Waves: Sound

What is sound? Sound is a longitudinal wave In air, sound waves cause air particles to move back and forth In areas where the particles are pushed together, compressions form In between the compressions, particles are spread out. These are rarefactions.

Sound requires a medium While light waves do not require a medium, sound waves must travel through a medium Remember that the material through which a wave travels is called a medium. For sound to be heard, sound vibrations must have air or some other kind of matter to travel through.

Fun fact: You cannot hear sound in outer space because there is no air or other matter to carry sound vibrations. Q: How do you think astronauts are able to talk to each other in outer space? The astronauts have devices in their helmets which transfer the sound waves from their voices into radio waves and transmit it to the ground (or other astronauts in space). This is exactly the same as how your radio at home works. Remember that radio waves are NOT sound waves - they are a form of electromagnetic radiation, and therefore can propagate through a vacuum.

What causes sound? Sound is caused by a vibrating object Examples: The vocal cords in your throat vibrate when you speak; tuning fork; musical instruments

How do we hear sound? Object vibrates Sound waves are sent. The outer ear “catches the sound waves”. The middle ear takes the sound waves and “vibrates” the eardrum. The inner ear sends the messages to the brain. The brain puts together the messages and you hear sound! Middle Ear Inner Ear Brain Outer Ear Source of Sound Waves

Sound travels through matter Sound can travel through solids, liquids, and gases The speed of sound depends on elasticity, density, and temperature of the medium the sound travels through

Sound travels through matter Elasticity Sounds travel fastest in solids because the particles of a solid bounce back and forth quickly as the waves pass by (molecules have greater attractive forces between them)

Sound travels through matter Density In materials in the same state of matter (for example: 2 liquids), sound travels more slowly in denser mediums Would sound travel faster in syrup or water?

Sound travels through matter Temperature Sound travels slower at lower temperatures than at higher temperatures Because at lower temperatures the particles of a medium move slower

Two important properties of sound: 1) Loudness 2) Pitch

Loudness Loudness means the volume of a sound Loudness is dependent on 2 factors: 1) Amount of energy it takes to make the sound Loud sounds use a lot of energy. Soft sounds use little energy. Example: The harder a drum is hit, the more the drum will vibrate. The more an object vibrates, the louder the sound it makes. 2) Distance from sound source The closer the sound source, the louder the sound

Pitch Pitch refers to the frequency of the sound waves Sound waves with a high frequency have a high pitch (For example a soprano singer) Sound waves with a low frequency have a low pith (For example a bass singer) Soprano: Bass:

Difference between light and sound waves: Characteristics Light Waves Sound Waves Type Transverse Longitudinal Medium Doesn’t always require a medium (Can even travel through a vacuum) Requires a medium such as solid, liquid, or gas (Cannot travel through a vacuum) Speed Higher Speed (3 x108 m/s) Lower speed (340 m/s in air) Wavelength Has a characteristic wavelength Depends on the medium which determines the speed and frequency Frequency Are of different frequencies, resulting in different colors Are of different frequencies, giving notes of different pitches Distance Traveled Can travel through a much greater distance Do not travel far as their energy is dissipated easily