Eli5 How do speakers turn signals into full sound, it could sound like you’re in front of a concert but it’s just a box
In: 10
Sound is just a pressure wave in the air.
When it comes to complex sounds, like an entire orchestra, you’re hearing the *sum* of all the pressure waves in the air, not each one individually.
Microphones act like your ear, and record the same summed wave you would hear if you were there.
Speakers simply reproduce that final summed wave that was recorded.
Even though we can’t see them air is made up of tiny little parts. A speaker disrupts and pushes these parts towards you. Think of ripples in water and how they make these circles that get larger. The smaller the room the less it takes to push these ripples. So it seems as if it’s as loud as a large room.
You have a speaker with a cardboard cone and a magnet attached to it. The magnet is inside a coil. Sound pushes the speaker cone which moves the magnet and this movement induces a varying voltage on the coil. To replay a sound do the opposite by applying the exact same varying voltage to the coils which moves the magnet and the speaker cone the same exact way producing the original sound again.
How does sound work?, how do speakers work?, how can the sound of an orchestra be contained in a single waveform? how can one vinyl record groove have so many sounds? … Variations of your question are asked here every week or two, and usually are removed with a “search first” admonition.
[This animated explanation of how sound & speakers work](https://youtu.be/RxdFP31QYAg?t=156) is probably is all you need. It’s a video companion to the [explanation on the Animagraffs website](https://animagraffs.com/loudspeaker/).
Anyway, since a top-level comment has to be an answer:
Everything you hear is just vibrations of the air molecules at your eardrum (plus some vibrations conducted through your skull & jaw). The sounds of different instruments are combinations of relatively simple vibrations, and they all merge into one very complex vibration as they arrive at the same time in your ears. Your eardrum converts the sounds to electrical signals for your brain, which eventually learns to identify separate sound sources and roughly where they are coming from.
To envision two frequencies combining into one complex motion, wave your hand slowly moving back and forth, mimicking the motion of the air as it transmits a low bass tone. Keep doing that, and add a fast tremor to your hand, mimicking a higher-pitched tone “riding on top of” the lower tone. That’s all there is to it; it’s what the instrument, the speaker, the air, and your eardrum are doing: making unique combinations of simple vibrations.
How far your hand moves back and forth, relative to a neutral center position, is the amplitude, and it correlates with sound intensity, which we perceive as “volume” or “loudness”. How fast your hand moves back and forth is the frequency (expressed in Hertz, e.g. 20 Hz means 20 times per second), and it correlates with pitch. Actual sound sources just vibrate the air much faster and with much less amplitude than your hand ever could, and they normally contain many pitches combined.
A speaker only has to make the air at your ears vibrate like that, and most full-range speakers can do a pretty good job. Your brain is what recognizes and distinguishes between instruments and other consistent sources of sound. How *that* works in the inner ear and brain is more difficult to explain, but is not really what you asked.
I’m going to change the question slightly, but the principle is the same. How can we listen to two earphones, just two sources of sound, but it seems like a whole range of different sources?
The answer is that we’re *always* listening to just two sources of sound. All your brain cares about is the vibrations of your eardrums. When you listen to an orchestra, all the sound waves (which are just wobbles in the air) combine as they enter your ear canal. This combination is what makes your eardrums wobble a particular way. If you replace your eardrums with a pair of microphones, you can record what the wobbles should be. Then, you can pump that through some speakers or earphones, and they just have to recreate the wobble of each eardrum and it should sound the exact same.
There are nuances to this, in terms of the position of your ears, the width of your head, and the specific swirls of your pinnae (the flappy bits of your ears that you can see) which affect the precise feeling and your ability to localise each of the different instruments. But generally speaking, you’ll be able to hear the combined sound because that’s what you’d be hearing anyway!
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