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21.1 Properties of Sound pp. 454 - 458 Mr. Richter

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Agenda Introduction to sound. Notes: Frequency and Pitch Amplitude and Loudness The Speed of Sound The Doppler Effect Tests and Labs Back

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Objectives: We Will Be Able To Explain how pitch, loudness and speed are related to waves. Explain the Doppler Effect.

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Frequency and Pitch

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Humans interpret the frequency of sound waves as pitch how high or low the sound is perceived to be. High pitch = high frequency (soprano) Low pitch = low frequency (bass) Humans have been able to hear sound waves between frequencies of 20 Hz and 20,000 Hz. Pitch is subjective, meaning it is only our perception of what is happening. Sound waves frequencies are objective (can be measured).

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Frequency and Pitch: Inaudible Waves As we age, humans can hear less of the range of frequencies. Enjoy it while you can! https://www.youtube.com/watch?v=9g0yThhJcxY https://www.youtube.com/watch?v=9g0yThhJcxY Ultrasonic waves (~ 10MHz) can be used to produce images of the insides of objects like: wombs (ultrasounds!) bridges and airplane wings

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Amplitude and Loudness

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Humans interpret the amplitude of a sound wave as loudness (volume). The greater the amplitude of a wave, the louder it sounds. However, volume (human perception of amplitude) is not directly proportional to amplitude. Humans measure volume on the decibel (dB) scale. If the amplitude is 10 times greater (multiplied by 10), the decibel level increases by 10 dB (10 is added).

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Amplitude and Loudness For example: an orchestra has 6 violins. If the conductor wants the violin section to be 10 dB louder, how many more violins should he hire? 54 more (54 + 6 = 60. Ten times as many as the original)

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The Speed of Sound

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Sound is a mechanical wave. Reminder: a mechanical wave is a wave that passes through a physical medium (material). The speed of sound depends on what medium the sound is traveling through.

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The Speed of Sound Sound waves, and all mechanical waves, travel by collisions of molecules (more in 18.2-18.3). In general, sound travels faster through solids than liquids, and more quickly through liquids than gases. The more closely packed the molecules are, the more quickly collisions can happen.

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The Doppler Effect

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Relative motion creates a change in frequency.

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The Doppler Effect As an object moves toward an observer the sound waves arrive more frequently, resulting in a higher pitch. The actual frequency of vibration of the object (car horn) does not change. As the object moves away from the observer, the sound waves arrive less frequently. Lower pitch. Note: the Doppler Effect only occurs with moving objects, not stationary objects closer or farther away.

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Wrap-Up: Did we meet our objectives? Explain how pitch, loudness and speed are related to waves. Explain the Doppler Effect.

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Homework p. 458 #1, 3, 5 p. 474 Reviewing Concepts #1, 4