# Chapter 13 Sound Properties of Sound – the source of all sound waves is vibration  Sound waves – longitudinal waves – the particles in the medium are.

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Chapter 13 Sound Properties of Sound – the source of all sound waves is vibration  Sound waves – longitudinal waves – the particles in the medium are moving (vibrating ) parallel to the direction of wave motion.  Compressions crowded areas of the wave  Rarefactions are the stretched out areas of the wave.  Compression to compression = one wavelength

Peaks = compressions Troughs = rarefactions

Speed of a sound wave depends on … The medium, density, pressure and temperature  Medium: Speed of sound = Gases (air) < liquids < solids  Higher the temperature – greater the speed

 Speed of sound = 331 m/s. At 0 0 C  The speed is increased by 0.6 m/s for every degree above 0 0 C.  Speed of sound in air at room temp = 346 m/s or 760 mi/h  Depends on how well the particles transmit the compressions and rarefactions of sound waves.  Breaking the speed of sound in freefall Breaking the speed of sound in freefall

Sound barrier A sonic boom is the sound from shock waves traveling through the air faster than the speed of sound. Felix Baumgarter breaking sound barrier - free fall

Stationary Moving to the right Observer in front of the source will hear a higher frequency Mach 1 Speed of sound at sea level in air 340 m/s or about 750 mph. jets breaking the speed of sound

Wave Interactions 3 Types: Reflection, Diffraction, and Refraction 1) Reflection of sound : bouncing back of a wave. Reflection of sound waves off of surfaces can lead to one of two phenomena - an echo or a reverberation.

Sound waves are diffuse – small amount of reflected sound from many parts of the wall Many grooves of a concert hall

Law of Reflection: The Angle of Incidence ϴ i = the Angle of Reflection ϴ r ϴ i = ϴ r ‘Normal’ – line perpendicular to the reflective surface

Pitch is determined by frequency  Higher the frequency – higher the pitch  Humans can hear between 20 – 20,000 Hz  Below the range of human hearing is called infrasound  Above the range of human hearing is called ultrasound AnimalFrequency Range (Hz) Human 20 to 20,000 Dog 20 to 40,000 Cat 80 to 60,000 Bat 10 to 110,000 Dolphin 110 to 130,000 supersonic dolphins sound echos and dolphins Properties of Sound

Ultrasound –ultrasonic (high frequency) sound waves used to produced computerized images - 10 MHz range Echocardiogram

3D (pictures) and 4 D (video) images now available from ultrasound technology and computers.

Echolocation – used by dolphins and other marine mammals Sonar - - sound navigation and ranging system.  uses sound waves to ‘see’ in the water.  Determine distance by measuring time it takes for sound waves to reflected back from a surface. Formula: D = v x t

2)Diffraction of sound: bending of a sound wave around small obstacles Low frequencies diffract easily- hear the low bass of car stereos

3)Refraction of sound: bending of waves as they pass from one ‘medium’ to another. (air to water)  Frequency does not change – determined by source  Wavelength and speed do change At night, when the air is cooler over ground or surface of lake, (travel slower) sound is refracted towards the ground and carries unusually well – hear sounds better at night than during the day

Doppler Effect The pitch of sound waves change as sound approaches an observer or as sound moves away from an observer. Pitch is related to frequency – higher the pitch – higher the frequency. Doppler Effect – an observed change in the frequency of a wave when the source or observer is moving.

 Light waves behave similarly – as light waves are approaching an observer they become shorter and when they are moving away from an observer the wavelength increases.  Red end of spectrum = longer wavelengths  Blue end = shorter wavelengths

13-2 Sound Intensity and Resonance  Intensity is the rate of Energy flow through a given area – as sound travels energy is transferred  Intensity and frequency determine which sounds are audible  Intensity of wave determines its loudness, or volume.  Relative intensity in measured in Decibels (dB)  (dB) - unit of the intensity of sound  Logarithmic scale - based on multiples of 10. Example: 20 dB sound is 10 times as intense as a 10 dB sound 40 db is 100 times more intense as 20 dB

Source of SoundSound Level (dB) Hearing Threshold (I o ) 0 Normal Breathing 10 Close Whisper 20 Library 40 Normal Speech 60 Busy Street Traffic 70 Average Factory 90 Old Subway Train 100 Loud Rock Music 115 Threshold of Pain 120 Jet Engine, at 30 meters 140

Resonance – When the frequency of a vibrating object matches a second object’s natural frequency - resulting in an increased amplitude.  A form of constructive interference.  Example: child on swing – pump legs in rhythm with the natural frequencies of the swing - producing larger amplitudes  Wine glass shattering - Sound Waves breaking glass Sound Waves breaking glass http://www.youtube.com/watch?v=17tqXgvCN0E Forced Vibration and Resonance Demo

Tacoma Narrow’s Bridge - high winds caused the bridge to oscillate at one of its natural frequencies

Bay Bridge – San Francisco 1989 Upper Deck of Freeway collapsed – Oakland California - 1989 Structural Resonance

13-3 Harmonics  Fundamental frequency – the lowest frequency (longest wavelength) of vibration of a standing wave  Length = ½ wavelength  Wavelength = 2 x length λ = 2 x L

Calculating the Speed of Sound Lab:  Use the principle of resonance to determine the wavelength of a sound wave.  If a known frequency of a tuning fork is known then using the speed = frequency x wavelength S = f x λ  The first harmonic of a closed end pipe is always 1/4 of the wavelength of the sound. L = ¼ λ λ = 4 · L

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