Concert hall acoustics
As a special field of room acoustics, concert hall acoustics focuses in the design of spaces for live music events, typically unamplified. Concert halls are usually intended for classical music performances (mainly symphonic works) with audience capacity ranging from 300 to 2500 seats.
Some concepts that have already been described in our article about room acoustics will be mentioned here, so it is recommended to have read that article first.
Basic room shape
There are only a few shapes that serve as a starting point for the design of a concert hall, and of these, the most typical tend to be shoebox (rectangular), fan-shaped, and vineyard.
The shoebox shape is simply a rectangular room, typically with some balconies. Thus, the basic design is simple, but if not careful enough, this type of room can have problems with flutter echoes (explained in our article about room acoustics). A well-known example of a shoebox concert hall is Musikverein in Vienna. It is generally agreed that the various structures in this specific room, such as the lateral statues, introduce scattering that helps avoiding the problem of flutter echoes.
Fan-shaped rooms are perhaps the more common, as they are able to accommodate a large number of attendants while keeping a frontal view of the performers. At the same time they are not prone to flutter echoes, simply because of the non-parallel walls. Additionally, the width of the room at the rear seats allows for good spaciousness of sound. Although not specifically a concert hall, an example of a fan-shaped room is the Chaktomuk Conference Hall in Cambodia.
Vineyard concert halls are named this way because the seating sections resemble slopes in a vineyard. This type of room has several advantages: 1) It is visually interesting. 2) The irregular pattern helps to avoid acoustic issues, such as flutter echoes and focusing (explained in our article about room acoustics). The disadvantage, however, is that the design is very complicated and expensive. Examples of famous vineyard concert halls include the DR Concert Hall in Copenhagen, Denmark, the Berlin Philharmonic’s Concert hall in Berlin, Germany and the Philharmonie de Paris in Amsterdam (kidding – in Paris!), France.
Other geometrical considerations
Aside from the general shape of the room, there are a few other design considerations that will have an impact on the (subjective) quality of the concert hall.
An important objective of a concert hall is to deliver the sound to different audience sections as uniformly as possible. The room’s actual ceiling is unlikely to be able to accomplish this. Therefore, it is common to install suspended reflectors off the ceiling, at angles that will reflect the sound towards different audience sections.
By taking into consideration the angles of the walls at both sides of the audience, and adjusting reflections of the walls using reflectors, it is possible to redirect the sound from the stage back into the audience at a very lateral angle. The result will be a perception of the music source being much wider than it really is, compared to the case where a listener only receives the direct sound from the stage.
Room acoustics must be designed with not only the audience in mind, but also considering the musicians. A concert hall without high enough sound strength will make the musicians feel like their instrument is not producing strong enough sound, leading to frustration and unbalanced overall sound. It is also important that the musicians are able to hear themselves, so it is helpful to have reflectors at their sides and/or above them.
Room acoustic parameters
Acoustical effects such as the ones described above can typically be measured objectively, and these measureable room acoustic parameters are specified in the ISO 3382-1 standard for performance places.
It is possible to define optimal value ranges and Just Noticeable Differences (JND). JND’s refer to the minimum increment in a parameter in order for it to be noticeable by the human ear. That is, increments smaller than the JND specified for a specific parameter, are not expected to be noticeable. Below, some important room acoustic parameters will be described in a general manner. For more detailed descriptions and the specific equations, check Chapter 8 in ODEON manual. Target values and JND’s for different parameters can also be found on page 146.
Reverberation time (RT)
This parameter has already been described in our room acoustics article. Out of the many types of RT listed in that article, the two mostly used in concert halls are EDT and T30. Occasionally, T20 is preferred if the background noise during the measurement is so high that it doesn’t allow sufficient signal-to-noise to derive T30.
Typical values for reverberation time in Concert Halls are 1.7 to 2.3 sec.
Sound strength (G)
The G parameter investigates how much the acoustics/reflections contribute to the SPL level. In a very reflective space/room, there will be a high contribution, in an absorptive space there will be a low contribution. In other words, G measures the amplification of the room to a sound source.
In short, G is measured by comparing the SPL from the sound source in the room with the SPL from the same source at 10 m distance in free field. In practice this procedure is not really convenient. Therefore several methods have been developed, based on calibrating the measuring system. See more about these procedures on the Application Notes page. Two similar parameters, Gearly and Glate, are used in auditorium acoustics to see how much the room contributes to the early and late SPL level, respectivelly (sound arriving before and after 80 ms since the sound was emitted). It is also used in equations within the field of “spatial impressions” of a room.
Typical range of G for concert halls is 3 to 10 dB. The lower values are those measured further away from the source, while the higher ones are those measured closer to the source.
Clarity (C80) and Definition (D50)
Clarity, C80 compares the amount of sound energy arriving within the first 80 ms to the total amount of energy until it ceases. In simple words, C80 compares the early with the late energy.
This parameter is based on the fact that later reflections in the room acoustics have a greater “sound-blurring effect than early reflections”. The sound will be experienced as less clear, when later reflections are more dominant in the space than early reflections. Therefore, Clarity typically grows in opposite direction than Reverberation time (RT). Lower C80 is a result of longer RT, while higher C80 means shorter RT. It should be noted that much acoustical research has been made for classical music, but not for rhythmical music (pop, rock, etc.). Therefore these findings might not apply the same way.
Typical range for C80 in concert halls is -1 to 3 dB.
Definition (D50) is very similar to Clarity, but focuses on an even earlier part of the response, up to 50 ms.
Lateral energy fraction (LF)
This parameter describes the spaciousness of the room, which was described earlier above. It is the ratio between energy arriving from the sides and the total (early) energy.
In concert halls, this is recommended to be greater than 0.25.