Chapter 13 Sound

Section 13.1 Sound Waves

The Production of Sound Waves Sound is a result of vibrations or oscillations. Ex: As the prong in the tuning fork swings to the right, the air molecules in front are forced together. As the prong moves to the left, the molecules to the right spread apart, and the density and air pressure in this region became lower than normal.

A region of lower density and pressure is rarefaction. A region of high molecular density and high air pressure is called compression.

Sounds waves are longitudinal waves A longitudinal wave produced by a vibrating object can be represented by a sine curve. The crests in the curve correspond to compressions in the sound wave, and the troughs correspond to rarefactions.

Characteristics of Sound Waves Speed of Sound Video Frequency = the number of cycles per unit of time. Infrasonic waves < 20Hz Ultrasonic waves > 20,000 Hz 20 Hz < audible sound waves (humans) < 20,000 Hz Dogs can hear ultrasonic waves, and elephants can hear infrasonic waves.

Cont.- Frequency determines pitch The frequency of an audible sound wave determines how high of low we perceive the sound to be, known as pitch. As the frequency of a sound wave increases, the pitch rises. As the frequency decreases, the pitch falls.

Ultrasonic waves can produce images When wavelength decreases frequency increases. Infrasonic waves have longer wavelengths than audible sound waves and ultrasonic waves have shorter wavelengths. Audible Ultrasonic Infrasonic

FYI A typical wave used in an ultrasonic device is about 10 MHz. Ultrasonic waves have widespread medical applications like to produce images of objects inside the human body. Sound waves are partially reflected when they reach a boundary between two materials of different densities. They are clearer and more detailed than other those of a lower-frequency sound wave because the short wavelengths of ultrasonic waves are easily reflected off small objects. Audible and infrasonic sound waves are not as effective because their longer wavelengths pass around small objects. A typical wave used in an ultrasonic device is about 10 MHz.

Speed of sound depends on the medium Sound waves can travel through solids, liquids, and gases. Solid particles respond more quickly to vibrations than gas particles because those molecules are closer together. Therefore, sound travels faster in solids.

Temperature In a gas, the disturbance can spread faster at higher temperatures than at lower ones because of an increase in collisions. The temperature differences between liquids and solids is less noticeable due to closer particles.

Sound waves propagate in three dimensions Sound waves travel in all three dimensions. In class, we work from the premise that all sound waves are spherical unless stated otherwise.

The circles represent the centers of compressions, wave fronts.

The distance between adjacent wave fronts is equal to one wavelength. The radial lines perpendicular to the wave fronts are rays. They indicate the direction of the wave motion. Each wave front crossed by a ray corresponds to a crest of the sine curve.

At great distances from the source, wave fronts appear almost parallel and are called plane waves. Plane waves can be treated like a series of identical linear waves.

The Doppler Effect Doppler Effect Video 2:01 Pitch is higher as the object moves towards you and lower as it moves away.

Relative motion creates a change in frequency The frequency of the car horn is constant, but because the source is moving towards observer A, the wave fronts hit Observer A sooner and sooner. As a result, Observer A hears a greater frequency sound resulting in higher pitch. Remember we are discussing frequency and not speed. The speed of the sound waves remains constant.

Relative motion creates a change in frequency The sound waves don’t reach point B, behind the car as frequently as in front of it. Therefore, the frequency heard by the observer is less than the source frequency. The Doppler effect is a shift in frequency due to motion between a wave source and an observer.

The Doppler Effect Applet The Doppler effect will occur when the source or the observer is moving. Even though the it is used mostly with sound waves, the Doppler effect can be used with electromagnetic waves like visible light. The Doppler Effect Applet

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