Early reflection design

Early reflections in concert halls contribute to the sense of clarity, intimacy and loudness and, if they arrive from the side, to the sense of source broad-ening. All these characteristics are required for the best acoustics. For an acoustic reflection to occur is principally a matter of simple geometry. There will always exist one, but only one, path for reflec-tion from an infinite surface. For reflecreflec-tion from a finite surface, the surface has to be tangential to an ellipse, which has the source and receiver posi-tions at its foci (Figure 3.12). Since an ellipse has the property that the sum of the distances from the foci to any point on the ellipse is constant (this allows an ellipse to be drawn with two pins and a piece of string), a particular ellipse defines a certain reflec-tion delay. In three dimensions, one is dealing with a rotated ellipse, known as an ellipsoid. Reflector size influences the range of frequencies over which the reflector is efficient. As discussed in section 2.6.2, small reflectors are effective only at high frequencies.

To achieve a sense of source broadening, one or more strong reflections are required from the side. ‘Lateral’ reflections from above and behind are acceptable, so that reflections off side wall/ceiling

cornices and side balcony cornices can be of value.

In detail, a reflection is lateral when there is a time difference between its arrival at the near and far ear: in other words when there is an inter-aural time delay. There should be a sufficient angle between the reflection path and the plane bisecting the head when the listener faces the sound source (the plane is known as the median plane). The larger the angle, the greater is the spatial effect. This has a bearing on the behaviour of simple plans (Figure 3.13). With the fan-shape plan the angle of the side wall reflection to the direct sound is small and source broaden-ing tends to be small, whereas in the reverse-splay plan the angle is high and source broadening is correspondingly greater. Though the reverse splay presents unrealistic problems for stage design to be used as an overall plan, it can be used with great effect as the envelope for remote seating.

F F

ф ф

Figure 3.12 A surface has to be tangential to an ellipse for reflection to occur. The source and receiver are at the foci, F, of the ellipse

0 80 200 0 80 200

Time (ms)

Level

Time (ms)

(a) (b)

50 Acoustics for the symphony concert hall

Low frequencies should be present in lateral reflec-tions for optimum spatial effect (section 3.2). Since low-frequency sound is attenuated as it passes over seating planes, this requires lateral reflections on paths remote from audience seating.

While the actual delay of the first reflection (the initial-time-delay-gap discussed in section 3.2) may not be of great importance, this is only within certain limits. A certain maximum value applies to allow for sufficient early energy within 80 ms; a delay gap in excess of 50 ms would be borderline for concert use.

Application of this criterion leads to some interest-ing design plans. In a simple symmetrical plan form, the seats most distant from reflecting surfaces lie along the centre line of the hall. In Figure 3.14, a reflecting surface needs to be within or tangen-tial to ellipse k to reflect sound from the source to position K. However for the remote seat N but with the same reflection delay, the relevant ellipse n is

much larger. From this diagram, the most effective location for a reflector able to serve all receivers is behind the source, such as reflector A in Figure 3.14.

Such a reflector placed above the stage was used in the Salle Pleyel, Paris, of 1927, the London Royal Festival Hall of 1951 and others. Though it suc-ceeds in maintaining loudness, the approach has two serious drawbacks which have deterred many subsequent consultants: an overhead reflector near the stage creates tone colouration (section 3.2) and a narrow source image. Reflections arriving more laterally are preferable in both respects.

The question of location for reflecting surfaces deserves consideration, Figure 3.15. Placing surfac-es above the stage mentioned above was common around 1950, but has now fallen out of favour, at least as a means of providing additional reflections to distant seats. With the building in 1963 of the first terraced concert hall, the Berlin Philharmonie (section 4.9), the alternative of reflecting surfaces near to listeners was introduced. There are two advantages of surfaces near listeners: there is much greater control of reflection conditions throughout the audience seating areas and, with the situation in plan as shown in Figure 3.15(b), the side reflec-tions arrive more laterally, creating greater source broadening.

For small halls a simple plan form can be used (Figure 3.16). The rectangular plan has a consistent reputation for good acoustics; the fan-shape on the other hand is not generally recommended. Figure 3.17 shows a disadvantage of a wide angle of fan

Source Source

Receiver

(a) (b)

Receiver

Figure 3.13 Angle of a lateral reflection in (a) a fan-shape plan and (b) a reverse splay plan (after Marshall, 1968)

Figure 3.14 Ellipses superimposed on a half-plan. Ellipse k is relevant for a 50 ms reflection to receiver K, etc. Reflect-ing surfaces need to be within the relevant ellipse for the delay to be within the 50 ms limit

Stage

k l m n

A

Source K L M N

10m

Acoustics for the symphony concert hall 51

Source on stage (a)

(b)

Source on stage

Figure 3.15 Comparison of the early reflection situation with (a) surfaces near the stage and (b) surfaces near the listener

in that it provides image sources only in a concen-trated region; for the listener this will produce only small source broadening. The narrow angle fan or parallel-sided hall generates reflections coming from lateral positions and this is thought to be one of its virtues. The relative merits of the rectangular and fan-shaped plan are further discussed in sec-tions 4.3 and 4.5.

The horse-shoe plan, so popular in theatres, has little to recommend it for music; when enlarged

enough to achieve a satisfactory reverberation time, the reflection pattern becomes poor, and there are focusing problems associated with the concave rear wall. The elongated hexagon offers a compromise with the visual advantages of the fan shape and the acoustic advantages of the reverse splay. A further compromise scheme is the gross fan-shape plan but with stepped parallel walls (Figure 3.18). This offers an improvement relative to the simple fan shape but probably still carries some of its faults.

(a) (b)

(c) (d)

Figure 3.16 Simple plan forms for concert halls

Image 1

Image 2

Image 3 Image 1

Image 3

Image 4 Image 2

Source

on stage Source

on stage

(a) (b)

Figure 3.17 Image positions for two surfaces. For a wide angle fan in (a), the images are on a small circle; for near parallel surfaces (b), the image positions cover a large area

Figure 3.18 Stepped parallel walls with a gross fan-shape plan

52 Acoustics for the symphony concert hall

Beyond a certain size of hall, some subdivi-sion of the audience space becomes desirable. A design which can be rationalized on the basis of the expanding ellipse model is the large De Doelen Concert Hall of 1966 (2230 seats, section 4.7), Figure 3.19. The elongated hexagonal envelope provides reflections to remote seats, but these reflections arrive too late at seats in front of the stage. These seat locations are serviced by a subsidiary smaller hexagonal form surrounding the stage and front stalls seating.

The expanding ellipse model also leads directly to another stepped plan scheme, formalized by Cremer (1986) as a trapezium terraces room, Figure 3.20. At each seating level, there is a reverse splay surface to provide a lateral reflection. It is neces-sary to tilt balcony fronts downwards so that these reflections reach seating areas.

While lateral reflections are particularly desira-ble, some reflected energy from the ceiling is appro-priate and often necessary. The ceiling is usually the largest room surface. The main concern is to avoid tone colouration effects. If the ceiling reflection is either first or prominent in the reflection sequence, it should preferably be diffused. Concert hall cross-sections with inclined ceiling cross-sections to render their reflections lateral have been used in some recent concert halls, though it is debatable whether the net reflection pattern including cornice reflec-tions from a horizontal ceiling and vertical wall is in fact inferior (Figure 3.21). A similar reflection path to that in Figure 3.21(b) can be provided by side walls and balcony soffits (Figure 4.5) offering additional early reflections to audience in the stalls.

Manipulating early reflections is not however without its risks, particularly if large plane surfaces

Figure 3.19 Half-plan of De Doelen Concert Hall, Rotterdam, with superimposed ellipses corresponding to a 50 ms delay reflection. Ellipse k is relevant for receiver K, etc

Figure 3.20 The trapezium terraces half-plan (after Cremer, 1986). To achieve geometrical side reflections, surfaces A, B and C have to be inclined slightly down from the vertical

Stage

K L M N

k l

m n

10m

A B

C

Stage

Source

Stalls 1st Tier 2nd Tier

Acoustics for the symphony concert hall 53

are used. There is always a risk of tone colouration.

Although the potential for colouration is greater for overhead reflections, it can still be perceived by the sensitive listener receiving a strong lateral reflec-tion from a plane surface. Much more obvious sub-jectively is the case of false localization, where the sound of a particular instrument suddenly appears to come from the reflector instead of from the stage (a case of extreme image shift in Figure 3.2). This can occur, for instance, when trumpets playing in their upper register (where they are highly directional) point at such a reflector. A remedy for both these faults is to add scattering treatment to the reflector, though this increases the spread of the reflection at the expense of reflection level on the original path.

One design strategy for concert halls is to have surfaces oriented so that they direct reflections down onto the audience; these are often called directed reflection designs. One by-product of directing early reflections onto audience is that it also influences the late sound. Because the audi-ence is highly absorbent, less energy is available in this case for the later reverberant sound. It has been noted that the subjective sense of reverbera-tion is best related to the early decay time (EDT), and one finds in these directed designs that the EDT is often shorter than the reverberation time. If the reverberation time itself happens to be somewhat short, the sense of reverberation can then become

subjectively inadequate. The only solution appears to be to extend the reverberation time (probably by increasing the volume), in order to leave an EDT which is long enough, even if the reverberation time becomes longer than the recommended maximum of 2.2 seconds (see section 4.10).

Manipulating early reflections is a necessary part of the acoustic design of concert halls beyond a certain size. Seats which are furthest from useful reflecting surfaces generally require most attention.

But there remains the question of how many reflec-tions are necessary, to which there is no qualitative answer. This is the point at which acoustic design can become an art, though acoustic models provide a valuable guide. The revised theory approach offers the guideline of average behaviour, against which model measurements can be compared (section 3.10.5).