CONCERT HALLS HAVE A VERY SPECIFIC acoustic intention: to provide the appropriate environment for good music quality. As opposed to theaters, concert halls do not put emphasis on speech intelligibility because their main purpose is musical performance. However, there is a wide range of music genres requiring different acoustic characteristics. There is, for example, Gregorian music and the qualities of the space necessary for that type of music are on one end of the music-genre scale (long reverberation times = live big rooms). On the other end we can find contemporary music (i.e., rock) with music qualities that have very different requirements (short reverberation times, amplification = dry/dead rooms). This chapter intends to give general guidelines regarding the design of spaces for music performance, without getting down to the details that are specific to the music genres.
11.2.1 Reverberation time
Reverberation times for concert halls include a wide range of activities. Figure 11.1 provides a good guideline. In general, longer reverberation times are desired than for speech-based spaces, because musical notes in a composition do not need to be strictly separated to be understood. In fact, it is desirable that there is a small overlap between the tones to aid the flow or mellowness of the musical piece. Reverberation also increases the sense of envelopment in a room, which improves the experience.
After defining the appropriate reverberation time for a venue, it is necessary to decide where to put the absorption. Construction materials tend to be reflective, so even for long reverberation times it is often necessary to add absorptive materials. Furthermore, there are surfaces in a venue that you always want to cover with absorptive material simply to eliminate other acoustic issues (e.g., absorption in the back wall to avoid echoes).
11.1 Optimum reverberation times for music venues
Audience as the main absorber It is very common to find that the greatest area for absorption of sound in a venue is the audience area. Audiences will be absorbing when the area is fully occupied, mostly due to people’s clothing. However, we cannot count on a venue always having a full house. For this reason we need to provide seating that resembles the acoustic properties of a full audience. Fully upholstered seats are a good choice; however, this is not always the design intent. In any case, calculating the amount of absorption that the audience at different levels of occupancy will provide is the first thing we need to consider before adding additional absorptive materials to the room surfaces. Variable acoustics can be considered to compensate for unoccupied seating designed to have very low absorption.
11.2.2 Clarity, C80
C80, or musical clarity is the equivalent of C50 from theaters. It defines a ratio between early energy (before 80 ms) and late energy (after 80 ms). Music clarity is measured with a limit of 80 ms as opposed to 50 ms for speech clarity, because, as mentioned before, music needs less separation of individual sounds, and this means some reverberation is accepted as a positive quality. For the calculation of C80, see Eq. A.27.
11.2.3 Subjective evaluation of concert halls
Particular terminology is often used to describe the perception of sound in concert halls. In acoustic evaluation it is as important to use the ears as it is to use measurement instruments.
For reverberation, it is common to say a room is dry/dead or live: Rooms which are acoustically dry tend to have short reverberation times, whereas rooms which are acoustically live tend to have long reverberation times. Acoustically drier rooms are generally more suitable for speech activities, while rooms which are more acoustically live are favorable for unamplified orchestra, choral music, or pipe organ music.
For clarity, we classify rooms as clear or muddy/blurry: Rooms which are acoustically clear usually have well-controlled reverberation levels so that echoes do not interfere with the gaps or lulls between words or syllables. Muddy rooms can have long reverberation times, causing interference in intelligibility. Clarity is also affected by room volume and background noise levels.
CONCERT HALLS HAVE EVOLVED TOGETHER WITH MUSIC and their shapes have changed many times, trying to adapt to the music of the time as well as the architectural typologies and interior design. Halls during the Renaissance, for example, had ample decorations on the walls, which created a rich texture that facilitates sound diffusion. While this is not always the desired acoustic quality, it does eliminate many acoustic defects (e.g., echoes and focalizations). Table 11.1 and Figure 11.2 explain the main characteristics of some common typologies.
Table 11.1 Concert hall typologies
Typology | Description | Advantages | Disadvantages |
Fan shape | The narrow part of the fan is towards the stage | Good visuals from every point | Lack of early reflections in front and center audience |
Large capacity | Lack of spatial impression to the absent lateral reflections | ||
Shoebox | Rectangular, relatively narrow | Good early reflections | Small balconies |
High sound envelopment and spatial impression | |||
Bad visuals from the front sides. | |||
Reverse fan shape | The large part of the fan is towards the stage | Great early reflections | Bad visuals for a large part of the audience |
High spatial impression | |||
Fan shape and reverse fan shape, or hexagonal | The room begins narrow, widens toward the center and narrows to the back again | Good visuals | |
Horseshoe | Typically used for the Italian opera houses. | Large capacity | Poor early reflections |
Multiple hexagonal shapes | Positions the audience at two levels, creating side walls for the smaller hexagon | Great lateral reflections | |
Good visuals | |||
Terraced | Audience is located in smaller sections terraced around three or four sides of the stage | Good early reflections | Poor stage acoustics |