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Properties of Sound Waves Sound has many properties of waves: Transmit energy Travel around corners (diffraction) Requires a medium in which to travel: Is sound a longitudinal or a transverse wave? Reflection, Transmission & Transmission Reflection When waves “bounce back” from a surface Transmission When waves pass through a surface Diffraction Spreading of a wave as it passes through a pinhole or by an obstacle Reflection: Echoes and Reverberation Reverberations are perceived when reflected sound waves reach your ear in less than 0.1 s. •Almost no delay is perceptible between the original and reflected wave. We hear only one very long sound wave. Diffraction: Waves from a Pinhole Amount of bend depends on the size of the opening relative to the wavelength. Bending is most noticeable when the width is approx. same as the wavelength Light and sound are types of waves. Why can we hear around corners, but not see around them? Speed of Sound The speed of sound in air is temperature dependent: v = 331 + 0.59 Tc where v is the speed of sound(m/s) Tc is the temperature of the air (˚C) Sound travels fastest in solids, slower in liquids and slowest in gases Example The temperature was 4.0˚C one morning, as Sara walked through the parking lot at the back of Thomson. She shouted at a wall of the school, and heard an echo. How far away was the wall? Interference of Sound Waves If sound is a wave, then it must exhibit interference. E.g. When the arms of a tuning fork vibrate, each arm acts like a vibrating source. Sound waves radiate outward from these sources. Interference of Sound Waves: Beats When two waves with slightly different frequencies interact (i.e. interfere), beats are produced. Interference of Sound Waves: Beats The number of high intensity points that occur per second is called the beat frequency. The beat frequency is the absolute value of the difference of the frequencies of the component waves: f beat = |f1 – f2| where f beat is the beat frequency (Hz) f 1 is the frequency of one wave (Hz) f2 is the frequency of the other wave (Hz) Beat Frequency continued… Example A tuning fork of unknown frequency is sounded at the same time as one of frequency 440 Hz. As a result, beats are heard. Over 15 s, 46 beats are produced. What are the possible frequencies of the unknown frequency tuning fork? Refraction: Bending of Waves The speed of a wave depends on what medium it’s in. Speed of sound in air: 331 m/s Speed of sound in water: 1482 m/s Refraction When a wave enters a different medium at an angle, the change in speed causes the wave to bend.