Demystifying the Science of Sound: Practical Acoustics for ...

Demystifying the Science of Sound: Practical Acoustics for Building ServicesDr Emmet English

G2 The Steelworks, Dublin 1

www.resonate-consultants.com

Dr Emmet English vs Dr Emmet Brown

Overview

An introduction to sound

Sound transmission through partitions

Building services noise

Sound power vs sound pressure

Controlling noise and vibration

Good and bad practice

What is sound

Sound is simply a pressure wave in air

travelling at approximately 340 m/s.

Frequency refers to ‘how often’ the air

particles vibrate

Range of audible sound is approximately

20 Hz to 20,000 Hz

Hertz (Hz) = 1 cycle per second

What is Acoustics?

Sound (Good) Noise (Bad) Vibration (Bad)

Effects of Noise

Noise Induced Hearing

Loss

Environmental Health

Hazard

Sleep Disturbance

Hypertension

Cognitive Impairment in

Children

Focus, Stress, Privacy,

Annoyance

Standards and Guidelines

EN ISO BS Standards (Recommendations)

ASHRAE Handbook (Chapter 48)

EPA Policy

BREEAM/LEED/HPI Criteria

Project Specific Criteria – i.e. Universities, Hotels, Hospitals etc

Acoustic Criteria• Environmental noise level

• Internal ambient noise level

• Sound isolation between spaces

• Reverberation time within the spaces

The Basics

Decibels

The human ear has incredible sensitivity and

dynamic range!

2 to 3 dB change is barely perceptible

5 dB change is easily noticeable

10 dB change is a doubling or halving of

loudness

Ref.

+1dB

+2dB

+3dB

+5dB

+10dB

Adding Noise Sources: 1+1=3

+= 1010 1010log10

BA

pL

Adding Noise Sources: 1+1=3

40dBA

Adding Noise Sources: 1+1=3

43dBA

Adding Noise Sources: 1+1=3

46dBA

Adding Noise Sources: 1+1=3

49dBA

Adding Noise Source:

40 + 40 = 43 (equal numbers add 3)

40 + 37 = 42 (difference of 2 – 4, add 2)

40 + 32 = 41 (difference 5 – 10, add 1)

40 + 25 = 40 (difference >10, add 0)

40 + 40 + 40 = 45 (3 equal numbers add 5)

40 + 40 + 40 + 40 = 46 (4 equal numbers add 6)

What is frequency?

0

-20

-10

-30

-45

-5

-25

-40

-15

-35

-50

31.5

One-third octave band centre frequency (Hz)

Level w

eig

hin

g (

dB

)

63 1k250 4k125 2k500 8k 16k

A

B

C

100Hz 1,000Hz 10,000Hz

Noise Descriptors

Standard Noise Descriptors

• A-weighting (dBA), C-weighting (dBC),

Building Services CIBSE/ ASHRAE Guides

1. Perceived Loudness

2. Sound Quality

• Noise Criteria Curve (NC)

• Noise Rating (NR)

• Room Criteria (RC)

• Room Criteria Mark II (RC 2)

Building Acoustics

© RICHARD BARRETT

Acoustics in Building Design

Architecture

ServicesStructure

Acoustics

&

Vibration

Sound wave interaction with structures

Noise impinging on a wall

Transmitted

Absorbed

Reflected r

IncidentStructure-borne

to other connected

elements

Incident

Diffraction

Shadow zone

Sound transmission

The extent sound is transmitted between adjacent spaces is defined both airborne and impact (structure-borne) sound

Airborne Sound Insulation is defined in:

▪ Rw (Laboratory Value used for Design),

▪ DnTw (Site Value, includes Flanking and Build Quality).

Impact Sound Insulation defined as

▪ Ln,w - Lab Value

▪ L’nt,w - Site Value

All values have units in dB!

Sound transmission factors

• Mass, density

• Stiffness

• Thickness

• Vibration isolation of skins

• Size of air gap between skins

• Structure borne noise

• Flanking

Flanking Transmission

© TMSoundproofing

Auralisation –

Walls To illustrate the impact acoustic decisions

play on project costs and outcomes.

Dntw Voice through

acoustic wall

35

40

45

50

55

Acoustics rated in DnTw ratings:

- Mass, density

- Stiffness

- Thickness

Example Partitions

DnTw 35 DnTw 45 DnTw 55

Reverberation

In an enclosed space, reflections will occur from the surfaces and a reverberant

field will be established in addition to the direct field

The absorption coefficients of the enclosing walls will determine the strength of

the reverberant field.

Reverberation

Glass/Concrete

Fully Absorptive

Plasterboard and acoustic tiles

Absorptive

Treatments

Sound Absorption

Sound Absorption is important in reducing the reverberation in a Room.

It can be introduced by Acoustic Ceiling Tiles and Wall Panels

Low reverberation times are important when clear communication is desired

e.g. Schools, Open Plan Offices, Conference rooms

Soft furnishings and carpets also help reduce the reverberation.

Note: Sound Absorption is different to Sound Insulation!

Building Services Noise

HOW BUILDING SERVICES AFFECT THE ACOUSTIC

ENVIRONMENT

• Services noise inside

• Services plant outside

• Hydraulic noise

• Sound insulation of walls (penetrations and

connecting aircon)

Power vs PressureSound Power - The rate at which a sound source emits energy.

Sound Pressure - The noise level measured at a distance

Both use the unit dB!

Understanding Data SheetsSound Data available for most mechanical plant:

Tips and Tricks

• Always specify a level that was used for design

• Specify a criteria Noise Rating & dB(A)…..

• Check if it is SWL or SPL?

• Look at the frequency content?

• Check if it is inlet, outlet, radiated?

• Never accept SPL without a distance!

Duct Acoustics

This Photo by Unknown Author is licensed under CC BY-SA-NC

Ducts and AcousticsWhen designed correctly ductwork can provide good attenuation.

Longer duct runs and internally lined ducts work best.

Ductwork needs to have flexible connections.

In-Duct Attenuation – Rough Guide

Element Size Low Freq High Freq

Duct 600mm sq x 1200, 50mm insulation 1 - 2 dB 10 dB

Bend 600mm sq, 50mm insulation 0 - 1 dB 10 dB

End Reflection 600mm sq 4 - 8 dB 0 dB

Branch Depends on takeoff size 1 - 10 dB 1 - 10 dB

Plenum 1.2 m cube, 600 x 600mm openings 7 - 8 dB 11 dB

Proprietary attenuator 600mm sq x 1200mm 5 - 10 dB 15 - 30 dB

Lined ducts and diffuser plenums

Acoustic Attenuators

Plenum Chamber

What to watch out for:• Size

• Regenerated noise

• Backpressure?

• Where is it located (i.e. above a meeting room)?

Piping

• Excessive fluid flow in pipes

• Primarily a structure-borne issue

• Inadequate vibration isolation

• Unsealed penetrations

• Direct contact with wall or floor

Piping

Pipework Noise Control

Service Penetrations

• Penetration gaps are a common

cause of sound transmission.

• Gaps less 10mm can be filled with

non setting flexible mastic.

• Larger holes require packing or a

pipe sleeve

Penetration Detail

Services IsolationMechanical plant needs to be isolated from structures to reduce

structure-borne noise.

Tips

• Vibration isolate all reciprocating and rotating equipment

• Specify inertia bases as required

• Vibration-isolate all ducts and pipes for at least 15 metres

from vibration isolated equipment

Sensitive Spaces

Lab Equipment

EMI, Noise and Vibration can affect

sensitive equipment, manufacturing

processes or specialist spaces

MRI Machines

This Photo by Unknown Author is licensed under CC BY-NC-ND

Site Examples

Isolator Types

Isolator Types

Cross talk

Isolator Types

Summary

Building acoustics design standards and regulations normally specify:

Reverberation times for different spaces

Sound Insulation Values for partitions, facades etc

Internal Ambient Noise Levels for different spaces

Sound insulation controls sound transmission through partitions and floors.

Sound absorption controls the reverberation (echo in a room).

Single number ratings are commonly presented in dB which can cause confusion.

Sound Power and Sound Pressure are different! Never accept a sound pressure level without a distance.

The noise level of a system can be predicted at design stage and noise control specified for the individual system

EMI, noise and vibration can affect sensitive equipment, manufacturing processes or specialist spaces

Questions

[email protected]

G2 The Steelworks, Foley Street, Dublin 1,

p+353 1 442 8991

[email protected]

Isolator Types