To help simplify the subject, most of this information and criteria can actually be distilled to four key goals of acoustic requirements. Knowing these goals and the metrics used to measure the effectiveness of each can help designers, architects and specifiers navigate the acoustic requirements in today’s standards and guidelines.
1. Prevent excessive reverberation and loudness inside occupied spaces.
The first goal of acoustic standards is to reduce excessive reverberation and loudness. The best way to do this is to apply the appropriate amount of sound-absorbing surface materials inside occupied rooms on the ceilings, walls or floors.
How sound absorption is measured:
- Noise Reduction Coefficient (NRC) – Percent of the noise absorbed by the material
- Reverberation Time (T60) – Time required for the sound level to decrease 60 decibels
When NRC values are specified in the standards and guidelines, they generally apply to ceilings in conference rooms and open offices and are in the mid to high range of 0.80 to 0.90. T60 is the more common criterion and values of 0.60 seconds or shorter are typical.
Suspended ceilings are used primarily, and most effectively, to achieve this goal through sound absorption. Additional absorption on the walls and floor may only be required if the ceiling does not provide enough sound absorption; for example, when the NRC is less than 0.70, or when parts of the ceiling are intentionally left sound reflective to project sound.
2. Limit occupant noise transmission between interior rooms.
The second goal is to ensure there is minimal noise travelling between rooms. This is achieved through the sound blocking elements around a room, along with preventing or plugging any penetrations, noise leaks or holes. Specific solutions can include glass wall systems and windows that are limited in size and insulated and/or laminated to increase their sound blocking capacity. Installing doors that swing, rather than slide, and have full perimeter seals will also help achieve sound blocking performance.
How interior sound blocking is measured:
- Sound Transmission Class (STC) – The sound blocking capacity of a wall or other assembly.
- Noise Isolation Class (NIC) – The total sound blocking capacity between two rooms as measured in the field after construction is complete.
Most standards and guidelines require sound isolation levels of STC/NIC 40, 45 or 50+.
Where the Ceiling Attenuation Class (CAC) measurement used to play a role, specifically in the ceiling panel’s ability to block noise, experts have found that this is no longer an effective strategy. Modular acoustic ceiling panels alone, based on their lack of mass and holes created by lights, fixtures etc., cannot provide the levels of isolation required by today’s standards and guidelines. This is why CAC is not found within the acoustic standards and guidelines.
Instead, with the goal of achieving true STC, it’s important to look above the ceiling system as well, at plenum barriers- extending walls vertically from slab to slab and in some cases sealing all penetrations where noise can pass through.
3. Limit exterior environmental noise transmission into the building.
Once the internal sound qualities are accounted for, it’s time to think about how the outside world is impacting the acoustic experience of the building. The roof, façade, windows and doors, all play a key role in this consideration.
How sound blocking is measured:
- Outdoor Indoor Transmission Class (OITC) – The sound blocking capacity of a roof, window, building façade or façade component.
OITC values required for the building shell vary greatly based on the use of the building and the noise levels on and around the site. OITC values range from 35-40 for a relatively quiet site, up to 60 for a very noisy site.
To achieve higher OITC values, buildings can be constructed with more massive exterior walls and roofs. Selecting acoustically rated windows or curtainwall systems is also effective in limiting exterior or environmental noise.
4. Limit noise levels generated by building mechanical, electrical and plumbing systems inside occupied rooms.
The last goal of acoustic standards is to minimize noise generated by a building’s systems. In addition to serving their immediate purpose, mechanical, electrical and plumbing systems should ideally not contribute additional noise. Selecting quiet equipment and implementing noise and vibration control measures to limit the background noise levels can help achieve this.
How system noise control is measured:
- Noise Criterion (NC) – Building system background noise level classification.
- Weighted decibels (dBA or dBC) – broadband sound level that is either A-weighted or C-weighted to better represent how people hear sound.
NC values for most rooms vary between NC-25 (quiet) to NC-35 (normal). Some open offices, corridors, waiting rooms and lobbies may have higher values (NC-40).
Complying with the background noise levels in the standards and guidelines relates mostly to the design of the building systems themselves. However, having sound absorption inside occupied rooms can also help to decrease perceived background noise that may result from any of these systems.