Acoustics: Design Principles for Ceilings

The importance of acoustics is found in the ways that poor acoustics can affect human health and well-being. According to the National Institute for Occupational Safety and Health (NIOSH), high ambient noise levels in a room or space affect people's health by increasing general stress levels. Further, they find that continued exposure does not lead someone to adjust and “get used to it,” rather, the effects worsen.

Photo courtesy of ROCKFON

Indoor environments that are prone to generating noise need to address acoustics as a matter of human health and welfare.

The World Health Organization (WHO) (www.euro.who.int/en/health-topics/environment-and-health/noise) has noted that “noise seriously harms human health by causing short- and long-term health problems. Noise interferes with people's daily activities at school, at work, at home, and during leisure time. It can disturb sleep, causes cardiovascular and psychophysiological effects, hinder work and school performance, and provokes annoyance responses and changes in social behavior.” Peaks in sound or noise and a high level of average sound are what have been shown, over time, to damage an individual's health. Therefore, attention to acoustics is important for all environments, from factories to kindergartens, as a matter of the health and welfare of those who use those environments.

Sound is measured in decibels of pressure to indicate how loud it is in a room. The actual level in any particular room depends on the strength of the sound source (or sources) and the ability of a room or space to help control that sound. Sound sources can be desirable such as a person speaking or music playing but they can also be undesirable if undue echoes are caused or background noise is encountered. Since sound radiates from a source and moves through space in all directions, how it acts within a room will be a function of what it encounters. If there are a lot of sound-reflecting surfaces, then echoes and reverberation become noticeable. If there are a lot of sound-absorptive surfaces, then the sound is deadened and echoes are reduced. The size and shape of the room will similarly come into play as sound moves through it and create additional impacts.

Once airborne, sound doesn't necessarily stay within a room—it can readily pass through openings, transfer through ceilings, etc. In this case, it is the ability of a space to insulate itself from unwanted sound that is important. Background noise that enters a room can interfere with the desired sounds, thus creating unwanted noise. By the same token, desirable sound created within a given space can become unwanted noise once it passes through to other spaces. Hence, it is important for rooms to be appropriately designed to contain sound where it is desired and prevent its passage to places where it is not desired.

One of the measures of good acoustical performance is “speech intelligibility” which is defined simply as how well speech can be heard and understood in a room. Many factors influence speech intelligibility. These include the strength of the speech signal, the direction of the source sound, the level of background noise, the reverberation time (RT) of the room, and the shape of the room. A good reverberation time will enable a listener to hear and understand the first word, allowing that sound to die out before the sound of the next word reaches the listener.

Longer reverberation times can impair speech intelligibility since word sounds will overlap, creating garbled sounding words and poor verbal communication. Instructional spaces, such as classrooms, are best with short RTs—less than 0.6 second to ensure clarity and high speech intelligibility. Auditoriums, theaters, and other musical spaces will typically benefit from longer RTs, typically greater than 1.2 seconds. In addition to RT, if desirable sound is drowned in background noise, the listener will have difficulty understanding what is being said. The normal reaction is to speak louder, leading to more reverberation and poorer speech intelligibility.

Recognizing the importance of good acoustical design in schools in particular, ANSI Standard S12.60 “Acoustical Performance Criteria, Design Requirements and Guidelines for Schools” has been developed. This national standard is used as a basis for determining currently acceptable levels of acoustic performance in schools including appropriate RT and speech intelligibility thresholds. Stone wool ceiling tiles can be used as part of an overall acoustical design to work with the principles of sound and demonstrate compliance with the ANSI standard.