The acoustical design of concert halls has previously been regarded largely as an art form reliant on the experienced acoustician's intuition. However, with growing information it has lost much of its mystique and become a set of generally known rules and numbers that can be calculated. At the same time, the architect's range of possibilities is continuously narrowing, as the acoustician's computer constantly draws attention to what cannot be drawn in a good hall.
This article rewiews recent trends in the design of concert hall acoustics and modelling with the computer, which has become the most important tool of the acoustician. The design of the glass-walled Sigyn Hall, Turku, is presented as an example. Its design included two noteworthy firsts in a Finnish context: the contemporary idea of sound field and the use of the computer.

ACOUSTICAL TRENDS AND NEW POSSIBILITIES
Two decades ago the last major turning point for generations in concert hall acoustics took place. For the first three quarters of the century, the governing idea was to use directed reflections. The aim was to position walls and other surfaces so as to reflect sound in the desired direction. The most obvious indication of this can be seen in the shaping of the ceiling. By the mid 1970s, research in hall acoustics had shown that the most important qualities of a good hall are that the sound field is diffuse (well mixed) and that the sound first reaches the listener's ears laterally (from the sides). With the design of the Sigyn Hall this present international trend arrived in Finland.
In a similar manner as, say, weather forecasts have grown considerably more reliable in recent years, also the prediction of concert hall acoustics has become remarkably more accurate. Determining the acoustics in advance has changed from an art into a controlled and precise design process. The reasons are the same as in meteorology; the evolution of mathematical models and the ever-continuing growth of the power of computers.<

As the most fascinating new possibility, the computer has brought auralisation within the designer's reach. The concept forms an analogue pair with visualisation, more familiar to an architect. With auralisation a virtual illusion of the designed acoustics is created. The computer calculates how music, originally recorded anechoically, would sound at a certain seat in the hall, and builds a sample recording which can be reproduced with earphones. This can happen when the hall still exists only on the drawing board - or more precisely in the architect's CAD file.

HALL FORMS: FROM FAN TO VINEYARD
It is known that certain basic hall forms provide more advantageous starting points than others for designing good acoustics. Fine halls are generally of the type of either the shoebox or the vineyard. The third acceptable basic form, which requires more elaborate adjustments, is the arena. The earlier fan, common until the 1960s, is, however, acoustically poor.
The classical nineteenth-century halls are narrow, high, rectangular shoeboxes. The best modern halls are vineyards made of layered terraces or arenas with prominent diffusing upper-reflector surfaces. For the moment, the shoebox shape seems to be experiencing a renaissance, following a pause of almost a century. Unsatisfactory fans are found not only in Finland but also everywhere else.
The shoebox is the oldest of basic forms and, looking back, the only "safe" choice which guarantees a good result. The shoebox class includes concert halls regarded as the very best, such as the Vienna Musikvereinssaal (1870), the Amsterdam Concertgebouw (1888) and the Boston Symphony Hall (1900). New shoebox halls of this decade, also rated remarkably good, are for instance the halls of Dallas and Birmingham.
The fan type, common between the 1920s and 1960s, had a fan-shaped plan and a curved, folded ceiling. Fan halls have proved unsuccessful everywhere. The acoustical disappointments led to research in the 1960s, with the eventual discovery of the paramount importance of the lateral sound field and reflections arriving from the sides.
The audience in a vineyard type of hall is subdivided into smaller, asymmetric rising blocks of steps. The vertical surfaces of the divisions create early lateral reflections into all sections of the audience. The best-known representative of the type is the Berlin Philharmonie (1963). Many successful halls of the last three decades belong to this class. Another celebrated modern form is the arena, with the Christchurch Town hall, New Zealand (1972), as the primary example. The elliptical base form is in itself acoustically unfavourable, and in order to avoid shortcomings strong, modulated reflecting panels are needed above the stage and balconies.

ACOUSTICS HEARD AND EXPRESSED IN NUMBERS
A hundred years ago W.C. Sabine took the first step in evaluating the acoustical qualities of concert halls in numbers with his famous equation of reverberation time. The first generation of investigations dealt with the monoaural (monophonic) properties, such as fullness of tone, clarity, loudness, and timbre. No earlier than the late 1960s was it discovered that thebinaural (stereophonic) qualities are critical when trying to achieve excellent acoustics. The characterisation of acoustics involving two ears is usually called spaciousness or spatial impression. This has been described using such concepts as "apparent broadening of the sound source" and the sense of being "enveloped" by sound. Strong spaciousness has been said to be especially characteristic of the most famous classical halls.
The impression of the listener can be formulated as a number of subjective concepts which have their objective physical counterparts. These acoustical parameters can be not only measured in a completed hall but also calculated beforehand with a computer model. The foremost parameter has traditionally been the reverberation time. Depending on the size of the hall, its value should preferably fall between 1.7 to 2.2. s. Perhaps the most important single quality of a hall is its spaciousness; a rough measure for this is the portion of sound energy coming from the sides in relation to the total energy, called the lateral energy fraction. A more sophisticated parameter is theinteraural cross-correlation, which indicates the difference of the signals entering the listener's ears.

COMPUTERS TAKE OVER IN ACOUSTICAL DESIGN
The traditional tool in hall design is the scale model. With the growth in computer power, numerical models started to take over from scale models by the end of the 1980s. During the last few years, computer modelling has matured from a supplemental tool to become not only a full substitute but actually a superior design method. Computer models have a number of advantages over scale models. Building a model is considerably faster, and introducing version changes is reasonably easy. "Measurements" over all seats and subsequent analysis take a relatively short time.
A computer model is built numerically from corner points and plane surfaces, which are given material data, including absorption. Several sound sources are inserted.Tens of thousands of sound rays are sent from the sources and each of these is traced for about the duration of the reverberation time. Of the emitted rays those which hit a receiver position are extracted. The earliest ray paths are modelled precisely using the mirror image method. The reverberation process at the listener is built from the collection of the ray paths; information on the arrival time and direction as well as the spectrum stays available. The possibility of being able to calculate rapidly the acoustics of a hall at all seats is one of the leading advantages of the computer models over scale models. The whole hall can be computed and analysed in 6 tp 12 hours.
The computer is about to make the popular topic of virtual reality into an acoustician's everyday tool: auralisation is becoming a routine part of hall design. Moreover, the connection to the architect's CAD programs will soon be fluent. The pictures illustrate the many possibilities offered by a computer model to an acoustician. The example halls were designed with ODEON, today's leading hall acoustics design program.

EXAMPLE CASE: THE SIGYN HALL, TURKU
The new facilities of the Turku Conservatory of Music were completed in late 1994. The complex is situated in the harbour of Turku, in a renovated shipyard hall and rope factory. The glass-walled, 400-seat recital hall is in many respects an extraordinary hall. Apart from the breathtaking architecture, the acoustical design was also exceptional in many respects in the Finnish context. The contemporary concept of preferable sound field was introduced and the design was prepared, for the first time, entirely with a computer.
The starting point was a hall with glass walls, meant for classical music, without any acoustical compromises. Glass is an extremely rare material for the walls of a concert hall, due to the burden of tradition. The only predecessor known was the small 200-seat AGA Hall in Amsterdam, also located inside an old renovated industrial building.
The primary acoustical quality of glass is that it is perfectly smooth. However, its characteristics as to reflection, absorption and insulation are not seriously inferior to other more common materials. The smoothness was the property which was given the most attention in the design. Relying on the fact that the important factor in a concert hall is not the material but the form - both the gross form and the details - glass began to feel a possible choice.
The Sigyn Hall has otherwise the basic form of a shoebox, but the barrel-vaulted ceiling, which was to be preserved visually, was an acoustician's nightmare. The concave motif was recreated using an acoustically transparent net, above which is the new acoustical ceiling consisting of large hanging pyramids. The side walls were originally sketched as smooth and parallel. In order to avoid both flutter echo and other glass-related difficulties, the walls were folded to form "accordion" pleats and fitted with horizontal shelves and cylindrical metal tube diffusers. This elaboration was in response to fear of that strange material, glass. One could expect in advance that the extensive glass walls might appear subjectively less diffusing and absorbing than more familiar materials. Especially at high frequencies, though, it was wise to be wary of too clear, "glass-like", side-wall reflections. It was felt that the listeners may easily find the acoustics more brittle than what could objectively be heard ("what you see is what you hear").
Measurements conducted in the completed hall showed that the design goals were met exceptionally well. The balance between reverberance and clarity was confirmed by the measured results. The tonal colour of the hall is warm, as designed and predicted, but contrary to the preconceptions of some now slightly surprised listeners. The success of prediction for the bass reverberation means that the acoustical data for the glass panels was reliable. In fact, the glass type used seems to be a more suitable wall material than many common panel materials with a higher absorption at low frequencies.

The properties of the auditorium can be calculated for all listening points. The early damping time (EDT) for reverberation in the Sigyn Hall.

ArchiCAD visualisation of the Sigyn Hall.