Introduction

Noise is always an issue. Typically it is most encountered either from the exterior environment to inside, or from room to another room within a structure. This page details how to control sound from a room and strategies you can use. One of the primary purposes of architectural acoustic design is to reduce sound transmission from one space to another to improve user comfort.

Three Main Strategies

When you need to control sound from a room, there are three main ways to approach solving the problem.

1. Reduce the Level of Sound from the Source.

As the most obvious solution, just simply reduce the sound from the source. However, reducing the noise level is not always possible. This is especially true if the sound is coming from an outside environment source not related to the ownership or your property. It could also be coming from a piece of equipment in the room that needs to be in there for the space to function.

• Sometimes a noisy machine can be modified (settings) to produce less noise, or enclosed within a smaller space to produce less noise.
• Machines and other equipment may also be programmed to run at times when the space is not occupied.

2. Modify the Absorption in the Space

This strategy is most effective when the room has lots of hard, reflective surfaces already existing in it. 

3. Introduce Non-Intrusive Background Noise

White Noise / Random Noise / Perfume Noise

Introducing what is commonly referred to as ‘white noise’ can help mask and dull out other noises and our perception of them. There are many types of noise and frequencies that can be produced for different effects.

This is a common strategy when the occupants are unable to change the interior level noise. For example in open office environments where it would be impossible to try and control other people’s meetings and conversations, introducing a white noise machine can help drown out some of the distracting sounds people experience, while also providing privacy for all users. 

• It should be noted, it is almost always better to first try and reduce noise levels, rather than add unwanted noise such as this strategy employs.

Site White Noise

Site elements such as gusty wind can reduce sound by introducing “white noise”. Elements such as fountains produce white noise and help block out other noises.

• Example: Fountains also produce this effect by the sound of rushing water

Frequency of Sound

Depending on what the sound is, the frequency pitch will dictate how the sound should be best handled. High pitch frequencies need a different sound mitigation strategy vs low frequency sounds.

High Frequency

Whistle, child’s voice

Acoustic panels are effective best at high frequencies, but will also work for mid frequencies as well.

Low Frequency

Road vehicles, aircraft, machinery

Controlling low frequency requires construction elements or products that can trap the longer low frequency sound wave. Usually requires thicker partitions with a build up of air space/gaps through furring or another method such as resilient channels.

Low frequency sounds are extremely hard to control… we’ve all been deep inside buildings and heard the rumble of a truck from outside. This is true regardless of the STC and OITC ratings.

Panel Resonator / Bass Trap: Open box mounted to the face of a wall, it absorbs low frequency sounds (however it is designed to) and then reflects mid to high frequency sounds.

Cavity Resonator / Heimholtz Resonator: A large airspace with a small opening. The gap allows a sound in and traps it based on the design of the cavity. 

Exterior Materials

One of the best ways to try and mitigate exterior sound, is to stop it before it enters the building. Glass can be designed with laminated layers and set within resilient framing. Laminated glass provides more mass, and the plastic barrier improves the damping even further. Combining a laminated panel with an IGU (and another panel) is a great way to improve sound performance.

• In some urban codes, the glazing must meet minimum sound design criteria close to street level.

Room Design Planning Criteria

As an Architect, you should have in the back of your mind sound and its implications at all times. Everything we do on a day to day basis will create sound, and it has the potential to affect others. Many of the planning strategies to mitigate sound can be reduced down to common sense.

• On a very high level, you should plan similar areas of use next to each other so that the loudness levels are the same. It typically makes little sense to plan bedrooms sporadically around a building, instead of grouping them all together.
• Use buffer spaces such as closets and hallways in between separate noise similar spaces whenever possible.
• Place mechanical equipment as far away from the end sources as possible, while still maintaining good proximity to the spaces to not incur unnecessary effort and sizing needed to the system.
• Stagger openings such as windows and doors along with other elements that may penetrate sound barriers. Additionally, locate openings far away from each other as much as possible.
•  Locate moveable (noise producing) items such as furniture as far away from walls between spaces.
• Minimize the common area of walls between spaces. If a major wall is needed, think about the STC and IIC of the wall.

Flutter Echoes: Repeated echoes

Sound Absorption

In a small room, sound absorption is an important aspect to consider. While doubling the distance will decrease noise by about 6dB outside, this is not the case for an enclosed room. The size of the room means that sound will bounce and decrease legibility of the sound. The best strategy is to decrease the amount of reflections in the room by increasing the absorption of the wall materials. This will improve speech privacy and decrease reverberation.

• Lightweight, porous materials are effective for sound absorption.
• Disconnect assemblies to create better sound absorption and disallow the transmission of sound to another space on the other side of the walls. See resilient channels for one strategy.

Small vs Large Room

The larger the room, the larger the area of the ceiling and floor is compared to the area of the walls. These do not grow proportionally as a room gets larger. Therefore, for larger rooms, it’s best to increase sound absorption on the ceiling as well as on the floor. For smaller rooms, an effective strategy is to focus the strategy on the walls.

Sound Absorption Coefficient

To better gauge how much sound reverberations you will experience, the Sound Absorption Coefficient (SAC) was developed. SAC is the ratio of the sound intensity absorbed by a material to the total intensity reaching the material.

• 0.0 to 0.2 = Reflecting sound material
  • Low value materials are suitable for large spaces. Such as metals, hard stone, etc.
• 0.2 to 0.5 = Range of values to shoot for
  • Avg SAC value should be at least 0.2
  • Above 0.5 is usually not desirable or economically viable unless for a sound critical program like a recording studio.
  • Carpet has roughly 0.2 SAC. Heavy carpet around 0.3 SAC.
• 0.2 to 1.0 = Sound absorbing material
  • Higher values are better for smaller spaces
• 1.0 = Maximum (theoretical) value (all sound is absorbed).

The coefficient varies with frequency of sound. A range of frequencies needs to be tested/checked for critical applications. Only check frequencies of expected levels of volume per the program activities.

• Rooms that contain several materials with different areas, the total absorption can be measured by the noise reduction calculation (see below).

(SAC) Sound Absorption Average: Starting to replace the SAC, the SAA is the average of the absorption coefficients for the 12 one-third-octave bands from 200 Hz to 2500Hz when tested in accordance with ASTM C423.

Increasing SAC

Doubling the amount of absorption in a room results in a noise reduction of only about 3dB (not significant). Therefore, it’s advisable that if spending the money to increase the absorption of a room, the total absorption should be increased by at least 3x. This change results in a decrease of approximately 5dB, which is noticeable.

• Each doubling of the absorption in a room reduces the reverberation by half (1/2).
• When adding extra absorption, an increase of 10 times is the approximate practical limit.

Absorption generally increases with an increased thickness of a porous absorptive material. Foam is a common and cheap material frequently used for this application. Low-frequency noises may need special design treatment. The factors affecting a materials absorption is the thickness, density of material, porosity, and the orientation of the fibers.

Isolation (Soundproofing)

Heavy impervious materials are effective for sound isolation. A room with a lot of sound isolation will help ‘isolate’ the room’s sound from other areas around it. However, this means that the sound will be loud in the room and may be perceived as being ‘louder’ than it actually is. Reverberations and echoes may occur.

• To increase sound isolation, seal the space off with sealing such as sealant, insulation, spray foam, etc. Leave no gaps for the sound to escape.

Noise

Noise is a general name given for any unwanted sound. External noise is particularly difficult to control, whereas internal noise can be confined to its place of origin. Normal city noises can affect the heart and blood pressure, and severe noise (above 85 decibels) can cause lasting and permanent damage.

Transmission Loss (TL)

Transmission Loss: The difference, in decibels, between the sound on a barrier in a source room and the sound radiated into a receiving room on the opposite side of the barrier. 

• The higher a value, the better the construction element will perform in dampening sound.
• Transmission loss varies with different frequencies
• This measurement is typically derived in a testing laboratory. Test reports often give transmission loss over 6 octave bands
• Transmission Loss through a barrier generally increases with the frequency of sound increasing.

• Transmission is primarily retarded by the mass of the barrier.
• Stiffness of the barrier is also important. (less stiff the better)
  • Given two barriers of the same weight per unit area, the one that is less stiff will perform better.

Example (TL)

Having a 50dB value in one room, which is measured at 32dB in the adjacent room, through wall A. Wall A has a Transmission Loss value of (50 – 32) = 18 dB

Rules Of Thumb And Notes:

• A wall with .1% open area (from cracks, holes, undercut doors, etc) will have a maximum transmission loss of about 30 dB.
• A wall with 1% open area will have a maximum transmission loss of about 20 dB.
• A hairline crack will decrease a partition’s transmission loss by about 6dB.
• 1 in² opening in 100ft² of gypsum can transmit almost as much sound as if the entire partition did not exist.

Noise Reduction (NR) / Noise Reduction Coefficient (NRC)

Noise Reduction (NR): The difference in decibels, between the intensity levels in two rooms separated by a barrier of a given Transmission Loss.

• Noise reduction (NR) is dependent on the transmission loss of the barrier, the area of the barrier, and the absorption of the surfaces of the receiving room.

Noise Reduction Coefficient (NRC): The average Sound Absorption Coefficient (SAC) to the nearest .05

• Measured at (4) one-third octave band center frequencies of 250Hz, 500Hz, 1000Hz, 2000Hz.

Noise Reduction (NR) Calculation

NR = TL + 10log(A/S)

• NR = Noise Reduction
• TL = Transmission Loss
• A = Area of the Barrier (Metric Sabins)
• S = Absorption of the Surfaces in Receiving Room

Ways To Increase Noise Reduction:

(any combination of the three strategies)

1. Increasing the Transmission Loss of the barrier
2. Increasing the Absorption in the receiving room
3. Decreasing the Area of the barrier separating the two rooms

Noise Reduction Within A Space

Increasing sound absorption within a space will result in noise reduction according to this equation

NR = 10log (A₂ / A₁)

• A₁ = original total room absorption in sabins

A₂ = the total room absorption after the increase.