A CAC (Ceiling Attenuation Class) rating is a single number that describes how well a ceiling system blocks airborne sound from traveling between adjacent rooms through the shared plenum above. The higher the number, the better the privacy: below CAC 25 is very low performance, and CAC 35 or higher is considered high performance. While ratings like NRC (Noise Reduction Coefficient) and SAA (Sound Absorption Average) measure how much sound a ceiling surface absorbs within a room, CAC measures something entirely different: how effectively a ceiling system prevents sound from traveling between adjacent rooms through the plenum. The plenum is the cavity between a suspended ceiling and the structural deck above it, and in open-plan offices, clinics, schools, and multi-room residences, it is one of the most common pathways for unwanted sound to migrate from one space to another. If a wall between two rooms does not extend all the way up to the structural deck, sound can travel up through the ceiling of one room, across the plenum, and back down through the ceiling of the next room, bypassing even a well-constructed wall entirely.

CAC is a lab-tested rating measured according to ASTM E1414, the Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum, and calculated under ASTM E413. During testing, a sound source is placed above one ceiling and microphones record what passes through to the space below an adjacent ceiling, across 16 frequencies ranging from 125 Hz to 4,000 Hz. Those measurements are then converted into a single numeric value, the CAC rating. Acoustical ceiling panels typically carry CAC values ranging from 25 to 40, while a 5/8-inch Type X gypsum board can exceed a CAC of 50. For sensitive environments such as medical exam rooms, HR offices, or confidential meeting spaces, a CAC of 40 or higher is frequently recommended by acoustic designers.

Fonctionnement des systèmes BASWA

BASWA DTG carries a CAC rating of 39, well above the CAC 35 mark for high-performance sound isolation between rooms. DTG installs directly on a standard suspended drywall grid with no separate substrate, so it fits projects where a suspended ceiling is already specified. It also absorbs sound within the room, with an NRC of 0.80 in Prime Base, so a single seamless ceiling limits sound traveling through the plenum and controls echo and reverberation below it. DTG is a 30 mm system rated Class A for fire per ASTM E84, and it typically installs one to two days faster than comparable systems. Where a ceiling is built as a continuous rated assembly instead of on grid, BASWA Phon applied over it adds 5 to 7 STC points to that assembly while delivering NRC ratings from 0.80 to 1.00. Visit our Technical Data section to review tested performance specifications for each BASWA system.

Understanding the real-world stakes of CAC performance begins with understanding how consequential noise is in occupied buildings. A peer-reviewed analysis of 42,764 occupant surveys from 303 office buildings found that sound privacy, the ability to hold a conversation without being overheard, was the most common source of dissatisfaction in open-plan offices, with more than half of occupants in partitioned workstations dissatisfied. Even in enclosed private offices, nearly one in five occupants were dissatisfied with sound privacy, which the authors linked to partitions and construction that fall short of expectations. That is exactly where plenum paths matter: a private office is only as private as the ceiling above its walls. It is also worth noting that CAC is closely related to, but distinct from, STC (Sound Transmission Class): STC measures how well walls, doors, and partitions block lateral sound transmission, while CAC applies specifically to the vertical and plenum-path transmission through ceiling systems. When walls with a high STC rating do not extend to the floor above, sound can arc over the wall through the plenum, which is why the ceiling's CAC should ideally be matched to the STC rating of the surrounding walls.

It is also important to recognize that a ceiling's published CAC is a laboratory value, and real-world performance depends heavily on installation quality and system integrity. Common through-ceiling penetrations such as recessed light fixtures, HVAC diffusers, and fire sprinklers can significantly reduce the attenuation performance of an otherwise high-CAC ceiling system. For architects and designers specifying high-performance acoustic environments, it is worth evaluating not just a product's rated CAC but how the full installed assembly, including light fixtures and mechanical penetrations, affects that value in the field. BASWA's acoustical plaster systems are finished as one continuous surface, so the ceiling itself adds no tile joints, and careful detailing around fixtures and diffusers helps the installed ceiling stay close to its rated performance.

Si vous en êtes au début du processus de compréhension de la façon de répondre aux préoccupations acoustiques dans un espace, le CAC est l’un des plusieurs indicateurs que vous voudrez évaluer ensemble. Un CAC élevé favorise la confidentialité entre les pièces grâce au plénum, mais il fonctionne mieux dans le cadre d’une stratégie acoustique en couches qui prend aussi en compte le NRC pour la qualité sonore en pièce, le STC pour la performance des murs et des cloisons, et l’IIC (Impact Insulation Class) pour le bruit d’impact plancher à sol dans la construction à plusieurs étages. Vous pouvez explorer comment ces évaluations s’appliquent aux projets réels de notre portefeuille, ou contacter un installateur certifié BASWA pour discuter des objectifs acoustiques pour votre espace spécifique. Pour ceux qui poursuivent des crédits de formation continue, nos ressources éducatives offrent un apprentissage structuré sur les principes de la conception acoustique pour les architectes et les spécificateurs.