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Lesson 4: Sound

# Speed of Sound

How fast does sound travel? Explore the nature of sound waves and how they travel. Discover why sound is a longitudinal wave and how its speed is influenced by the medium it travels through. Learn about the relationship between wavelength, frequency, and speed, and how changing the medium affects the speed of sound. Created by David SantoPietro.

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• How can the amplitude be changed?
• Once the wave has started, you can only dampen the sound (reduce the amplitude), and this happens naturally because sound energy is lost in transmission, for the same reason you cannot hear people whispering when they are very far away. If you wanted to increase it, you would need to add additional energy at the start but you cannot make something louder once the sound wave has started
• Does increasing amplitude change the frequency or wavelength of a wave?
• No, the amplitude is how loud something is, and you can increase the height of a wave without influencing the length of each wave.
Similarly, the frequency or wavelength determines the pitch (how high a note is) and you can play different pitches at the same volume.
• Does speed of sound in a gas increase with pressure?Why or why not?
• Yes, because speed of sound is dependent on the medium's density. As you apply more pressure to a compressible medium (gas works here!) the medium is compacted into a smaller space, and have many more interactions with the other molecules. If you're a sound wave, this is your bread and butter because you rely on intra-medium interactions, so to speak. Sound propagates because of compressions and transfers of energy through a medium.

This is why sound is so much faster under water (and why the speed under water in the ocean is so variable due to different densities because of different salinities and temperatures).
• So what exactly is going on when you speed up or slow down a video on YouTube, for example? It seems the pitch goes up if you speed up the settings (higher frequency), but does that mean the speed of sound is still unchanged and it's all due to a change in wavelength?
• Yes, speed of sound relies only on the medium it is traveling through. The higher pitch is due to the shorter wavelength, and, therefore, higher frequency.
• If you change the speed of the sound will the frequency change and why
• The frequency would not be impacted. Frequency is determined by the frequency of the source, and the properties of the medium (stiffness and density) determine the speed of sound. The wavelength would change however since λ = c/f where c is the speed of sound.
• can sound and light travel in vacum? why?
• no,sound cannot travel through vacum because -
vacum is a place with no air n there will be no matter for the vibrations to work in but light can travel through vacum as light is a electro-magnetic wave n it is produced when there is a disturbance in the electro magnetic field n a electro magnetic field is present in the vaccum.
• Helium was used as an example here for changing the speed of sound. Why helium?
• Helium atoms are much lighter than oxygen and nitrogen molecules, so the speed of sound is greater (by about a factor of 3). It's like putting a lighter mass on a spring: it will oscillate at a higher frequency.
• If I stand at a distance where when the person speaks slowly is not audible, why then if the person speaks louder in the same distance is audible
• As the air molecules moves they experience Friction ,Therefore at some distance it stops and doesn't reach our ear drum . So because no vibration in our eardrum , we doesn't hear any sound . But when we speak louder because of more disturbance in air molecule it stays in chain for long range and easily overcome friction. It also depends where you are speaking , in a very empty room or a room filled with sound absorbing materials.
• Is there any differences between the graph of Transverse waves and the graph of Longitudinal waves?