Pitch is the highness or lowness of a sound. When objects vibrate they produce sound waves. If the sound wave is long it makes a low pitched sound and if the sound wave is short it makes a high pitched sound. 

Pitch is related to frequency. Frequency is how fast the sound wave is oscillating or vibrating. It’s measured in hertz (Hz). The faster the sound wave oscillates, the shorter the wavelength and the higher the pitch. If you pluck the strings of a guitar you will hear that the big, heavy strings make a low pitched sound. That’s because they vibrate slowly. The thinner, lighter strings of the guitar vibrate faster and make a high pitched sound.

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Humans can only hear sounds within a certain pitch: 20-20,000 Hz. They cannot hear sounds below 20 Hz, called infrasounds, and they cannot hear sounds above 20,000 Hz called ultrasounds. Different animals can hear different frequencies, like bats. They can hear up to 120,000 Hz! Even dogs and cats can hear higher frequencies than we can. 

Ultrasounds are used by doctors to create a scan of a baby while it’s still in the womb. The high-frequency sound waves are sent through the mother’s body and when they reflect off the baby and bounce back, the computer creates the picture of the baby.


The Doppler effect

In 1842 a scientist named Christian Doppler discovered something very curious about sound. To understand it, let’s imagine you’re on the street and you hear a fire engine from afar. As the fire engine comes closer, you notice the sound is different, the pitch is higher. For the people inside the fire engine, the sound of the siren was always the same, so why does it sound different from the outside? This strange event is called the Doppler effect

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The Doppler Effect happens when the sound waves from a moving object (the fire engine) are moving toward the observer (you). The closer the object gets, the more the sound waves compress, because there is less distance between you two. As the fire engine moves away, the distance increases and it takes longer for the sound waves to reach your ears. The sound seems lower. The frequency of the sound wave never actually changes; it just seems that way to the observer.

Frequency seems to increase when the sound source comes closer or we move towards it. Frequency seems to decrease when the sound source moves away or we move away from it.