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