The acoustics of concert halls

1. Introduction 

La music it is a sound that changes continuously; it usually sounds rhythmic, changing tone, timbre, clarity, in numerous ways. The criteria for the design of concert halls are studied in parallel with music, because music itself cannot be considered separate from the acoustics of the space where it is performed. The auditorium transmits music to the listeners, trying to preserve the qualities of tone, clarity and timbre; the hall must contribute to the fullness of tone, clarity, and spatiality. The systems for designing acoustic halls have changed and evolved very rapidly, especially in recent years.

These new methods developed in the field of design have led to greater flexibility regarding the shape of different architectures. This freedom can also be considered as a constraint, since even more attention must be paid to ensuring good acoustic qualities, in a manner consistent with new creative solutions.

In the past, concert halls were mostly designed with types of shapes traditional, which mainly fall into four categories: shoe box (rectangular rooms), fan-shaped rooms, hexagonal rooms and vineyards. Some particulars must be considered acoustic qualities, of an objective and subjective nature, when designing a concert hall.

The most relevant objective measures are the early decay time (EDT), clarity (C80), subjective intensity of the direct sound (G) and the fraction of the first lateral energy (LF). These indices must have optimal values, to guarantee good acoustic quality inside the room, coherently with the chosen design form. The study of the best indices occurs with the use of some specific software, such as Rhinoceros 5.0, a virtual modeling software and multiple plug-ins, such as Python, Pachyderm Acoustic Simulation and Grasshopper.

2) The acoustics of concert halls

The needs of a concert hall are different from other spaces designed for acoustics. The basic requirements for good acoustics in an auditorium are:

• the sound must be loud enough;
• the components of a complex sound must maintain an appropriate ratio between their intensities;
• Excessive sounds must be clear and distinct.

The guidelines to follow for a good design of a room are:

• minimize background noise
• amplify the direct sound

Imagine sitting near the stage of an open-air stadium; when the artist begins to play, we will hear the notes of the instrument with great precision; the sound that reaches us is a direct sound, as there are no closed and delimited spaces or reflective surfaces. In a concert hall, the sound that first reaches the listeners is called direct sound. These early reflections contribute to clarity, intimacy and intensity, characteristics that improve the acoustics.
The following are also important: further reflections, which simply depend on the geometry of the room and the acoustic characteristics of the surfaces themselves. For good acoustics it is also essential to consider the inclination of the stalls and the possible presence of balconies, which are used in large rooms to reduce the distance between the most distant seats and the sound source, and to increase the capacity of the room; however, the arrangement of the balconies must be studied very carefully because if they are not correctly sized, they can cause acoustic defects such as echoes or shadow areas.

As mentioned above, when designing a concert hall, it is necessary to give uniformity to some Acoustic index values, such as the early decay time (EDT), clarity (C80), strength (G, loudness of the direct sound), the fraction of the first lateral energy (LF, lateral fraction), studying the best shape of the room so that the aforementioned indices can be suitable for the distribution of acoustic quality of the room itself.

2.1) The impulse response 

It is possible to determine all the physical properties of a concert hall that are contained in its impulse response.
In a closed environment, the impulse response is the time recording of the sound energy arriving at the reception point following the emission of an impulsive signal on the stage.
It is given by the sum of the energy that progressively reaches the receiver, that is the direct sound, the first reflections and the sound tail.

2.2) The main objective measures

The first reverberation time or early decay time

Reverberation time is the most important parameter for determining the quality of a sound environment, since it is the most evident that is perceived in a closed environment. Inside a closed environment, the sound produced in one point reaches the listener not only directly, but also after reflections on walls and floors. The direct sound typically has a greater intensity, and arrives before the reflected sound. For example, when clapping your hands in a room you hear a prolonged sound that fades more or less quickly depending on the nature of the room; in very large environments the sound tail is perceived for a few seconds. 
It is the physical parameter describing the perception of sound reverberation, defined as the time required for the sound level at a point in the room to decay by 60 dB from the moment a sound source emitting a stationary signal is turned off. Consequently, this parameter depends on the volume of the room and the acoustic absorption characteristics of the surfaces present (such as the seats in the room).
Typically, environments intended for speech require short reverberation times, less than a second, while music requires longer times, on the order of two seconds. 
There are reverberation times relating to decay times, not only referred to 60 dB, but to 10 dB (T10), 20 (T20) and 30 (T30), etc. All these values ​​are however equivalent to each other, since even if they are measured on shorter decays, since it is difficult in normal situations to have a sound decay of 60 dB, they are extrapolated to the corresponding T60. 
The early decay time (EDT or T10) is the time required to obtain a 10 dB decay of an impulsive signal. 

Clarity (C80)

It is the parameter that indicates the ability to clearly perceive sounds. Normally a distinction is made between vertical clarity, that relating to sounds that reach the ear simultaneously, and horizontal clarity, referring to sounds articulated in time. Clarity is linked to the structure of reflections and vividness, to the distance of the listener from the orchestra and to the subjective intensity of the direct sound.

Strength (G)

The parameter expresses the level of perceived sound in relation to its position and the power of the sound source.
Outdoors, a sound emitted by an orchestra is perceived as weaker than the same sound perceived in a hall. The robustness index G describes the perception of the intensity of a sound. 

The fraction of the first lateral energy (LF)

La spatiality it's the feeling of feeling inside the sound event.
The first lateral energy fraction (LF) is the ratio of the sound energy reflected laterally in a room to the sound energy arriving from all directions including the direct sound energy from the source. It is a measure of the spaciousness of a room.

3) Types of salt

The design of concert halls depends on the definition of the shape, from dimensionsand at our materials, and the purpose is to create the better acoustic conditions across the entire audience area, to best meet the listeners' personal preferences.

Reflected sounds, usually from concave surfaces, can be perceived as separate sounds if they are much louder than the rest of the reflections, and if they are too far from the direct sound. Therefore, special attention is also given to the shape of the room, to avoid sound concentrations and strong late reflections.

Concert halls are classified, based on their shape, into four main types: shoe box, fan room, hexagonal room and vineyard.

Rectangular rooms

In the second half of the 19th century, the demand for large concert halls developed, and especially those with rectangular plans proved to have particularly favourable acoustics. The proportions are approximately those of a double cube, and are called shoe box. They are mostly characterized by their rectangular shape with the orchestra at one end of the room, the parallel and vertical side walls and the flat ceiling (hence the name shoebox). shoe box It is the most important type of concert hall, and also the one with the highest number of examples. Most of these historical halls are characterized by the presence of ornaments, such as sculptures, bas-reliefs and frames. These tend to make the hall assume a complex and irregular volume, ensuring a good level of sound diffusion or sound dispersion: most of the sound waves are not reflected specularly by the surfaces, but are spread in many directions.

One of the most renowned shoe box and the Large Music Hall, in Vienna, built between 1867 and 1869, by the Dane Theophil Ritter von Hansen (1813-1891) and opened in 1870. It is long and narrow, 56,3 by 19,8 meters, quite small by today's standards, it can accommodate 1680 people, but it is tall for its width (17,8 meters), with a volume of 14600 m³. The interior is mainly finished in stucco: the ceiling, which is decorated with gilded stucco panels, shows the lower part of the structural beams on the sides; the side walls are irregular as there are more than 40 large windows, 20 doors above the balconies and 32 majestic female statues below the balcony. The narrow width and the interrupted surfaces of the hall mean that an immediate reflected sound reaches every seat.

Another important rectangular room is the Symphony Hall Boston, designed by architects McKim, Mead and White. It is the first hall where the science of acoustics was applied and was opened on October 15, 1900. The sound inside the hall is clear, warm, bright and strong, but there are also a series of negative characteristics. The seats in the corners and under the balconies are reached by an unnatural sound due to a shadow area. There is also an echo effect coming from the rear wall. There are two levels of balconies that draw a "u" profile. The balconies, with the front part decorated and gilded, are shallow to avoid sound reflections. Symphony Hall has a high diffusion, thanks to the presence of the coffered ceiling, and numerous ornaments, such as statues, niches, recesses and decorations on the side walls, and the presence of two levels of balconies, also having the front rich in decorations.

The third among the major rectangular European halls is the Concert Hall, in Amsterdam, designed by AL van Gendt and opened in 1888. Its interior is very large, with a curved wall surrounding the stage and a 15-meter high coffered ceiling. The hall is 29 meters wide, with a volume of 18700 m³, and this causes a great reverberation, and the sound waves reflected by the side walls, given the width, arrive relatively late in the center of the hall. All this gives the music a mixed, warm tone, lacking clarity. It seats 2200 on the large flat floor that are removable. It has a high and narrow balcony with a very ornate front, which extends along three sides of the hall, and is supported by decorated columns.

Fan-shaped rooms

This type of room had a great development between the 1920s and the 1960s.

It offers better visibility, but one of its major problems is echo, especially when the wall that delimits the rear part is characterized by a concave curve, which produces an echo on the stage. The only remedy for this problem is to try to reflect the sounds downwards and therefore on the audience. Acoustic imperfections are also noticeable with regard to subsequent reflections, since inside these rooms you do not have the perception of being surrounded by sound. The sound in the fan-shaped room is more frontal than lateral and the reverberation is lower.

One of the most famous fan rooms is the Northern Alberta Jubilee in Edmonton, characterized by a low roof. The material used inside is mainly French walnut wood, which breaks the continuity of the lines of the internal side walls. It was built in 1957 and accommodates 2678 people. The musicians on stage, during the performances, have noted that the sound is not lively enough. Some acoustic consultants think that the reverberation is really satisfying and wonderful. The hall has balconies that are very particular, since they adapt to the shape of the fan: they are arranged laterally on one level, and at the back of the hall on two levels.

Another one concert hall internationally renowned fan is the Great Hall of Caracas. The auditorium with 2.666 seats, and inaugurated in 1954, boasts the best acoustics in the country. The project is the work of architect Carlos Raul Villanueva, with the help of sculptor Alexander Calder, who is responsible for the hall's distinctive feature: the famous "flying saucers". The architect, for the design of the hall, proposed a fan shape, with a curved roof and a rear wall with its center of curvature at the back of the stage. When the initial drawings were completed, the curved ceiling and the curved rear wall presented echo problems and in general there was a lack of uniformity in the distribution of the sound; improvement was needed, but by then the project had been established. An alternative proposal was thus developed: the addition of reflective panels to cover a surface equal to 70% of the ceiling: initially these panels were to be rectangular, but then the intervention of sculptor Alexander Calder was asked to try to improve the proposed solution. The result was surprising: a series of “flying saucers”, with unusual colors and shapes, suspended on the roof. The hall is used for conferences and music. Although its shape is not favorable for music, it has received positive comments from conductors and musicians; the music played is clear, distinct and the sounds are brilliant.

Hexagonal rooms

Hexagonal rooms have the notable advantage of providing sound reflections in the last seats of the stalls, if the side walls are not parallel to each other but inclined. In this way almost all positions of the stalls exploit the first reflections, except the area in front of the stage.

The hall of St. David's Hall of Cardiff is a major hexagonal hall. The main challenge of this Hall was to design a large concert hall with 2.000 seats in the narrow space available and the building had to be inserted into a project, partly already built, inside an existing shopping center. The hall was opened in 1982; it had to not be too high, and initially a square plan was considered. But then a hexagonal plan was chosen, so that the audience could wrap around the stage. It was influenced by the hall vineyard of the Berlin Philharmonic and seats 1952 people. A survey of the top eleven concert halls in England, supported by a group of acoustic consultants, concluded that the Cardiff hall claims that the aforementioned concert hall it was close to having the best characteristics in terms of clarity, reverberation, intimacy, and enveloping the sound by the listeners. All this because the reverberation is high, and the sound is spread in an optimal way.

Also the The Doelen Concert Hall Rotterdam is a famous hexagonal hall, built in 1996. It has 2242 seats. The central part of the stalls is a sort of enclosure surrounded by walls of different heights. The diffusing elements in the hall and in the walls of the stage allow the incident sound to be spread in an excellent way and to send the first reflections to all the seats. There is no echo and the sound on the stage and in the main part of the hall is excellent.

Salt vineyards

The vineyard room is usually characterized by a very high number of seats. 

This type of room seems to offer many acoustic advantages, of the parallel sides and the shoebox room: good balance between clarity and reverberation, the possibility of early lateral reflections and a diffuse sound field. In addition, another advantage is the possibility of having simultaneously a greater width of the room and an audience arranged in such a way as to surround the stage. In addition, the room vineyard offers greater flexibility to designers, as some details can be changed independently from the rest of the room.

The most renowned hall vineyard and the Philharmonic Hall Berlin. “Putting music at the center” is the basic principle of the project, by architect Hans Scharoun; he argued that the normal arrangement of the orchestra at the end of the hall would have prevented the audience and musicians from communicating easily. The result is a spectacular hall, inaugurated in 1963, with 2215 seats; no listener is further than 30 meters from the stage. The center of everything is the orchestra space, the place from which the music spreads; since this is the lowest area of ​​the hall, Scharoun configures the convex roofing to ensure that the music spreads uniformly, as if it were coming from above. Seen in plan, the hall is symmetrical, with the seats surrounding the stage space; however, if observed from one of the lateral positions, the large auditorium does not appear symmetrical at all, as it is dynamic and with a large variable space. The orchestra area is not fixed, but can be lowered or narrowed according to needs. The Philharmonie Hall has become a model of acoustic design. In the audience section in front of the orchestra, the sound is clear and balanced. The main disadvantage is that spectators sitting in the back of the stage hear a different sound. But fortunately, the view is excellent, which has encouraged comments on the shape of the hall.

4) Acoustic design software

There are many software that can be used to develop models of concert halls. Of course there must be a first part of three-dimensional modeling of the hall, to then move on to specific programs that study the sound effects inside the hall itself.

Rhinoceros

It is a basic CAD software, for three-dimensional modeling, capable of managing complex surfaces and calculating different geometric characteristics of them. It is capable of managing complex shapes and geometries thanks to the NURBS (Non Uniform Rational B-Splines) technology; all geometric entities are represented by NURBS. Very simply, NURBS are a mathematical representation through which it is possible to accurately define 2D and 3D geometries such as lines, arcs and free-form surfaces.

Python

Python, a Rhinoceros plug-in, is a programming language, suitable for creating applications quickly, and is used for a wide range of purposes: scripting, creating complete programs, creating prototypes of applications that will then be rewritten in other languages. The script is a small program or application, written within other more complex software, capable of giving instructions and commands to the program in a specialized and personalized way.
In the case of acoustic design, scripts can be written within Rhinoceros using Python, in order to best customize the shape you intend to give to the room being studied.

Pachyderm Acoustic

It is another plug-in that provides professional-level tools for acoustic analysis and simulation with Rhinoceros. Perhaps the most innovative feature of Pachyderm compared to other software comes from the fact that it was designed as an integrated component of Rhino. If the modeling phase and the acoustic simulation phase, usually separated between two different software, remain both within the same platform, the workflow will be positively affected, in terms of time and model quality.

Grasshopper

Grasshopper is a further plug-in performed inside Rhinoceros and it is a visual programming language, mainly used to build parametric models.
Grasshopper allows you to organize projects in parametric systems, based on relationship logics between parts, offering the possibility of altering the overall configuration of the system, acting on the variables at the basis of the design process.
Grasshopper also allows to integrate variables and aspects of heterogeneous nature in a single process, focusing on the network of relationships, flows and exchanges that the project establishes with the surrounding systems. These aspects constitute a real evolution in architectural research.

Music Park Auditorium

THEMusic Park Auditorium is a multifunctional center located in Rome, built to host musical and cultural events. The complex was opened on December 21, 2012, occupies an area of ​​55.000 square meters and was designed by Renzo Piano.
The main rooms of the structure are three, of different sizes, and they compose a shape that recalls a scarab. These volumes are delimited by a roof of lead sheets, worked on a wooden structure. The composition revolves around the open amphitheater, the cavea. The cavea thus becomes the fulcrum of the entire architecture; the bar, the foyer and the filter areas overlook it: it therefore functions as a solution of continuity between the various spaces. The cavea, when necessary, also transforms into an open-air theater that hosts up to 3000 spectators.
Each room has suitable acoustic characteristics depending on the type of music played inside it.
Hall 700, known as the Petrassi Hall, consists of a movable proscenium, a dedicated orchestra pit and a balcony; it is possible to enlarge the stage by eliminating four rows of seats.
Hall 1200, called the Sinopoli Hall, is rectangular and was designed for symphonic music for choir and orchestra and for chamber music. The primary objective of the hall is the adaptability of the space depending on the event. Its volume can be modified, thanks to the variation of some of its parts: the stage, the choir, the orchestra and the seats can be completely cleared, thus allowing the stage in the central area to be available for particular performances.

The largest of the three halls is the Santa Cecilia hall, designed for classical music. This space has natural acoustics (i.e. without the use of amplification systems) and therefore its acoustic parameters are at the limit. The hall hosts up to 2700 people, the base surface measures 3000 m19 and the height reaches up to 31000 meters: this volume (of XNUMX mXNUMX) is specifically designed to have a sufficiently long resonance. The idea was precisely to have a concert hall where all listeners are equally exposed to the free sound field. The cladding in the podium, stalls and connections between the floors and galleries is made of cherry wood panels, whose shape and inclination have been studied according to acoustic needs. These elements function as sound diffusers, so they have different curvatures and widths. The roofing represents a true architectural and engineering work, thanks to the ceiling elements, with surfaces of up to 250 mXNUMX. Above the podium there are reflective elements that ensure the orchestra can hear the sound better; these elements can be inclined differently for all orchestral needs. Thanks to all these measures, the environment has very good reverberation characteristics.

Sidney Opera House

The Opera House is a concert hall huge, with a height of 25 meters. The building was designed by the Danish architect Jørn Utzon, starting in 1958, but it was inaugurated only in 1973. The complex is a great example of contemporary architecture, especially thanks to the original shape of the roof, which resembles gigantic sails. These sail-shaped shells are composed of prefabricated elements of reinforced concrete and covered with more than a million white tiles.
The work has been controversial on numerous occasions, due to its complexity. In 1966 Utzon abandoned the project, and was replaced by a group of architects led by Peter Hall.

La concert hall It can accommodate up to 3000 spectators and the hall is covered with wooden panels, which have the task of absorbing low frequencies.
The auditorium has often been criticized for its poor acoustics. Initially, functional and acoustic problems led to the creation of an additional hall that could accommodate up to 1547 listeners. The initial acoustic project could only accommodate up to 2000 spectators, but the increase in seating was necessary for an improvement in sound levels.