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

Pattern and orientation of diffusers in rooms
with an absorbent ceiling
Emma ARVIDSSON1; Erling NILSSON2; Delphine BARD3 Ola KARLSSON41,32,4Lund University, SwedenSaint-Gobain Ecophon, Sweden

ABSTRACT
Lately have the demands on good room acoustics in public ordinary rooms increased. Traditionally has only
reverberation time been considered in this type of rooms but complementary parameters as sound strengthand speech clarity are more commonly used today. This trend naturally implies higher demands on theacoustic treatment.
Typical for public ordinary rooms is to use an absorbent ceiling. One way to further improve the acoustic
properties is to complement the ceiling treatment with diffusers. To investigate the effect of combining these
two types of acoustic treatments have measurements been carried out combing different setups of diffusers
in a reverberant room with an absorbent ceiling. The impact on reverberation time, speech clarity and soundstrength has been studied.
In this paper we present how diffusers can be used in order to improve the acoustic properties in a public
ordinary room with an absorbent ceiling and how the pattern and orientation of diffusers can be used to adjust
different acoustic parameters.
Keywords: Absorbent ceiling, Diffusers, Speech clarity

1. INTRODUCTION
Contribution of early reflections is of importance in rooms for speech. In acoustic design ofperformance spaces as auditoria and theaters has parameters considering the early reflections been
included for many years. Several researchers recommends higher effort on considering theseparameters also in rooms for speech [1] [2] [3].
The traditional acoustic treatment for public ordinary rooms, such as classrooms and offices, is anabsorbent ceiling. Nilsson has showed how furniture contributes to a more diffuse sound field and bythat improving the acoustic parameters reverberation time and speech clarity [4]. Choi has made a
study, in 1/10 scale, on the combination of absorbers and diffusers, showing that the combination ofthese two different types of acoustic treatments were the most preferable for reverberation time,speech clarity and sound strength [5].
An advantage by treating the acoustics also with diffusers in these type of rooms is that the soundenergy is conserved in the room [6]. By choosing the right design and pattern of the diffusers could
certain frequencies be treated and room acoustic parameters be improved. The use of diffusers could
also open up for more esthetical freedom for interior designers.
This paper presents the results from an investigating using adaptive diffusers specially designed forrooms with absorptive ceilings. It is investigated how the amount, orientation and pattern of

1

Emma.arvidsson@construction.lth.se2

Erling.nilsson@ecophon.se

3

Delphine.bard@construction@lth.se

4

Ola.karlsson@ecophon.se2281

diffusers effects the equivalent scattered absorption area (Asc). Further is the relation between Asc
and the room acoustic parameters reverberation time, speech clarity and sound strength investigated.

2. METHOD
2.1 Room configuration and measurement method
Measurements have been carried out in a reverberation chamber. A highly absorptive ceiling, αp of 1, for thefrequency range 500 Hz to 4000 Hz, was installed at height 2,70 m. The area of the chamber was
3,6 x 4,00 m2.

Figure 1 Sketch of reverberation chamber used for the measurements.

Reverberation time [7] and speech clarity [8] has been evaluated from the impulse response measured
from an omni directional sound source. A constant sound power source was used to evaluate the soundstrength [8].
2.2 Diffuser design
The design of the diffusers used in the study is showed in Figure 2. The frequencies of interest in thiscase were 2000 Hz and 4000 Hz. The results for reverberation time, speech clarity and sound strength
were thereby evaluated for these frequencies.

Figure 2 Sketch of diffuser used in the test.

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2.3 Equivalent scattering absorption area (Asc)
A Statistical Energy Analysis (SEA) model [9] developed for characterization of the energy decay in rooms
with absorbent ceiling treatment has been applied in the analysis. In these type of rooms can the sound field
be subdivided into a grazing and non-grazing part where the grazing field comprises waves propagating
almost parallel to the ceiling.
To quantify the scattering effect of furniture or other sound diffusing objects a parameter called equivalentscattering absorption area (Asc) is introduced. In an SEA terminology the Asc is interpreted as a couplingloss factor describing the energy transfer between the grazing and non-grazing sound field. The Asc is givenby‫ܣ‬௦௖ ൌ ߨܸ߱Τܿ ‫ߟ כ‬௡௚ǡ௚

(1)whereV is the room volume,ω the radian frequency andc the speed of sound.
Assuming that the subdivision into a grazing and non-grazing part still is justified after introducing the sounddiffusing objects the Asc is approximately given by‫ܣ‬௦௖ ൌ Ͳǡͳʹ͹ܸ ‫ כ‬൫ͳΤܶଶ଴ǡ௪௜௧௛ െ ͳΤܶଶ଴ǡ௪௜௧௛௢௨௧ ൯(2)whereV is the room volume,
T20, with is the reverberation time with diffusers andT20, without is the reverberation time without diffusers
Equation (2) is a two-dimensional equivalent to Sabine formula.
2.4 Diffuser configurations
17 configurations were evaluated. These configurations include different directions of diffusers, different
amount of diffusers and diffusers installed in different patterns. Description of all configurations can be findin Table 1 and are visualized in Figure 3 (configurations with both vertically and horizontally orienteddiffusers) and Figure 4 (configurations that were evaluated for only vertically oriented diffusers).

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Table 1 Description of the configurations.

Configuration

Number of diffusers

Orientation

Pattern123456789101112
1314151617121224
24
24
24
48
48
121224
24
20
10224
8

VerticalHorizontal
VerticalHorizontal
VerticalHorizontal
VerticalHorizontal
VerticalHorizontal
VerticalHorizontal
VerticalVerticalVerticalVerticalVerticalVertical rows
Vertical rows
Vertical rows
Vertical rows
One full wall
One full wall
Two full walls
Two full walls
ChessChess
Chess
ChessChessChess
Chess
Horizontal rows
Horizontal rows

1256

91034781112

Figure 3 Configurations with vertically and horizontally oriented diffusers.

1314151617

Figure 4 Configurations with vertically oriented diffusers.

The effect on the acoustic parameters reverberation time, speech clarity and sound strength was related to thenumber of diffusers and to Asc, calculated according to Eq. (2) in order to investigate the impact of different
diffuser directions and the installation pattern.

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1234

3. RESULTThe equivalent scattered absorption area (Asc) was calculated according to Eq (2). The Asc values increase
with the number of diffusers but the result shows also that same amount of diffusers can give different Asc
depending of the pattern of which the diffusers are positioned. This is seen for both vertically and horizontallyoriented diffusers. It should be observed that the same number of diffusers gives significantly higher Asc
values for vertically oriented diffusers compared to horizontally oriented diffusers. Asc related to the numberof diffusers is shown in Figure 5 and Figure 6. Figure 5 presents the values for vertically oriented diffusers
and Figure 6 presents the results for the horizontally oriented diffusers.

Equvivalent Scattered Absorption Area
14,00
12,00

Asc (m2)

10,00
8,00
6,00
4,00
2,00
0,00
0

1020304050

No of Diffusers
Vertical diff 2000 Hz

Vertical diff 4000 Hz

Figure 5 Equivalent scattered absorption area, vertically oriented diffusers, depending on the number of diffusers.

Equvivalent Scattered Absorption Area
14,00
12,00

Asc (m2)

10,00
8,00
6,00
4,00
2,00
0,00
01020304050

No of Diffusers
Horsizontal diff 2000 Hz

Horizontal diff 4000 Hz

Figure 6 Equivalent scattered absorption area, horizontally oriented diffusers, depending on the number of diffusers.

The relation between Asc and the acoustic parameters reverberation time and speech clarity has been
evaluated. This evaluation indicates that similar behavior are achieved in T20 and C50 when relating theseparameters to Asc even if the pattern or orientation of the diffusers differs. The results of T20 and C50 related
to Asc is presented in Figure 7 and Figure 8. The graphs to the left are the results for vertically oriented
diffusers and the figures to the right presents the results for horizontally oriented diffusers. It is important toobserve the different scale on the x-axis and that the number of diffusers to reach a certain Asc value differs
between the left and right figures.

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1,0

0,80,8T20 (s)T20 (s)

Reverberation Time
1,0

0,6
0,4
0,2

Reverberation time

0,6
0,4
0,2

0,0

0,0
0,00

5,00

10,00

15,00

0,00

Vertical diff 2000 Hz

2,00

4,00

6,00

Asc (m2)

Asc (m2)

Horsizontal diff 2000 Hz

Vertical diff 4000 Hz

Horizontal diff 4000 Hz

Figure 7 Reverberation time depending on Asc for vertically, to the left, and horizontally, to the right, oriented diffusers.

Speech Clarity

12,012,0

10,0

10,0

8,08,0C50 (dB)

C50 (dB)

Speech Clarity

6,0
4,0

6,0
4,0
2,0

2,0

0,0

0,0
0,00

5,00

10,00

Asc (m2)

Vertical diff 2000 Hz

0,00

15,00

2,00

Asc (m2)

Horsizontal diff 2000 Hz

Vertical diff 4000 Hz

4,00

6,00

Horizontal diff 4000 Hz

Figure 8 Speech clarity depending on Asc for vertically, to the left, and horizontally, to the right, oriented diffusers.

The acoustic parameter sound strength (G) was also evaluated in relation to Asc. The results show that just asmall effect on G is achieved by using diffusers. Further is no obvious trend seen in the relation between Gand Asc, as for T20 and C50. The result of G related to Asc is shown in Figure 9. This figure includes bothvertically oriented diffusers and horizontally oriented diffusers.

Sound Strength
20,0G (dB)

19,5
19,0
18,518,0
17,50,00

2,00

Vertical diff 2000 Hz

4,006,00

Vertical diff 4000 Hz

8,00

Asc (m2)

10,00

Horizontal diff 2000 Hz

12,00

Horizontal diff 4000 Hz

Figure 9 Sound strength related to Asc for vertically and horizontally oriented diffusers.

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14,00

4. DISCUSSION
The results show that the orientation, in terms of vertically and horizontally, of the diffusers as wellas the pattern of how these are installed influences the equivalent scattered absorption area (Asc).
Vertically oriented diffusers gave significantly higher Asc than horizontally oriented diffusers whensame amount of diffusers were compared. This can be explained by the direction of which the soundwaves are reflected. The vertically oriented diffusers contribute with a higher extent to the diffuse
sound field by disturbing the grazing field. Further will the vertically oriented diffusers direct thesound to the absorbent ceiling.
Concerning the patterns was it found that Asc, for configurations with same amount of diffusers butwith different patterns, was higher for the chess pattern compared with vertical row pattern. Further
gave diffusers organized in horizontal rows higher values than the chess pattern. This was valid forboth 2000 Hz and 4000 Hz and can to some extent be explained by how the diffusers are exposed
related to the sound field. The design, in terms of the construction of edges etc may have an influence.
A correlation can be seen between the Asc values and the acoustic parameters rev erberation time andspeech clarity, in both 2000 Hz and 4000 Hz. Similar values are found for vertically oriented diffusers
and horizontally oriented diffusers when relating it to Asc values. Comparing Asc values seems to bea good indicator for the effect on the acoustic parameters reverberation time and speech clarity. Moreresearch is needed on how to estimate the effect of a diffuser depending on the pattern. The re lation
between the pattern, Asc, C50 and T20 may also be investigated in different room dimensions withmore realistic conditions.
Concerning sound strength, just a small change was seen when using diffusers. Since strength,
opposite reverberation time and speech clarity, is related to the steady state condition it is closer
determined by the diffuse field assumption. Thereby is strength more influenced by the totalabsorption in the room and less by the scattering. Combining diffusers and absorbent treatment isthereby of interest to investigate further in order to improve the acoustics in public ordinary rooms.

5. CONCLUSION
The direction, amount and pattern of diffusers will have an impact on the efficiency of the diffuser. Further
is the design of the diffuser important, in terms of the frequencies in which the diffuser should operate but
also regarding choices of how it is accomplished.
The Asc value seems to be an appropriate method for quantifying the effect of the diffusers when comparing
the values to the room acoustic parameters reverberation time and speech clarity. Sound strength will be more
dependent on the amount of absorption.
This study has been based on measurement in a reverberant chamber, it will be of importance to investigate
the influence of diffusers in combination with an absorbent ceiling in a real sized room.

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6. REFERENCES[1] J. Bradley, H. Sato och M. Picard, ”On the importnace of early refelctions for speech in rooms,”
The journal of Acoustical Society of America, 2003.
[2] E. Nilsson och C. Svensson, ”Room Acosutic Evaluation of Different Room Types,” BNAM, 2008.
[3] C. Campbell, E. Nilsson och C. Svensson, ”The same reverebration 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,” IBPC, 2015.
[4] E. Nilsson, ”Input Data For Acoustical Design Calculations,” ICSV, 2017.
[5] Y.-J. Choi, ”An Optimum Combination of Absorptive and Diffusing Treatments for Classroom
Acosutic Design,” BUILDING ACOUSTICS · Volume 21 · Number 2 · 2014 , p. Pages 175–180, 2014.
[6] T. J. Cox och P. D'Antonio, Acoustic Absorbers and Diffusers, CRC Press, Third Edition, 2017.
[7] EN ISO 3382-2: Acoustics - Measurements of room acosutic parameters- Part 2: Reverberation
time in ordinary rooms, 2008.
[8] EN ISO 3382-1: Acoustics - Measurement of room acoustic parameters - Part 1: Performance
spaces, 2009.
[9] E. Nilsson, ”Decay Processes in Rooms with Non-Diffuse Sound Fields Part II: Effect ofIrregularities,” Building Acosutics, vol. 11, pp. 133-134, 2004.

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