noise control

Noise control measures

R.Narasimha SwamySenior consultant

[email protected]

Contents

• Effect of noise on human beings

• Basic measures of noise control

• Case study of a HVAC system

Effect of noise on humans

• Affects peace of mind, social behaviour and concentration.

• Causes discomfort, irritation and annoyance.

• May cause hearing impairment.• Affects other physiological functioning of

the body.• In totality, disturbs the life style and

affects the productivity.

Effect of noise and vibration on humans

Four noise control mistakes to avoid1. Thinking that there is no noise problem! NL greater than

85dB in factory environment and level as low as a 55dB in a class room/Studios should be considered as a problem. In general, if conversation is difficult, treat it as a serious noise issue.

2. Not considering noise control during the design stage: Although all source of noise can be treated after installation, it’s generally twice as expensive and half as effective compared with designing proper noise control into the system before the noise source is installed.

3. Not sealing the air leaks: Sound always takes the easiest path around or through a barrier. Construction gaps or air leaks are the easiest way for sound to pass from one space to another.

4. Not using a systems approach to noise control: A common mistake in noise control effort is the failure to consider all possible noise paths. All airborne and structure borne noise paths must be studied and treated accordingly.

Basic measures of noise control

• Absorption• Damping• Decoupling• Mass• Flow control

• Absorption by installing appropriate sound absorbing solutions in the form of wall panels, false ceiling, carpeting. This is necessary for any serious soundproofing project.

• Decoupling to isolate walls from the studs, thereby breaking the direct path of sound, by using resilient channel and sound clips. This decoupling technique adds resilience to the walls.

• Damping is the process of sound insulation by installing the non vibrating dead panels as barriers. Damping is improved by applying suitable compounds in between two constrained layers.

• Mass simply means creating a heavier wall by using more (another layer) and/or thicker material.

• Flow control measures to avoid turbulence and resulting noise and vibration in free flow of fluids in pipe lines, ducts etc.

Noise Reduction by AbsorptionIncreased absorption reduces ambient noise

Noise reduction by absorption and decoupling

Ceiling tiles

Fabric-wrapped glass wool panels absorb sound

Noise Reduction• Combined effect of TL and absorption

• NR = TL - 10 log (S/AR) NR: noise reduction (db) TL: transmission loss (db) S: area of barrier wall (ft2) AR: total absorption of receiving room (Sabins, ft2)

Decoupling measures

Directivity index

Pressure increases at walls

Resilient Channel

Resilient Clip

Resilient Duct Hangers

Elastomeric Hanger

From Kirkegaard Associates

Resilient Duct Hangers

Spring-and-Neoprene-in Series Isolator (Hanger)


Pre-compressed Spring-and-Neoprene-in-Series Isolator (Hanger)

Dampers

Isolator Types

Elastomeric Pads

Isolator Types

Neoprene-In-Shear Floor Mount

Isolator Types

Open Spring Floor Mount

Isolator Types

Restrained Open Spring Floor Mount

Flow control measures

Plenum chamber

Wind generated tones can be avoidedby profile changes or spoilers

Smooth ducts and pipes create less

turbulence noise

LF exhaust noise transformedto HF is easier to attenuate

Flow noise in pipes is formed bysudden pressure changes

Case study of noise control in HVAC system

48

Typical Sound Paths AirborneSound that travels through supply ductwork, return ductwork, or an open plenum

Can travel with or against the direction of airflow

BreakoutSound that breaks out through the walls of the supply or return ductwork

TransmissionSound that travels through walls, floors, or ceilings

Main HVAC Noise Sources

• Fans (for air circulation)– Axial– Centrifugal– Propeller

• Compressors (that convert gas to liquid)– Piston– Rotary– Scroll– Centrifugal– Screw

• Pumps (to circulate liquids)

• Diffusers and Ductwork (to distribute air)– Turbulent aerodynamic

noise– “Break-out” noise

Noise Control Approaches in HVAC

• Location• Sealing penetrations • Resilient mounting & connected services• Flexible connections to equipment to lower fluid

velocities• Internal duct lining and attenuators• Routing of ductwork and piping• Enclosing ductwork and piping

Fan Coil Units• Opportunity for

significant noise issues:– Fan and coil in close

proximity: high turbulence

– Applications: typically close to “listeners” (hotel rooms, etc.)

– Water flow noise

Typical Air-Handler Design

Equipment Location: Rooftop

Equipment Location: Mechanical Equipment Room

• Noise inside the MER• Noise outside the

MER• Duct Breakout• Active Noise Control

Fan Noise Components

• 1 duct length• 3 duct length• 5 duct length

• Aerodynamic noise• Blade-passage noise

fB = (RPM/60) ·N N = number of blades

Fan Noise

Fan noise depends on the fan operation point on the fan curve

Fan and Compressor Noise

Diffuser Noise

• Flow sets the noise level at a given static pressure level forcing the flow

• Good aerodynamics are important to lower the noise from air terminals

From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (Long Fig. 13.23, p. 474)

How Ductwork Radiates Noise (Break Out)

Duct Shape and Noise Control

From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

• Stiffness of round ductwork reduces break-out noise since motion of the duct walls is restricted

• However, this means that more noise energy stays within the duct and may produce higher noise levels at the outlet

Duct Liner

MJR Figure 9.5 and 9.7, pp. 193 and 194

Duct Lagging

Make the ducts stiff using lagging, typically fire-rated drywall.

Duct Lagging

MJR Figure 9.14, p. 200

Duct Lagging


Resilient Hangers


Flexible Duct Connections


Application of Duct Liner in Underfloor Plenum


Lined Plenum(For under-floor air supply)

Silencer Location


Duct Sound Attenuators


Air Plenums, Passive and Active Silencers

• Plenum used near equipment outlet; promotes laminar airflow and provides acoustical insertion loss (< 12 dB)

• Passive silencers used when large insertion loss is required; must account for pressure drop

• Active silencer has no pressure drop.


Improvements in Design for Noise Performance

Poor Design

Better Design

Duct Liner Data

Airflow: Turbulent Noise in Ductwork

Long, p. 486

Frequency (Hz)

63 125 250 500 1000 2000 4000

Loss (dB/ft)Circular

0.03 0.03 0.03 0.05 0.07 0.07 0.07

Loss (dB/ft)Square

0.36 0.20 0.11 0.06 0.06 0.06 0.06

• Data for circular duct from Long, Table 14.1• Data for square duct from previous equations with P/S = 4

Duct Shape and Noise Control

A Sample Interior Noise Prediction

Active Noise Control in Ducts

MJR Figure 9.19, p. 205

Using data from the input microphone, the controller generates a signal to be played by the loudspeaker which is out of phase (180⁰) with the duct-borne noise at the loudspeaker position. Feedback from the error microphone (which ideally senses no noise) helps fine tune the process.

Active silencer• Good LF attenuation.• HF attenuation remains a challenge.• Normally combine active/passive for full

range attenuation.• Lower pressure loss.• Smaller size and weight than passive for

similar LF performance.• Higher initial cost?

Conclusion• All measures discussed viz Absorption,

damping, decoupling, adding mass and flow control needs to be carefully considered while designing the potential noise emitting equipment.

• Active silencer is attractive measure but, not an alternative for the classical measures, but, can supplement these efforts.

Thank you

Air-Borne Sound

Transmission Loss (TL): sound energy lost through a construction assembly

noise control

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