Balcony design

Balconies are a common feature of large concert halls. They allow the vertical dimension to be exploited, which in concert halls tends to be large to satisfy the reverberation time requirement. Bal-conies allow more people to be accommodated within a certain distance from the performers. They can also offer acoustic advantages to listeners not underneath them; balcony soffits and balcony fronts represent additional hard surfaces that can provide useful reflections.

Seats under a balcony overhang are however disadvantaged both visually and acoustically. In both senses the audience under the overhang can feel cut off from the main volume of the audito-rium, with a loss of intimacy and sense of detach-ment from the acoustics of the main space. Seat prices frequently reflect this. In some cities there are

Acoustics for the symphony concert hall 55 examples of auditoria where the decision has been

taken to have no overhangs at all. But the penalty of this strategy can be that many listeners are so far from the performers that the intimacy of the whole performance space is seriously compromised. One of the major design problems for a large auditorium is to balance the advantages and disadvantages of balcony overhangs.

To understand the acoustic effect of an over-hang, it is useful to consider the early and late sound separately. Contrary to what one might first expect, the early sound is often little influenced by the presence of an overhang. Side-wall reflections are usually unaffected and the presence of a nearby rear wall is likely to compensate for the absence of other early reflections which are obscured. The late sound on the other hand is significantly modified by an overhang. In an exposed seat away from bal-conies, reverberant sound arrives at a listener from most directions. Under an overhang however the vertical angle from which sound can arrive at a seat is greatly reduced (Figure 3.22).

In terms of the subjective qualities considered important for music listening, there appear to be three noticeable effects under overhangs that are all linked to reduced late sound arriving at listeners (Barron, 1995a). With reduced late sound level the balance between clarity and reverberance shifts to less sense of reverberation and greater clarity.

Objective clarity (the early-to-late sound index) increases and early decay time (a measure of rever-berance) decreases under overhangs. There is also a reduction in sound level under an overhang but in general the reduced sense of reverberation is likely to be more noticeable than the reduced loudness.

The third effect of a balcony overhang is clear in qualitative terms but difficult to quantify: listeners are aware of the reduced solid angle from which they are receiving sound. They are likely to detect the absence of sound from above as well as sound from behind; both probably contribute to the sense of detachment felt under overhangs. Listener envel-opment (section 3.2) can also reduce under balcony overhangs.

Gap?

Figure 3.22 Long section through a balcony showing how the angle from which late sound can arrive is reduced below an overhang. A gap behind the balcony can provide some compensation

56 Acoustics for the symphony concert hall

For balcony design, Beranek (1962) suggested a rule-of-thumb for concert halls that the depth beyond the overhang should not exceed the height of the opening (that D/H ≤1 in Figure 3.23). More recent study (Barron, 1995a) suggests that the verti-cal angle of view (θ in Figure 3.23) may be a better parameter, with a recommended minimum value of 40º. The vertical angle of view approach takes account of the rake of the floor under the overhang.

Beyond restricting the degree of overhang, there is little that can be done to mitigate the effects of an overhang on the reverberant sound. It is possible to leave a gap behind the balcony to allow sound to filter round, providing a sense of some sound from behind for those below the overhang (Figure 3.22). A rare example of such a gap is to be found in the Michael Fowler Centre, Wellington, New Zealand (section 4.10). The sound level under an overhang can be increased by profiling the balcony soffit to provide extra reflections, but using geom-etry to boost late as opposed to early reflections is problematic. Maintaining a high opening at the overhang helps avoid reducing the reverberant sound, but that can of course remove one of the main design gains of the balcony. Balcony design in

Segerstrom Hall, California (section 10.6 and Barron, 1995a) is particularly interesting with heights at the openings to the overhangs of 6.1 and 7.2 m.

While Figure 3.23 considers geometry in just two dimensions, it is clear that the third dimension also influences sound behaviour under balcony over-hangs. Such details as hall width, location of reflect-ing surfaces and the degree of diffusion in the main body of the hall must also be significant. These aspects remain to be investigated.

3.8 Design for the performers (written by A.-C. Gade)

It is only since around 1975 that the acoustic needs of the musicians have been subject to systematic investigation. Knowledge of performers’ needs and how to fulfil them is not as well developed as knowledge about audience preferences. Yet certain guidelines have been developed, together with objective measurement procedures relating to stage conditions. The sound field inside the orches-tra is extremely complex and probably impossible to describe in detail because of dependence on a wide interplay of factors: the orchestration of each piece (which varies from one bar to the next!), the directivity and sound powers of each instrument, the arrangement of the orchestra on the platform as well as the acoustics of the room. It is therefore wise to view the still immature scientific results against the background of many years of practical experi-ence of stage conditions. It is in this spirit that the following practical design guidelines are given.

Meeting the needs of the musicians is primarily a question of proper design of the platform itself and of surfaces in its vicinity. These elements of the hall are of utmost importance for the two major acoustic concerns of performers: ease of hearing each other (necessary for good ensemble playing) and the feeling of support for sound from their own instrument. Other aspects of the musicians’ acous-tic experience, such as timbre and reverberance, are probably related to the design of the hall as a whole, so that if the audience is happy with these aspects the musicians are likely to be satisfied too.

H

D

θ

Figure 3.23 Basic balcony overhang proportions for a concert hall. Criteria are based on the ratio of depth to height, D/H, or θ, the vertical angle of view.

Acoustics for the symphony concert hall 57 Of course the surfaces around the platform may

also be used to provide sound reflections to the audience. But such reflections have two serious drawbacks for listeners: potential tone colouration and a narrow source image (section 3.2). Adequate reflections for the audience should be available from the main body of the hall, leaving the plat-form area of the hall designed primarily to satisfy the musicians’ needs. If the players are unable to perform at their best, the listeners’ experience will definitely suffer.

3.8.1 Floor space, layout and risers

Selecting the platform floor area is a compromise between acoustic needs and comfort. As with the seating standard for audience, the area demanded per musician on the platform has also increased in recent years. While the platform in the Vienna Musikvereinssaal has an area of 130 m² and is still used successfully for modern full-sized orchestras, a newer hall like the Philharmonie Gasteig in Munich has a platform area of as much as 250 m². A variety of Parkinson’s Law seems to afflict orchestral musi-cians, who will spread out to occupy the available area as well as tend to move closer and closer to the front edge of the stage (unless told otherwise by the conductor). But spreading out on a large floor space means greater distances and reduced acous-tic communication between musicians. It has been found (Gade, 1989a) that beyond a distance of 8 metres, the delay of the direct sound becomes large enough to reduce the ease of ensemble playing.

While aiming at a close spacing, one should not forget to consider the rise in sound level as one moves close to the more powerful instruments.

Excessive levels near tympani or in front of brass sections may mask any other sound in the orches-tra and even be dangerous for musicians placed in these areas. Local problems such as these should be avoided by adjusting the layout, by the use of risers or if all else fails by setting up small screens within the orchestra.

The tendency to move forwards is also danger-ous for several reasons. The orchestra itself will get

less benefit from the back wall and/or ceiling reflec-tions. A late reflection off the stage back wall may also upset clarity for the audience. An orchestra on the edge of a stage also loses a reflection from the empty stage apron, which is especially useful to project sound from a soloist or a weak string section into the hall volume.

Beranek (1962, p. 498) says ‘musicians like an area of about 20 sq. ft each’, or 1.9 m². This is cer-tainly too generous for violinists, for instance, but it may be used as an average figure to estimate the total platform area required: for a 100-piece orches-tra almost 200 m² is necessary. More recent studies indicate the following net areas per player for differ-ent groups of instrumdiffer-ents:

1.25 m² for upper string and wind instruments;

1.5 m² for cello and larger wind instruments;

1.8 m² for double bass;

10 m² for tympani, and up to 20 m² more for other percussion instruments.

For a full 100-piece orchestra (with a usual per-cussion section) this means a net covered area of about 150 m². If the platform is built at 200 m², this leaves ample extra space for soloists and extra percussion, space to compensate for losses due to risers, access routes, including a 1  m wide empty strip along the front of the platform. Also a narrow empty zone along the side and rear walls can be advantageous, as for certain instruments, for example the French horn, it is not comfortable to sit right against a wall.

For seated choirs, 0.5 m² per person is needed, so that a 100-person choir requires a further 50 m² of space. Choir seating may be placed at the rear of the stage with the stage front extended further out-wards into the audience, as is done in the Barbican Concert Hall, London (section 5.10). Alternatively it is preferable to have a separate elevated choir balcony such as in the Royal Festival Hall, London (section 5.5) and St David’s Hall, Cardiff (section 5.11). When not required as choir seating, these seats are sold to concertgoers, who appreciate the close visual contact with the orchestra.

In order to minimize distances within the orches-tra, the platform should be neither too wide nor

58 Acoustics for the symphony concert hall

too deep. With a 200 m² stage, the average width should not exceed 18 m, resulting in a depth of about 12  m (for full orchestra but without choir).

It is important for smaller groups that the effective floor area can be reduced. Ideally movable bound-ing walls, such as those on lifts at the Gulbenkian Great Hall, Lisbon, should be used (Barron, 1978).

Where these are absent, the musicians should be persuaded to move backwards in order to limit dis-tances between musicians and maximize the effect of reflections from the surrounding walls.

The provision of risers on the platform is impor-tant for good ensemble playing in large orchestras, because direct sound will propagate more freely between distant players when they are elevated rather than being hidden behind players sitting in between. For large distances between players (a value exceeding 8  m was mentioned above as causing time delay problems), it is clear that a weak direct sound cannot be fully compensated for by reflections, which will always be further delayed.

Risers towards the sides of the platform for the back

rows of strings are therefore also recommended, a feature which is often implemented as semicircular risers, e.g. in the Berlin Philharmonie (Figure 3.24) or the Kitara Concert Hall in Sapporo, Japan.

If a separate choir balcony is not used, the riser system should allow for the full orchestra to be moved back and forth depending on whether a choir is present or not. This flexibility may be diffi-cult and expensive to obtain if a hydraulic system is used, while a system of small, loose box elements (as in Derngate, Northampton, section 11.8) is cheaper and more flexible, but can be time- and cost-con-suming to operate. In any case, it is important that the riser system allows for the ensemble, whatever its size, to be placed in a position on the platform where the benefit of reflections from surrounding surfaces is optimal. If the system lacks flexibility, it will result in wasted space, with steps in the wrong places for many of the required orchestral layouts.

To avoid loss of space, the horizontal depth of each riser is important: 1.25  m for upper strings and woodwind, 1.4 m for brass and cellos. Double

Figure 3.24 Stage of the Philharmonie Concert Hall, Berlin

Acoustics for the symphony concert hall 59 bass players require slightly more depth, unless

the music stand is placed on the step in front of them. If the rear of the platform is to double as choir seating, the 1.4 m can be split into two risers of 0.7 m. However 0.8 m is needed if the choir is to be seated. Percussion and tympani need a depth of about twice 1.4  m at uniform height. Riser height may be kept modest, at about 100 mm per step for woodwinds and slightly higher further back. But it is always advantageous if the heights are adjustable to suit the particular orchestra arrangement and the desired balance for the audience. It is often feared that risers will favour the audibility in the audience area of the wind and brass. But unless one is dealing with small halls and regional orchestras with severe-ly undersized string sections, this is a factor which the conductor should be able to control.

Finally there is the question of the stage height above the stalls floor. The height should be more than 0.5 m, otherwise soloists’ ‘command’ over the audience tends to be lost. Values in excess of 1.0 m lead to screening from view of the centre section of the orchestra from the front rows of the stalls.

The stage height determines the setting-out point for sightlines and is thus influential for much of the seating layout (section 2.3).

3.8.2 Floor material

Musicians always speak unambiguously in favour of wood over an airspace as the appropriate con-struction for the platform floor, because it provides a ‘warm’ sound. The physics behind this notion has been the subject of few investigations (Askenfeld, 1986). Basically two processes with opposite effects are involved. The floor may be capable of acting as a sounding board for low-register instruments in direct contact with the floor, but it will also absorb sound reaching it through the air. Another possible effect may be the sensation of structural vibration set up in the floor adding positively to communi-cation within the orchestra. The physical studies do not as yet suggest conclusive advantages, yet it would be unwise to contradict the request for this type of floor. In order to allow the floor to be as

acoustically ‘alive’, it should be chosen as thin and with the joist spacing supporting it being as large as possible, within limitations provided by loading requirements (especially grand pianos), rigidity (e.g.

for television cameras) and local fire regulations. A platform thickness of 25 mm and a joist spacing of 600 mm may be regarded as a desirable minimal construction.

3.8.3 Side and back walls of the platform

As already mentioned, the wall surfaces in the vicinity of the platform should be mainly oriented so as to send reflected energy back to the musi-cians themselves. Where possible in order to avoid attenuation of reflections, non-horizontal reflection paths should be provided. This may be done either by tilting the upper part of the walls downwards, by tilting balcony fronts down or by providing balcony soffits around the stage to give reflections off the soffit and adjacent vertical surface.

In halls which contain a stagehouse, an orches-tral shell is necessary. Yet the shell must be designed in such a way that the shell volume forms part of the same acoustic space as the hall. If it acts like a separate space the musicians will experience a lack of contact with the main hall acoustics and, with a high degree of enclosure, they may have so much support that they are tempted to play too softly. In other words, a rather critical balance exists between recessing the platform into an orchestra shell and exposing it in the main hall. (To confirm the situa-tion objectively, the early decay time on the stage should not be less than 70 per cent of the value in the hall.) The shell itself should be made of fairly solid materials to avoid low-frequency absorption.

A surface density of 20 kg/m² is recommended.

At this point it should be mentioned that orches-tras, with members who have unfortunately expe-rienced hearing loss and tinnitus due to excessive sound levels, are often reluctant to accept any advice for their new or existing hall. We are used to thinking in terms of sound absorption as a way to reduce sound levels, whereas more efficient sound

60 Acoustics for the symphony concert hall

reflection on stage may be a better solution. It is important to remember that musicians produce sound according to what they hear and if you only hear your closest neighbours, you are tempted to play louder – like when we shout in a telephone with poor transmission. In other words, the orches-tra is more likely to cultivate a playing style with modest, balanced sound levels if the acoustic con-ditions allow them to communicate via efficient propagation of direct sound and early reflections. In fact measurements – even in orchestra pits – have shown that absorption added to wall surfaces near the orchestra only reduces support, but not the sound levels, which are always dominated by direct sound from the nearby instruments.

3.8.4 Ceiling reflectors

The most effective surface for providing early reflec-tions on the platform is the ceiling. In halls with exposed platforms and high ceilings, it will often be necessary to suspend a single large reflector or an array of smaller reflectors over the platform. In the latter case, depending on the fractional area of the reflector array compared with the floor area beneath (the degree of perforation), the average height of the reflectors should if possible not exceed 8–10 m above the stage for them to be fully effec-tive. The degree of perforation should be optimized.

If it is too high, inadequate sound is reflected down and if it is too small, sound will be prevented from

If it is too high, inadequate sound is reflected down and if it is too small, sound will be prevented from