How Everything Works
How Everything Works How Everything Works

Question 910

How does a magnetic tape record the difference in timbre or sound quality of the sounds? How does it represent a piano versus an electric guitar? Also, how does more than one tone get recorded (e.g., an entire band or symphony)?
Even a single instrument playing a single note produces a complicated sound. The air pressure fluctuations produced by the instrument aren't as simple and smooth as you might think. While the instrument may produce mostly the fundamental tone—the main pitch associated with the note being played—it also produces other tones that are usually integer multiples of the fundamental tone. These higher pitched "harmonics" contribute to the sound we hear and allow us to determine what instrument is playing that sound. We also hear the temporal shape of the sound—the sound envelope. A piano produces a sound that starts loud and gradually becomes softer while a violin produces a sound that starts soft and gradually becomes louder. An electric guitar offers its player even more control over the pitch and sound envelope. The tape recorder detects the pressure fluctuations associated with all these tones and volume changes and records them all as the magnetization of the tape's surface. When many instruments are playing at once, the pressure fluctuations are even more complicated and they add together to create a complicated pressure pattern at the microphone. Nonetheless, the recorder simply detects the air pressure changes at the microphone and records them on the tape, and that's all it needs to do to keep an accurate record of the sound. When the magnetization of the tape is used to reproduce sound, you again hear all the instruments playing.

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