The acoustics of concert halls

by Denise Barbaroux

 

1. Introduction

Music is a sound that changes continuously; it is usually rhythmic, changing tone, timbre, clarity, in numerous ways. The criteria for the design of the rooms are studied in parallel with the music, because the music itself cannot be considered apart from the acoustics of the space where it is performed. The auditorium transmits the music to the listeners, trying to preserve the qualities of tone, clarity and timbre; the room must contribute to the fullness of tone, clarity, spatiality. The design systems of acoustic rooms have changed and evolved very rapidly especially in recent years.
These new methods developed in the design field 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 guaranteeing good acoustic qualities, in a manner consistent with new creative solutions.

In the past, concert halls were mainly designed with traditional types of shapes, which fall mainly into four categories: shoe box (rectangular rooms), fan rooms, hexagonal rooms and vineyards. We must consider some particular acoustic qualities, both objective and subjective. , when designing a concert hall.

The most relevant objective measures are the early decay time (EDT), the clarity (C80), the subjective intensity of the direct sound (G, loudness of the direct sound) and the fraction of the first lateral energy (LF , Lateral fraction) .These indices must have optimal values, to ensure good acoustic quality inside the room, consistently with the chosen design form. The study of the best indices takes place with the use of some specific software, such as Rhinoceros 5.0, 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:
 

1. the sound must be sufficiently loud;
2. the components of a complex sound must maintain an appropriate relationship between their intensities;
3. excessive sounds must be clear and distinct.

The guidelines to follow for a good room design are:

• minimize background noise
• amplify the direct sound

Let's imagine we are seated near the stage of an outdoor 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 listeners is called direct sound. These early reflections contribute to clarity, intimacy and intensity, characteristics that improve acoustics. The subsequent reflections are also important, 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 furthest places and the sound source, and to increase the capacity of the room; However, the arrangement of the balconies must be carefully studied because if they are not correctly sized, they can cause acoustic defects such as echoes or shaded areas.

As mentioned previously, in the design of a concert hall it is necessary to give uniformity to some values of acoustic indices, such as the early decay time (EDT, early decay time), clarity (C80), strenght (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 adapted to 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 recording over time of the sound energy that arrives at the receiving 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.

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