1. Chapter 16 Outline Sound and Hearing Sound waves Pressure fluctuations Speed of sound General fluid Ideal gas Sound intensity Standing waves Normal modes Instruments Interference and beats

2. Sound Sound is a longitudinal wave. We normally think of sound in air, but it can travel through any medium. The displacement of the medium is along the direction of propagation, so we can also describe the wave in terms of pressure fluctuations. The maxima in displacement magnitude correspond to the minima in the gauge pressure magnitude.

3. Sound Wave Pressure Equation

4. Speed of Sound in a Fluid

5. Speed of Sound in an Ideal Gas

6. Speed of Sound in Air

7. Sound Intensity

8. Decibel Scale

9. Sound Intensity Level Example

10. Standing Sound Waves Just as we had standing waves on strings, we can set up standing waves in air columns. This is the basis of wind instruments. As we showed earlier, we can describe a sound wave in terms of the displacement of the medium or the pressure. A pressure node is always a displacement node, and vice versa.

11. Standing Sound Waves Consider a wave traveling down a pipe. When it reaches an end, it will be reflected. If the end is closed, the displacement at the end must be the zero. This is a displacement node (pressure antinode). If the end is open, the pressure must be the same as the atmospheric (constant) pressure. This is a pressure node (displacement antinode). In the following diagrams, the waves drawn will represent the displacement.

12. Pipe Open at Both Ends

13. Pipe Closed at One End (Stopped Pipe)

14. Standing Waves Example

15. Interference and Beats Standing waves are an example of interference of waves with the same frequency. What if we have two waves with slightly different frequencies? The superposition of the waves will look sinusoidal but with a varying amplitude.

16. Interference and Beats

17. Beats Example

18. Chapter 16 Summary Sound and Hearing