21.1 Properties of Sound
pp. 454 - 458
Mr. Richter
Agenda
 Introduction to sound.
 Notes:
 Frequency and Pitch
 Amplitude and Loudness
 The Speed of Sound
 The Doppler Effect
 Tests and Labs Back
Objectives: We Will Be Able To…
 Explain how pitch, loudness and speed are related to waves.
 Explain the Doppler Effect.
Frequency and Pitch
Frequency and Pitch
 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).
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
 Ultrasonic waves (~ 10MHz) can be used to
produce images of the insides of objects like:
 wombs (ultrasounds!)
 bridges and airplane wings
Amplitude and Loudness
Amplitude and Loudness
 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).
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)
The Speed of Sound
The Speed of Sound
 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.
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.
The Doppler Effect
The Doppler Effect
 Relative motion creates a change in frequency.
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.
Wrap-Up: Did we meet our objectives?
 Explain how pitch, loudness and speed are related to waves.
 Explain the Doppler Effect.
Homework
 p. 458 #1, 3, 5
 p. 474 Reviewing Concepts #1, 4