How the hell are we using ohms law for speakers?

[DIGIDIGIBOMBO 10+ year member]

Hi, how are we saying that P=V.I =v^2/R and that V=R.I, well we have AC current not DC and also the speakers voice coils are inductors not resistors right? . With AC current the voltage and current are not in phase when we have voice coil ( inductor) its not a simple resistive circuit. Can we consider the speakers coil as a simple resistor? just take the resistance value in ohms and use simple ohms law? why??? Thx

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[DIGIDIGIBOMBO 10+ year member]

anyone?

 

[SAG3 5,000+ posts]

the question should be, why the hell would we not use ohms law for speakers?

 

[snoopdan 5,000+ posts]

Hi, how are we saying that P=V.I =v^2/R and that V=R.I, well we have AC current not DC and also the speakers voice coils are inductors not resistors right? . With AC current the voltage and current are not in phase when we have voice coil ( inductor) its not a simple resistive circuit. Can we consider the speakers coil as a simple resistor? just take the resistance value in ohms and use simple ohms law? why??? Thx

it sounds like you answered your own question....it is a resisitive value, lol

 

[DIGIDIGIBOMBO 10+ year member]

How can we consider the inductor ( voice coil) a simple resistor? in AC current the power P=V.I.Cosine(Phi) and the cosine(phi) is equal to 1 only when we have a pure resistor not inductor:S there is the apparant power, real power etc...so heh i am lost baby. we are considering the inductor as if it is a simple resistor used in a DC current system. Thx

 

[christiankills 10+ year member]

sorry buddy... the only reason i passed physics was because i was in PC support 3...

we were pretty good with computers //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

really wanted to learn physics, except the way the teacher taught.. I tried to learn.. it was just so hard to learn from him. 90% of the class pretty much agrees

 

[PESteele 10+ year member]

because we like to listen to music not think about it

 

[thylantyr 10+ year member]

How the hell are we using ohms law for speakers?

In regards to what specific issue? Are you trying to match speaker power

ratings to amplifier ratings ?

 

[thch 10+ year member]

Hi, how are we saying that P=V.I =v^2/R and that V=R.I, well we have AC current not DC and also the speakers voice coils are inductors not resistors right? . With AC current the voltage and current are not in phase when we have voice coil ( inductor) its not a simple resistive circuit. Can we consider the speakers coil as a simple resistor? just take the resistance value in ohms and use simple ohms law? why??? Thx

thats actually why. because it is simple and "good enough" to give the results we want.

in engineering there is a philosophy of "close enough". basically everything is a model. if you buy a 100ohm resistor, its not really a 100ohm resistor. it has a temperature coefficient, and parasitic inductance and capacitance. it also has length, making it into a distributed element.

but for most applications this doens't matter. just assuming all the parasitics are small will allow useful and accurate analysis.

i will tell you that the simple inductance isn't the core issue. if you look at the impedance plot, you will notice a huge peak due to the mechanical elements. in free air, the peak should have a Q of Qms, IIRC. i'm not 100% sure on that, but the charts i've seen seem to indicate such. this even makes sense. the mechanical elements can push or pull the coil through the gap. this allows mechanical energy to be transformed into electrical energy.

the inducatance is actually nonlinear, as are some of the mechanical elements which get transformed into things that affect the electrical model. nonlinear differential equations are generally not analytically solvable. this makes it even harder to find a "better" solution.

you are 100% correct in saying that actual power losses will not be extremely accurate if the speaker is modeled as a simple resistor. but its still useful for analysis.

i encourage you look up some of the T/S models for various boxes. they are in the help section of winISD IIRC.

 

[bogie418 10+ year member]

-An inductor has something similar to resistance. The term is reactance and it is use when calculation non linear power factors such as induction and capacatance.

The speaker varies its polarity +- then -+ etc to generate the sound waves. In this expanding/collapsing magnetic field is a 'resistive' force, but because its 90 degrees out of phase we have to call it something else -reactance. Depending how poor (inductance) or great (capacitance) the power factor angle is will depend on the amount of reactance. This is the short and dirty version, because it only gets deeper and deeper. I think its such a small power factor, with the magnet helping the situation, that the reactive portion is considered negligent. Either that or its just to **** confusing for everyone so they've simplified it for us.

 

[helotaxi 5,000+ posts]

A speaker is a reactive load. We've established that. As a reactive load it has an impedance rather than a resistance value. The value of the impedance varies with frequency. If you want to plot the impedance curve of the driver in its enclosure (because the enclosure affects the curve) and then plan your system according to that, go right ahead. It won't be worth the trouble for a normal system. Use the nominal impedance as a resistance value (it will most always represent the min impedance iving you a margin of error for amp stability) and call it a day.

There are two situations where the "good enough" approach isn't. When you are building passive filter networks, the impedance curve has to be considered and a Zobel network added to make a nice filter. SPL competetiors also have to consider impedance rise and account for any variances in impedance around their peak SPL freq. If you do not fall into one of these two categories, I wouldn't worry about it.

 

[DIGIDIGIBOMBO 10+ year member]

Thx for all of u who have replied...i only set my amplifier gains by ear( listening to music) i always had very very poor results when i have used those calculations+ test tone cd of 0dB, i always set my gains very far from what the DMM method requires....and i have never burned any speaker.Thats when i started to think that it may not be appropriate to use these calculations Thx again

 

[thch 10+ year member]

@DIGI

exactly. the ear is the final judge. the DMM/Oscope are just tools.

@bogi

normal inductance is linear, as is capacitance. basically, inductance has reactance based only on frequency. to be nonlinear, it would need a reactance based upon something else as welll, noramlly based upon amplitude.