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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