Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 165

refurbishment. Point of departure in this respect is a

purposeful influencing control of the primary structure

of the performance room. This concerns, among other

things:

• The size of the room.


• The shape of the room.


• Functional-technological circumstances, for instance
  the platform or stage arrangement, the installation of
  balconies or galleries, lighting installations, and the
  arrangement of multimedia equipment.


• The topography regarding the arrangement of per-
  formers and listeners, like for instance the sloping of
  tiers or the proscenium area in front of the stage
  opening.

Based on these premises, the secondary structure of
the room will be acoustically determined. This struc-
ture concerns essentially:

• The arrangement and distribution of frequency-
  dependent sound-absorbing as well as sound-
  reflecting faces.


• The subdivision of the surface structure for direc-
  tional and diffuse sound reflections.


• The frequency-dependent effect of uneven surfaces.


• The architectural-stylistic conformation of all bound-
  ary surfaces of the room.

7.3.2.2 Room Form and Sound Form

There exists a correspondence between the shape of a
room (room form) in its primary structure and the
resulting sound. The term sound form refers in this
context to the reverberation timbre which is herewith
divided into its low-frequency portion (warmth) and its
high-frequency portion (brilliance).
The method used for assessing the acoustical quality
of concert halls is based on a paper by Beranek^46 in
which one finds a list of seventy concert halls arranged
in six subjective categories according to their acoustical
quality. Of all these there are three halls listed in the
category A+ as outstanding or superior and six halls in
the category A as excellent. Eight of these are
shoebox-shaped, a fact that gives rise to the question as
to whether a good roomacoustical quality is linked to a
rectangular shape of the room.
The subjective assessment parameters used are, on
the one hand, the warmth of the sound pattern and, on
the other hand, the brilliance of the same. Warmth and
brilliance refer in this context mainly to the influence of
the sound energy density on the lower- and the higher-
frequency ranges, respectively. Questions concerning

initial reflections will be left out of consideration for the

time being; only the timbre in the decay process will be

considered.

The criterion bass ratio, BR (Beranek^9 ), provides

indisputable evidence on the warmth of the sound (see

Section 7.2.1.2). The desirable optimum value range for

music performances is between 1.0 to 1.3. According to

Beranek for rooms having a reverberation time lower

than 1.8 s, it is permissible to have a bass ratio of up to

1.45.

For objective assessment of the timbre Schmidt^23 has

defined the timbre measure. By analogy with the BR

and the timbre measures there was an equivalent

measure deduced and introduced as TR1 (Timbre

Ratio). It is used only for evaluating comparative

aspects of brilliance

.

This numerical relationship is used to evaluate

timbre as the ratio of the reverberation time at high

compared to low frequencies. Thus the value TR1>1

stands for a longer reverberation time at higher frequen-

cies rather than at lower frequencies, and hence in this

context higher brilliance in the sound pattern.

As regards the primary room structure of a concert

hall, there are four basic forms considered: rectangle

(shoebox), polygon, circle, and various trapezoidal

forms.

The concert halls selected by Beranek for the catego-

ries A+ and A allow the following pairs of values to be

ascertained in an occupied hall, Table. 7-6.

Table 7-7 shows that, in shoebox-shaped rooms, the

brilliance is lower in comparison to rooms of polygonal

primary shape. However, on the basis of the brilliance

ratio TR1, no such significant difference can be shown

between rooms of a quasi-circular ground plan (five

halls) and those having diverse trapezoidal primary

shapes (nine halls).

7.3.3 Primary Structure of Rooms

7.3.3.1 Volume of the Room

As a rule, the first room-acoustical criterion to be deter-

mined as soon as the intended purpose of the room has

been clearly established, is the reverberation time (see

Section 7.2.1.1). From Eq. 7-6 and the correlation

between reverberation time, room volume and

equivalent sound absorption area, graphically depicted

TR 1

T2000 Hz+T4000 Hz

T125 Hz+T250 Hz

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