10 Design of Mufflers Silencers .edu
[Pages:43]Design of Mufflers and Silencers
D. W. Herrin, Ph.D., P.E. University of Kentucky Department of Mechanical Engineering
Types of Mufflers
Design of Mufflers and Silencers 1. Dissipative (absorptive) silencer:
Duct or pipe
Sound absorbing material (e.g., duct liner)
Sound is attenuated due to absorption (conversion to
heat)
Dept. of Mech. Engineering
2
University of Kentucky
Noise and Vibration Short Course
Types of Mufflers
Design of Mufflers and Silencers
2. Reactive muffler:
Sound is attenuated by reflection and "cancellation" of sound waves
Compressor discharge details 40 mm
Dept. of Mech. Engineering
3
University of Kentucky
Noise and Vibration Short Course
Types of Mufflers
Design of Mufflers and Silencers 3. Combination reactive and dissipative muffler:
Sound absorbing material
Perforated tubes
Sound is attenuated by reflection and "cancellation" of sound waves + absorption of sound
Dept. of Mech. Engineering
4
University of Kentucky
Noise and Vibration Short Course
Performance Measures Transmission Loss
Design of Mufflers and Silencers
Wi Wr
Muffler
Wt
Anechoic Termination
Transmission loss (TL) of the muffler:
TL(dB)
=
10
log10
Wi Wt
Dept. of Mech. Engineering
5
University of Kentucky
Noise and Vibration Short Course
Performance Measures Insertion Loss
Design of Mufflers and Silencers
SPL1
SPL2
Muffler
IL (dB) = SPL1 ? SPL2
Insertion loss depends on :
G TL of muffler G Lengths of pipes G Termination (baffled vs. unbaffled) G Source impedance
Note: TL is a property of the muffler; IL is a "system" performance measure.
Dept. of Mech. Engineering
6
University of Kentucky
Noise and Vibration Short Course
Example TL and IL
Design of Mufflers and Silencers
Expansion Chamber Muffler
Inlet Pipe
Source
2" 24"
20
6" 12"
10
TL and IL (dB)
0 0
-10
200
400
600
800
1000
-20
-30 -40 -50
Pipe resonances
Insertion Loss Transmission Loss
Frequency (Hz)
Outlet Pipe
12"
Dept. of Mech. Engineering
7
University of Kentucky
Noise and Vibration Short Course
Acoustic System Components
Design of Mufflers and Silencers
Source
Su Su
Any acoustic system
P
Zt
(sound pressure
reaction)
Input or load impedance
z = P = r + jx Su
Termination impedance
zt
=
Pt Sut
= rt
+
jxt
Dept. of Mech. Engineering
8
University of Kentucky
Noise and Vibration Short Course
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