slough library - room acoustics of large spaces

Jeremie DufaudMACH Acoustics 81-83 Stokes Croft, Bristol, BS1 3RD

[email protected] mobile 07960 256299

Slough Library - Room acoustics of large spaces

Jeremie DufaudMACH Acoustics 81-83 Stokes Croft, Bristol, BS1 3RD [email protected] mobile 07960 256299

Contents

SUMMARY..............................................................................

INTRODUCTION.....................................................................

CHAPTER 1 – Case Studies...................................................

Bristol Library....................................................................

Cardiff Library...................................................................

White Chapel Library.......................................................

Peckham Library...............................................................

CHAPTER 2 - Presentation of Slough Library.........................

CHAPTER 3 - Acoustic Assessment........................................

CHAPTER 4 - Acoustic Design Targets...................................

CHAPTER 5 - Acoustic Design Tools......................................

Distance and Layout.........................................................

Soft treatment...................................................................

Screens............................................................................

Acoustic Sculpture............................................................

APPENDIX – Reverberation Times results............................

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Summary

Based on the study of four existing libraries: Bristol, Cardiff, White Chapel andPeckham, MACH Acoustics has defined three main acoustic targets for Sloughlibrary. These are Quietness, Softness and Privacy. To achieve these targets asbest as possible, the acoustic design tools have been set out below.

Layout – Page 11- Noisy activities should not be located below the voids,- Study areas and community areas should be kept apart,- Activities which require concentration should be carried out in a separate room,- Noisy activities should be screened and surrounded with acoustic treatment.

Soft Treatments – page 12- The floors should be carpeted as much as possible,- 50% of the ceiling of the Ground Floor should be covered with Class Aabsorption,- Treatment should be particularly above noisy areas,- The top ceiling should be absorbent.

The table below represents the ideal amount of acoustic treatment acoustictreatment:

Screens – page 18- Screens should be used to separate the different activities of the ground floor inorder to limit noise build-up. In general the each different coloured area of theground floor (see page 9) should be separated from each other with screens.- For more details about the screens, please see page 18.

Acoustic Sculpture – page 19- An acoustic sculpture could be hung down within the voids to reduce noisetransfer from the ground to the first floor. The bigger the sculpture is, the moreefficient it will be.

Floor / ceilingGround floor

Ground floor ceiling

First floor

First floor ceiling

Recommended treatment

At least 50% carpet

70% Class A absorption

100% carpet

50% Class A absorption

Jeremie DufaudMACH Acoustics 81-83 Stokes Croft, Bristol, BS1 3RD [email protected] mobile 07960 256299 Page 1

MACH Acoustics are to provide the acoustic design for the new development ofSlough Library. This report will specifically present advice regarding the roomacoustics of the large open plan spaces within the library.

There is currently no standard on which to base the acoustic design of libraries.Therefore, MACH Acoustics have visited a number of libraries in order to gaininspiration and an understanding of their general acoustic feeling of thesebuildings. The main purpose of these visits was to determine the acoustic benefitsand drawbacks of different designs and ideas, in order to provide a template tobase the design of Slough Library.

Half a day was spent at each library such to understanding these spaces. Themajority of the text within this report was undertaken during these visits.

With the agreement of the manager at Cardiff Library, MACH Acoustics carriedout measurements to determine reverberation times and the decay of sound overdistance within this building. The results of these measurements have been usedas a basic criteria to design Slough Library.

Based on the design criteria extracted from the visits of the libraries and themeasurements, the following acoustic targets have been set up: “Quietness,Softness and Privacy”, which seen to be the fundamental acoustic requirementsfor a library.

As Slough Library is a large open space, the acoustic design of such a space islimited. Hence to achieve the acoustic targets above, a series of tools to controlacoustics in large open spaces are listed and detailed.

Generally, the noise generated by a person is proportional to the backgroundnoise level surrounding that person. Indeed, within a quiet and peacefulenvironment an individual would tend to be calm and would not feel the need toshout to make himself understood. Whereas in a noisy environment, the sameindividual would likely be noisier and would have to speak louder to make himselfunderstood. It is therefore critical to keep the potential noisy areas as quiet aspossible in order to avoid noise build-up.

Introduction

Bristol Library

Bristol Library is divided into three different parts. Each part was built at adifferent time and shows a different acoustic behaviour. The study area (theoldest part) has a very high dome made of plaster and glass, and with its creakywooden floor this space is particularly sensitive to noise and could beconsiderably noisy. However, because of its use, people respect it and keep asquiet as possible which makes the room quiet. On the other hand, it seemsdifficult to have a conversation as sound can easily travel and bounce within thislarge space. Hence, privacy is low and as such, people working in groups tend togo work in other, newer areas which have a better acoustic design.

The most recently built spaces include the IT equipment and the kids area. Thebackground noise level is higher as people tend to discuss more, their voice inthese areas are louder than in other parts of the library. To reduce noise levelsthere is a reasonable amount of acoustic treatment, floor areas are carpeted andwalls are finished with perforated plasterboard.

Within the main library space, people walk through the shelves and maintain aquiet manner. There is no addition of acoustic treatment, with most soundappearing to be absorbed by the bookshelves and carpet. The table belowprovides general comments regarding the acoustics of the old part of the library.

CHAPTER 1 - Case Studies

Acoustic Design Tools

General Comment Subjective Acoustic Appreciation

Layout Large volume open plan Reasonably quiet

Soft treatment None Very reverberant

Screens Low level desk screens Reasonably efficient


Cardiff Library

Cardiff Library is the most recent library of the four case studies and certainly themost similar to Slough Library. It is a four floor open space with a main void linking thefour floors. It has a concrete ceiling on each floor apart from the top floor where aperforated wood sheet covers acoustic absorption. The absorption within this library isprovided by the carpet, books, areas of acoustic treatments to the ceiling and anumber of very large wall panels.

The building is particularly well laid out with a lot of different study spaces allowingpeople to be in groups or in a quieter area on their own. Most noise is generated bypeople chatting. Noise from people chatting can be perceived as two different noises.The first is as a sort of background noise where speech is perceived as a generalhubbub. This type of noise is generally not disturbing as long as it stays under acertain level. The second type of noise from people chatting is classified as“intelligible speech”, which occurs when a group of people are chatting within a fewmetres of the listener. This type of noise can be considerably more disturbing if thelistener is trying to concentrate. The solution is to place group tables at some distancefrom each other or to use screens between the workstations.



General CommentSubjective Acoustic Appreciation

Layout Large volume open plan Reasonably quiet

Soft treatmentCarpet, wall panels, perf‐orated wood ceiling panels

Sounds soft

Screens Desks, group screens Efficient


Cardiff Library – Acoustic Measurements

This page presents the results of the sound test carried out at Cardiff Library.MACH Acoustic carried out 2 type of tests:

Decay of sound over the distance:The graph on the right shows that the sound decays by about 25dB over about 20metres. The speakers were set up at head level on top of a book shelf and themicrophone stayed in line of sight with the speakers during all the measurements.

Reverberation times:The table below shows the reverberation times measured at different positionwithin the library. The average reverberation time of Cardiff Library is 0.8seconds.


Measurements M1 M2 M3 M4 M5 M6 M7

Reverberation Times(Seconds)

0.96 0.88 0.66 0.82 0.76 0.86 0.79


White Chapel Library

White Chapel Library is certainly the noisiest library of the four. The building is on 4floors which laid in a circular “corridor” around a central stair-core. The central core isacoustically independent as there is a lobby with 2 doors on each floor. However thelast three floors are linked together by a main void/atrium at the end of the building.

Compared with Cardiff Library, the main source of noise seemed to come from thelibrary itself rather than from the people within the library. The lift was particularlynoisy, as were the hand dryers of the toilets which are disturbing outside of the toilets.All of the floors were covered with lino and therefore people walking and cateringtrolleys were also significantly noisy. Because of its layout, it can be difficult to find aplace out of the way of people wandering around the library.

White Chapel Library includes a few learning labs, which are rooms for teaching or forpeople who want to study in a quiet environment. These rooms have doors and cantherefore be acoustically separated from the rest of the library.

An interesting point of White Chapel Library is the separation of the ground floor to therest of the library. The ground floor is a communal space which can be quite noisy,and therefore to stop sound travelling to the upper floors, the void linking the groundand the first floors is blocked with glazing.




Layout Four semi open plan areas around the main stair core

Works in certain areas but people traffic is a real issue

Soft treatment Very few wall panels Noisy, hard, harsh environment

Screens Low level bookshelvesshould be higher

Minimal levels of acoustic screening


Peckham Library

Peckham Library is one large open plan space. It has two “nests” which arepartially isolated from the rest of the library and are quieter than the rest of thelibrary. The layout is clever with a row of tables along the window side which donot get distracted by people passing. The shelves break down to central partwhich have common tables and computer tables.

Peckham Library certainly has the largest amount of treatment of all four librariesas the whole ceiling is covered with perforated plasterboard and all floors arecovered with carpet. As a result, it sounds very soft. The library itself does notgenerate noise, most of the noise comes from groups of people chatting.




Layout One main open plan Pleasant

Soft treatment A lot Soft and cosy

Screens Bookshelves Works reasonably well


Presentation of Slough library

Slough Library spans over a four floor building (ground, ground mezzanine, firstand first mezzanine). Floors are interlinked by large voids and balconies whichresult in large volumes interconnecting the different floors. This layout brings astrong visual impact to the building but makes the acoustics of the building difficultto control, as large spaces can get considerably noisy due to reverberation, peopletraffic and the ease of sound bleeding from one space to another.

The ground floor (in red) with the ground mezzanine floor (in orange) create onesingle large volume as well as the first and first mezzanine floors (in blue). Theceiling height of these two large volumes is 5.8 metres so long reverberation timeare expected.

The ground floor will certainly be the busiest and noisiest. The central corridor willbe used as a public passage and is therefore expected to be noisy. The youthgames area, children's library and cafe are also expected to be noisy. Thesespaces are also seen to be well suited together. Additional levels of soft treatmentwill be required in these areas.

The first floor is proposed to be a large open plan library and is expected to bemuch quieter and will require a certain degree of silence due to its general use.

One of the main concerns is therefore noise build-up on the ground floor beingtransferred to the first floor through the voids.

The purpose of this document is to propose solutions to acoustically separatethese two floors whilst keeping the voids opened. Ground Floor

Ground Floor Mezzanine

First Floor

First Floor Mezzanine

CHAPTER 2 – Presentation of Slough Library


Acoustic Assessment of the Library

Potential sources of noise

As explained in the previous section, the ground floor is seen as the noisiest areawithin the library. This report will therefore concentrate on limiting the noisegeneration of the ground floor.

The image on the right shows the different open plan areas of the ground floor.The green areas are expected to be quiet, whereas the reddish areas areexpected to be noisy. The type of noise disturbances expected are:

Noise from people:- People walking through the library / footfall- large groups of people chatting- Babies crying- People on mobile phones

Noise from the building:- General lift noise- Doors shutting- Catering trolley- Toilet, air hand dryers- Cafe noise, cutlery noise, cafe machines....

Transfer of noise from ground to first floor:

Noise at the source is controllable to a certain point. Therefore, it is also critical tolimit the transfer of sound from the source to the receptor. In the case of SloughLibrary, the source is the ground floor and the receptor the first floor.

The image on the right shows potential sound paths from the ground to the firstfloor.

Reflection of sound from Ground to First Floor

Different noise areas of the Ground Floor.

Expected noisy areas Expected quiet areas

CHAPTER 3 – Acoustic Assessment



Layout - The layout of the space will affect the distance between areas andpotentially the snowball effect of large groups contained within a small area.

Distance - Sound is reduced over distance, therefore different types of activitiesplaced further apart will benefit from increased levels of separation.

Soft Treatments - Soft treatments are required both to promote speechintelligibility but also to reduce the level of noise within open plan spaces and toreduce the spread of noise across these spaces.

Screens - Breaking the line of sight between two points is often an effective wayof providing a small but effective acoustic break between two spaces. When highlevels of separation are required, full partitions are seen to be the only solution.

Acoustic Sculpture - Acoustic sculptures could add absorption to reduce thenoise transfer and reverberation.

Acoustic Targets

There are no standards available to base the acoustic design of a library on.However based on the case studies above, two main acoustic criteria are seennecessary within a library and are listed below:

Quietness - A library is a place to learn, study, relax... It is therefore a criticalrequirement for a library to be quiet in order to provide an atmosphere suitable tostudying.

Privacy – A library is also a place to share and meet. People will also havediscussions and a certain degree of privacy will be required. This will allowgroups of people to work together without disturbing others.

In an open plan space, design options to control the acoustics are fairly limited,the list below presents the different parameters which can have an beneficialimpact on the overall acoustics of the building.

Peckham Library – Mini interview room


Layout - Distance

The layout of noisy activities can have considerable impact on the backgroundnoise levels and the propagation of noise. The areas in green on the figure to theright, show the openings interconnecting the ground and first floors. A first andsimple principle is to avoid the localisation of noisy activities underneath the voidand to locate them in enclosed, acoustically treated spaces.

Different types of activities should be spread out to a maximum, such that thesound separation between both is maximised and one does not interfere withanother, thus avoiding sound build up.

The photos below were taken at Cardiff, White Chapel and Peckham Library theyprovide examples of layouts where different types of activities are kept away fromeach other.

CHAPTER 5 – Acoustic Design Tools


Room acoustics and reverberation times refer to the behaviour of sound withinthe room. Sound takes longer to decay in a reverberant room and this has twoconsequences: firstly speech sounds become more difficult to hear as the longdecay blurs successive syllables into each other and secondly, a build-up of noiseoccurs as the sound takes longer to be absorbed. When this build-up of noiseoccurs, there is a snowball effect as voices are raised to be heard above thenoise.

The amount of reverberation in a room can be described by the reverberationtime and is specified in terms of seconds. Within this open plan space, MACHAcoustics is mainly concerned about this second effect i.e. the build up ofsound/background noise.

An example of this effect can be seen in a café/restaurant with maybe 50occupants, sitting at tables of 2, 4, 8 and 10. If the finishes within this space arehard, there will be little or no absorption of sound, which will result in a loud andharsh space. In this space, people will raise their voices over the backgroundnoise. As a result, speech/conversation becomes limited to groups of 4, even ontables of 6 or more.

Positioning the café outside prevents sound bouncing/reflecting off walls, and assuch, reducing noise levels. This reduction in noise promotes speech intelligibility,potentially enabling all 10 members of a single table to hold the sameconversation. This external effect is therefore desired within Slough Library and isachieved by ensuring the correct levels of soft treatments.

The mathematical expression for reverberation, provided above, shows that thereverberation time of a space is directly linked to the volume of a space and thelevels of acoustic absorption (soft finishes) within this space (acoustic ceiling tiles,carpets, acoustics panels and so on). The equation above shows that if thevolume of a space is doubled, the level of acoustic treatment will also be requiredto be doubled, to maintain a constant reverberation time.

Soft Treatment – Theory CHAPTER 5 – Acoustic Design Tools


Soft Treatment – Acoustic Model

In order to accurately estimate the amount of acoustic treatment required to lowerreverberation time and increase the decay of sound over distance, a computermodel (CATT model) of the library has been built up.

The figures on the right show the decay of sound over distance when there is notreatment. The sound source was set up at 100dBA, the noise map shows thatthe sound decays from 100dB to 85dB at the opposite end of the library.

When the ceiling is treated as shown on the figures below, the decay of soundover distance is increased. With the sound source set up at 100dBA, the sounddecays to less than 70 at the other end of the library. This shows the importanceand the impact of the addition of treatment. It is also important to understand thatthis additional 15dB decay, due to the absorption, will have a significantsubjective effect which is likely to result in noise levels being reduced further.

The overall reverberation time is also decreased which helps to reduce noisebuild up. Without any treatment, the reverberation time achieved is about 5seconds, whereas with the addition of treatment on the ceiling of the first floor, thereverberation time drops to 3 seconds.

No treatment –Decay of sound over distance

With treatment –Decay of sound over distance

No treatmentConcrete finish

With treatmentAbsorbent finish



Soft Treatment – Acoustic Model CHAPTER 5 – Acoustic Design Tools

For the acoustic treatment to be efficient, it needs to be reached by the sound. Itis therefore important to place absorption in a location where it is likely to be hitby the sound. The computer model allows a visualisation of the soundpropagation from a point source through the open space of the building.

The figures below show how sound expands away from a given sound source.From these figures, it can be seen that the first surfaces to be hit by the soundare the floor, followed by the ceiling and finally the walls.

It is easy to treat the floor as it can be covered with carpet, however carpet is notideal in place with high foot traffic. It is proposed to have a stone floor on thewalkway of the ground floor. MACH Acoustics advise that other areas of theground floor be covered with carpet if possible.

The ceiling of the first floor is to be a concrete coffer. This type of ceiling usuallyhas a strong visual impact. MACH Acoustics therefore proposes to treat theceiling with innovative treatment options integrated within the concrete beams(see following pages) .

The first floor ceiling presents an area of about 700m2, MACH Acoustics advise tocover about 70% of the ceiling with Class A absorption. Ideally, treatment shouldbe located above the noisy area to maximize its efficiency. The top ceiling shouldalso be treated with 50% of Class A absorption and the first floor should becovered with carpet.

1 2 3 4 5 6

7 8 9 10 11 12

No treatment –Propagation of sound to the first floor

With treatment –Propagation of sound to the first floor


Soft Treatment – Example 1

These illustrations present examples of acoustic treatment integrated within theconcrete coffered ceiling. The drawing on the bottom left represents the originalbare concrete coffered ceiling. The drawing on the top right shows an example ofacoustic treatment. Here, the acoustic treatment is integrated and flush with theconcrete beams. The overall design represents giant patterns of random shapes.

The sketch on the bottom right gives a quick indication of the mounting of thestructure.



Soft Treatment – Example 2

These illustrations present examples of acoustic treatment integrated with theconcrete coffered ceiling. The drawing on the bottom left represents theoriginal bare concrete coffered ceiling. The drawing on the top right shows anexample of acoustic treatment. Here, the acoustic treatment is integrated andflush with concrete beams. The overall design represents repetitive waveshapes which are understood to be proposed for the facade of this building.

The sketch on the bottom right gives a quick indication of the mounting of thestructure.



Soft treatment – Example 3

These illustrations present examples of acoustic treatment integrated with theconcrete coffered ceiling. The drawing on the bottom left represent theoriginal bare concrete coffered ceiling. The drawing on the top right shows anexample of acoustic treatment. Here, the acoustic treatment is integrated andflush with the concrete beams. The overall design represents beams wavingbetween the concrete beams.

The sketch on the bottom right provides a brief idea of the mounting of thestructure.



Screens

Screen size, height and position (cases 1 – 2 – 3):The performance of a screen is dependant upon the extra distance sound has to travel over thescreen. It can therefore be said that the larger the screen the better. As an absolute minimumrequirement, the screen must block the line of sight between the source and receiver. This meansthat as an absolute minimum, the screen heights should be no shorter than head height, to preventthe spoken voice from passing over the screen (see cases 1, 2 and 3).

Maximising the effect of screens (cases 4 and 5):The acoustic performance of an acoustic screen is always limited by the fact that sound can passover or around a screen. On the other hand, it is generally accepted that a well positioned,appropriate sized screen, can provide as much as 15 dB(A) of sound reduction, when located in afree field condition i.e. outside in an open field.By bringing a screen indoors, reflections off hard surfaces are likely to compromise the screenfurther. At this stage, it is assumed that the maximum performance of a screen placed in the library,is 10 dB(A). In simple terms, the performance of a screen is linked to the extra distance in whichthe sound is required to take, due to the position of the screen. In other words, the larger thescreen, the better the performance

Cases 4 and 5 show the effect of placing a screen adjacent to a hard surface. Such to ensure theintegrity of the screen in Case 1, the screen must be placed adjacent to a hard surface i.e. a wall,table or other hard object and sealed with mastic. There should also be no air gaps between thescreen and any other surfaces. The alternative to Case 2 is to place an absorbent surface alongthe length of the reflective area.

Poor screening effect High screening effect

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Materials:The simplest requirement for an acoustic screen is that it should have a mass of at least 15kg/m2.Ideally, the screen would be finished with an absorbent covering, i.e. mineral wool covered withcloth. This requirement is only likely to slightly increase the performance of the screen. On the otherhand, finishing the screen with a soft covering will prevent the passage of sound as a result ofreflections off this surface.



Acoustic Sculpture

An acoustic sculpture, i.e. a 3 dimensional object made of an absorbentmaterial, could be suspended in the voids to limit the noise transfer from theground floor to the first floor. The photos to the right, provide potential examplesof acoustic sculptures. This principle could be effective and efficient for providingacoustic absorption whilst adding an artistic touch to the building. If the sculpturesare hung within the void between the ground and first floors, they could help toreduce the transfer of noise.


Acoustic Sculpture reducing noise transfer to the First

Floor


Appendix – Reverberation times results

No treatment Ground floor ceiling 100% Class A + top ceiling 100% Class A

Ground floor ceiling 50% Class A + top ceiling 50% Class A + 50% carpet onground floor and 100% carpet on first floor

Ground floor ceiling 100% Class A + top ceiling 100% Class A + First Floor Carpet


slough library - room acoustics of large spaces

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