1. 21.1 Properties of Sound pp. 454 - 458 Mr. Richter

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

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

4. Frequency and Pitch

5.  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).

6. 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

7. Amplitude and Loudness

8.  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).

9. 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)

10. The Speed of Sound

11.  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.

12. 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.

13. The Doppler Effect

14.  Relative motion creates a change in frequency.

15. 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.

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

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