Basics of Sound and Noise. David Herrin, Ph.D., P.E. University of Kentucky Department of Mechanical Engineering

Transcription

1 Basics of Sound and Noise David Herrin, Ph.D., P.E. Department of Mechanical Engineering

2 Ø Public University Ø 16 Colleges Ø 93 Undergraduate Programs Ø 99 M.S. Programs Ø 66 Ph.D. Programs Ø 28,000 Students 2

3 Faculty Ø Ø D. W. Herrin, Ph.D., P.E. T. W. Wu, Ph.D. Students Ø Ø Ø Ø Ø Ø Ø Ø Ø Yitian Zhang (Ph.D.) Gong Cheng (Ph.D.) Kangping Ruan (Ph.D.) Peng Wang (Ph.D.) Keyu Chen (Ph.D.) Weiyun Liu (Ph.D.) Wanlu Li (M.S.) Shishuo Sun (M.S.) Huangxing Chen (M.S.) Vibro-Acoustics Group 3

4 Vibro-Acoustics Consortium HVAC and Refrigeration Industry Ø Emerson Climate Ø Ingersoll Rand Trane Ø JCI York Ø Transicold Carrier Heavy Equipment Industry Diesel Engines Ø Caterpillar Inc. Ø Cummins Inc. Ø Deere and Company Ø Southwest Research Institute Ø Universal Silencers Small Engines / Generator Sets Ø BASCO Ø Kohler Corp. Sound Absorbing Materials Ø 3M Company Ø American Acoustical Products Ø Blachford Inc. Ø Commercial Vehicle Group Ø Federal Foam Technologies Ø Insul-Coustic Corp. Ø Technicon Acoustics Inc. Automotive Supplier / Motorcycle Ø Dana Corp. Ø Eaton Corp. Ø Harley-Davidson Motor Co. Ø Mann+Hummel Group Others Ø Bechtel Marine Propulsion Corporation Ø Ebco Inc. Ø General Electric Appliances Ø Lexmark International

5 Structural Dynamics Lab 5

6 Hemi-Anechoic Chamber 6

7 Acoustic Materials Characterization 1.00 ABSORPTION COEFFICIENT FOR SINGLE AND DOUBLE LAYER ABSORBING MATERIALS ALPHA Double layer Single layer FREQUENCY (Hz) 7

8 Wallin, Carlsson, Åbom, Bodén, and Glav, Sound and Vibration, 2 nd Edition, Marcus Wallenberg Laboratoriet, Stockholm, Sound and Vibration Fields Sound and vibration waves are mechanical elastic waves, and thus the conditions for their existence are that the medium possess mass and elasticity (i.e., stiffness). If a mass particle is displaced from its equilibrium position, the elastic forces will seek to return it to its original position. The particle influences the surrounding particles and in this way, a disturbance (i.e., wave) propagates through the medium. Wallin et al. Longitudinal Transverse 8

9 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 λ = 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 9

10 Wallin, Carlsson, Åbom, Bodén, and Glav, Sound and Vibration, 2 nd Edition, Marcus Wallenberg Laboratoriet, Stockholm, Sound and Vibration Fields An acoustic field implies a small disturbance. Sound pressure disturbances are only on the order of 1 Pa for 94 db. 10

11 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 d( t) = Dsin 2πft Particle displacement amplitude D 11

12 Particle Velocity u( t) = ( 2πfD) cos2πft Particle velocity amplitude (m/s) u increases with frequency for a constant displacement Particle velocity is like current, sound pressure like voltage Particle displacement amplitude D 12

13 Field Quantities p, u = sound pressure and particle velocity in the field. How do we determine these? u n = velocity of surface in normal direction must be known 13 Numerical Acoustics

14 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 = IndS S (watts) 14

15 An Analogy Like temperature, the sound pressure depends on the source power level AND the environment in which the source is placed. 15

16 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, close fitting enclosures) 16

17 Special Cases 1. Plane Waves with no reflection Oscillating Piston u n λ Plane waves in a duct u = u p = n + a phase shift ( ρ c) u = z u o o I = pu z o = characteristic impedance p = p = oc ρ W = IS 17 2 p ρ c o

18 Special Cases 2. In the far field* of a source in a free field p, u p = ρ cu o I = p p ρoc = 2 p ρ c o (like plane waves in a duct except the sound pressure decreases with distance) * The far field is where the SPL decreases by 6 db for a doubling of the distance to the source 18

19 Combining Acoustic Sources Addition of Sound Sources p tot For Two Sources p 2 tot = 1 T T 0 2 p tot N ( t) = p n t ( t)dt = p p T n=1 T 0 = 1 T p 1 ( ) T 0 ( p 1 ( t) + p 2 ( t) ) 2 dt = ( t) p 2 ( t)dt 19

20 Uncorrelated Sources p 2 tot = p p p n Addition by Sound Pressure Level p 2 = p 2 ref 10 L p /10 p 2 tot = p p = p ref ( 10 L p1 / L p2 /10 ) L ptot =10 log( 10 L p1 / L p2 /10 ) In general N n=1 L ptot =10 log 10 L pn /10 20

21 Example A sound source causes a sound pressure level of L p1 at a certain point. What increase in SPL is provided by a second source, equal in strength to, but uncorrelated to, the first? L ptot =10log( 10 Lp1/ L p1/10 ) =10 log( 2 10 L p1 /10 ) = L p1 +10 log( 2) = L p1 + 3 db 21

22 Example A machine causes a SPL of 90 db at a certain point while the background noise is 83 db. What is the SPL of the machine with the background noise removed. p 2 tot = p 2 2 M + p B p M 2 = p 2 2 tot p B L pm =10 log( 10 L tot /10 10 L pb /10 ) = 89 db 22

23 Adding Frequency Components p tot N 2 = p n 2 n=1 L ptot =10log 10 L pn /10 N n=1 The 1000 Hz octave band includes the 800, 1000, and 1250 Hz third-octave bands. Determine the octave band level if the thirdoctave band levels are 79, 86 and 84 db, respectively. N n=1 ( ) L ptot =10 log

24 Those Amazing db s Sound Pressure Level: Sound Power Level: L p L w p rms ( db) = 10 log pref = 20 pref W (db) = 10 log10 Wref = 1 10 W ref 2 10 µ 12 Pa watts 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 µpa) sound pressure level 100 db (re 1x10-12 W) sound power level 24

25 But they are related L = L 10 log S S in m 2 p W 10 (no reflections) r S S 2 = 4π r (Spherical source) 2 = 2π r (Hemi-spherical source) S = cross-sectional area (duct) r I 25

26 An Example 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. L p = 2 ( 10) 59 db (re 20 Pa) 90 log10 4π = µ b. c. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. 26

27 Two Vacuums Shopvac Bosch 27

28 Bosch Vacuum Exhaust flows through foam 28

29 Shopvac Vacuum 29

30 Sound Power Exhaust Deflected Upward I S W = Σ IS (Watts) = Avg Sound Intensity (W/m 2 = Surface Area (m ) 2 ) 30

31 Time and Frequency Domains 31

32 Sound Power Comparison Bosch, All Sides Front Back Left Right Sound Power (db) Top Total Frequency (Hz) 32

33 Sound Power Comparison Narrow Band 80 A-Weighted Sound Power (db) Bosch w/ Foam (77.6 dba) Bosch w/o Foam (81.2 dba) Shopvac (85.0 dba) Frequency (Hz) 33

34 Sound Power Comparison at Low Frequency A-Weighted Sound Power (db) Low Frequency Tone Bosch w/ Foam Bosch w/o Foam Shopvac Frequency (Hz) 34

35 Sound Power Comparison 1/3 Octave A-Weighted Sound Power (db) Bosch w/ Foam (77.6 dba) Bosch w/o Foam (81.2 dba) Shopvac (85.0 dba) Frequency (Hz) 35

36 Sound Quality Bosch (original) Bosch (w/o foam) Shopvac Bosch (w/o 1 st peak) 36

37 Sound Quality: Jury Test Average Rating BOSCH Foam BOSCH No Foam 4.61 Other Vacuum BOSCH No 1st Peak Note: Rate each vacuum on a scale from 1 to 10 where 1 is very quiet and 10 is very loud. 37

38 Foam Inside Bosch Vacuum 38

39 Sound Absorption Coefficient α sound energy absorbed sound energy incident 0 = = 1 R 2 39

40 Sound Absorption Coefficient of Foam 1 Absorption Coefficient Frequency (Hz) 40

41 Sound Intensity (Shopvac) 41

42 Sound Intensity (Bosch) 42

43 Sound Intensity (Bosch) 43

44 Summary q Absorption of foam in BOSCH significantly reduces sound power q Sound exhaust is better directed on BOSCH q Recommend altering design to reduce/shift first peak 44