Sound Notes

Sound Waves Sound is a disturbance that travels through a medium as a longitudinal wave. Sound waves are longitudinal waves that begin with a back and forth vibration. If you pluck a guitar string, it vibrates back and forth. The vibrating string sends a longitudinal wave through the air.

Sound Waves Sound waves need a medium to travel through - gases such as air, liquids such as water, and solids such as wood.

Interactions of Sound Waves As discussed before, sound waves reflect off objects, diffract through narrow openings and around barriers, and interfere with each other.

The Speed of Sound The speed of sound depends on the elasticity, density, and temperature of the medium through which it travels. Sound travels at different speeds in different mediums. For example, sound usually travels more quickly in solids than in gases. At room temperature (20°C), sound travels through air at about 343 m/s.

The Speed of Sound Sound travels faster in mediums that are elastic, or stretchy. A rubber band is very stretchy. Therefore, sound travels quickly in a rubber band.

The Speed of Sound Sound travels more slowly in denser mediums. Something that is dense feels heavy for its size. Lead is a very dense solid. Therefore, sound travels more slowly in lead than in most other solids.

The Speed of Sound Sound travels more quickly when the medium is warm. For example, sound travels more quickly in air that is 20°C than in air that is 0°C.

Loudness The loudness of sound depends on two factors: the amount of energy it takes to make the sound and the distance from the source of the sound. Loudness is how loud or soft a sound seems to the person who hears it.

Loudness The more energy it takes to make a sound, the louder the sound. For example, pounding hard on a door with your fist makes a louder sound than tapping gently on a door with your fingers. The amount of energy a sound wave carries per second through a unit of area is its intensity.

Loudness The closer you are to the source of the sound the louder the sound. The whisper of a person next to you is louder than the whisper of a person across the room. The loudness of a sound is measured in decibels (dB). The loudness of a whisper is about 20dB. The loudness of a rock concert is about 120dB.

Pitch The pitch of a sound that you hear depends on the frequency of the sound wave. Pitch is how high or low a sound seems to a person who hears it. The sound of a whistle has a high pitch. The sound of thunder has a low pitch.

Pitch The pitch of a sound depends on the frequency of the sound waves. A sound wave with a higher frequency makes a sound with a higher pitch. Conversely, a sound wave with a lower frequency has a lower pitch.

Pitch Humans hear only a certain range of pitches. A sound that is too high for humans to hear is called ultrasonic (ultra- means above). A sound that is too low for humans to hear is called infrasonic (infra- means below). In music, different pitches are usually referred to as notes. Singing, an individual can change your pitch by stretching or relaxing your larynx, or vocal chords.

The Doppler Effect When a sound source moves, the frequency of the waves changes because the motion of the source adds to the motion of the waves. The Doppler effect is a change in pitch that happens when a sound source is moving. For example, when a motorcycle races by you, the sound of the engine changes pitch.

The Doppler Effect When a motorcycle is coming toward you, the bike is moving in the same direction as the sound waves you are hearing. This makes the waves closer together. The waves have a higher frequency, so the sound has a higher pitch. When the motorcycle is going away from you, the bike is moving in the opposite direction from the sound waves you are hearing. This makes the waves farther apart. The waves have a lower frequency, so the sound has a lower pitch.