sound power level:
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1Dept. of Mech. EngineeringUniversity of Kentucky
Sound Power Level: watts101 log10)dB( 1210
refref
w WWWL
Sound Pressure Level:
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 level100 dB (re 1x10-12 W) sound power level
Pa 20 log10)dB( ref
2
ref
rms10
p
ppLp
2Dept. of Mech. EngineeringUniversity of Kentucky
r
r
But they are related…
I
(no reflections)
SLL Wp 10log10 S in m2
24 rS (Spherical source)22 rS (Hemi-spherical source)
S = cross-sectional area (duct)
3Dept. of Mech. EngineeringUniversity of Kentucky
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?
An Example
Pa)20(redB59104log90 210 pL
Pa)20(redB62102log90 210 pL
Pa)20(redB901log90 10 pL
a.
b.
c.
4Dept. of Mech. EngineeringUniversity of Kentucky
Field Quantities
un = velocity of surface in normal direction – must be known
p, u = sound pressure and particle velocity in the field.
How do we determine these?
Numerical Acoustics
5Dept. of Mech. EngineeringUniversity of Kentucky
Two Vacuums
Shopvac Bosch
6Dept. of Mech. EngineeringUniversity of Kentucky
Bosch Vacuum
Exhaust flows through foam
7Dept. of Mech. EngineeringUniversity of Kentucky
Shopvac Vacuum
8Dept. of Mech. EngineeringUniversity of Kentucky
Sound Power
(Watts) SIW )(W/mIntensity Sound Avg 2I
)(m Area Surface 2S
Exhaust Deflected Upward
9Dept. of Mech. EngineeringUniversity of Kentucky
Time and Frequency Domains
10Dept. of Mech. EngineeringUniversity of Kentucky
Sound Power Comparison – Bosch, All Sides
10
20
30
40
50
60
70
80
0 1000 2000 3000 4000 5000
Frequency (Hz)
Sou
nd P
ower
(dB
)
Front LeftBack RightTop Total
11Dept. of Mech. EngineeringUniversity of Kentucky
Sound Power Comparison – Narrow Band
10
20
30
40
50
60
70
80
0 1000 2000 3000 4000 5000
Frequency (Hz)
A-W
eigh
ted
Sou
nd P
ower
(dB
)
Bosch w/ Foam (77.6 dBA)
Bosch w/o Foam (81.2 dBA)
Shopvac (85.0 dBA)
12Dept. of Mech. EngineeringUniversity of Kentucky
Sound Power Comparison at Low Frequency
10
20
30
40
50
60
70
80
100 200 300 400 500
Frequency (Hz)
A-W
eigh
ted
Sou
nd P
ower
(dB
)
Bosch w/ Foam
Bosch w/o Foam
Shopvac
Low Frequency Tone
13Dept. of Mech. EngineeringUniversity of Kentucky
Sound Power Comparison – 1/3 Octave
40
45
50
55
60
65
70
75
80
100 1000 10000
Frequency (Hz)
A-W
eigh
ted
Sou
nd P
ower
(dB
)
Bosch w/ Foam (77.6 dBA)
Bosch w/o Foam (81.2 dBA)
Shopvac (85.0 dBA)
14Dept. of Mech. EngineeringUniversity of Kentucky
Sound Quality
Bosch (original) Bosch (w/o foam)
Shopvac Bosch (w/o 1st peak)
15Dept. of Mech. EngineeringUniversity of Kentucky
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.”
5.74
7.70 8.00
4.61
0123456789
10
BOSCH Foam BOSCH NoFoam
Other Vacuum BOSCH No 1stPeak
Ave
rage
Rat
ing
16Dept. of Mech. EngineeringUniversity of Kentucky
Foam Inside Bosch Vacuum
17Dept. of Mech. EngineeringUniversity of Kentucky
Sound Absorption Coefficient
20 1
incidentenergy soundabsorbedenergy sound R
18Dept. of Mech. EngineeringUniversity of Kentucky
Sound Absorption Coefficient of Foam
0
0.2
0.4
0.6
0.8
1
0 1000 2000 3000 4000
Frequency (Hz)
Abs
orpt
ion
Coe
ffici
ent
19Dept. of Mech. EngineeringUniversity of Kentucky
Sound Intensity (Shopvac)
20Dept. of Mech. EngineeringUniversity of Kentucky
Sound Intensity (Bosch)
21Dept. of Mech. EngineeringUniversity of Kentucky
Sound Intensity (Bosch)
22Dept. of Mech. EngineeringUniversity of Kentucky
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
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