Step 3: Design the background sound level
Quieter is not necessarily better, especially in focus rooms. Some background sound is necessary to mask annoying or distracting noise and to help achieve speech privacy, and to aid concentration or rest and comfort. This background sound can be from music, nature, mechanical systems, or electronic sound masking. Ensure the proper background sound, measured in A-weighted decibels (dBA), is achieved to deliver adequate sound privacy.
While designing focus rooms, use the table in Figure 3 to check the background noise level is in the right range relative to the STC rating of the walls and slabs selected in step two above.
Remember that for speech and music rooms, the background sound level should be as low as possible to aid in speech intelligibility, music clarity, and dynamic range.
Finalizing the acoustic design: speech
Optimal acoustics for speech rooms, such as classrooms, should include the correct amount of sound-absorptive surfaces anchored by a suspended, acoustic panel ceiling with a moderate to high NRC of 0.75 to 0.90. Larger rooms with higher ceilings or those with no other absorption will require ceilings on the higher side of that range, while smaller spaces with other absorptive surfaces might be able to meet the maximum RT requirements with a ceiling of only NRC 0.75. Absorption on the floors and walls can be avoided if the room is sized correctly and the ceiling is kept low enough. This saves cost initially and throughout the life of the facility, as the less durable acoustic materials are located overhead, out of harm’s way.
Noise inside a speech room can come potentially from many different sources, including those located on the exterior of the building, in other interior spaces, or the building systems themselves. The building envelope should be designed to attenuate the level of environmental noise specific to the site. Interior partitions around speech rooms should always be full height, extending from floor slab to slab or the roof above. Stopping the partitions at ceiling level and leaving an open plenum above adjacent rooms is not recommended.
The building’s mechanical, electrical, and plumbing systems should operate quietly by locating noisy equipment remotely from speech rooms and by routing ducts and piping around, not over, speech rooms. The mechanical systems should meet maximum permissible noise levels regardless of whether the acoustic ceiling panels are installed or not. This provides design and functional flexibility in the future. This does not affect cost if the design team plans for this criterion as it lays out the floorplan and locations of the mechanical equipment.
As focus rooms can be oversized and creatively shaped per non-acoustic drivers, they often require a large extent of high-performing sound absorption overhead and on the walls and floor. Overhead sound absorption should have a minimum NRC of 0.90 and cover 75 to 100 per cent of the room. Walls should be absorptive as well with a minimum NRC of 0.70 for 50 per cent of the wall area or more. Ideally, the floor would be carpeted. This abundance of absorption keeps sounds from propagating through the space to aid in privacy.
In addition to implementing high-performing sound absorption, focus rooms may benefit from designed background sound to mask or cover up transient sounds that can disturb, distract, or annoy. Background sound provides speech privacy by making words hard to hear and understand outside close proximity. Selecting background sound depends on the room’s purpose and the desired acoustic experience. As examples: Music rejuvenates and energizes; nature sounds soothe and relax; electronic sound masking is benign, and is best at contributing to speech privacy.
The designed background sound level should be significantly louder than that which is permissible in speech and music rooms. Levels of 35 to 50 dBA or more are required compared to levels of just 20 to 25 dBA in large speech and music rooms. The approach is to create a low signal-to-noise ratio by increasing the noise level and by absorbing any reflections off the architecture so the signal is diminished.
Sound-absorbing treatments in activity rooms often need to have impact-resistant surfaces and be up high, where they are less likely to get damaged. While more sound-absorbing materials are usually required for these rooms due to their large sizes, it is less in relation to their size compared to speech or focus rooms. Additionally, the performance level for the ceiling can be decreased to NRC 0.70 because less overall control is required. Ceiling panels with impact-resistant surfaces are likely to have lower NRC performance.
Activity rooms should not have too much sound absorption. These spaces often work better when there is a raised energy level supporting the activity. For example, an overly absorptive basketball gymnasium can make an audience of 500 people sound like an audience of just 50.
Background noise in activity rooms is typically less critical than in any others because speech intelligibility and music appreciation are not the primary acoustic goals. Generally, keep the background sound levels below 45 and 50 dBA.
Rooms for music
When a dedicated music performance space is sized and shaped correctly, very little, if any, permanent sound absorption is required. Rooms for music practice and rehearsal require more sound absorption than properly designed performance spaces. Multipurpose auditoriums for speech and music also typically require more sound absorption to meet their varied functions.
The audience and seating in performance spaces can provide adequate absorption in music rooms. If a lot of permanent sound absorption is required, either the space is oversized and one is trying to decrease the maximum RT, or the room is incorrectly shaped and one is trying to attenuate echoes. Both can be avoided with good acoustic design.
Rooms for music practice and rehearsal require more sound absorption at higher NRC ratings above 0.80 than do properly designed performance spaces. This is true especially in early education because young musicians have not yet mastered volume control. A marching band rehearsal room in a middle school requires most surfaces to be covered with NRC 0.90 or higher absorption to help control the power of the band. Another reason for having more sound absorption in music practice and rehearsal rooms is so the music student and teachers or directors can accurately hear subtleties of intonation and embrasure.
Figure 4 provides a summary of the primary acoustics design guidelines and performance metrics for speech, focus, activity, and music rooms in learning facilities.