~ Sound ~ The Nature of Sound • Speed of Sound • Human hearing • Doppler effect • “Seeing” with sound
Speed of Sound • 344 m/s in air at 20°C • Depends on: • Type of medium • travels better through liquids and solids • can’t travel through a vacuum • Temperature of medium • travels faster at higher temps
Human Hearing sound wave vibrates ear drum amplified by bones converted to nerve impulses in cochlea
Human Hearing • Pitch • highness or lowness of a sound • depends on frequency of sound wave • human range: 20 - 20,000 Hz ultrasonic waves subsonic waves
Human Hearing • Intensity • volume of sound • depends on energy (amplitude) of sound wave • measured in decibels (dB)
Human Hearing DECIBEL SCALE 120 110 100 80 70 40 18 10 0
Doppler Effect • Doppler Effect • change in wave frequency caused by a moving wave source • moving toward you - pitch sounds higher • moving away from you - pitch sounds lower
Medical Imaging SONAR “SoundNavigationRanging” “Seeing” with Sound • Ultrasonic waves - above 20,000 Hz
Sound Problems • vs = D ÷ T where vs = speed of sound 344 m/s D = distance in m T = time in s • vs = f x λ, just like a wave problem!
Ex #1:Find the wavelength of a 200 Hz sound. GIVEN: vs = 344 m/s f = 200 Hz λ = ? WORK: vs = f x λ λ = vs ÷ f λ = 344 m/s ÷ 200 Hz λ =1.7 m
Ex #2: If you hear a sound 3 s after you see the motion, how far away is it? GIVEN: vs = 344 m/s T = 3 s D = ? WORK: vs = D ÷ T D = vs x T D = 344 m/s x 3 s D =1032 m
~ Music ` Music vs. Noise Resonance Harmonics Interference Acoustics
~ Music vs. Noise ` • Music • specific pitches and sound quality • regular pattern • Noise • no definite pitch • no set pattern
= Resonance = • Forced Vibration • when one vibrating object forces another object to vibrate at the same frequency • results in a louder sound because a greater surface area is vibrating • used in guitars, pianos, etc.
= Resonance = • Resonance • special case of forced vibration • object is induced to vibrate at its natural frequency
= Resonance = “Galloping Gertie” The Tacoma Narrows Bridge Disaster Wind through a narrow waterway caused the bridge to vibrate until it reached its natural frequency.
} Harmonics } • Fundamental • the lowest natural frequency of an object • Overtones • multiples of the fundamental frequency
Constructive - louder Destructive - softer | Interference | • Interference • the ability of 2 or more waves to combine to form a new wave
= Resonance = • Beats • variations in sound intensity produced by 2 slightly different frequencies • both constructive and destructive interference occur
Anechoic chamber - designed to eliminate reverberation. _ Acoustics _ • Acoustics • the study of sound • Reverberation • echo effect produced by the reflection of sound