Science Teaching Kit for Senior Secondary Curriculum Wave ...

Noise Control in Architecture

Science Teaching Kit for Senior Secondary Curriculum

[Teacher notes]

Wave Motion

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ContentsPreamble

Teaching plan i

DisclaimerCreate Hong Kong of the Government of the Hong Kong Special Administrative Region provides funding support to the project only, and does not otherwise take part in the project. Any opinions, findings, conclusions or recommendations expressed in these materials/events (or by members of the project team) do not reflect the views of the Government of the Hong Kong Special Administrative Region.© 2012 Hong Kong Institute of Architects

Lesson 1 : Noise Control in Architecture

1.1 What is Noise?

1.1.1 Sound Intensity (I) and Sound Pressure Level (L)

1.1.2 Difference Between Noise and Sound Pressure Level

1.2 Sound Transmission in Architecture

1.2.1 Sources of Noise

1.2.2 Noise Transmission Paths

1.3 Noise Control through Architectural Design

1.3.1 Reducing Transmitted Sound Pressure Level

1.3.2 Sound Reduction Index

Exercise: Calculate the Appropriate Sound Insulating Performance for a

Room Design

1.3.3 Redirecting Sound away from Receivers

1.3.4 Case Study — Kwai Tsing Theatre

Summary, Key words and Further reading

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Science | Noise Control in Architecture

Topic 05Noise Control in Architecture

Major teaching areasPhysics: Chapter III Wave Motion• Wave Nature of Sound

• Noise

Learning objectives• To distinguish noise from Sound Pressure Level

• To learn the equation for calculating Sound Pressure Level

• To understand noise control in architecture through scientific approaches

Teaching planLesson Contents

Lesson 1

Noise Control in Architecture

• 1.1 What is Noise

• 1.1.1 Principle and equation of Sound Pressure Level

• 1.1.2 Difference between noise and Sound Pressure Level

• 1.2 Sound Transmission in Architecture

• 1.2.1 Sources of noise in architecture

• 1.2.2 Noise transmission paths in architecture

• 1.3 Noise Control through Architectural Design

• 1.3.1 Rationale of sound intensity reduction

• 1.3.2 Equation and calculation of Sound Reduction Index

• 1.3.3 Rationale of redirecting sound transmission paths

• 1.3.4 Case study — Noise control in Kwai Tsing Theatre

Teachers can refer to the video ‘Noise Control in Architecture’ as introduction in the class.

For further references on Acoustic Design in architecture, please see Science Topic 6 ‘Noise Control in Architecture’.

Interdisciplinary teaching areasDesign and Applied Technology• Strand 2 Technological Principles

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Lesson 1Noise Control in Architecture

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Lesson 1Noise Control in Architecture1.1 What is Noise? In daily life, noise means unwanted sound. As you have no doubt experienced, unwanted sounds can include distractions (your neighbour playing the drums while you try to study), sounds that are loud enough to damage hearing (construction sounds, concerts) and even sound leakage that could affect privacy (overhearing a conversation through a door).

In architecture, one of the many goals of the designer is to create rooms and buildings that maximize sound performance. This means that unwanted sounds should be prevented, and desired sounds should be enhanced.

1.1.1 Sound Intensity I and Sound Pressure Level LSound can be detected when the sound power imposes pressure on the receiver. Human ear is one kind of receivers. Sound pressure is a scientific distinction from the more subjective “loudness”. The higher the power a sound wave carries, the higher the sound pressure it is on the receiver. For outdoor sound propagation, the relationship between sound pressure and sound power is distance between sound source and receiver. For indoor sound propagation, the relationship between sound pressure and sound power is room absorption. A loud noise usually has a larger pressure variation and a weak one has smaller pressure variation.

Sound intensity is the measurement of sound power per unit area (Wm-2) therefore it can be used to describe sound pressure. A logarithmic scale is used to represent different levels of sound pressure. It is called sound Pressure Level (or Sound Intensity Level) and is defined as:

where,L = Sound Pressure Level (dB); I = Measured sound intensity (Wm-2); and I0 = Standard sound intensity (Wm-2), the softest sound intensity that human ear can hear.The standard reference of I0 is 10

-12 Wm-2.

1.1.2 Difference Between Noise and Sound Pressure Level

Noise is a subjective and relative perception, but Sound Pressure Level is a scientific measure of the power of sound. Even if a sound source has a relatively low Sound Pressure Level, some individuals might find a certain frequency of sound wave annoying because the human ear reacts differently to different frequencies of sound (pitches) and the hearing sensitivity of individuals is subjective. Sound Pressure Level can only be used as a standard to identify the volume of sound that could cause hearing damage. It does not represent the exact perception of the sound to individuals.

Teaching TipsTeachers can refer to the video “Noise Control in Architecture” on website as an introduction to class

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1.2 Sound Transmission in Architecture

1.2.1 Sources of Noise

The two major sources of noise in architecture are:1. Airborne noise, or sound waves transmitted from a source to a receiver through the air.2. Structure-borne noise is caused by vibration from within the building caused by footsteps or

machines. The vibration is directly transferred through the building’s structures and materials.

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1. The sound of a plane’s engine is a kind of exterior airborne sound that may affect communities near airports.2. The MTR’s operating systems are quiet, but the vibration of the train rail generates exterior structure- borne noise.3. In a gymnasium, cheering spectators may cause interior airborne noise to the surrounding rooms, while the athletes’ activities generate interior structure-borne noise.

For outdoor sound propagation, the simple relationship between sound pressure and sound power is distance between sound source and receiver. For indoor sound propagation, the simple relationship between sound pressure and sound power is room absorption.

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1.2.2 Noise Transmission PathsSound waves travel from the source to the receiver via multiple paths. Altering the paths of sound transmission is a common way to control noise in architecture. When sound strikes on a surface, part of the sound reflects off the wall surface back into the space, and part of it is transmitted through the wall to the space beyond.

Teaching TipsMore information on acoustic design for desired sound quality can be found in Science Topic 04: ‘Acoustic Design for Auditoria’.

Sound Pressure Levels from various sound sources are as follows:

Sound source Decibel dB(A) Sound source Decibel dB(A)Softest sound that young people can hear

0 Average road traffic at 25 metres from busy primary distributor road

70

Ticking of a watch 10 Diesel freight train running at high speed at 25 metres

80

Unoccupied broadcast studio 20 Breaker at 10 metre 90Country park 30 Loud music in discotheque 100Audible but unobtrusive conversation (e.g. in library)

35-40 Percussive piling at 10 metres 110

Moderate but unobtrusive conversation

40-45 Ship’s engine room 120

Unobtrusive conversation with low levels of surrounding activities

45-50 Sound that causes pain 130

Unobtrusive conversation with high levels of surrounding activities

50-60 The dB(A) is the A-weighting scale of Sound Pressure Level. The dB(A) is often used as it reflects more accurately the frequency response of the human ear. Further reference please visit : h t tp : / /www.epd .gov.hk /epd/no i se_educa t ion /web/ENG_EPD_HTML/m1/ in t r o_5 .h tm l

(Source: Environmental Protection Department and VIPAC Engineers & Scientists Ltd.)

1 How is unwanted sound transmitted in the following diagram?

[Discussion]

Flanking transmission of sound

Flanking transmission is the propagation of sound wave via building elements. An air gap can stop the transmission of sound effectively.

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1.3.1 Reducing Transmitted Sound Pressure Level

When sound wave travels through air or materials, the transmitted Sound Pressure Level is reduced due to sound power loss.

Almost every material exhibits sound insulating properties. They only vary in their level of acoustic performance, which is measured in terms of the Sound Transmission Loss or Sound Reduction Index.

1.3.2 Sound Reduction Index

where,SRI = Sound Reduction Index (dB); Wi = Sound power incident on one side of a sound barrier (W); and Wt = Sound power transmitted into the air on the side of the partition (W).

When two layers of sound barriers are installed independently, the sound resisting ability of the entire wall will be increased.

The Sound Reduction Index of a material as it affects a specific frequency of sound can be found using the following equation:

where,SRI = Sound Reduction Index (dB); m = The mass per unit area of the material (kgm-2); and f = Frequency of the sound (Hz).

The sound insulation performance of any homogenous material depends mainly on its mass.

1.3 Noise Control through Architectural Design

Calculate the Appropriate Sound Insulating Performance for a Room Design

The school principal wants to convert one of the school’s activity rooms into a meeting room. The meeting room should be insulated from the noise of the nearby sports courts. He wishes the room to be well insulated so that meetings conducted in the room will be kept confidential.

After going through the following statistics and materials, calculate the appropriate sound reduction level of the noise control installation required. Suggest a suitable surface material for the room.

Sound Pressure Level inside the unoccupied meeting room = 25 dBSound Pressure Level inside the meeting room in use = 45 dBSound Pressure Level of audible sound = 35 dBSound Pressure Level at the nearby sports court = 70 dB

[Media Corner]

The Environmental Protection Department disseminates information on environmental noise ath t tp : / /www.epd .gov.hk /epd/no i se_educa t ion /

[Extended Knowledge]

[Exercise]

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Sound reduction required from the meeting room= 45 - 35 dB = 10 dBSound reduction required from the nearby sports courts= 70 - 25 dB = 45 dB

The wall of the meeting room should have a Sound Reduction Index ranging from 10 dB to 45 dB.

According to the graph of Sound Reduction Index, three building materials could be suitable for the meeting room:• 50 mm Plastered Breeze Block (a light concrete

building block made with cinder aggregate)• 9 mm Glass• 0.7 mm Sheet Steel

Sound Reduction Index of common building materials(Source: “Noise Control in Mechanical Services”, 1972)

After obtaining the data from an acoustic consultant, architects decide what materials should be used for the functions and aesthetics of the space. Other considerations include structural tolerance, environmental concerns and universal design for all users.

Teaching TipsMore information on: • Environmental concerns of building

envelopes under Science Topic 06 ‘Calculation and Application of OTTV and U-value’;

• Aesthetics of materials and textures under Arts Topic 08 ‘Visit: Ting Shui Wai Municipal Services Building’;

• Considerations for school design under Design and Applied Technology Topic 08 ‘Design Project: Village School’; and

Possible perspectives

1 The school principal prefers using plywood as the wall surface. He comments that a glass wall is not good for privacy but he wants to introduce natural lighting and ventilation to the new meeting room.

What possible solution would you suggest?

[Discussion]

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t Double-partition wall insulation When two layers of sound barriers are installed independently, the sound resisting ability of the entire wall will be increased. © NAP Acoustics (Far East) Ltd.

Double-glazingAn operable double-glazed window provides horizontally offset openings that allows natural ventilation while preventing direct propagation of traffic noise. The narrow path between the double window panes dissipates sound energy and lowers noise levels relatively.

Suggested AnswerA double-leaf plywood wall could be constructed with a row of double-glazed operable clerestory windows for natural lighting and ventilation.

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Noise barriers can redirect the paths of noise away from receivers. They can be artificial (for example, a wall) or natural (a forest). One common application for noise barriers is to prevent traffic sounds from penetrating nearby neighbourhoods or habitats.

Sometimes noise barriers are integrated with the building design. Noise sensitive buildings like residential blocks may be built above a podium or a shopping mall that inhibits the transmission of noise to the apartments.

Re�ection on the noise barrier

Re�ected by the forest

Direct transmission

Direct transmission through the barriers

SecondaryNoise Barrier

Primary Noise Barrier

p Artificial noise screen

1.3.3 Redirecting Sound away from Receivers

1 Can you identify the noise sources and barriers in the images and trace the sound transmission paths? Mark them on the following picture.

[Discussion]

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Kowloon’s Kwai Tsing Theatre illustrates how architectural design can be used to protect quiet spaces against airborne and structure-borne noises from inside and outside the theatre. Its location next to the MTR railway and heavy traffic is particularly challenging given the requirements of the theatre’s multifunctional spaces, which sometimes host simultaneous events.

1.3.4 Case Study — Kwai Tsing Theatre

Isolating structure-borne noise from the building Springs placed on the pile caps of the building can absorb exterior structure-borne noise before it is transmitted to the interior. Theatre and music rooms are built inside an isolated shield to reduce flanking transmission of noise.

p Noise-isolating springs © NAP Acoustics (Far East) Ltd

p Photographs showing how the springs integrate with the structure of Kwai Tsing Theatre © VIPAC Engineers & Scientists (HK) Ltd

© WAAGNER-BIRO AG

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Spring mounting locations

© VIPAC Engineers & Scientists (HK) Ltd

Improving the Sound Reduction Index of the StructureThe ceilings, walls and floors of the theatre are isolated from building structure by springs or resilient pads. An air gap around the floating floors and suspended ceilings also improves the Sound Reduction Index of the structure. The air gap is the most important factor – increasing it (rather than increasing the mass of the floors) would achieve greater reductions in noise as compared to increasing the mass of the structure or the floating floor.

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Further reading1. The Environmental Protection Department disseminates information on environmental noise at h t tp : / /www.epd .gov.hk /epd/no i se_educa t ion /

2. Harris, Cyril M. Noise Control in Buildings: A Practical Guide for Architects and Engineers. New York: McGraw-Hill, 1993.

3. Neufert, Ernst and Peter. Neufert Architects’ Data. Oxford: Wiley-Blackwell, 2012.

Summary

1. Noise means unwanted sound. It is subject to human perception. 2. Sound can be detected when the sound power imposes pressure on the receiver. Sound pressure is

a scientific distinction from the more subjective “loudness”. 3. Sound intensity is the measurement of sound power per unit area (Wm-2). A logarithmic scale of

sound intensity is used to describe different levels of sound pressure, and is expressed as:

4. Noise control in architecture is dealing two sources of noise: airborne and structure-borne.5. Two ways to control noise: reducing transmitted Sound Pressure Level and redirecting sound

transmission paths to the receivers.6. Sound Reduction Index is used for the purpose of choosing the appropriate sound insulating

performance for a building. It indicates the degree of sound intensity reduction when a sound wave passes through a material.

7. Noise barrier redirects noise transmitting paths to the receivers.

NoiseDecibel (dB)Sound PressureSound Pressure LevelSound IntensitySound Reduction IndexNoise barrier

Key words

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