Electromagnet impedance vs transformer impedance

[nix85]

at the some kHz and over 10 kHz the impedance may increase much higher of course.

Do you think this is the main reason for amp power drop of 1.5-2 decibels at ~20KHz?

[gyulasun]

I am not an expert on loudspeakers,  so cannot give you a correct answer for your question. If you google for    loudspeaker coil inductance   or   voice coil inductance,   you will find some good papers on the behaviour of speaker coils at the higher audio frequency ranges.  It seems a complex problem.   
Consider the effect of cross over networks (if used such) too. 
Amplifier power drop : if you use a dummy load instead of loudspeaker(s),  then you can check the amplifier behaviour first, you surely know this.

[nix85]

Consider the effect of cross over networks (if used such) too. 
Amplifier power drop : if you use a dummy load instead of loudspeaker(s),  then you can check the amplifier behaviour first, you surely know this.

Sure, there are other factors too why amp power drops, some mentioned here. https://answers.yahoo.com/question/index?qid=20190510203041AAZbp3b

Coil Inductance Calculator
https://www.allaboutcircuits.com/tools/coil-inductance-calculator/

[nix85]

What citfta replied above is simplistic and does not answer the question. Of course it draws more current cause it is doing work, as it is in a sense a linear motor. But that is not the answer to why most of speaker coil's resistance is ohmic resistance.

As i wrote in both cases, transformer and the speaker, just like in all kinds of motors, reason for increase of current when doing work is due to demagnetization aka lenz, backemf across the working inductor falls and more current rushes in. Formula for this is Ia=Ve-Eb/Zs where Ia is the current, Ve is the input voltage, Eb is the backEMF and Zs is the impedance. Reason why most of speaker coil impedance is DC resistance aka heating is due to the fact speakers do not need much power to produce relatively loud sound, so relatively high resistance wire is used to limit the current, otherwise it would be like a short at lower frequencies. Not to get into the fact that at lower frequencies power must be increased for human ear to hear the bass at same level as higher frequencies (higher energy) considering higher frequencies are more energetic and also our hearing is logarithmic.

Similar example for light would be LED where big resistor is often used to limit the current and 1W LED may have a 10W resistor limiting the current to acceptable value, so 90% of energy is wasted into heat. Of course there are more efficient ways like inductive current limiter, so inductive reactance is used instead of ohmic resistance.

[floodrod]

Deleted.   I should have known better. LOL