Room Geometry

Prior to setting any acoustic criteria, it is important to know the use of each room.

A room could be for:

• teaching theory with no instruments played.
• single musicians or singers to practice.
• small ensemble or group practice.
• loud instruments. 
• large ensemble or group rehearsal.
• performances. 

With this in mind, the acoustician can help you set the right dimensions, volume and shape for each room.

Room dimensions and volume

As mentioned inAcoustic design of schools for performing arts – Part 1: Design aspects, finding a balance between the sound reverberation and the loudness of a room is key to obtain ideal acoustic conditions.

This is done by setting the dimensions and the volume depending on:

• the number of musicians practicing, rehearsing or performing
• the type of music played (amplified or acoustic)
• the instruments likely to play inside (ex: pure singing, quiet instruments or loud instruments) 
• the size of the audience or the orchestra (for performance spaces or rehearsal rooms)

For early guidance on ideal room dimensions and volume, check Acoustic design planning of music spaces. 

Room shape

The shape of a room is a basic feature that allows you to direct the reflected sound where and when you want.

Acoustic experts are unanimous about the shoebox shape for most music spaces. For performance spaces, that shape is useful to reflect sound, coming from the stage, back to the side of the audience. This is proved particularly beneficial for the acoustic comfort of the audience and the perception of the musical performance. 

Other shapes are also well known to be a “no-no” for acoustics. Any domes, curved shapes or hipped roofs can focus sound and should be avoided.  

It is also common practice to angle the opposite walls of small rooms and recording studios. This is avoid any flutter echos and standing waves to be created between parallel walls. 

Building layout

The sound insulation design of a school for performing arts generally requires more consideration and higher performances, leading to higher construction costs. 

You can minimise this by optimising the layout of the school. 

Here is what you could do:

• Position the sensitive spaces away from the loud areas.
• Create acoustic buffers around or adjacent to the sensitive spaces (like the rehearsal and performance spaces). They can be quiet corridors, quiet spaces or lobbies.  
• Position the sensitive spaces away from the external sources of noise. That way, you minimise the need for high performance building façade to control the external noise getting in the building. Buffer areas, like corridors, can also be used in this case.
• Position the loud areas away from the external noise sensitive receptors. This can avoid the need for a high performance façade.

Internal finishes and furniture

For music rooms, especially for the larger spaces dedicated to performances and formal rehearsals, choosing the right finishes and furniture will be one of the most important design tasks. It is also a very specialist, scientific and (even) artistic task.

Although a lot could be said about room acoustic design, below are some general trends that are worth thinking about early.

Sound reverberation at low frequencies

As mentioned in Acoustic design of schools for performing arts – Part 1: Design aspects, if you want large spaces to accommodate acoustic music rehearsals or performances, you need to consider their reverberation characteristics at low frequencies (below 500Hz).

The starting point is to work on the right density and stiffness of the surfaces (walls, floors, ceilings, joinery, sound reflectors/diffusers, etc). You need to find the right balance between:

• The surfaces being too light and flexible, so they absorb too many low frequencies and will cause the music to sound too dry.
• The surfaces being too heavy and stiff, so they don’t absorb enough low frequencies and will cause the music to sound too “boomy”.

Variable acoustics

In some spaces, you may want to be multi-purpose by organising various types of activities and events. These could be acoustic music, quiet music, loud music, amplified music, pure teaching or drama performances. 

Each of these types have different optimal reverberation conditions. You therefore need to be able to change reverberation conditions.

You can do this with variable acoustic systems. Their general principle lies in revealing or hiding an acoustic absorber. So that, a room is less reverberant when the absorber is revealed and more reverberant when the absorber is hidden.

Such systems are:

• Thick acoustic curtains extended along some walls or stored in dedicated cupboards.

• Acoustic banners hung along the walls.

• Rotating acoustic systems that usually have one absorptive surface and another (flat) one reflective. Some systems have a third surface that diffuses the sound.

• Retractable seating that make a room less reverberant when pulled out and more reverberant when stored.

• other systems exist like openable timber slats, panels with variable perforation or even inflatable absorbers (aQflex and aQtube from FlexAcoustics). 

Seating

If you choose to have a performance space for acoustic music (i.e. non amplified), you may want to invest in a high-end seating system. 

In this type of space, most of the acoustic absorption is provided by the audience. Therefore, you run the risk to have very different reverberation conditions depending on the size of the audience.

To avoid this, you can design the seating system to have similar absorption characteristics whether the seats are full and empty.

Also, a good seating system should not generate noise when operated. You don’t want to hear the noise from people walking to or off their seats in the middle of a performance. Neither do you want to hear the noise from the seats tipped up or down.

    Procurement tip: You should purchase the seating system early in a project. And ideally, you should do it outside of the main building contract. This is because the potential laboratory tests of the system selected by the contractor could be delayed and data not available prior to the completion of the design.

Acoustic diffusing surfaces

Diffusing surfaces can be of many forms and shapes. They allow to scatter the reflected sound energy in many directions. 

For some spaces for performing arts, you could benefit from using diffusing surfaces to: 

• avoid flutter echoes and room mode effects from sound waves bouncing back and forth between two hard parallel surfaces. You could also use absorbing materials, but some rooms might sound too dead with the additional absorption. 
• spread the sound energy evenly on an audience or a large ensemble

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