Topic

21.1 Properties of Soundpp. 454 - 458Mr. Richter

AgendaIntroduction to sound.Notes:Frequency and PitchAmplitude and LoudnessThe Speed of SoundThe Doppler EffectTests and Labs Back2Objectives: We Will Be Able ToExplain how pitch, loudness and speed are related to waves.Explain the Doppler Effect.Frequency and PitchFrequency and PitchHumans 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). Frequency and Pitch: Inaudible WavesAs we age, humans can hear less of the range of frequencies. Enjoy it while you can!https://www.youtube.com/watch?v=9g0yThhJcxYUltrasonic waves (~ 10MHz) can be used to produce images of the insides of objects like:wombs (ultrasounds!)bridges and airplane wings

Amplitude and LoudnessAmplitude and LoudnessHumans 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).Amplitude and LoudnessFor 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)

The Speed of SoundThe Speed of SoundSound 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.

The Speed of SoundSound 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.

The Doppler EffectThe Doppler EffectRelative motion creates a change in frequency.

The Doppler EffectAs 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.

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

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