted on Saturday, August 13, 2005 - 12:57 am:

thanks for the apology. peace!

Bronze Member

Username: Ghowards

Clark, SD
USA

Post Number: 31
Registered: Jul-05

Posted on Monday, August 22, 2005 - 12:21 am:

I am looking for a set of component speakers that will hit hard for rap songs but will sound good for rock songs. The two im looking at in my price range are kicker ks series or eclipse. What do you guys reccomend and do different brands work better for rap vs. rock songs?