I have an auxiliary cable with 3.5mm jack plugs at both ends.

I want to cut one jack plug off to connect the audio signal to an external speaker.

When I strip the outer sleeve, there are three sleeved cores: red, white and black.

I assume right channel, left channel and ground?

Which one is which and how can I connect these to a speaker.
I mean a naked loudspeaker, with magnet and terminals etc. exposed.

TiA

Depends. Which channel do you want to listen to – Left or Right?

Not bothered!

It also assumes that it is a headphone output rather than a signal (line) level output.

Normally connect black to one terminal and red to the other

To make it clear, I’m trying to make the homemade speaker from a bottle, neo magnet, coil of wire. I want to connect my phone to it but don’t know what to connect to the coil.

Connect black and red to two ends of coil

@leffeboy

I’ve tried that and it didn’t work.

Are you following some instructions for your construction? Size of wire, number of coils, etc?  And do these instructions suggest your source is suitable?

Do you have a multimeter or continuity tester that you can test from the jack? On a three-connector stereo jack, tip and ring are left / respectively and the body is ground.

White / red would be left / right in the phono world and black is logical for ground so that would be my guess, But the simple fact is that the wire colours are whatever the cable manufacturer decided to use.

I’ve tried that and it didn’t work.

Far too many variables to suggest why beyond guesswork. Is your speaker actually known to work? Was the cable a known good before you butchered it? Is the source level sufficient to drive a speaker that size or do you need an amp? Etc, etc.

YouTube videos.

I’ve tried that and it didn’t work

Most likely then your phone isnt strong enough to drive the coil.  It will depend on the impedance of the coil and that will depend on where it is placed relative to the magnet

To check if you have the correct two wires just take a normal set of headphones that work with your phone and touch the red wire to the tip of the headphone connector and the black wire to the metal part of the connector nearest the plastic if that makes sense. You should get sound if you have the wires correct

You need an Amplifier. or a very easily driven speaker.

Would an aux cable have ground? Third wire could possibly be mic?

But I’d second (third, whatever) your phone not having enough oomph to power the coil.

Would an aux cable have ground? Third wire could possibly be mic?

Yes. A 3.5mm stereo jack has three connectors, a headset with microphone has four (tip, two rings and sleeve) and for added LOLs there’s different standards.

Ah, pretty pictures:

https://www.cablechick.com.au/blog/understanding-trrs-and-audio-jacks