Noise Reduction Silencers

Overview

Engineering reference data for Noise Reduction Silencers in acoustics.

Key Formulas

Speed of Sound

$c = \sqrt{\gamma R T}$

Speed of sound in an ideal gas.

Sound Level

$L = 10 \log_{10}(I/I_0)$

Decibel level.

Wavelength

$\lambda = c / f$

Wavelength = speed / frequency.

Variables

Symbol	Description	Unit
$c$	Speed of sound	m/s
$L$	Sound level	dB
$\lambda$	Wavelength	m
$f$	Frequency	Hz

Silencer Performance Data

Silencer Classifications

Absorptive (Dissipative) Silencers

• Use sound-absorbing materials (fibrous linings) to attenuate sound.
• Energy is transformed to heat via fiber motion.
• Baffle thickness is selected based on the predominant noise frequency.
• Common types: lined ducts, packaged attenuators, acoustic louvers, lined plenum chambers.
• Widely used in HVAC systems.

Reflective (Reactive) Silencers

• Reflect sound waves back toward the source using tuned cavities or membranes.
• Operate on principles of lambda/4- and Helmholtz-resonators.
• Excel at attenuating low-frequency, pure-tone noise.
• Non-fibrous, cleanable, with low pressure drop.
• Best for fixed-speed machinery; rarely used in HVAC.

Diffuser (Depressive) Silencers

• Use perforated "pepper pots" to slow flow velocity and reduce low-frequency noise generation.
• Divide total pressure drop across multiple stages (e.g., nozzle, valve, diffuser).
• Primarily used for nozzles, control valves, and jet engines.

Active Silencers

• Employ electro-acoustic means (microphones, speakers, electronics) to generate an anti-phase sound field that cancels noise.
• Effective for steady, low-frequency noise (< 300 Hz) from sources like fans and engines.
• Not suitable for broadband noise reduction.

Interactive Charts

References

• Original Source