[ELI5] How can we hear multiple frequencies at once?

They interfere with each with other to create a new waveform entirely.

The reason different instruments have their own unique sound despite playing the same notes, is because they resonate differently with the notes you play. The sound of an instrument is basically made of two parts. The formant and the overtones. The formant is the note you're playing, the overtones are the character of the instrument.

Think of the formant and each overtone as their own sine wave at a different frequency. The middle of the wave is 0, above the wave is positive and below the wave is negative. When two waves meet, they are added to each other. Two positives or two negatives make it louder, a positive and a negative cancel each other out. Each sine wave at a different frequency will lead to different results each second rather than a consistent sine wave like they are on their own. This is what makes everything sound like it does.

There are more waves than sine waves too, so this can get quite complex and produce some very interesting sounds. This is the basis of synthesisers.

Once an instrument has produced it's note and it has made its unique sound. Each instrument will interact with each other. Every sound you ever hear for that matter. Your two ear drums vibrate with this wave form made of all vibrations interacting with each other.

/r/explainlikeimfive Thread