Sound

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Everyday we talk and sing, do you know how the sound is produced.?
Sound is kind of longitudinal wave, so it means when something vibrates, the longitudinal wave is transmitted in the air that produce sound, there are some compressed air motions and rarefaction air motions.
Sound is a family of mechanical wave, so sound needs medium to transfer, without medium, there will be no sound heard.
Medium will influence how quick the sound is transmitted too!
Try to hear through a wall, water and air, you can feel the difference, the speed of sound transmitted through wall is higher than water and air, the speed of sound transmitted through water is higher than air.

Not all sound can be heard by human, only the one at our range of audibility that is 20 - 20,000 Hz. Below 20 Hz is called infrasound and above 20,000 Hz is called ultrasound.
Both kind of sound can be heard by certain animal, but not to us.

When we scream in a wide room (as a reflecting surface), some sound is reflected, this is what we call ECHO.
Echo is widely used in daily life, for example in detecting fetus (USG), SONAR, even for insect (echolocation).

When we listen to a music, we notice many notes. What create different notes is related to pitch (related to frequency), it could be higher or lower.
And so does louder sound, can be reached by increasing its amplitude.
While the same note can be sung at different waveform to create a quality (timbre), which will enrich the sound of a music.

Doppler, an Austrian scientist, notes that as one moves nearer to the source of sound, he will get the higher frequency of sound, while he moves away from the source of sound, he will get the lower frequency of sound.