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# Pitch

Introduction
When you are in school, you become aware of when the period gets over. The sound of a doorbell easily reaches us when someone arrives at our doorstep.  Similarly, if someone is calling your name, you can make it out by hearing the sound. Sound plays a crucial role in our daily lives as it influences various aspects of our lives. We encounter numerous types of sounds in our surroundings. Have you ever wondered, how sound becomes audible to us? It depends on many properties such as speed, pitch, and loudness. In this article, we will explore what is pitch, and how pitch is influenced by the frequency and the Doppler effect.

What is Pitch?
Let us consider an example of a guitar. We use two rubber bands with varying thicknesses as the strings of a guitar. You likely observed the thicker rubber band producing a lower sound compared to the thinner one. How low or high we hear a sound is to be the pitch of the sound.

How Pitch Depends on Frequency
The pitch of the sound is influenced by the frequency of its corresponding sound wave. Frequency refers to the number of waves produced in one second. It is measured in hertz (Hz), where 1 Hz represents one wave per second.

Frequency and Hearing
Some people use dog whistles to summon their dogs. However, you probably noticed that when someone blows a dog whistle, it appears silent to your ears. This is because the frequency of the sound wave lies beyond the range of human hearing. But the dogs can perceive a very high pitch from the whistle and respond by coming to the caller. The following table illustrates a comparison between the frequency ranges those humans and animals can hear.

Audible Frequencies
The average human ear is capable of perceiving sounds that have frequencies ranging from 20 Hz to 20,000 Hz. Examples of sounds within this range include the lowest one that is produced by a pipe organ (approximately 40 Hz) and the high-pitched screech of a bat (10,000 Hz or higher). It is important to note that hearing range can vary from person to person. Young children often possess the ability to hear sounds with frequencies above this range, whereas many elderly people face difficulties perceiving sounds above 8,000 Hz.

Inaudible Frequencies
Sounds that fall outside the audible range for humans are assigned specific names. Sounds with frequencies below 20 Hz are classified as infrasonic, while sounds with frequencies above 20,000 Hz are known as ultrasonic. These designations are derived from the combination of "sonic" referring to sound, "infra" meaning "below," and "ultra" meaning "beyond."

Ultrasonic waves find diverse applications. For instance, they are utilized in the cleaning of jewelry and removing ice from metal surfaces. Scientists envision using this technology to de-ice airplane wings, car windshields, and freezers.

The Doppler Effect
Have you ever experienced a car driving by with its horn blaring? If so, you noticed the sudden change in pitch__ a sound resembling “EEEEEOOoooowwn” as the car passed by you. The pitch appeared higher as the car approached, but it lowered once the car had passed. This phenomenon is known as the Doppler effect.

For sound waves, the Doppler effect refers to the apparent change in frequency of a sound caused by the motion of either the listener or the sound source. The figure illustrates the Doppler effect. It is important to note that the frequency of the car horn does not change; it merely sounds like it does. The driver of the car perceives a consistent pitch because they are in motion alongside the car.

Summary
• The human ear’s perception of sound frequency within the audible range is referred to as pitch.
• A sound with a higher frequency corresponds to a higher pitch, while a lower frequency results in a lower pitch.
• Frequency represents the number of cycles of compression and rarefaction that occur per second as the sound wave travels through a medium.
• The Doppler effect refers to the apparent change in frequency of a sound caused by the motion of either the listener or the sound source.

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