Four Key Goals of Acoustic Standards and Guidelines

Managing or even understanding, the acoustic demands of a building can be a challenging task. Even a quick look at acoustics sections in building standards or guidelines can leave anyone aside from acousticians scratching their
heads.

To help simplify the subject, most of this information and criteria actually can be distilled to four key goals of acoustic requirements. Knowing these goals and the metrics used to check compliance with each can help design and construction professionals 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 materials over occupied rooms. Additional sound absorbing treatments on the walls or floors also may be required.

How sound absorption is measured:

  • Noise Reduction Coefficient (NRC) – Per cent of the noise absorbed by the material.
  • Reverberation Time (T60) – Time required for the sound level to decrease 60 decibels (dB).

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. As the NRC ratings of materials are increased, reverberation time is  decreased and compliance with the standards is achieved with less material. This results in cost savings and is a more sustainable approach.

Suspended, acoustic ceilings are used primarily, and most effectively, to absorb sound. Additional absorption on the walls and floor only may 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.

Shorter reverberation time, achieved through higher NRC values, results in better speech intelligibility. In other words, the room reflects sound less when there is higher absorption and improves speech intelligibility, lowers noise levels, and enhances sound privacy. NRC ratings can be grouped into good, better, and best categories.

Good absorption (NRC 0.70) is appropriate for corridors, waiting areas, and private offices. Better absorption (NRC 0.80) is appropriate for classrooms, restaurants, and meeting/ seminar rooms. Best absorption (NRC 0.90 and higher) is appropriate for patient care areas and open offices.

2. Limit occupant noise transmission between interior rooms.

The second goal of acoustic standards is to ensure there is minimal noise transferring between rooms. This is achieved by the sound blocking elements enclosing 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 having 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+.
Blocking can be categorized as best at STC 50 or higher, better at STC 45 or higher, or good at STC 40 or higher. Avoid STC below 40 as it does not provide adequate sound privacy. Standards most frequently require a sound blocking level of STC 45. This means a listener in a quiet room would hear raised speech in adjacent rooms, but would not be able to understand the conversation.

Full-height interior walls can achieve STC 45, 50, or higher. Extending the demising walls’ full height up to the slab or roof above is the approach required by most standards and guidelines, and leads to the highest level of blocking. As an added benefit, the noise leaks or flanking paths through the ceilings are mostly inconsequential, and do not need to be remediated.

If full-height walls are unavailable or impractical, recent research shows that even when the main demising wall between two rooms stops at the height of the suspended ceiling, STC 40, 45, and 50 levels of blocking can be achieved by adding lightweight plenum barriers that begin at the top of the wall and extend up to the underside of the floor or roof above. These plenum barriers can be made of stone wool insulation with foil face, standard gypsum board, or limp mass loaded vinyl. Other materials are possible as well. The studs from the lower wall do not necessarily need to extend past the ceiling level either.

The plenum barriers can be friction- fitted against the top of the wall and underside of the slab above or fastened in place by being screwed to metal channels or angles. While higher levels of isolation (e.g., STC 50) require that penetrations through the plenum barriers by pipes, ducts, and conduits be sealed airtight, lower levels of isolation (STC 40) can be achieved by just stuffing large openings with stone wool insulation and leaving gaps and cracks open. Taping and caulking, the time- consuming part of the installation, is not always required. Lastly, the plenum barriers only need to be used between the two rooms that require privacy. They do not need to extend horizontally all the way around the perimeters of both rooms. This permits return air to still move freely in the plenum in un- ducted systems

As examples:

  • Using just one layer of standard 5⁄8-inch gypsum board as a plenum barrier in combination with a stone wool, suspended, modular, acoustic ceiling, can result in STC 45, assuming any penetrations through the plenum barrier are sealed. And, the noise leaks or flanking paths through the ceiling system do not need to be remediated. This is a significant benefit because it saves on costs and allows the ceiling to be optimized for aesthetics and absorption, and permits easy access through the ceiling for maintenance.

3. Limit exterior environmental noise transmission into the building.

Once the internal sound qualities are accounted for, think about how the outside world is impacting the acoustic experience inside 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. Limiting the amount of glass facing exterior noise sources is a key design strategy. Selecting acoustically rated windows or curtainwall systems is also effective in limiting exterior or environmental noise. But above all, selecting a quiet site is the wisest and most cost-effective option.

4. Limit noise levels generated by building mechanical, electrical, and plumbing systems inside occupied rooms.

The last goal of acoustic standards is to control noise generated by a building’s systems. In addition to serving their immediate purpose, mechanical, electrical, and plumbing systems ideally should not generate excessive additional noise. Selecting quiet equipment and implementing noise and vibration control measures to limit the background noise levels can help achieve this, but noise control of these systems should not be over designed either. When background sound levels are too low, sound privacy and disturbances can negatively affect occupants.

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). The louder background sound can be beneficial.

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 also can help to decrease perceived background noise that may result from any of these systems.

Understanding these four key goals will help achieve an effective, no- compromise approach that complies with the industry’s latest standards and achieves an optimal acoustic experience at a competitive price.