People experience buildings not just with their eyesight, but with all their senses including the sense of hearing. That means the quality of their experience is directly related to the quality of the acoustic design of a space they encounter. If there are too many surfaces that create echoes, then speech is frustratingly difficult to understand. If sound passes from one interior space to another, then privacy, comfort, and wellness are compromised. If noise enters from outside the building or from mechanical equipment, then the overall quality of the indoor environment suffers. It has been well documented by numerous sources that poor acoustics or exposure to noise in buildings negatively impacts the ability of people to concentrate, be productive, learn, converse, or heal. Further, those exposed to these conditions over the long term can see a deterioration in their health related to physical and psychological degradation. This course focuses on how the design of buildings can directly and intentionally address these acoustical concerns. It looks at some specific strategies related to common interior surfaces including walls and ceilings as well as some special concerns related to doors, architectural louvers, and healthcare cubicles. Overall, different strategies and techniques are explored to create positive indoor environments with desirable acoustical results.

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Acoustical experiences in buildings are directly influenced by the materials used, including ceilings, walls, doors, and other types of enclosures.

Acoustics Overview

The study of acoustics is defined as the branch of physics concerned with the properties of sound. That means that there are basic scientific principles that have been studied, observed, documented, tested, and ratified by specialists for decades and even centuries. The application of this science is what design professionals often become involved in since all building interiors will be subject to sound which acts in accordance with those scientific characteristics. By understanding how sound acts upon or reacts against walls, ceilings, doors, and other parts of buildings, then the behavior of that sound in a building is more predictable.

Acoustics have garnered more attention recently because of numerous studies, surveys, and other efforts that look at the impact on people of good and bad acoustical design in buildings. Many national building design standards contain a variety of acoustics requirements that include attention to the way that sound is absorbed inside rooms, how to block sound from passing between rooms or between inside and outside, and the control of background noise such as from mechanical equipment. Some of the metrics that are found in different standards are shown in the following chart and discussed in the subsequent paragraphs.

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Table 1

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Using high-performance ceiling panels made of stone wool or perforated metal with NRC ratings of at least 0.90 can lead to compliance with the required maximum reverberation times even when there are no other sound-absorbing surfaces in the room.

Sound Absorption

The building standards cited typically prescribe minimum sound absorption requirements in terms of either minimum ceiling Noise Reduction Coefficient (NRC) or maximum sound reverberation time (T). This absorption is required so that speech is intelligible in enclosed rooms used for assembly/ communication (such as classrooms). It is also needed so that auditory privacy is achieved in open spaces (like open offices) and between enclosed spaces (patient recovery areas). The standards typically require minimum ceiling NRC to be a very absorptive 0.90 in open and noisy (high occupancy) spaces and in patient care areas or medication safety zones inside healthcare buildings. They require 0.80 or higher in medium to large rooms where people gather/assemble to communicate (classrooms, meeting rooms, training rooms, lecture halls). They only permit low NRC of 0.70-0.75 in small private rooms such as private offices. NRC below 0.70 is not permitted by the standards because it is seen as detrimental to wellness in humans.

Sound Isolation

The building standards typically have minimum sound isolation requirements between adjacent rooms (horizontal and vertical adjacency) in terms of Sound Transmission Class (STC). This isolation is needed so that noise from adjacent rooms/floors does not annoy people or interfere with what they are trying to do. It also provides privacy or confidentiality when sensitive information is being discussed. The standards typically require STC 50 or greater for the floor-ceiling assembly located between two vertically adjacent rooms. STC 45 for the partitions is the most widely used performance level between two horizontally adjacent rooms.

The use of STC indicates that demising walls extend vertically from floor to floor and all penetrations are sealed airtight. Relying on any modular acoustic ceiling alone, for example, when the wall stops at the ceiling and does not completely block off the plenum, cannot provide the levels of isolation required by the standards and guidelines. A modular, acoustic ceiling does not have enough mass to block sound. The lights, air devices, and other elements also result in noise leaks, worsening performance. For these reasons, CAC (Ceiling Attenuation Class) is not a part of most standards and guidelines and may no longer be necessary in project specifications.

Background Sound

The building standards typically have maximum background sound level requirements in terms of Noise Criterion (NC). Excessive background noise can interfere with speech intelligibility which can cause stress, fatigue, and discomfort. The standards typically require that the background noise does not exceed NC 30 (e.g., conference rooms), NC 35 (e.g., classrooms) or NC 40 (e.g., lobbies, open offices).

Stone Wool Acoustic Ceilings

Acoustic ceilings are one of the few building products and systems that contribute to compliance with multiple performance categories including sound absorption, sound isolation and background noise levels. Therefore, it is easy to see why the use of acoustic ceilings is a design decision that produces a high benefit relative to their low cost. The main acoustic purpose and strength of suspended ceilings is 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.

While different materials are used to make acoustic ceiling products, not all ceiling panels can meet the high NRC requirements in the standards. High performance choices use stone wool for the core of the acoustic ceiling panels or position stone wool absorptive backers on top of perforated metal ceilings. These acoustic ceilings suspended inside enclosed rooms and throughout open spaces are a cost-effective, easy, and efficient way to comply with the absorption requirements in the standards. An architect or specifier only needs to ensure that the manufacturer’s NRC rating from their data sheet meets or exceeds the minimum NRC in the applicable standard(s). If the absorption requirement is given in Reverberation Time, then the NRC is entered into a simple equation with the room’s volume and surface areas to check compliance.

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In open spaces, where sound blocking between rooms is not important, using ceiling panels made of stone wool or perforated metal with a sound absorbing backer and with a NRC rating of 0.90 or higher can lead to compliance with maximum mechanical system and exterior noise levels by absorbing noise that has already entered the room.