Lecture 11 Wave Motion. Sound Waves. Chapter 6.1  6.7 Outline Waves in General Wave Parameters Standing Waves Sound Waves

Two main categories of waves: World of Waves Water waves Sound waves Radiation (electromagnetic) waves A wave is a periodic disturbance that spreads out from a source and carries energy Two main categories of waves: Mechanical waves Electromagnetic waves

Water Waves

Transverse and Longitudinal Waves Particles move perpendicular to the wave motion direction in transverse waves. In longitudinal waves, the particles move in the wave motion direction. Transverse waves occur only in solids, because each particle has to drag a tightly bound adjacent one. Longitudinal waves may propagate in any substance, because each particle has to just push on the neighbors. Demonstration

Wave Parameters Demonstration The distance between 2 adjacent crests (or troughs) is called wavelength (). The number of crests that pass a given point each second is called frequency (f). The time needed for a complete wave to pass a given point is called period (T). The rate at which each crest moves is called speed (v). The height of a wave above the undisturbed level is called amplitude (A).

Relations between the Wave Parameters v = f   = v/f f = v/ T = 1/f The frequency f is measured in cycles/second, a unit called the hertz (Hz). Demonstration

Examples of standing waves: Demonstration1 Demonstration2 Examples of standing waves: Vibrating strings in musical instruments Vibrating air columns in whistles, flutes, organ pipes, etc.

Sound Waves Sound waves are longitudinal, compression waves in the air. The frequency range of sound waves is 20  20,000 Hz. Humans’ ears are most sensitive to the range 3,000  4000 Hz. Frequencies below 20 Hz are called infrasound. Frequencies above 20,000 Hz (20kHz)  ultrasound. The power of sound is measured in decibel (dB). ~0 dB  whisper, ~130 dB  airplane engine. A range of 100dB corresponds to an amplitude range of 105.

Doppler Effect An approaching sound has a higher pitch. A recessing sound has a lower pitch. This effect was first explained by Christian Doppler in 1842. It is due to a frequency (and wavelength) change, resulting from a source relative motion with respect to the observer. The Doppler effect occurs in light waves and is a tool to measure motion of the stars in astronomy.

Summary Waves carry energy with them, not particles. There are two major types of waves by nature: mechanical and electromagnetic. There are two major types of waves by the particle motion direction: longitudinal and transverse. Sound waves are mechanical, longitudinal waves, propagating in the air. The Doppler effect represents a change in frequency of a moving source and is used to measure wave speeds.