Dept. of Mech. Engineering University of Kentucky 1 Sound Power Level: watt 10 1 log 10 ) dB ( 12 10 ref ref w W W W L 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 level 100 dB (re 1x10 -12 W) sound power level Pa 20 log 10 ) dB ( ref 2 ref rms 10 p p p L p
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 - PowerPoint PPT Presentation
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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
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.
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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
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Two Vacuums
Shopvac Bosch
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Bosch Vacuum
Exhaust flows through foam
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Shopvac Vacuum
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Sound Power
(Watts) SIW )(W/mIntensity Sound Avg 2I
)(m Area Surface 2S
Exhaust Deflected Upward
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Time and Frequency Domains
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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
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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)
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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
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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)
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Sound Quality
Bosch (original) Bosch (w/o foam)
Shopvac Bosch (w/o 1st peak)
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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
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Foam Inside Bosch Vacuum
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Sound Absorption Coefficient
20 1
incidentenergy soundabsorbedenergy sound R
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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
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Sound Intensity (Shopvac)
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Sound Intensity (Bosch)
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Sound Intensity (Bosch)
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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