PHYS 2211 Module 15

15: Sound

Hearing is an important human sense that can detect frequencies of sound, ranging between 20 Hz and 20 kHz. However, other species have very different ranges of hearing. Bats, for example, emit clicks in ultrasound, using frequencies beyond 20 kHz. They can detect nearby insects by hearing the echo of these ultrasonic clicks. Ultrasound is important in several human applications, including probing the interior structures of human bodies, Earth, and the Sun. Ultrasound is also useful in industry for nondestructive testing. (credit: modification of work by Angell Williams)

Of all the mechanical waves that occur in nature, the most important in our everyday lives are longitudinal waves in a medium—usually air— called sound waves. The reason is that the human ear is tremendously sensitive and can detect sound waves even of very low intensity. The ability to hear an unseen nocturnal predator was essential to the survival of our ancestors, so it is no exaggeration to say that we humans owe our existence to our highly evolved sense of hearing.

In Module 14 we described mechanical waves primarily in terms of displacement; however, because the ear is primarily sensitive to changes in pressure, it’s often more appropriate to describe sound waves in terms of pressure fluctuations. We’ll study the relationships among displacement, pressure fluctuation, and intensity and the connections between these quantities and human sound perception.

Sound waves travel through the air and other media as oscillations of molecules. Normal human hearing encompasses an impressive range of frequencies from 20 Hz to 20 kHz. Sounds below 20 Hz are called infrasound, whereas those above 20 kHz are called ultrasound. Some animals, like the bat shown above, can hear sounds in the ultrasonic range.

Many of the concepts covered in Module 14 also have applications in the study of sound. For example, when a sound wave encounters an interface between two media with different wave speeds, reflection and transmission of the wave occur.

Ultrasound has many uses in science, engineering, and medicine. Ultrasound is used for nondestructive testing in engineering, such as testing the thickness of coating on metal. In medicine, sound waves are far less destructive than X-rays and can be used to image the fetus in a mother’s womb without danger to the fetus or the mother. Later in this module, we discuss the Doppler effect, which can be used to determine the velocity of blood in the arteries or wind speed in weather systems.

15.1 Sound Waves

Explain the difference between sound and hearing
Describe sound as a wave
List the equations used to model sound waves
Describe compression and rarefactions as they relate to sound

15.2 Speed of Sound

Explain the relationship between wavelength and frequency of sound
Determine the speed of sound in different media
Derive the equation for the speed of sound in air
Determine the speed of sound in air for a given temperature

15.3 Sound Intensity

Define the term intensity
Explain the concept of sound intensity level
Describe how the human ear translates sound

15.4 Interference of Sound Waves

Explain the mechanism behind sound-reducing headphones

17.5 Sources of Musical Sound

Describe the resonant frequencies in instruments that can be modeled as a tube with symmetrical boundary conditions
Describe the resonant frequencies in instruments that can be modeled as a tube with anti-symmetrical boundary conditions

17.6 Beats

Determine the beat frequency produced by two sound waves that differ in frequency
Describe how beats are produced by musical instruments

15.7 The Doppler Effect

Explain the change in observed frequency as a moving source of sound approaches or departs from a stationary observer
Explain the change in observed frequency as an observer moves toward or away from a stationary source of sound

15.8 Shock Waves

Explain the mechanism behind sonic booms
Describe the difference between sonic booms and shock waves
Describe a bow wake