1. 1. Sound Waves Learning Object (LO4) Naysilla Dayanara Section L2H
2. 2. Longitudinal Waves Waves in which the displacement of the medium is in the same direction, or opposite to, the direction of travel.
3. 3. Medium The molecules of the medium oscillate as sound wave passes through Stretched = Rarefraction PRESSURE IS LOWERED Compressed=Compression PRESSUREISELEVATED
4. 4. Different Ways to Describe Sound Wave P vs. position (x) Displacement (y) vs. Position (x)
5. 5. How much mass is oscillating Stiffness in 2D How much the length of the string changes when we exert a force on it Speed of Sound Recall textbook Sec 14-4 (p. 388) on Wave speed on a String Depends on INERTIAL & ELASTIC properties of the medium Linear mass density () Tension of String which gives us the equation: Stiffness in 3D waves Measure by what fraction the volume changes when we change the pressure exerted on the material
6. 6. Speed of Sound The 3D equivalent of Stiffness is called the Bulk Modulus Ratio of (P) and fractional change in volume (V/V) Negative (-) sign: because V/V is always opposite of the sign of P Similar to this equation How much mass is oscillating Density of medium, how individual particle oscillates.
7. 7. Displacement, Pressure, Intensity At High pressure: Particles are pushed into it from left and right. Hence, at Pmax, displacement must be 0 Left Side (+) displacement Right Side (-) displacement Positive Negative Similarly, at Pmin, displacement is also 0
8. 8. Amplitude of pressure variation Comparing equations for P and s(x,t), we see that although the wave function has a cosine function and the pressure is a sine function, the arguments are the same in both cases. They have the same wavelength, period, and wave speed but are /2 out of phase (between sin and cos) This relationship is plotted in the next slide.
9. 9. Comparison
10. 10. Displacement, Pressure, Intensity We now examine the energy that a sound wave delivers (INTENSITY): Power delivered per unit area Where P is the rate at which the wave delivers energy, and A is the area that the wave is impinging upon. As shown previously (one dimension) For a sound wave, replace the (linear mass density) with rho (mass density). This substitution gives a new unit of W/m2 (Power/area = I) Therefore