Acoustics
Sound insulation/ blocking
Sound absorption
Soundproofing

How to soundproof a room: your guide to best practices in interior wall acoustic insulation

Brendan Knapman, Technical Services Manager with the ROCKWOOL Building Science Department
Brendan Knapman
16 September 2019

Helping you minimize interior noise to achieve sound isolation with your next project

Version 2: Your guide to room soundproofing solutions - header for the blog article - updated version for cropping to use as the hero image.

This article, the third in our deep-dive series on acoustics, focuses on optimizing the Indoor Environmental Quality (IEQ) on your next project through sound isolation or sound blocking. We explain the difference between blocking and absorption and address the function that blocking plays in ensuring indoor speech privacy.

You will learn about sound transmission class (STC) and how it is calculated and used in acoustic room design and building acoustics planning. We’ll also cover how sound blocking is applied in different types of buildings to meet specific acoustics goals.

Most importantly, we include specific direction on how to use different construction assemblies and materials hand in hand to manage interior noise and ultimately contribute to a healthy and productive experience for building occupants.

What is acoustic insulation?

Acoustic insulation is one aspect of soundproofing your space; it is typically a component of a floor or wall assembly designed to ensure sound comfort for occupants. Managing the acoustics of a built environment, by its most simple definition, is done by either blocking or absorbing sound (one example being a sound barrier wall with acoustic insulation inside).

Most acoustic home and building design plans include a combination of both to reduce the transmission of unwanted direct sound from the source. When it comes to blocking interior sound (we will also refer to this interchangeably as “sound isolation”), the materials selected to support the acoustics objectives matter as much as the design of the space itself.

What is sound isolation?

While “blocking” is the more commonly used term, sound isolation is the ability to block sound transmission from one room or area to another (between the source and the receiver). This may also be needed in instances where the room is upstairs or right next door.

Blocking (sound isolation) occurs when sound bounces off of hard, reflective materials in a space, whether on the ceiling, walls or partitions, windows, or the floor.

Absorption, conversely, is what happens when a material has the ability to absorb and dampen the sound. Softer materials, such as carpets, fabric panels or perforated ceiling tiles, are most often used for this purpose.

The design approach to be considered for sound isolation is separating (or decoupling) assembly materials to stop the transfer of sound energy. This soundproofing technique is often used in “floating” walls or floors but doesn’t have to be done by constructing a massive concrete slab or wall.

There are several types of sound isolation including room-to-room, one-way sound, vertical airborne sound, and impact sound.

PNG - what is sound isolation? The ability to block sound transmission from one room or area to another. Four types: room-to-room, one-way, vertical airborne, and impact.

Sound isolation is the ability to block sound transmission from one room to another (between the source and the receiver).

Why should you consider acoustic insulation?

Acoustic insulation can be applied to almost any area to prevent noise from transmitting through the walls and floors - serving as somewhat of a sound barrier. As acoustic comfort in buildings becomes a greater priority for renovation, retrofit or new construction projects, acoustic insulation in walls, floors, and ceilings takes on greater importance in minimizing noise levels for occupants.

According to the National Institute of Building Sciences, providing a comfortable environment for employees contributes significantly to their optimum performance and reduced absenteeism. They also find that the factors contributing to worker productivity such as acoustic room design are still often overlooked.

That being said, there is a steadily increasing demand for acoustic comfort, driven by occupants and building professionals who recognize the importance it plays in our daily lives – at home, at work and in sensitive environments such as schools and hospitals. The other consideration is the increasing compliance required with building codes and voluntary guidelines surrounding acoustics which we examine in our article, ‘Defining the acoustic requirements and regulations for noise control in buildings’.

In residential applications, ensuring the right acoustic room design including sound-blocking walls is essential, especially if you’re looking for how to muffle sound through a shared wall. Acoustics planning is also an important component if you’re looking to convert your basement into a home theatre or music room where a sound barrier to the floor above may be needed. Acoustics room design can also help to minimize distractions in a home office or reduce the impact noise from noisy neighbors.

For commercial construction, you may want to consider the value of acoustic insulation to help in several areas, including:

  • Office buildings: People working in offices – be they open-plan environments with cubicles, private rooms, or large meeting spaces – will likely encounter sound from mechanical equipment in adjacent spaces, copy machines and printers, and phones and voices from other people within the workplace. Creating office privacy and ensuring open plan office noise reduction will lead to a happier, healthier and more productive workforce,
  • Healthcare facilities: Noise in many healthcare facilities is a 24/7 phenomenon; the source could be anything from loud equipment used in patient care, conversations between patients and care teams, visitors, making hospital acoustics and protecting patient confidentiality in a busy healthcare setting a valuable part of your design process, and
  • Schools and educational institutions: Students and teachers alike will experience noise in the form of hallway foot traffic, conversations, other classrooms, mechanical equipment, and conversations inside the classroom. School acoustics are an important component of creating a positive and productive learning environment.

There are several different ways to achieve the sound isolation goals of a space. The construction assemblies used and the building materials that are specified and selected are only two examples. Sound blocking or isolation is a key component of any acoustics plan and there are numerous product options available to help you achieve the desired STC rating for your space including stone wool insulation.

In order to help you take the best approach for the project at hand, let’s first look at what sound isolation actually is and how it’s measured.

Acoustic requirements in building codes

Learn more about the industry standards that have been developed for each of these environments by visiting our in-depth blog post.

Acoustic requirements in building codes

PNG - creating acoustic comfort in commercial construction across offices, schools and healthcare facilities requires unique considerations for sound isolation.

When creating acoustic comfort in commercial construction, know that sound isolation requires a different set of considerations for space planning across office buildings, schools, and healthcare facilities.

How is sound isolation measured?

To understand how sound isolation is measured, first get to know more about sound transmission class (STC) ratings. STC speaks broadly to the role of building components to reduce sound transmission through wall assemblies, floors/ceilings, windows, and doors, from one space to another.

STC ratings are calculated by measuring sound transmission loss at 16 data points in a frequency range of 125 to 4,000 Hz – with Hertz being the frequency of a sound wave.

Once these 16 points have been charted they are compared against contour curves of established STC ratings. The contour that most closely matches the test data is then used to define the STC rating that is awarded to the assembly. STC ratings must be certified by a qualified lab such as the Intertek Group, Element Materials Technology, and Alion Science and Technology Corporation.

The below visual outlines assembly STC ratings and what it means in terms of speech privacy for occupants including STC 50 where 'very loud sounds such as musical instruments or a stereo can be faintly heard'.

Where else is sound isolation needed?

HVAC systems and equipment are a problematic source of noise for commercial buildings. Members of the building design and construction team should focus on background noise reduction to ensure a comfortable occupant experience regardless of the building type.

PNG - how does STC rating translate to what occupants actually hear? the STC rating quantifies how much a wall assembly reduces sound. Scale of STC from 25 to 60+.

The assembly STC rating quantifies how much a wall assembly reduces sound and the table above shows the impact of each STC rating on what occupants of the space actually hear.

STC ratings of assemblies and systems

The STC rating quantifies how much a particular soundproofing assembly or system such as an interior wall assembly, reduces sound.

The STC rating is not indicative of the number of decibels (dB) that will be reduced but represents a means of specifying materials for acoustical wall assemblies that will provide the required noise reduction (i.e. an assembly comprised of ⅝” gypsum board, steel studs and ROCKWOOL SAFE’n’SOUND® insulation will produce an STC rating of 52 while an assembly comprised of ⅝” gypsum board, wood studs, resilient channels, and ROCKWOOL SAFE’n’SOUND® will produce an STC rating of 45).

STC ratings are most commonly used for interior sound transmission transfer as the frequency range used lacks certain low frequencies that is more commonly found in the exterior environment.

Understanding STC and STC ratings

The International Building Code (IBC) requires lab-tested STC 50 between dwellings (i.e. for walls, floors, and ceilings that separate dwellings in new construction, also known as “party walls”. Under the acoustic legislation, this is a minimal amount of sound isolation and at this level, you may still be disturbed by neighbors in adjacent dwellings.

So what is a good STC rating and how is it achieved? In general, that would vary based on the building materials – walls, for example, as compared to windows. In general, higher STC ratings are achievable with acoustical wall assemblies than those of windows. For wall assemblies, we would recommend that you consider STC 55 to STC 60 assembly rating for an improved occupant experience.

Higher STC ratings are always indicative of greater sound absorption or blocking, however, as the calculation follows a logarithmic scale, the STC number is not necessarily the sum of its parts. As an example, if you have an STC 33 wall assembly and decide to add another sheet or layer of drywall, you do not necessarily get a finished wall with a higher STC rating. This new assembly would have to be tested to confirm, but you may expect to get an STC of approximately 35.

Additional sound ratings

Noise Isolation Class (NIC) is another method for rating a partition material’s ability to block airborne noise transfer. Similar to an STC field test, it is conducted in the actual environment in which it will be used rather than a lab. The difference is that NIC only measures the transmission loss between 125 and 4,000 Hz and doesn’t account for loss from reverberation time, room size, or the size of the test partition. The benefit of this testing system is that a rating is assigned in the actual environment in which the partition will be used. The downside is that it is highly dependent on the conditions in that environment, and therefore the NIC rating might not be exactly replicated in other spaces.

Sound attenuation is the energy loss that noise experiences as it transfers through a material. The sound is dampened because the material’s properties reduce its quality and volume. Different materials will have different sound attenuation properties and should be carefully considered when developing the acoustics plan for a space.

How to use sound isolation for speech privacy

Speech privacy requires optimizing the sound isolation between rooms to ensure an inability to understand unintended speech. Factors that contribute to speech privacy include sound amplitude, background noise level, attenuation, and listener language skills. Examples of unintended speech include a patient in the neighbouring room, a private call in an office, or noise from the adjacent classrooms.

Good speech privacy has a Privacy Index (PI) > 95% or Speech Privacy Class (SPC) > 80, where PI is calculated using the following formula: PI = (1 – Articulation Class) x 100%. It’s basically the opposite of speech intelligibility. The measure has gained widespread adoption among acousticians in North America and ASTM standards have identified different types of speech privacy, which vary by the need:

  • Normal Privacy – when approximately five to twenty percent of anything that is said may be understood by a casual listener within an open plan environment. This is considered the baseline for achieving acoustic comfort in most “open” offices and corresponds to a PI of 80% to 95%
  • Confidential Privacy – when five percent or less of anything that is said may be understood, but that the meaning of conversation cannot be determined by a casual listener. It is extremely difficult to achieve this level of privacy in an “open-landscape” office. This corresponds to a PI of 95% or greater.
  • Secure Privacy – when zero percent of what is said can be heard or understood. This level of speech privacy is the most difficult, and therefore expensive, level to achieve within an open plan space.

Depending on the level of speech privacy required, different construction techniques or assemblies and materials will be required. It’s no surprise that some building materials are better suited to help manage sound than others.

PNG - speech privacy requires optimizing the sound isolation between rooms to ensure an inability to understand unintended speech. Privacy index measure with scenario graphics.

Speech privacy requires optimizing the sound isolation between rooms to ensure the inability to understand unintended speech. Different construction techniques and wall assemblies and materials can help increase the privacy index (PI) of a space.

What materials can block sound?

Regardless of the type of building and its unique needs and challenges, acoustics plays a critical role in supporting Indoor Environmental Quality (IEQ). IEQ is a fundamental principle in many voluntary certification programs, such as LEED, the FGI Guidelines Institute (FGI), and the WELL Building Standard. Acoustics is one of the components that will significantly affect occupant comfort and wellbeing.

It may surprise some to learn that sound travels easily through wood framing and the layer of drywall or sheetrock – usually the largest point of connection between you and your neighbors, whether in a home or commercial building. From the perspective of acoustics, it means planning properly with the right construction assemblies and insulation materials. While the best time to insulate and soundproof walls is during new construction, it is possible to add sound isolation to existing walls without tearing off the drywall.

With proper insulation and mass, walls can definitely help block out noise. But this still isn’t enough. In fact, closed offices often give the illusion of privacy. Even with appropriate insulation, noise will still find a way through other, less obvious, open spaces called flanking paths. For example, ceiling systems will always have noise leaks – created by installing light fixtures and HVAC ventilation air devices, among others – which make them even less effective at blocking or insulating sound. Since ceiling tiles are also usually poor at blocking sound because they don’t have sufficient mass, sound-absorbing tiles are a better option. Wall assemblies are also susceptible to flanking paths such as doors, windows, and outlets will also have open spaces, which allow the transmission of noise between rooms.

As a more comprehensive soundproofing solution, we recommend the construction of full-height walls, or at the very least wall assemblies that can be modified with plenum barriers to provide additional sound-blocking benefits. From an acoustics perspective, full-height walls are the preferred design approach in almost all scenarios. They also should be used when the building design must comply with STC ratings in a standard, guideline or rating system.

Interior wall insulation for energy efficiency

Did you know? Interior wall insulation can also save on utility costs, providing energy efficiency in the form of temperature control within a space. 

Many homes and buildings waste energy because they are not insulated to the latest standards. This means they can be uncomfortable to live in as well as cost more to heat in winter or to keep cool in summer.

Room acoustics - solutions for sound isolation

While every room in a home or building and the surfaces within it will react differently to different frequencies of sound, the insulation within the assemblies has a significant impact on the room acoustics and the ability to block or insulate noise.

Insulating walls is one part of the equation that is essential for any level of speech privacy, as it creates the necessary mass to reduce airflow. But not all interior insulation products are created equal, which makes finding the right solution an important decision during the design and construction processes.

How to prevent sound from entering a room

Limiting the transmission of sound from one area to another requires the use of materials that have the ability to enclose the source of the noise and form a barrier that absorbs vibrations between the source and the receiver.

There are different forms of insulation products that may be specified and applied to help minimize sound levels and enhance acoustic comfort in homes and buildings. These include cellulose, fiberglass, foam and stone wool (or mineral wool as it’s often referred).

Consider stone wool if you are tired of noise in your apartment, office, home, or classroom, and are looking to lower the distractions for building occupants. We suggest acoustic insulation such as ROCKWOOL SAFE’n’SOUND®.

Stone wool insulation solutions from ROCKWOOL will help you soundproof and insulate your space with the proven durability to last for the long term without seeing a decrease in performance. When your walls are properly insulated with stone wool, you will reduce airborne noises coming from other rooms, resulting in a much quieter space.

ROCKWOOL SAFE‘n’SOUND®

This stone wool insulation product is designed for use in interior wall partitions of residential wood and steel stud construction, providing excellent acoustical dampening properties for wall, ceiling and floor applications.

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Rockfon® Plenum Barrier Board

The noise-cancelling plenum boards install quickly above interior partitions to provide additional sound blocking benefits. For optimal performance, combine with CAC 20-40 ceilings.

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Using interior wall insulation for sound control

When soundproofing an interior wall, you must first determine whether your intended goal is to block out noise and sound (speech privacy), or whether you’re trying to improve the quality of sound within a space (speech intelligibility). Interior wall insulation can then serve as a sound barrier that improves soundproofing performance of the space keeping noise from traveling from one room to another.

It’s important to know that soundproofing materials for walls, floors, ceilings, and doors are often denser materials that add mass and reflect noise – this can have the effect of “trapping” noise within a space. Sound absorbing materials, on the other hand, are more often porous and lightweight and keep noise from bouncing around inside a room. So if speech privacy is the only goal, blocking alone would suffice. If speech intelligibility is also required, then a combination of blocking and absorption is necessary.

A key feature in any wall assembly for reducing low-frequency sound transmission is the use of air gaps combined with acoustic insulation. Air gaps effectively “decouple” the main wall assembly components from each other, reducing the ability of low-frequency sounds to easily transmit through the full assembly.

This table illustrates the differences that can be achieved in STC rating using air gaps, staggered rows of studs, and single rows of studs. In all cases, studs are standard 2” x 4” wood and each assembly has a ⅝” sheet of gypsum board on either side. Two examples of wall assembly construction techniques including 'double leaf' or mass-air-mass construction - used to improve sound transmission loss, and using resilient channels and air sealing when designing acoustic systems for your walls and roofs.

Stud SpacingSpacing ConfigurationInsulationAssembly STC
16"Double Row w/ 1" Air Gap3.5" COMFORTBATT per row60
24"Staggered Rows5.0" COMFORTBATT50
16"Single Row3.5" COMFORTBATT36
PNG - mechanical vibration noise and air sealing, wall assemblies designed the right way using resilient channels and air sealing can support improved room acoustics.

While wall assemblies designed to reduce a wide spectrum of noise frequencies are extremely important, reducing air and vibration transfer through your system should not be overlooked.

PNG - double leaf consturction for sound isolation is a common way to improve sound blockage without greatly increasing weight - defined as creating cavity construction with two layers of weight and an air space between.

Double leaf construction for sound isolation is a common way to improve sound blockage without greatly increasing mass where you use two layers of weight with an air space between.

Summary

Managing interior noise doesn’t have to be complicated. It begins by understanding how sound isolation or blocking, and absorption, work differently to help achieve the objectives of a space. While sound isolation is best achieved by insulating interior walls, there are different options to help achieve the desired outcome, so knowing which construction materials to use in different assemblies is essential.

STC ratings of assemblies can help with the decision process by identifying a product’s ability to reduce sound transmission between rooms. With all of the options available, building design and construction pros will benefit from understanding the performance advantages possible with products such as stone wool insulation for soundproofing.

Ultimately, selecting the best construction materials to create wall assemblies that deliver acoustic comfort in buildings and homes at the beginning of a project to meet those objectives will help ensure the finished product has a high degree of IEQ.

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