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Noise Control in HVAC Active Passive or None

Dr Geoff Leventhall

Noise and Vibration Consultant

[email protected]

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Noise Sources in HVAC - B5

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7Fan Noise Propagation

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Control

ofNoise Transmission in Ducts

Attenuation of Travelling Noise

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All duct systems have

Frequency dependent

attenuation

Frequency

dB

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Lined Ducts used widely in the USA.Lined Ducts used widely in the USA.

Gives noise attenuation and thermalinsulation, but raises concerns for

health. Not now recommended for

sensitive locations except with specialprecautions. Older systems taken out,

for example in schools and health care.UK uses more localised silencers than

USA. UK ~100%. USA ~50%

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Lined Duct

Rectangular duct attenuation depends on

Perimeter/area, (Pd /Ad), and Lining Thickness

Attenuation is not as well known for circular

ducts

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Johns Manville Superduct RC Air Duct Board

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[From CIBSE B5]

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Significance of Pd

/Ad

Duct

Dimensions

Pd /Ad

3m x 3m 1.3

1m x 1m 4

1.2m x 0.3m 8

0.3m x 0.3m 13

0.15m x 0.15m 27

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Localised Attenuation

Parallel baffle (passive) silencer

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20Parallel Baffle Silencer

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Duct SilencersDuct Silencers -- parallel baffleparallel baffleConsiderations

• Length, width, height….. weight

• Baffle thickness, Air gap

• Air velocity

• Pressure loss - varies as (velocity)2

• Location wrt fan, bends etc

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Silencer attenuation 200mm/200mm

Variation w ith length

0

10

20

30

40

50

60

63 125 250 500 1000 2000 4000 8000

Octave band frequency Hz

A t t e n u a t i o n

d B

2400mm

2100mm

1800mm

1500mm

1200mm

900mm

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Limitations of Passive SilencersLimitations of Passive Silencers

• Poor low frequency attenuation

• LF attenuation improved by

narrow air gaps and longerlength

• This leads to higher pressure loss

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Energy loss in silencers

Power (W) used against pressure loss

Air flow (m3/s) x pressure loss(Pa)

fan-motor efficiency

10m3/s into 100Pa > 1KW

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Energy use by fans in the UK

(Lockwood - FETA Magazine December 2005)

Fans in the range 1.1kW to 400kW consume

33.5TWh per year of electricity33.5TWh per year of electricity

Potential saving of

22.5% (7.5TWh)22.5% (7.5TWh)

by improved fan and system efficiency.

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Signal

mic

Loudspeaker

ControllerControlmic

Active silencer

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Active silencers Active silencers• Good low frequency attenuation

• Normally combine active/passive

for full range attenuation

• Lower pressure loss

• Smaller size and weight than

passive for similar LF performance

• Higher initial cost?

A ti Sil tt tiA ti Sil tt ti

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Active Silencer attenuation Active Silencer attenuation

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Silencer Requirements:

1.5 m length;

<50 Pa pressure loss;

>25 dB attenuation at 125 Hz

Non-porous media

Not possible by passive silencer

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Silencer Dimensions:

1500 L x 1400 W x 1600 H

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1400 mm

1600 mm

View from reverb room into end of silencer from discharge side

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Gas Platform AHU Active-Passive Silencer

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0

10

20

30

40

50

60

70

80

90

100

63 125 250 500 1000 2000 4000 8000

Octave Band Frequency

S o u n d

P r e s s u r e

L e v e l

Passive Active

Vibro-Acoustics

Wise Associates

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Passive silencing

Active silencing

Is no silencing possible?

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Is no silencing possible?

• Very quiet fan operating optimally• Good duct design

• Modify our noise level requirements

- less demanding criteria

[One of the necessary lifestyle changes

in the future?]

Fan is quietest at maximum efficiencyFan is quietest at maximum efficiency

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38[ASHRAE 2003]

Fan is quietest at maximum efficiencyFan is quietest at maximum efficiency

Fan outlet configuration - 1

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g

Slope 1 in 7

A > 1.5B

From SMACNA Guide - HVAC Sound and Vibration Manual 2005

F tl t fi ti 2

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Fan outlet configuration - 2

Full radius turningvanes

Minimum

150mm

radius

From SMACNA Guide

Reducing air flow generated noise

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From SMACNA Guide

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42From SMACNA Guide

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43From SMACNA Guide

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44From SMACNA Guide

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Most duct elements have

attenuation which increases withfrequency, although attenuation

may drop at higher frequencies.Lined bends simple and effective

Hz

dB LinedUnlined

Residual low frequency attenuation

requirements obtained by active silencers

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