Chap-17 Waves-II Sound Waves are longitudinal mechanical waves that can travel through solids, liquids, or gases. Point S represents a tiny sound source, called a point source, that emits sound waves in all directions. Wavefronts are surfaces over which the oscillations due to the sound wave have the same value. Rays are directed lines perpendicular to the wavefronts that indicate the direction of travel of the wavefronts. Near the point source the wavefronts are spherical and spread out in three dimensions. As the wavefronts move outward and their radii become larger, their curvature decreases. Far from the source, we approximate the wavefronts as planes (or lines on two-dimensional drawings), and the waves are said to be planar.

The Speed of Sound, v Speed of sound in a stretched string is given by: (T = tension, μ = linear density) Medium Speed (m/s) Air (0 °C) 331 Air (20 °C) 343 Helium 965 Hydrogen 1284 Water (0 °C) 1402 Water (20 °C) 1482 Seawater 1522 Aluminum 6420 Steel 5941 Speed of sound in a medium with bulk modulus B and densityρ:

Sources of Musical Sound

The Doppler Effect The Doppler effect is a change in the observed frequency of a wave when the source or the detector moves relative to the transmitting medium (such as air). The Doppler effect holds not only for sound waves but also for electromagnetic waves. The observed frequency for sound waves is given by: When the motion of detector or source is toward the other, the sign on its speed must give an upward shift in frequency. When the motion of detector or source is away from the other, the sign on its speed must give a downward shift in frequency.

Detector Moving, Source Stationary

Source Moving Towards Observer