Sound insulation and blocking should be left to walls, not to an acoustic ceiling.

January 1, 1

Before you stop the walls under a suspended acoustic ceiling instead of extending them all the way up, think about if you’re making a shortsighted, budget-driven compromise or a sound decision for the usability of that building throughout a long lifetime.

The impact of poor acoustics

Post-occupancy building surveys show that the number one dissatisfaction with buildings is their acoustics – and more specifically, sound privacy. Those low scores relate to the use of open spaces, but they relate to poorly designed, private rooms, too.

People don’t expect speech privacy when they are in a large open area full of people. But, when they enter a private room and close the door, their expectations for sound privacy change a lot. The problem is that whether or not they will have their expected privacy is completely out of sight. It depends on if you decided to compromise the architecture – inside the walls and above the ceiling.

Before you do that, think about the weight of a wall compared to that of an acoustic ceiling panel. Think about how solid and massive that wall is compared to a lightweight acoustic ceiling full of holes for lights and air devices. The ceiling is no match. That’s why building standards, guidelines, and rating systems require full-height walls with high Sound Transmission Class (STC) ratings.

What is sound insulation?

Sound insulation, also referred to as sound isolation, occurs when a massive and nonporous architectural surface or assembly, such as a concrete slab or multi-layer gypsum board wall, reflects sound energy.

Because the sound reflects off the surface, very little sound transmission into adjacent rooms occurs. Sound insulation helps to provide sound privacy and speech privacy.

Rockfon’s Optimized Acoustics™ design process utilizes full-height walls with the appropriate noise blocking sound capabilities or lightweight acoustic plenum barriers combined with our stone wool ceilings to ensure speech privacy.


Sound insulation in building standards, guidelines and rating systems

 More and more building types and room types must now comply with more stringent sound insulation criteria in standards, guidelines and rating systems.

  • Guidelines for the design and construction of healthcare facilities, published by the Facilities Guidelines Institute (FGI) , require that "sound insulation shall be considered for all demising construction separating occupied spaces."
  • The WELL Building Standard typically used for commercial office buildings states that “noise from adjacent spaces can be disturbing to building occupants” (feature 81) and can negatively impact the human immune and nervous systems.
  • The Collaborative for High Performance Schools (CHPS) emphasizes that "student learning suffers in acoustically poor environments" where "excessive noise negatively affects speech communication" (EQ14.0). Minimum sound insulation performance for all learning spaces is required.

How is sound insulation defined and measured?

The amount of sound insulation required between rooms in building standards, guidelines and rating systems is defined in one of two ways – either by the minimum Sound Transmission Class (STC) of a full-height wall or by the minimum Noise Isolation Class (NIC) of the whole architectural room enclosure after construction. Ceiling Attenuation Class (CAC) is not recommended becuase it is associated with an unacceptable design approach.

Sound Transmission Class (STC):

STC is a laboratory-measured rating of the sound insulation performance of a wall assembly. An STC rating also can be used for doors, windows and floor/ceiling assemblies. Higher values indicate higher levels of sound insulation between rooms. To determine the appropriate STC of a wall, consider each room’s use, the occupant’s sensitivity to noise and the amount of noise being generated. Building standards, guidelines, and rating systems typically require STC ratings of 40, 45 or 50. Some more critical conditions require values above 50. If a wall has an STC rating requirement, it must be full-height from floor to floor or to the roof, and cannot stop at the height of a suspended ceiling leaving an open plenum. STC ratings are determined according to ASTM E90 standard.

Noise Isolation Class (NIC):

NIC is a field measurement of the total sound insulation performance between two rooms after construction is complete. Higher values indicate higher levels of sound insulation between rooms. NIC accounts for all sound paths through walls, doors, windows, and penetrations. NIC is used in some building standards, guidelines and rating systems instead of STC because NIC considers all sound paths and better represents what people will experience in the space. NIC values of 35, 40 or 45 are typical in normal applications. NIC ratings are determined according to ASTM E336 standard.

Ceiling Attenuation Class (CAC):

CAC indicates a suspended acoustic ceiling’s ability to block sound traveling through a plenum when the wall does not extend full height. High CAC values indicate higher levels of sound insulation between rooms. Suspended acoustic ceilings typically have CAC ratings between 20 and 35. Some can achieve CAC 40 or slightly higher. When light fixtures and supply- and return-air grilles are installed, the ceiling system CAC rating decreases ten points. CAC is not used in most building standards, guidelines and rating systems because suspended acoustic ceilings alone are not able to provide an acceptable level of sound insulation between rooms. CAC ratings are determined according to ASTM E1414 and ASTM E413 standards

Understanding the reality of sound insulation inside buildings and how to block sound between rooms

Sound insulation between rooms on the same floor mostly relates to the walls and doors. In the past, some interior designs had walls that stopped at the height of a suspended ceiling, leaving the plenum above the ceiling open. The designers did not realize that the sound insulation between rooms would only be as good as the weakest link – the acoustic ceiling. They did not realize that the sound insulation of the ceiling (meaning a CAC 20-35) should be the same as the sound insulation of the wall (meaning an STC 40, 45 or 50). In fact, the ceiling system typically performs 10-20 points below the wall’s performance. To learn more about this topic, read Acoustic Myth: Ceiling and wall blocking performance does not need to be equal.

While this practice is no longer permitted by most building standards, guidelines and rating systems, open plenums can still be found in some existing and new buildings.

Stopping the walls at the height of a suspended ceiling and relying on the acoustic ceiling alone to provide adequate sound insulation is a risky and unadvised design practice. This design approach still lingers because some ceiling manufacturers still claim that acoustic ceilings alone are adequate for blocking noise or sound between rooms. As you navigate the ceiling panel options available, you may notice products in the market that claim to absorb and block noise. The reality is that modular acoustic ceilings by themselves do not have the mass needed to block enough noise. To learn more about this topic, read Acoustic Myth: Ceilings are Effective at Blocking Sound and an original Rockfon article titled All-in-one acoustic ceiling solutions are not the answer.

Additionally, when acoustic sound reducing ceiling tiles or panels are placed into a standard suspension grid, along with other building system components, such as supply-air diffuser (left) light fixtures (center) and return-air grille (right), substantial flanking paths – or noise leaks – are created. Rockfon used a high-definition acoustic camera linked to a sound intensity probe to help designers actually see noise leaking through the ceiling system (red and yellow colors). These noise-flanking paths degrade the ceiling system’s acoustic performance substantially. In fact, these leaks can cut perceived noise or sound blocking performance in half, making speech from adjoining spaces intelligible.


By attempting to address both blocking sound and absorbing sound, those dual-purpose panels in the market actually compromise both. Designers mistakenly sacrifice Noise Reduction Coefficient (NRC) for Ceiling Attenuation Class (CAC), and the ceiling sound insulation or blocking is simply not good enough. To learn more about this topic, read Acoustic Myth: It’s OK to sacrifice absorption for blocking. Look to your ceiling panels to meet the high sound absorption requirements and look to your walls for blocking, when it is needed. The CAC approach is no longer compliant with most acoustic standards, guidelines and rating systems.

How can sound insulation be optimized between rooms?

Rockfon’s Optimized Acoustics™ aligns with building standards, guidelines and rating systems by requiring full-height, STC-rated, walls for blocking noise between rooms. Stopping the wall at the height of a suspended ceiling and leaving the plenum open permits too much sound to pass between rooms. This design approach is not permitted and there is no magic noise-cancelling system that building owners can buy afterward to correct the design mistake.


Plenum ceiling

Ceiling plenum design includes a growing number of standards, guidelines and rating systems that are encouraging designers to use lightweight plenum walls to block off the ceiling plenum when the partition between the rooms stops at the ceiling. This requirement is necessary due to the ceiling noise insulation properties, by themselves, being inadequate to achieve speech privacy. The combination of the acoustic ceiling and return-air plenum barrier working together matches the STC performance of the partition below the ceiling level.

Sound-blocking plenum barriers are an excellent sound barrier solution, beginning at the top of the wall and extending to the underside of the floor or the roof above. Rockfon® Plenum Barrier Board is made of pliable stone wool insulation to conform without gaps into tight spaces and around structural members and mechanical, electrical and plumbing elements that may be located in the plenum. The fiber-reinforced aluminum foil facing that is applied to one side increases its sound-insulating performance. When positioned above interior partitions that stop at the height of the suspended, acoustic ceiling, our Plenum Barrier Board stops noise and speech from traveling between rooms through the plenum above the ceiling.

NO, Otto Nielsens vei 12, Arcasa arkitekter, Office, Meeting Room, Open plan office, Rockfon Blanka dB42, A edge, 600x600, white, Chicago Metallic T24 Click 2890

Better blocking. More privacy.

Good sound blocking provides greater privacy between rooms and better concentration in each. Optimized Acoustics™ is key to room-to-room confidentiality.

How to select the right sound transmission class (STC)

Optimized Acoustics™ makes selecting the correct STC easy. First, consider how much noise will be generated by the people and equipment in the adjacent room. Next, consider what people will be doing in the space being designed, how sensitive they would be to disruptive noise and how private what they say must remain.

Sound insulation between rooms can be categorized as Best at STC 50 or higher, Better at STC 45, or Good at STC 40. Avoid STC acoustics ratings below 40 because they do not provide adequate sound privacy. Designers can achieve all three of these performance categories using either a full-height wall or a combination of Rockfon acoustic stone wool sound reducing ceiling tiles or panels and Rockfon® Plenum Barrier Board.


If the adjacent room has a lot of people or equipment, and transmitted noise would interfere with sensitive activities being performed in the room being designed, use the Best STC category of 50. An example is classrooms. Also use the Best STC category of 50 if there needs to be a high level of confidentiality for example, in executive offices.

If one person usually occupies the adjacent room, and the main function of the room being designed is not sound sensitive, use the Better STC category of 45. Some examples of the Better STC category are standard, private offices and patient rooms in healthcare facilities.

See the difference: Why plenum barriers combined with ceilings work better than ceilings alone

Rockfon used a high-definition acoustic camera linked to a sound intensity probe to show visually the difference between using a ceiling alone for sound insulation between rooms and boosting the performance of the ceiling with an acoustic plenum barrier. This visualization method shows noise leaking through a high CAC mineral fiber ceiling (CAC 37) in red and yellow colors. The worst leaks are the return-air grille on the right side and recessed light fixtures in the center. Other noise leaks include the supply-air diffuser on the left and the intersection of the wall and ceiling along the bottom of the images. Altogether, these leaks decrease the ceiling system rating to just CAC 27, a 10 CAC point drop.

When our stone wool ceiling panels and Plenum Barrier Board is used (bottom image), the noise leaks disappear. This Rockfon Optimized AcousticsTM approach performs at STC/CAC 40, 45 or 50 (STC/CAC 50 pictured) depending on the panel and plenum barrier combination. The low noise levels shown in purple are over 25 decibels lower than when the mineral fiber ceiling panels alone (top image) was being used for ceiling sound insulation.


When should you choose a plenum barrier versus using full-height walls?

From an acoustics perspective, full-height walls are the preferred design approach on almost all occasions. If the building design must comply with STC ratings in a standard, guideline or rating system, then full-height walls should be used. A growing number of standards, guidelines and rating systems now also are providing the designer with the option of using either a lightweight plenum barrier combined with an acoustic ceiling or full-height walls. In these cases, the use of plenum barriers should be considered for their potential cost savings and ease of installation.

For some designs, full-height walls just are not an option. The widespread use of pre-manufactured, modular, wall systems is a prime example. These walls stop at the underside of a suspended acoustic ceiling. There is no full-height wall option. Instead of constructing a gypsum board bulkhead above these walls or leaving the plenum open above the ceiling, Optimized Acoustics™ proves that a lightweight plenum barrier is the optimal design approach. Similarly, when a raised access floor creates a plenum below the floor, a plenum barrier below the wall can be used to maintain the sound insulation rating of the wall above.

Many interior fit-outs built in the 1980's and 1990's made the mistake of stopping the walls at the height of a suspended acoustic ceiling and relying on CAC 35 ceiling panels to block noise. We are still dealing with these problem buildings today. As knowledge about acoustics increases and expectations rise, more and more people are complaining about the lack of sound privacy. This is another great application of plenum barriers above the ceiling.  It is simply too difficult to go back after construction and extend the gypsum board walls up full height.

Final words on sound insulation

Made of naturally-absorptive stone wool, Rockfon acoustic ceiling panels combined with Rockfon® Plenum Barrier Board or full-height walls can be used as part of an overall sound insulation strategy to achieve Optimized Acoustics™ and for compliance with standards.

Did you also learn about these other parts of Rockfon’s Optimized Acoustics™?

Learn about the 1st part: Acoustic absorption of the ceiling

Learn about the 3rd Part: Background sound level

Check out our tools to help you design your next acoustics project

Explore our ceiling product range

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