How does audio get transmitted from my iPod to speakers?

Question

I need some help understanding a simple concept. I understand that my iPod/any other audio device sends electrical signals to my speakers/headphones.

From my research so far, an analog signal is sent. Now what I need help understanding is that what is the analog component of the signal? In other words, is it the current that is varied and the voltage remains the same, or the other way around?

I haven't found a definite answer, but it seems to me that the current is what fluctuates, and voltage remains constant.

Under that assumption, would that mean that resistance also fluctuates as a direct result to balance the equation V=IR?

Sincerest thanks :)

Answer 1

Audio signals are usually intended to be voltage mode. In other words, the ideal source for a audio signal has 0 Ohms, which means its voltage doesn't vary regardless of what the load does. Of course 0 Ohms is not possible in reality, but there are various levels of close enough.

Power amplifiers designed to drive speakers have very low output impedance. Speakers are usually rated at 8 Ω, so audio power amps have output impedance a fraction of that. This is done by feedback internal to the audio amp. The feedback adjusts what the final output stage does so that the voltage immediately leaving the amp is what it's supposed to be, largely not effected by the current the speaker draws. Again, 0 Ω is not possible, but good audio power amps have output impedance less than 1 Ω. It's not worth going less than a 100 mΩ or so since the wires between the amp and the speaker will add more than that.

Speakers are designed so that the sound output relfects the applied voltage, not the current they draw from that voltage. Another way of looking at this is that the speaker impedance is not a flat 8 Ω accross the frequency range, and it's not purely resistive either. If it were a purely resistive flat 8 Ω, then it wouldn't matter whether you drive it with a voltage or current signal since the two would always be proportional to each other. However, it's not a flat 8 Ω, so voltage and current aren't the same. The industry has converged on using voltage as the true audio signal, with current being whatever it ends up at roughly 8 Ω but with significant variation accross the frequency range and some reactive component. If you were to drive such a speaker with a current signal, you would get more distortion and poorer frequency response than with a voltage signal.

Answer 2

If you look at it from the standpoint of the wires going to the speaker, the voltage and current are both fluctuating, in a ratio given by R=V/I. (The speaker isn't a true resistor, but close enough for this explanation).

What you've described, holding the voltage constant and varying the resistance to produce a varying current, is actually the job of the transistor(s) inside the music player. Again, probably not that simple inside your iPod, but hopefully you get the point.