PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Optimising the Acoustic Design for Multi-Purpose Rooms used for aVariety of Speech Communication Activities
Colin CAMPBELL1; Helene SALLENHAG2; Erling NILSSON3 Emma ARVIDSSON4
1 ,2,3,4Saint-Gobain Ecophon AB, Sweden

ABSTRACTThe sound environment for a room designed for speech communication can be influenced by many factors.
In a newly completed multi-purpose lecture room, we wanted to see the effects of the different designelements during the process including; utilising predictive modelling, the surface materials in terms ofabsorption performance, discussion, amount and placement as well as other design details influencing theoverall sound environment. We have followed this project though the build and fit out process making
acoustic measurements at different stages. Typically, a room of this type might have acoustic criteria forsound insulation and reverberation time. However, this can be somewhat limited when we compare thetheoretical acoustic predictive modelling versus how the occupants might actually perceive the room in reallife situations. We have measured additional room acoustic parameters such as speech clarity and thedifference in sound levels. We will identify the acoustic differences as the room is built up with ceiling, walland floor finishes, in addition to the design elements and furniture. Finally, we will discuss design
configurations which can make a room fit for multi-purpose speech based activities.
Keywords: Reverberation Time; Speech Clarity; Sound Strength Sound Pressure Levels; SpeechCommunication

1. INTRODUCTION
The acoustic characteristics of a room can be calculated, measured and many room acoustic
standards have been set around one common parameter – Reverberation Time (RT). Over the years,
RT has been widely understood and referenced as the most practical measure to evaluate the qualityof acoustics in a classroom.
In recent decades it has also become increasingly common for the acceptable reverberation timevalues to be lowered (shortened) and indeed in many cases, the previous recommended lower limits
have been removed. Looking at the relevant and similar types of rooms also used for similar speechcommunication intensive activities; monologue lectures, dialogue in the form of group work anddiscussions in pairs. According to Rasmussen (1) in the Nordic countries the trend has clearly indicatedthat a shorter RT is assumed to be better. However, even when a speech communication intensive
room like a classroom, meets the required short objective RT values, it does necessarily mean thegiven classroom will be subjectively perceived as having good speech intelligibility or low soundlevels by the users when the room is occupied which cast doubt on whether the room is fit for purpose.
Measuring the RT means that we mostly consider the decay of the late reflections and we miss theoverall room response to a given sound and in particular the early reflections (Bradley(2) ) which arevery significant when it comes to the clarity of speech and how it will be perceived as found inlistening tests performed by Nilsson(3,7) also Marshall(4) .
The shortcomings of the classical diffused field theory(5,6) become quite clear in rooms with nonuniform distribution of absorption treatment. This has paved the necessity for introducing more1colin.campbell@ecophon.sehelene.sallenhag@ecophon.se3
erling.nilsson@ecophon.se4
emma.advidsson@ecophon.se25952

parameters as well as taking into account the scattering effect of furniture and interior fittings has
been investigated.

2. BACKGROUND
2.1 Variety of speech communication activities
Increasingly, there is a requirement for multi-purpose speech communication rooms which canhold a variety of different speech communication intensive activities. These activities might include;monologue lectures, watching knowledge films or streaming lectures and more diverse speechcommunication activities encouraging dialogue which include workshops, group work and discussions
in pairs.
2.2 Characterising the room acoustic conditions for speech communication
On this basis and from the increasing knowledge about how to characterise the acousticalconditions in these types of rooms used for multi-purpose speech communication activities can to an
extent be compared to modern classrooms has increased in recent years. Several investigations havehighlighted the necessity of including more acoustic parameters (Nilsson (7), Bradley(8) ) for a relevantcharacterisation of the acoustic environment. Parameters related to the noise levels and to speech
intelligibility have shown to be an important and necessary complement to the RT (Campbell (9) ).
2.3 The relevant room acoustic parameters
In Bradley’s paper(8), the use of the room acoustic parameters C 50 and Strength are examined bothexperimentally and theoretically. In Barron (10), Nilsson(7), a model is presented for calculating C 50.
There was particular focus on explaining the non-diffuse sound field in rooms with ceiling treatment
and how this influences these parameters. In national standards and regulations for classrooms
designed for similarly intensive and varied speech communication e.g. UK, Germany, (Nordiccountries see Rasmussen (1)) there is still a clear dominance of RT as the parameter for characterizing
the acoustic quality. In view of this, it is clear that the practice of only defining a single number
evaluation of RT potentially restricts development of optimal acoustic conditions to make these rooms
fit for purpose. (5)3. OUTLINE OBJECTIVE
3.1 Objective measurements which correspond to subjective perceptions
In this study we wanted to look into objective measurement data which we understood to have asignificant correspondence to the subjective perceptions of the room for the users. Today, the main
parameter in acoustic design, standards and regulations of ordinary room types is the RT. Nevertheless,
it is well known that the RT alone is not sufficient for a relevant characterization of the acousticalconditions in rooms. The purpose of this modest room acoustic investigation is to show the potential
benefit of including additional parameters alongside RT whilst measuring different acousticconfigurations. Optimal acoustic conditions should be specified by a balance of objectively
measureable parameters related to speech clarity, sound strength and reverberation time.
3.2 Objective room acoustic parameters
A key objective in this study is to establish the conditions for optimal room acoustics as manifested
in the room acoustic parameters; Speech Clarity C 50 (dB), sound pressure levels (measured with aconstant sound power source) and reverberation time, T20 (s). C 50 evaluates the effect of the room’s
response to given sounds and the balance of the early reflections in relation to the late reflections. Theparameters are defined in the standards ISO 3382-1 and 2 (11,12). In general, this additional parameter
C 50 , outlined in ISO 3382-1 (11) , were intended for large performance spaces rather than for speech insmaller rooms, however, they have been found to be good indicators regarding the room acoustic
quality as well as RT (4,5,6,7,8,9) . It has also already been identified that in typical classrooms that C50values can be different even when the RT is the same, Nilsson (5).

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4. ROOM DESCRIPTION
4.1 Room dimensions
The size of the rectangular room we used are length x width x height = 12.35m x 6.05m x 2.64m.
74.4m2 overall floor area 74.4m2 and 197m3 volume.
4.2 Room surface finishes
The room surfaces before the acoustic treatment on the ceiling and the additional wall absorption,
were 1000mm average overall depth of system in the ceiling void, sloped soffit.
Room acoustic treatment, a fully covering suspended ceiling 20mm “Absorption Class A” glass
wool absorber panels. 40mm “Absorption Class A” wall absorbers in accordance with ISO 11654.
(wall coverage 7.8m2).
Walls are a typical lightweight drywall construction with partition cavity insulation. One long glass
wall installed with around a 5degree outward tilt – so wider at the top.
Carpet Egetaepper 8mm heavy commercial, Alpha w 0.2 tufted cut pile with a woven textile back.

Figure 1 – Multi-purpose (Orion) room in use during a workshop session.

5. METHODOLOGY
5.1 Acoustic intervention study
The main part of this study was to measure and analyse data from the room before and during thethree acoustic intervention stages to understand the influence of additional surface materials on thefloor, walls and ceiling.
5.2 Impulse response measurements
The acoustic measurements were done using an impulse response in order to evaluate the following
room acoustic descriptors; T20 and C50 in accordance with ISO 3382-1 (11) and we measured the soundlevels Leq in dB. Analysis of the room acoustic parameters measurement data is gathered across 1254000Hz frequency band. In addition, the sound pressure levels (Leq) in dB were measured with aconstant sound power source.
5.3 Acoustic configurations
The Ecophon Orion multi-purpose room acoustic measurements:
Configuration No. 1 – room with fully covering ceiling absorption.
Configuration No. 2 – same as “No. 1” plus fully covering carpet on the floor.
Configuration No. 3 – same as “No. 2” plus the furniture (tables & chairs).

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Configuration No. 4 – same as number “No. 3” plus 7.8m2 of wall absorbers over 3 walls.

Figure 2 – Config. 1. Base conditions with a fully covering suspended ceiling.

Figure 3 – Config 2. Now with the carpet installed.

Figure 4 – Config. 3. Now with furniture in place.

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Figure 5 – Config. 4. Now with the additional 7.2m2 of wall absorbers distributed over three walls

Figure 6 – Config. 4. Six vertical wall absorbers at the front on the left wall as shown in Figure 5.

Figure 7 – Config. 4. Two circular wall absorbers on the right wall towards the middle of the room.

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Figure 8 – Config. 4. Nine circular wall absorbers on the back wall.

6. ACOUSTIC MEASUREMENT RESULTS
We measured the base conditions in the room (Confg.1) followed by three further alterations givinga total of four separate configurations outlined below. Below is a summary of the values achieved
with the configurations 1-4 for T20, C50 and SPL:
Configuration No. 1 – room with fully covering ceiling absorption.
Configuration No. 2 – same as “No. 1” plus fully covering carpet on the floor.
Configuration No. 3 – same as “No. 2” plus the furniture (tables & chairs).
Configuration No. 4 – same as number “No. 3” plus 7.8m2 of wall absorbers over 3 walls.

Figure 9 – T20 results for all room configurations.

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Figure 10 – C50 results for all room configurations.

Figure 11 – Sound pressure levels for all room configurations

7. DISCUSSION AND CONCLUSION
The effect of different acoustical treatment in a multi-purpose room has been studied.
Measurements of reverberation time, speech clarity and sound pressure level have been performed.
The sound pressure levels were measured with a constant sound power source. The acousticaltreatment has been performed in four stages, starting with a suspended absorbent ceiling in the nonfurnished room. Next stage was to add a sound absorbent carpet, then inserting furniture and finallyadding wall panels. During each step room acoustic measurements have been carried out. The results
are presented in figures 9 to 11. Some interesting conclusions can be drawn. Inserting the suspended
ceiling creates a non-diffuse sound environment which gives long reverberation times in the high5958

frequency region. This is due to the grazing sound field occurring between the walls and which is toa less extent influenced by the ceiling absorber. However, even if the reverberation times are long the(steady state) sound pressure levels have been significantly lowered. Adding the absorbent carpet evenfurther emphasises the grazing sound field and in fact gives even longer reverberation times at thehigher frequencies, but this in combination with and due to the decrease of the (steady state) soundpressure levels. Inserting furniture will add some sound scattering which mainly shortens thereverberation times and increases the speech clarity. The sound pressure levels are hardly at allaffected by the furniture. The furniture consists of non-absorbent tables and lightly upholstered chairs.
Thus, the main effect of furniture is to increase the diffusion in the room which affect the reverberation
times and the speech clarity however there is only a minor effect on the sound pressure level. An
efficient way to reduce the effect of the grazing sound field is to add wall panels. This gives a clear
improvement of reverberation times as well as speech clarity, even though the amount of wallabsorption coverage was quite small.
It’s clear from the measurement results that to understand the effect of different acousticaltreatment under non-diffuse conditions it’s important to analyse several room acoustic parameters.
For example, if only reverberation time were considered and assuming diffuse conditions, theincreased reverberation times at higher frequencies when adding the carpet would have led to theconclusion that the sound pressure level has increased, which would be wrong. Furthermore, the large
effect of wall panels on reverberation times and speech clarity values is another effect that would havebe hidden if we assume diffuse field conditions.

REFERENCES
1. B. Rasmussen, J. Brunskog, D. Hoffmeyer, ”Reverberation time in class rooms – Comparisonof regulations and classification criteria in the Nordic countries”, Joint Baltic – Nordic Acoustics
Meeting, BNAM, 2012.
2. J. S. Bradley, H. Sato, M. Picard, “On the importance of early reflections for speech in rooms”,
J. Acoust. Soc. Am. 113 (6), June 2003.
3. E. Nilsson, P. Hammer, “Subjective evaluation of speech intelligibility for normal hearing
persons and for persons with simulated minimal degrees of hearing loss”. Report TVBA-3083,
Engineering Acoustic, Lund University, Sweden, 1994.
4. L. G. Marshall, “Speech intelligibility prediction from calculated C50 values”, J. Acoust. Soc.
Am. 98 (5), November 1995.
5. E. Arvidsson, D. Bard, E. Nilsson, “Optimization of sound absorbing ceilings”, Inter-Noise 2018.
6. J. Harvie-Clark, N. Dobinson, R. Hinton, “Acoustic response in non-diffuse rooms”, Euronoise
2012.
7. E. Nilsson, “Input data for acoustical design calculations for ordinary public rooms”. 24th
International Congress on Sound and Vibration, 2017.
8. J. S. Bradley, “Review of objective room acoustic measures and future needs”, Applied
Acoustics 72 (2011) 713 – 720.
9. C. Campbell, E. Nilsson, "The Same Reverberation Time in Two Identical Rooms does not
Necessarily Mean the Same Levels of Speech Clarity and Sound Levels when We Look at
Impact of different Ceiling and Wall Absorbers" 2015 IBPC Science Direct.
10. M. Barron, L-J. Lee, “Energy relations in concert auditoriums 1”, J. Acoust. Soc. Am. 84 (2),August 1988.
11. ISO 3382 – 1: Acoustics – Measurement of room acoustic parameters – Part 1: Performancespaces.
12. ISO 3382 – 2: Acoustics – Measurement of room acoustic parameters – Part 2: Reverberation
time in ordinary rooms.

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