Speed of Sound | Mechanical waves and sound | Physics | Khan Academy

Summary

This script explains the concept of sound waves and their speed. It clarifies that the speed of sound, approximately 343 meters per second in non-humid air at 20 degrees Celsius, is not the same as the speed of individual air molecules. Sound is a longitudinal wave, moving parallel to the medium's oscillations. The speed of sound is determined by the medium's properties, such as temperature and density, and is calculated as the product of wavelength and frequency. Changes in amplitude do not affect speed, emphasizing that loudness does not equate to faster transmission.

Takeaways

• 🎵 Sound is produced by creating a pulse wave through a short burst from a speaker.
• 📏 The speed of sound in non-humid air at 20°C is approximately 343 meters per second or 767 miles per hour.
• 🔍 The speed of sound is determined by observing the movement of compressed regions through a medium.
• 🌊 The speed of sound does not equate to the speed of individual air molecules moving back and forth.
• 🌀 Sound waves are longitudinal, meaning they travel parallel to the direction of the medium's oscillations.
• 🌊 Transverse waves occur when the wave velocity is perpendicular to the medium's oscillations, like waves on a string or water surface.
• 📊 A graph of air displacement versus position shows the wave shape moving to the right as the wave travels.
• 🔢 The velocity of a sound wave is calculated as the wavelength divided by the period, or wavelength times frequency.
• ⚠️ Increasing the frequency of a sound wave does not increase its speed; instead, the wavelength decreases proportionally.
• 🔑 The speed of sound can only be changed by altering the medium (like air's temperature, humidity, or density) or by using a different medium altogether.
• 📢 The amplitude of a sound wave affects its loudness, not its speed, so yelling does not make you heard faster.

Q & A

What is a pulse wave in the context of sound?

A pulse wave is a single burst of sound that travels through a medium, such as air, and can be used to determine the speed of sound by observing how quickly the compressed region of the wave moves through the medium.

What is the speed of sound in non-humid air at 20 degrees Celsius?

The speed of sound in non-humid air at 20 degrees Celsius is approximately 343 meters per second or 767 miles per hour.

How is the speed of a sound wave related to the movement of air molecules?

The speed of sound is not the same as the speed of individual air molecules moving back and forth. Instead, it refers to the speed at which the disturbance or the wave pattern moves through the medium.

What is the difference between longitudinal and transverse waves?

Longitudinal waves, like sound waves, travel parallel to the direction of the oscillations of the medium. Transverse waves, on the other hand, have their oscillations perpendicular to the direction of wave travel, like waves on a string or water surface.

How can you visually observe the speed of a sound wave?

By looking at a graph of air displacement versus position, one can observe the speed of a sound wave by tracking the movement of the wave's peaks, valleys, or any single point on the wave shape.

What is the formula for the velocity of a sound wave?

The velocity of a sound wave is calculated by the formula speed equals wavelength times frequency, which is applicable to all types of waves, not just sound.

Why does increasing the frequency of a sound wave not change its speed?

Increasing the frequency of a sound wave results in a proportional decrease in wavelength, which keeps the speed of the sound wave constant, as speed is the product of wavelength and frequency.

How can the speed of sound be changed?

The speed of sound can be changed by altering the medium through which it travels or by changing the properties of that medium, such as temperature, humidity, density, or by using a different material like water, helium, or metal.

Does changing the amplitude of a sound wave affect its speed?

No, changing the amplitude of a sound wave does not affect its speed. A sound wave with a large amplitude will not travel faster than one with a small amplitude in the same medium; it will only be louder.

What does it mean when it is said that the speed of a sound wave is determined by the properties of the medium?

It means that the speed at which a sound wave travels is entirely dependent on the physical properties of the medium it is passing through, such as its density, elasticity, and temperature.