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1 Introduction David Herrin
2 Wave Animation 2
3 Wave Motion Some Basics Sound waves are pressure disturbances in fluids, such as air or hydraulic fluid as a result of vibration, turbulence, pumping, 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. The wavelength is the acoustic yardstick. 3
4 Particle Motion Particles oscillate (but no net flow) Waves move much faster than particles Particle Displacement sin 2 Particle Velocity 2 cos 2 Particle displacement amplitude D 4
5 Overview Sound Power and Decibels Measuring Sound Power Noise Paths 5
6 Sound Intensity and Power u, p I pu Sound intensity is the sound power radiated per unit area I To get sound power, we integrate the normal component of the sound intensity over a closed surface W I nds S (watts) 6
7 An Analogy Like temperature, the sound pressure depends on the source power level AND the environment in which the source is placed. 7
8 The Decibel Scale Sound Pressure Level: Sound Power Level: L p L w p rms ( db) 10log10 pref 20 Pa pref W 12 (db) 10log10 Wref 1 10 watts W ref 2 The main thing to remember is that 100 db sound pressure level and 100 db sound power level are completely different! 8
9 Sound Power to Sound Pressure L L 10log S S in m 2 p W 10 (no reflections) r S 4 r S 2 r 2 2 (Spherical source) (Hemi-spherical source) S = cross-sectional area (duct) r I 9
10 An Example NVH Training Workshop March 7, 2018 A source has a sound power level of 90 db (re 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? a. b. c. L p L p L p db (re 20 Pa) 90 log db (re 20 Pa) 1 90dB (re 20 Pa) 90 log log10 10
11 Overview Sound Power and Decibels Measuring Sound Power Noise Paths 11
12 Hemispherical Free Field Divide surface S into sub-areas S Measure sound pressure at a central point in each area Sum up mean-square sound pressures weighted by areas N sound pressure measurement points S W S 12
13 Munjal, 2013 NVH Training Workshop March 7, 2018 Hemispherical Surface Standard Surfaces Parallelepiped Surface 13
14 Overview Sound Power and Decibels Measuring Sound Power Noise Paths 14
15 Washburn and Wood, 2016 NVH Training Workshop March 7, 2018 Heavy Equipment Multiple Sources 15
16 Ivanov and Copley, 2007 NVH Training Workshop March 7, 2018 Database of Source Data 16
17 T.O.C. Template Source Side Forcing Functions (Source) Energy Paths (Source) Example: Intake / Exhaust Combustion Combustion Airborne Sound Emitting Orifices Intake Exhaust Example: Engine Structure-Borne Mechanical Dynamics Piston Slap Internal Gears Bearing Impacts Etc. Example: Hydraulics/ Hydrostatics Engine Structure Vibration Path Engine Mounts Etc. Radiating Surfaces Front Cover Engine Block Valve Covers Oil Pan Fluid-Borne Flow Ripple Kinematic Component Inertial Component Fluid-Borne Path Vibration Path Hose / Pipe Attachments Etc. Radiating Surfaces Reservoir Etc. The source energy paths are characterized by the whether the forcing function is airborne, structure-borne, or Fluid-Borne.
18 T.O.C. Receiver or Reactive Side Template Template Reactive / Receiver Side Energy Paths (Source) From Source Side Page Energy Paths (Reactive) Airborne Direct Air / Air Receiver (Reactive) Sound Emitting Orifices Intake Exhaust Exterior Air Holes/Leaks Cab Airspace Airborne Indirect Air / Structure / Air Operator s Ears Radiating Surfaces Front Cover Engine Block Etc. Engine Compartment Undercarriage Firewall Windows Quarter Panels Floor Cab Airspace SPL Sound Quality Structureborne Structure / Air Environmental Emission Vibration Path Hose / Pipe Attachments Etc. Rear Cab Panels Cab Airspace Sound Power Sound Pressure (Passby) Structure-Borne Vibration Vibration Vibration Path Engine Mounts Chassis Cab Mounts ROPS Floor Floor Isolation Seat
19 What is the Source Attached to? Noisy Better 19
20 Summary How are sound pressure and sound power different? What is the importance of each? How is sound power measured? Why is hydraulic noise difficult to treat? 20
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