When we speak out loud, we produce sounds. A sound is a wave, and sound waves can be formed from many sources, such as a dog barking, a tree falling, a thunder from a storm, or a person speaking. A sound wave travels through the air and you hear a sound when the sound wave reaches your ear and vibrates your eardrum. For instance, when a guitar is plucked, a sound wave is created and travels through the air. The fact that we perceive sounds of different pitch is because sound waves have different frequencies. When the string "A" of the guitar is plucked, it results in a low pitched sound. This is because the chord vibrates at low frequency. On the other hand, when the string "B" of the guitar is plucked, it results in a high pitched sound, because the chord vibrates at a higher frequency. When three chords are plucked at the same time, the resulting sound wave is the sum of all three individual sound waves. Speech production is similar. When someone speaks out loud, one sound is heard by the listener.

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