Acoust. Sci. & Tech. 26, 2 (2005)
The young architect s guide to room acoustics
Lau Nijs1; and Diemer de Vries2
1
Delft University of Technology, Faculty of Architecture, Building Physics Section,
Berlageweg 1, 2628 CR Delft, Netherlands
2
Delft University of Technology, Faculty of Applied Science, Laboratory of Acoustical Imaging and Sound Control
( Received 8 September 2004, Accepted for publication 23 October 2004 )
Keywords: Auditorium and concert hall acoustics
PACS number: 43.55.Fw [DOI: 10.1250/ast.26.229]
1. Introduction
V
At the Faculty of Architecture of our University, students RT ź 0:161 ð1Þ
S
develop (virtual) plans for concert halls of different sizes.
The loudness is calculated as:
They start with reading elementary books and lecture notes.
For those who want to dig into the theory more profoundly,
Q 4ð1 Þ 1
GðrÞ Åº10 log þ 10 log ð2Þ
Kuttruff s book on acoustics [1] appears very adequate.
4 r2 S 4 102
The next step is to use this knowledge for the first draft of
a concert hall. Three books, commonly preferred in this stage, where Q is the directivity of the source. The distance between
are Barron s, Beranek s and Lord s (in alphabetical order) source and receiver is given as r. For most positions in a hall
[2 4]. However, these books seem to be written for the the first term within the brackets can be neglected and hence
acoustic consultant or the architect having some experience in Eq. (2) turns into:
the field of room acoustics. One extra problem of Beranek s
4ð1 Þ
book is that it deals with big halls only, while in many student G ź 31 þ 10 log ð3Þ
S
plans the focus is on a hall with 100 to 800 seats.
In the next stage students use simulation computer
Equations (1) and (3) are valid under the assumption that
programs. It appears easy to input a hall into the computer,
the sound field within the concert hall is diffuse; then Eq. (3)
but then: how should the calculated values for the reverber- does not depend on r. In practice, however, G tends to
ation time, loudness, clarity, etc. be interpreted? What are the
decrease with r, but it can be proved that Eq. (3) is valid when
ideal values ? It is the aim of the research described here, to
r ź 4V=S, which is the mean free path [3,6]. At the early
help architectural students with these early steps in the design
stage of the design process, Eq. (3) gives a sufficient
process.
indication of the total hall. In a later stage of the design
The present paper was originally intended as a subject for
process, computer models will automatically produce the
discussion with the attendants of the RADS-conference in
different G-values through the hall.
Awaji, Hyogo [5]. Comments were given at the conference
itself, but before and after the conference itself more profound
3. The G-RT-diagram
contributions were madea). These comments had a rather big
To combine the two acoustical values with the three
influence on the present text.
building parameters, we developed a G-RT-diagram.
Examples are given in [6], where the method is explained in
2. Theory
somewhat more detail. However, in one graph only two
There is a big variety of acoustical values; Beranek s book
building parameters can be presented, so the surface S is
[3] gives an overview. However, the present method is meant
expressed in the volume by assuming a shoebox shape where
for students, so the theory is kept elementary. Two values are
length:width:height = 1.4:1.0:0.7. Fortunately the method
presented here for the first stages of the design process: the
appears surprisingly insensitive for other values. Only when
reverberation time RT and the loudness G (strength). A third
a cube is used or an extremely long hall, significant dif-
parameter (the clarity C80) appeared very useful as well, but it
ferences are found and the following diagrams need to be
is left out of this paper.
readjusted.
On the other side are the building parameters (shape,
In Fig. 1 Eqs. (1) and (3) are plotted with the hall volume
number of seats, materials, etc.) from which three have a
and the absorption coefficient as parameters, where the room
leading role: the volume V, the total surface S and the mean
volumes range from 400 to 25,600 m3 a. The values for
absorption coefficient .
(0.11, 0.20, 0.33, 0.50) are chosen to get 3 dB steps in Eq. (3).
The start is with Sabine s formula for the reverberation
RT is given along a logarithmic scale, because RT should be
time, given as:
considered as a relative factor.
Figure 1 also contains Beranek s rectangle. In Bera-
e-mail: L.Nijs@bk.tudelft.nl
nek s book [3] values can be found for the mid frequencies of
a)
The authors want to thank R. Metkemeijer and M. Barron for their
the ideal concert hall for symphonic music: RT should be
contributions.
229
Acoust. Sci. & Tech. 26, 2 (2005)
10.0
acoustics (if any) for halls smaller than those described by
25600
0.11
Beranek and to draw an ideal curve from Beranek s
12800
5.0 abs. coeff. Volume [m3]
6400
rectangle through the G-RT-diagram.
0.20
3200
Barron deals with halls for chamber music ([2], chapter 6)
1600
0.33
800
and he gives an ideal curve which is based on the work by
2.0
400
0.50
Cremer and Müller [7]. It is written as:
Beranek s rectangle
1.0
log RT ź 0:138 log V 0:349 ð4aÞ
0.5
This resulting curve is drawn in Fig. 2. Barron compared
0 5 10 15 20 this curve with results from some existing halls and found a
G (strength) [dB]
good agreement. Yet we prefer a slightly different curve,
defined as:
Fig. 1 Reverberation time and strength for a series of
hall volumes and absorption coefficients. log RT ź 0:21 log V 0:55 ð4bÞ
which is also drawn in Fig. 2.
The reason to deviate slightly from the Cremer-Müller-
10.0
curve is twofold:
25600
0.11
12800
1. The curve from Cremer and Müller does not run through
5.0 abs. coeff. Volume [m3]
6400
0.20
3200 Beranek s rectangle. It doesn t need to, since the Cremer-
1600
proposed
Müller-curve is made for chamber music and if this is
0.33
800
2.0
400
performed in big halls, a slightly lower RT may be preferable.
0.50
Cremer MÜller
Yet, we think, students should depart from Beranek s
1.0
Rectangle.
2. All rooms given by Barron are between 2,000 and
0.5
20,000 m3. If the curve is extrapolated to halls in the order
0 5 10 15 20
of 400 m3 the mean absorption coefficient is so low that only
G (strength) [dB]
small audiences are allowed.
Table 1 gives the same results. It has been used by
Fig. 2 Ideal curves drawn in the G-RT-diagram.
students and was found very useful for the establishment of
target values when using computer programs. This will be
between 2 and 2.3 s, while G should be between 4.0 and illustrated in a following section.
5.5 dB for the European hall. If these values are applied, the
ideal halls lie within a rectangle, as shown in Fig. 2. 5. Audience size
Of the three variables RT, G and V, only two can be RT and G are interesting values for the acoustician, but if
chosen independently. So only one volume ( 16;000 m3) can a hall is in its first stage of design, the architect needs building
be found if RT ź 2:15 and G ź 4:75 are chosen as ideal parameters like dimensions, audience size and absorption
values. On many occasions, this conclusion is a shock to coefficients.
architectural students. Many students of our faculty want to In almost any case the audience and the orchestra are the
have Mahler s 8th symphony played in a local gymnasium main absorbing surfaces in a concert hall. Kosten [8] used the
with a 3,200 m3 volume by choosing a 2.1 s reverberation (big) concert halls given in Beranek s book to derive a relative
time. Figure 1 shows them why this sounds like an inferno: it factor (1.07) to calculate the reverberation time from the
is much too loud. Smaller halls are meant for smaller volume and the total occupied surface Socc. This was a first
orchestras and if a large orchestra has to play in a small hall, attempt to combine acoustical values and building parameters,
both RT and G must be decreased to find a compromise. but Kosten s value 1.07 fails for smaller halls. Log-log-
Figure 1 also explains to students why some modern halls dependencies like given in Eqs. (4) are more likely, but this is
have sophisticated technical means to change the volume subject of further research.
considerably depending on the type of music to be played. Our somewhat different approach is found in Table 2. The
occupied surface is assumed as totally absorbing. For other
4. Ideal values for smaller halls surfaces a mean absorption factor is used. In the paper for the
It is the aim of the present work to find the ideal RADS-congress [5], this value was estimated as 10%, but in
Table 1 The values of RT and G from the lower curve in Fig. 4. C80 and are added.
Volume [m3] 400 800 1600 3200 6400 12800 25600
RT [s] 1.00 1.15 1.32 1.53 1.77 2.06 2.39
G [dB] 18.0 15.5 13.0 10.5 8.0 5.5 3.0
C80 [dB] 3.1 2.1 1.2 0.3 0:6 1:5 2:3
[ ] 0.19 0.21 0.23 0.25 0.27 0.29 0.32
230
RT [s]
RT [s]
L. NIJS and D. de VRIES: YOUNG ARCHITECT S GUIDE TO ROOM ACOUSTICS
Table 2 Audience size as a function of hall volume for two mean absorption coefficients for non-audience surfaces.
Volume [m3] 400 800 1600 3200 6400 12800 25600
Socc at 10% [m2] 34 65 122 224 407 736 1311
m3/pers at 10% 5.9 6.1 6.5 7.1 7.8 8.7 9.8
Socc at 13% [m2] 24 49 97 185 348 642 1173
m3/pers at 13% 8.3 8.2 8.3 8.6 9.2 10.0 10.9
the before-mentioned discussions on that paper Dr. the curve for 2,400 m3. Also six measured values are given in
Metkemeijer commented that this value is too low. He did the hall when it was completely empty. The six measured
many measurements in halls when the chairs were removed values show a rather small variation in RT-values; the
and a value of 13% appears more likely. This value even tends variation in G-values is rather big because they are taken
to increase since nowadays halls are filled with ever for different source-receiver distances. They agreed with the
increasing sets of lighting. Therefore the volume of modern values predicted with Barron s method to calculate decreasing
halls tends to increase as well in order to keep the right sound levels through the hall (see [6] for details about the
reverberation time. Table 2 gives the results for 10 and 13%. method).
It is interesting to calculate the volume per person from One specific measured value is denoted by a square dot. It
Socc. Table 2 gives these results if the number of seats is is for the mean free path distance and it is the aim of our
assumed as 2.0/m2, which is a common value for older halls. method to bring this value close to the target value of the hall
Some modern halls have values as low as 1.6, but there is a when renovated. This is not always easy since from the three
tendency back to older values in recent years. Table 2 gives variables RT, G and V, only two can be chosen independently.
only a very rough estimation, because values in practice vary The next step was to build a computer model and to
considerably. This is illustrated by the four examples given by calculate G and RT for similar source and receiver positions.
Barron ([2], chapter 6): Wigmore Hall has a small stage and In this case, bringing the values at the mean free path distance
the audience surface is big. Hence it has only 5.3 m3/person. close to the target value was not too difficult, since the extra
The other three halls Barron deals with are more according to audience plane appeared adequate.
Table 2 with values ranging from 8.3 to 9.2 m3/person. A similar G-C80-diagram can be made with C80 instead of
Beranek s book shows (for big halls) values from 6 12 m3/ RT. In theory this diagram gives no extra information.
person, but all his better concert halls are in the order of However, when interpreting computer output, it appeared
8 10 m3/person. useful as well. It is even possible to estimate the influence of
sound reflectors etc.
6. An example: Young architect at work Another task set for this particular student was to make a
One student s project is given here as an example. The design for a theatre in this existing hall as well. At that stage
task was to convert a machine hall from the 19th century into we made a first attempt to develop an ideal curve for speech as
a concert hall for chamber music. The given volume is well. It will be given in a future paper.
2,400 m3, so the size for audience plus orchestra as estimated Designing a theatre with the method appeared not too
(after interpolation) from Table 2 is about 140 m2 and the difficult as well, but, as always, the biggest task was to design
number of seats is 280. That does not fit on the floor surface of technical possibilities to change the theatre into a chamber
this particular hall, so the seat number was reduced to 260. music hall and vice versa.
Figure 3 shows the G-RT-diagram. It contains the target
value, which is on the intersection of the ideal curve and 7. Discussion
A simple scheme is possible in the first stages of the
design process and we believe that the curves of Fig. 2 and the
values in Tables 1 and 2 are appropriate. It took some time for
10.0
12800
0.07
students to get familiar with the system, but it proved to be a
6400
abs. coeff. Volume [m3]
3200
useful instrument, especially for understanding the output of
0.11
5.0
1600
computer models.
800
0.20
400
References
measured
2.0
0.33
[1] H. Kuttruff, Room Acoustics (Elsevier, New York, 1991).
[2] M. Barron, Auditorium Acoustics and Architectural Design
1.0
target value
(E & FN Spon, London, 1993).
[3] L. L. Beranek, How They Sound, Concert and Opera Halls
0.5
5 10 15 20 25 (Acoustical Society of America, Woodbury, 1996).
G (strength) [dB]
[4] P. Lord and D. Templeton, The Architecture of Sound;
Designing Places of Assembly (Architectural Press, London,
Fig. 3 Design scheme. The square dot from the meas- 1986).
ured values is at mean free path distance between [5] L. Nijs, D. de Vries and D. Petri, The young architect s guide
source and receiver. The example is for 500 Hz; for to room acoustics, Int. Symp. Room Acoustics: Design and
other octave bands slightly different values are found. Science (2004).
231
RT [s]
Acoust. Sci. & Tech. 26, 2 (2005)
[6] L. Nijs, P. Versteeg and M. van der Voorden, The combination Room Acoustics (Applied Science, London, 1982).
of absorbing materials and room shapes to reduce noise levels, [8] C. W. Kosten, A new method for the calculation of the
Int. Congr. Acoustics, Kyoto, Japan (2004). reverberation time of halls for public assembly, Acustica, 16,
[7] L. Cremer and H. A. Müller, Principles and Applications of 325 330 (1966).
232
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