Section 1 review
Sound is a longitudinal pressure wave that can travel through many substances, though humans normally hear it through air. The human sense of hearing responds to sound waves with frequencies that range from about 20 Hz up to 20,000 Hz. High frequencies are perceived as high pitch and large-amplitude sound waves are perceived as loud. The amplitude of a sound wave is measured using the logarithmic decibel scale: An increase of 20 dB means that the amplitude of a sound wave has been multiplied by a factor of 10. The Doppler effect describes how a sound wave’s pitch is altered when its source moves toward or away from the listener. Supersonic (faster-than-sound) motion through a substance creates a shock wave in that substance. Read the text aloud
pitch, speed of sound, decibel (dB), supersonic, Doppler effect

v=fλ
f= f 0 ( v s v s v )

Review problems and questions

Amplitude versus time graphs for three sound waves
  1. These three graphs show the relative amplitudes of three different sound waves, each as a function of time.
    1. Which of the three sound waves has the lowest pitch?
    2. Is that pitch high enough for the typical human to hear? Read the text aloud Show
  1. One valuable assistive technology is the hearing aid, an electronic device that contains a small amplifier to boost sound strength.
    1. Suppose that one hearing aid multiplies the amplitude of sound waves by a factor of 10. How many decibels will the hearing aid add to the sounds it detects?
    2. Another hearing aid adds 40 dB to the sounds it detects. By what factor does it multiply sound amplitude? Read the text aloud Show
  1. Return to the graphs near the top of this page. Suppose that Sound A and Sound B both came from the same source and had the same original frequency f0. Now consider that one of the sounds was heard as the source approached you, while the other was heard after the source passed you and receded into the distance.
    1. Which sound (A or B) was from the source when approaching?
    2. The source was moving at a speed of 114 m/s. What was the source’s actual sound frequency, f0? (Assume a sound speed vs of 343 m/s.) Read the text aloud Show

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