Amazing dB’s Sound Pressure Level: Sound Power Level: 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 Dept. of Mech. Engineering University of Kentucky 1
But they are related… S in m2 S = cross-sectional area (duct) (no reflections) (Spherical source) r (Hemi-spherical source) S = cross-sectional area (duct) r Dept. of Mech. Engineering University of Kentucky 2
An Example 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 m2? a. b. c. Dept. of Mech. Engineering University of Kentucky 3
Field Quantities How do we determine these? Numerical Acoustics p, u = sound pressure and particle velocity in the field. How do we determine these? Numerical Acoustics un = velocity of surface in normal direction – must be known Dept. of Mech. Engineering University of Kentucky 4
Two Vacuums Shopvac Bosch Dept. of Mech. Engineering 5 University of Kentucky 5
Bosch Vacuum Exhaust flows through foam Dept. of Mech. Engineering 6 University of Kentucky 6
Shopvac Vacuum Dept. of Mech. Engineering University of Kentucky 7
Sound Power Exhaust Deflected Upward Dept. of Mech. Engineering 8 University of Kentucky 8
Time and Frequency Domains Dept. of Mech. Engineering University of Kentucky 9
Sound Power Comparison – Bosch, All Sides Dept. of Mech. Engineering University of Kentucky 10
Sound Power Comparison – Narrow Band Dept. of Mech. Engineering University of Kentucky 11
Sound Power Comparison at Low Frequency Low Frequency Tone Dept. of Mech. Engineering University of Kentucky 12
Sound Power Comparison – 1/3 Octave Dept. of Mech. Engineering University of Kentucky 13
Sound Quality Bosch (original) Bosch (w/o foam) Shopvac Bosch (w/o 1st peak) Dept. of Mech. Engineering University of Kentucky 14
Sound Quality: Jury Test Note: Rate each vacuum on a scale from 1 to 10 where 1 is “very quiet” and 10 is “very loud.” Dept. of Mech. Engineering University of Kentucky 15
Foam Inside Bosch Vacuum Dept. of Mech. Engineering University of Kentucky 16
Sound Absorption Coefficient Dept. of Mech. Engineering University of Kentucky 17
Sound Absorption Coefficient of Foam Dept. of Mech. Engineering University of Kentucky 18
Sound Intensity (Shopvac) Dept. of Mech. Engineering University of Kentucky 19
Sound Intensity (Bosch) Dept. of Mech. Engineering University of Kentucky 20
Sound Intensity (Bosch) Dept. of Mech. Engineering University of Kentucky 21
Summary Absorption of foam in BOSCH significantly reduces sound power Sound exhaust is better directed on BOSCH Recommend altering design to reduce/shift first peak Dept. of Mech. Engineering University of Kentucky 22