Chapter 13 Sound Section 1 Sound Waves

Production of Sound Waves Sound begins with an object vibrating. Compression is the region of a longitudinal wave that has greater density and pressure than normal. Rarefaction is the region of a longitudinal wave that has less density and pressure than normal. Figure 13-2 (a) As this tuning fork vibrates, (b) a series of compressions and rarefactions move away from each prong. (c) The crests of this sine wave correspond to compressions, and the troughs correspond to rarefactions.

Sound Waves Sound waves have vibrations of air molecules parallel to direction of wave motion. Sound waves are longitudinal waves. Audible human hearing is between 20 and 20,000 Hz.

Characteristics of Sound Waves As frequency of a sound wave increases, the pitch rises. Ultra sonic waves can produce images because they have shorter wavelengths. Speed of sound depends on the medium. Figure 13-3 Ultrasound images, such as this one, are formed with reflected sound waves. This colorized image depicts a fetus after 2 1 weeks of development.

The Doppler Effect Relative motion creates a change in frequency. This frequency shift is known as the Doppler Effect, discovered by Austrian physicist Christian Doppler. Pitch depends on frequency. Figure 13-6 As this car moves to the left, an observer in front of the car, at point A, hears the car horn at a higher pitch than the driver, while an observer behind the car, at point B, hears a lower pitch.

Section 2 Sound Intensity and Resonance Chapter 13 Section 2 Sound Intensity and Resonance

Sound Intensity Intensity is the rate of energy flow through a given area. Intensity and frequency determine which sounds are audible. Softest sounds occur at 1000 Hz and an intensity of 1.0 x 10^-12 W/m^2 Loudest sounds have an intensity of 1.0 W/m^2 Figure 13-10

Relative Intensity is measured in Decibels.

Resonance Every system vibrates at a certain frequency. This frequency is known as its natural frequency. Vibration at the natural frequency produces resonance.

Resonance Example Figure 13-12 On November 7, 1940, the Tacoma Narrows suspension bridge collapsed, just four months after it opened. Standing waves caused by strong winds set the bridge in motion and led to its collapse.