Sound Longitudinal wave requires a medium (cannot travel in a vacuum)

Study of Sound is known as ACOUSTICS

Properties of Sound A sound is a vibration The vibrating causes the air molecules near the movement to be forced closer. This is called compression As the vibration moves on, the density and air pressure becomes lower than normal and is called rarefaction Pressure wave – longitudinal Frequency = pitch v = 334 m/s in air at room temperature Velocity is dependent upon the material

Do molecules move faster or slower as temperature increases? Can affect speed Do molecules move faster or slower as temperature increases? So would sound travel faster or slower as temperature increases?

Waves travel fastest in solids, slowest in gases. Type of Medium Solid Liquid Gas Waves travel fastest in solids, slowest in gases.

Speed of sound Fastest in solids, slowest in gases. Air = 340 m/s water= 1440 m/s steel = 5000 m/s Supersonic: faster than the speed of sound.

Sound travels faster in warm water than in cold water Speed of sound Sound travels faster in warm water than in cold water By measuring the time it takes for sound to travel a known distance through the ocean the average temperature of the water can be calculated = ATOC (acoustic thermometry of ocean climate)

Speeds of Sound at T = 20 C Air Helium Hydrogen Water Sea water Iron/Steel Glass Aluminum 343 m/s 1005 m/s 1300 m/s 1440 m/s 1560 m/s ≈5000 m/s ≈ 4500 m/s ≈ 5100 m/s

Pitch The highness or lowness of sound. Depends on the frequency of sound waves. High frequency = High pitch Low frequency = Low pitch

Depends on the amplitude of sound waves. (amplifier) Intensity Also called LOUDNESS Amount of energy Depends on the amplitude of sound waves. (amplifier) Large Amp. = Loud sound Small Amp. = Soft sound

Intensity of Sound Unit is the “Bel”. Named after Alexander Graham Bell More commonly used is the decibel (dB) = 0.1 Bel  (in dB) = 10 log(I/I0) I is the intensity ( A2) I0 = 1.0 x 10 -10 W/m2 the “threshold of hearing”

Some Intensities (in dB) Jet plane at 30 m Threshold of pain Indoor rock concert Auto interior Street traffic Conversation Whisper Rustle of leaves 140 120 75 70 65 1x10-10 1x10-11

Reverberation Sound waves reflecting from hard surfaces Ex.: Multiple echo resulting from the direct sound AND the reflected sound

Reverberation vs Echo Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Measurement of loudness Decibels Near total silence - 0 dB A whisper - 15 dB Normal conversation - 60 dB Lawnmower - 90 dB Threshold of pain - 120 dB A rock concert or a jet engine - 120 dB Gunshot, firecracker - 140 dB

Sound is a pressure wave Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Ear

Human Ear 20Hz--20 000 Hz Infrasonic: below 20 Hz (below our hearing) Ultrasonic Sound: above 20,000Hz (above our hearing)

Tuning fork creating a sound wave Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Ultrasounds Ultrasound can be used to create internal images of the human body Ex. Pregnant woman gets a “picture” of her unborn baby

White Noise An equal mixture of all frequencies of sound Used to calm stress Used in office buildings

Noise Pollution Yep! There is such a thing Causes: Damage to ear resulting in hearing loss Stress

Dead Spots Caused by destructive interference Result – hardly any or no sound Bad for concert halls – designers be careful!

Used to locate underwater objects and distances. sonar Used to locate underwater objects and distances. ***Reflection**

Doppler Effect Apparent change in frequency (pitch) of a sound from a moving source. Source moving toward observer: f’ = f / (1-vs/v) Source moving away from observer: f’ = f / (1+vs/v) f = frequency of source, f’=frequency heard by observer, vs = velocity of source, v = velocity of sound

Change in pitch due to motion. Doppler effect Change in pitch due to motion.

Figure 6. 11: The Doppler effect Figure 6.11: The Doppler effect. (b) The clanging bell on a moving fire truck produces sounds that move outward (black circles). An observer ahead of the truck hears the clangs closer together, while an observer behind the truck hears them farther apart.

Figure 6. 11: The Doppler effect Figure 6.11: The Doppler effect. (c) A moving source of light emits waves that move outward (black circles). An observer in front of the light source observes a shorter wavelength (a blue shift), and an observer behind the light source observes a longer wavelength (a red shift).

Doppler effect zoom *Moving towards increases the pitch *Moving away decreases the pitch *Think of sirens zoom

Guitar String creating a sound wave Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com