1. Dept. of Mech. Engineering University of Kentucky 1 Wave Motion – Some Basics Sound waves are pressure disturbances in fluids, such as air, caused by vibration, turbulence, explosions, etc. These disturbances propagate at the speed of sound c ( c = 343 m/s or 1125 ft/s in air at room temperature) The wavelength = c/f. For f = 1 kHz, the wavelength is approximately 0.34 m or 1.13 ft. As a sound wave passes a point, the fluid particles are displaced but return to their original position until the next wave passes. Wave animation

2. Dept. of Mech. Engineering University of Kentucky 2 Traveling and Standing Waves Traveling waves are waves that propagate away from a sound source at the speed of sound A traveling wave moving in the opposite direction, e.g., due to a reflection, combines with the original traveling wave to produce a standing wave Animation of a standing wave

3. Dept. of Mech. Engineering University of Kentucky 3 Particle Motion Particles oscillate (but no net flow) Waves move much faster than particles Surface displacement determines particle displacement and resulting sound pressure, as well as frequency Particle displacement amplitude D

4. Dept. of Mech. Engineering University of Kentucky 4 Particle Velocity Particle displacement amplitude D Particle velocity amplitude (m/s) u increases with frequency for a constant displacement Particle velocity is like current, sound pressure like voltage

5. Dept. of Mech. Engineering University of Kentucky 5 Sound Intensity and Power Sound intensity is the sound power radiated per unit area To get sound power, we integrate the normal component of the sound intensity over a closed surface I (watts)

6. Dept. of Mech. Engineering University of Kentucky 6 An Analogy Like temperature, the sound pressure depends on the source power level AND the environment in which the source is placed.

7. Dept. of Mech. Engineering University of Kentucky 7 Another Analogy A light bulb produces the same optical power (in watts) regardless of its environment – big or small room – but the intensity of light depends on the environment (reflectance of the walls) and the distance from the light bulb. A sound source produces the same sound power (in watts) regardless of its environment* – big or small room – but the intensity of sound and the sound pressure depend on the environment (reflectance of the walls) and the distance from the source. _____ * There are some notable exceptions to this (exhaust noise)

8. Dept. of Mech. Engineering University of Kentucky 8 Plane waves in a duct Oscillating Piston u n Special Cases 1. Plane Waves with no reflection z o = characteristic impedance

9. Dept. of Mech. Engineering University of Kentucky 9 2. In the far field* of a source in a free field p, u Special Cases * The far field is where the SPL decreases by 6 dB for a doubling of the distance to the source (like plane waves in a duct except the sound pressure decreases with distance)

10. Dept. of Mech. Engineering University of Kentucky 10 Sound Power Level: Sound Pressure Level: Those Amazing dB’s The main thing to remember is that 100 dB sound pressure level and 100 dB sound power level are completely different! To avoid confusion, use the reference values: 100 dB (re 20  Pa) sound pressure level 100 dB (re 1x10 -12 W) sound power level

11. Dept. of Mech. Engineering University of Kentucky 11 r r But they are related… (no reflections) S in m 2 (Spherical source) (Hemi-spherical source) S = cross-sectional area (duct)

12. Dept. of Mech. Engineering University of Kentucky 12 A source has a sound power level of 90 dB (re 10 -12 W). What is the sound pressure level at a distance of 10 m in (a) a free field, (b) in a hemispherical free field, and (c) in a duct of cross-sectional area 1 m 2 ? An Example a. b. c.

18. Konversi Tekanan Bunyi

19. Tabel dB Level

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