Sounds are vibrations in air, which can be thought of as waves of varying pressure. Air can only be one pressure at a given time and place, which means that multiple sound frequencies merge together through something called "interference". There can be destructive interference where a peak of a wave encounters the trough of another and cancel each other out to some extent, or constructive interference where two peaks or two troughs encounter each other and combine to form a higher peak or deeper trough.

Multiple sound frequencies then all combine to form one complex sound wave which a speaker can then produce.

Simple soundwaves (sinewaves) of various frequencies add up to a single complex waveform. A single guitar note, for example, is a complex.waveform made up of various simple waves of different frecuencies. This is the Fourier theorem.

So a speaker reproduces a single waveform, which is the sum of all the frequencies of whatever it is playing.

A speaker doesn't "know" frequencies. It is a device designed to create a pressure wave in the air that we interpret as sound. So it moves according to the signal given.

The signal itself is a single wave that consists of a combination of waves of different frequencies.

That's it.

For a non ELI5 explanation, this would require some knowledge of waves and possibly Fourier transforms etc.

How can a single ear drume in you ear hear multiple frequencies at the same time?

Sound is a pressure in air and when the pressure waves from multiple sources interact you get the sum of all those waves. So the pressure wave that reaches your ear is a combination of all sound sources and results in a single pressure on your eardrum at each moment in time. Over time the eardrum move following the combined pressure of all the sound waves.

If a force a speaker membrane to move just like your eardrum did it will produce a pressure wave just like the one that reached your ear and you will hear the same sound.

Mathematically you can show that any periodical movment that is just not a single sine wave can be described as the sum of multiple sine waves. So unless it will perfectly move as just a single sine wave it will produce multiple frequencies. You can show the same thing for no period signals but then you can never have a single frequency, if you did it would be a periodic sine wave.

Imagine dropping a single pebble into a puddle. Now imagine a handful. Now record that clip and play it back. Now just translate that to sound waves - the recording encodes the ripples, the speaker merely reproduces it.

All the speaker does is move forwards and backwards. Its a linear motor.

If you take 2 signals (say 2 simple frequencies) and add them together, you get a new, combined signal.

Picture

The above combined signal is created from 2 simple sine waves with a high and a low frequency. If the speaker moves in the combined pattern, it will create waves of air pressure which will stimulate 2 different length hair cells in your cochlea.

The same way your ears hear more than one frequency.

The sounds around us are composed of a complex range of frequencies mixed together. Those mixed together sounds impact your eardrum causing it to vibrate and you hear it.

If you were able to make your eardrum vibrate the same exact way, and amplify the movement, it would emit the same sound you just heard.

Electronic speakers and microphones are the same.

Very simplified, microphones are just speakers. Electricity applied to a speaker makes it move.

This works in reverse too, moving or vibrating a speaker manually makes tiny bits of electricity that match that movement or vibration.

The complex pattern of sound hitting a (mic / speaker) and the resulting complex pattern of electricity it makes can be recorded.

Then you amplify and play back that pattern of electricity to a much larger and louder mic/speaker and that same sound is reproduced

Your ears or a microphone actually move very very little with sound. So you can hear low and high frequencies at the same time.

Similarly most (not all) headphones are just a single tiny speaker that do a decent job of reproducing most frequencies that humans can hear. They can do this because they don’t have to move very much (and be very loud). Again, like my “your ears in reverse” example.

However large speakers have to move a LOT to make very loud low frequency sound. If a speaker is moving an inch back and forth 500 times a second, ALSO trying to make it vibrate a fraction of a millimeter’s distance, 20,000 times per second at the same time doesn’t sound very good.

So to make loud amplified sound, sound better they divide up the work. Small very tight speakers (tweeters) are good at making high pitched sounds. (But can’t move enough to make loud low frequency sounds)

Large loose speakers (woofers) that can move a lot are very good at making low frequency sounds. (But are not tight enough to make loud high frequency sounds)

Inside a speaker is something called a crossover that divides up the electric pattern such that high frequencies go to the tweeter and low frequencies go to the woofer.

That way you can more accurately reproduce loud sound at much higher volume levels.

Speakers work by moving back and forth to move the air which goes to your ears.The higher the sound the speaker is playing, the faster it wobbles.

If a speaker is playing a high sound and a low sound, the speaker kinda just wobbles fast in between the slow wobbles.

Anyways, thats a ELI5, there are other things that i kinda glossed over.

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