2 Vibrations & Waves & Sound When things vibrate, they move the air and create sound wavesMusic, talking, sirens, etc.
3 Sound Waves Sound waves begin as vibrations. As an object vibrates forward it pushes air molecules together forming a compression and as it moves backward the air molecules are less dense creating a rarefraction.As the vibrations continue they create a series of compressions and rarefractions in waves.Sound waves are longitudinal: the air molecules virbate parallel (in the same direction) as the wave moves.
4 Characteristics of Sound Waves Sound waves that can be heard by human ears are audible waves and have frequencies ranging from 20 Hz to Hz.Sound waves below 20 Hz are called infrasonic; elephants can hear infrasonic waves.Sound waves above Hz are called ultrasonic; dogs can hear ultrasonic waves.
5 Sound WavesMolecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.
7 Characteristics of Sound Waves (cont’d.) Pitch is the highness or lowness of sound; as frequency increases, pitch increases; as frequency decreases, pitch decreases.Pitch and frequency are conversely proportional. Pitch cannot be measured but frequency can be measured (in Hertz)Intensity-the rate at which energy is transferred from one air molecule to the nextIntensity decreases with distance-this is why the intensity of an echo is less than the intensity of the original sound
8 Characteristics of Sound Waves (cont’d.) Speed of sound depends on:Type of medium: sound travels faster in solids because the molecules are closer together, making it easier to “bump” each other.Temperature (of gases): As temperature increases molecules are moving faster, colliding more often, therefore sound travels faster in high temperatures in gases. Temperature makes little difference in solids and liquids.
9 Sound Now consider your ear Nerves tell brain “sound!” Eardrum vibrates
10 Speed of Sound Depends on the material of the vibrating medium Sound can vibrate water, wood (speaker enclosures, pianos), metal, plastic, etc.Faster in warm air, slower in coldWater 4 times faster, steel 15 times faster
11 Wave ReflectionWhen a sound wave reflects from a surface we generate an echoWave reflection from surfaces depends on the characteristics of the surfaceSmooth hard surfaces reflect bestRough soft surfaces reflect poorlyEnergy not reflected is absorbed or transmitted through the material
12 Wave RefractionIf there is a change in the characteristics of a medium, waves are bentThis occurs because different parts of the wave front travel at different speedsThink of a marching around a curved trackThe inside people have to move more slowly than the outside people to keep the lines straight
14 Natural FrequenciesObjects have “natural” frequencies based on their size and structureGuitar strings are an example
15 Forced Vibrations Can externally impose a vibration on an object Guitars and violins and pianosSet the wood into motion at the frequency of the stringThis provides a larger surface to interact with the airHarp vs. Piano
16 ResonanceWhen the forced vibration matches a natural frequency we get a “resonance” conditionThink about a swing on a playgroundYou go high when you pump the swing at its natural vibration frequency
17 What is Resonance?Many objects have a natural frequency – vibrates in a regular pattern.Resonance occurs whenever a sound wave has the same frequency as the natural frequency of an object. The sound will cause the object with the same natural frequency to vibrate.
18 What is sound intensity? Sound intensity is the energy that the sound wave possesses. The greater the intensity of sound the farther the sound will travel and the louder the sound will appear.Loudness is very closely related to intensity. Loudness is the human perception of the sound intensity. The unit for loudness is decibels.
19 How is frequency related to pitch? The pitch of a sound wave is directly related to frequency. A high-pitched sound has a high frequency (a screaming girl). A low-pitched sound has a low frequency (a fog-horn).
20 What is the Doppler Effect? The Doppler Effect is the apparent change in frequency detected when the sound is moving relative to the hearer.Video-Excellent example of Doppler Effect with car horn (26 seconds)Video-A Motorcycle does the Doppler Effect (27 seconds)