Since the 1950s, drop ceilings have been the preferred method for concealing HVAC vents, electrical wires, plumbing pipes, phone cables, and security lines in interior commercial buildings. These suspended, interconnected ceiling systems consist of a metal grid comprising cross-tees and main runners.
The main runners are suspended by hanger wires from the structure above, and wall channels or angles provide a clean look throughout the perimeter. Panels are used to conceal the plenum—hiding the visible structure, suspension system, HVAC, and other equipment, while providing simple access for future maintenance.
The suspension ceiling system is selected for aesthetics, maintenance, and specialized performance such as fire resistance, seismic mitigation, or limited accessibility in security applications. For all ceiling designs, specifiers should check the suspension systems are manufactured to ASTM International standards. On request, suspension manufacturers may provide reports from the International Code Council (ICC) and third-party seismic performance testing and certification reports.
Corrosion resistance is also a priority for metal suspension systems supporting stone wool and other ceiling panels. The industry standard is 23.8-mm (15/16-in.) galvanized steel for suspended metal ceiling grids; most may be specified with a minimum of 25 percent recycled content.
While the ceiling panel’s size, orientation, color, finish, and edge largely determine the overall aesthetic, changing the size of the grid’s face also changes the appearance. For example:
- a 14.28-mm (9/16-in.) narrow face diminishes the distinction between grid and panel for a more monolithic look;
- adding a 3.17-mm (1/8-in.) slender, center regress with a ‘bolt-slot’ design accentuates the shadow between panel and grid;
- mitered intersections provide crisp, continuous lines for a uniform ceiling plane;
- wide-face 34.92-mm (1 3/8-in.) ceiling suspension offers bolder expression of the ceiling grid modules, especially at high elevations; and
- in curved drywall applications, radius systems create concave and convex shapes, including barrel-vaulted ceilings.
Specifying acoustic comfort
According to the World Health Organization (WHO):
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, cause cardiovascular and psychophysiological effects, hinder work and school performance and provoke annoyance responses and changes in social behavior.1
Therefore, it could be argued design professionals have a duty to create acoustic comfort and well-being for the occupants of their buildings. Stone wool can help with two primary components of acoustic comfort: speech intelligibility and noise reduction.
The material’s airflow resistance and density contribute to its high noise absorption properties. The fibers’ size and non-directional orientation lead to stone wool’s inherent sound-absorbing qualities. The measures and concepts discussed in this article provide a foundation for understanding the relationship between stone wool’s characteristics as a material and achieving acoustic comfort.
One important component of acoustic comfort and sustainability, speech intelligibility refers to a listener’s ability to hear and understand a speaker in a room or space. It is measured as a signal-to-noise ratio, expressed in decibels (dB). For this application, the signal typically is speech and the noise usually is everything else in the background.
An important factor for creating speech intelligibility, it is defined as the time it takes for the sound pressure level to decrease 60 dB below its original level. In most situations (excluding unamplified music performance), a lower reverberation time improves speech intelligibility and acoustic comfort. For most rooms requiring speech intelligibility, mid-frequency reverberation time should be between 0.50 and 1.00 seconds when the room is unoccupied.
Noise reduction coefficient
The noise reduction coefficient (NRC) indicates a surface’s ability to reduce noise by absorbing sound. It is calculated by averaging the absorption coefficients from the 250-Hz, 500-Hz, 1-kHz, and 2-kHz octave bands. It varies between 0.0 (i.e. absorbs very little sound) and 1.0 (i.e. absorbs a lot of sound). NRC is one of two important variables in determining reverberation time (the other being room volume). A higher NRC indicates more noise reduction (or sound absorption) and leads to lower reverberation times and greater speech intelligibility. Stone wool ceiling products typically have an NRC of 0.85 or higher.
Undesired sound from various potential sources can include noise transmitted into the building from the exterior, or coming in from other interior spaces. It can also include sounds generated by the building’s systems or even those reverberating too long inside the room.
Factors influencing speech intelligibility include:
- speech signal’s strength and clarity;
- sound source’s direction;
- level of background noise;
- room’s reverberation time and shape; and
- listeners’ hearing acuity and attention span.
Reverberation time depends on two main variables: the volume of the room and the amount of sound-absorbing materials. As volume decreases or as the amount of sound-absorbing materials increases, reverberation time decreases and speech intelligibility generally increases. Since the volume of the room often depends on functional and aesthetic criteria, reverberation time is often solely dependent on the amount and efficacy of sound-absorbing materials.
In many cases, placing sound-absorbing materials on the walls is not desirable due to its tendency to get damaged, dirty, or worn because of occupant contact. As a result, whether speech intelligibility is poor, fair, or good can highly depend on the ceiling specified. This is why acoustic standards and guidelines for schools, hospitals, offices, and other types of facilities have minimum NRCs of 0.70 and up to 0.90. Stone wool ceiling panels, more than other panels made of less-absorbing materials, help ensure projects comply with acoustic performance requirements in these standards and guidelines.
Even if reverberation time is appropriate, speech intelligibility can be low if the background noise in the room is too loud. Speech intelligibility equates to a high signal to noise ratio. Consequently, it is also important to ensure noise from the exterior, other interior spaces, and from the building’s systems is controlled.
In other rooms or spaces like open offices, cafeterias, libraries, and gymnasia, speech intelligibility is not the primary acoustic goal; rather, the push is for overall noise reduction for stress relief and concentration. Noise reduction equates to an overall decrease in sound pressure level from loud continuous noise (e.g. traffic noise transmitting into the building), as well as event-specific noise (e.g. a crying baby). The sound pressure level in a room depends on the strength of the sound source, the room’s size, and the quantity and quality of sound-absorbing surfaces.
Just 30 decibels of periodic noise can be disturbing to sleep or concentration. Conversational speech is generally between 50 to 70 dB. Noise with sound levels of 35 decibels or more can interfere with speech intelligibility in smaller rooms. This is demonstrated by a phenomena known as the ‘cocktail party effect,’ whereby as noise levels get louder and louder, people try to talk louder and louder to be understood. Despite their efforts, speech intelligibility decreases and acoustic stress increases. It is not until someone leaves the ‘party’ that they realize just how agitated they were as their muscles begin to relax, heart rate slows, and respiration deepens. Stone wool, because of its high noise-absorbing characteristics, also helps achieve the overall noise reduction goals.
Whether sound reduction is needed for speech intelligibility or overall acoustic comfort, blocking noise that could be in the plenum above the ceiling can also be important in some instances. As more acoustics standards and guidelines place minimum noise control criteria on wall constructions (i.e. sound transmission class [STC]), the need for ceilings to block noise from adjacent spaces traveling via the overhead plenum is becoming less frequent. This is because achieving the minimum STC wall requirements necessitates the walls be extended up to, and sealed against, the underside of the deck above them. However, in the cases where the walls do not extend full height, or where there may be noisy mechanical equipment in the plenum, the ceiling also may need to block noise from transmitting into the space below them.