Sound Transmission

Sound Transmission Class (STC) is the rating of airborne sound transmission and describes the degree of sound isolation provided by a home construction and materials. The Sound Transmission Class of a wall assembly is measured in a lab where the wall composition is placed between two rooms (source and receiving). A measurement is taken to determine the sound loss between the two rooms.

The STC measures the sound-isolating capability of a wall. This sound reduction is measured in decibels: a higher class rating means that more sound is being stopped, thus indicating the quality and performance of the barrier. The minimum recommended STC for different residential applications, required by the National Building Code of Canada, is shown in the table below.

For instance, listening through a wall during a heated discussion:

• With an STC of 45, you would hear the conversation intelligibly.
• With an STC of 50, you would still hear voices but the conversation would be unintelligible.
• With an STC of 55, you would no longer hear the conversation.

STC is used in rating exterior and interior walls, floor/ceiling assemblies, doors and windows, and other room partitions.

Exterior noise through doors or windows

Most exterior noise enters the home through doors or windows. Two factors cause sound to enter the home: thickness of the material and air gaps. The first factor is obvious: the thicker and heavier the window glass or the more panes used, the better the window is in terms of sound reduction. The denser the door, and similar results will follow. Less obvious are the air gaps found in many systems. The more air that flows through gaps in the window or door frame, the more sound will be transmitted. An improperly sealed door will decrease STC by 3 to 8 dB, resulting in allowing over twice the amount of noise through.

There exist window and door systems with actual acoustical ratings, which help reduce noise. They are, however, quite expensive. Another option is to install sound rated interior windows, either in glass or plastic, that are placed on the inside frame. Installing storm windows over the outside of the frame is another option to decrease sound transmission, as long as the windows are sufficiently thick and gaps are properly sealed. Without having to make major changes to the window system, simply filling gaps around the window with caulking will also help abate your noise problem.

Interior Walls

The principles of insulation and mechanical decoupling play heavily in decreasing sound transmission through interior walls. For mid to high frequency noise, glass fiber batt or mineral fiber batt insulation should be used to fill the wall cavity between gypsum.

One effective approach for new wall construction is to alternate wall studs, so the 2x4 wall studs don't connect with the finished drywall on either side of the wall. The following image illustrates this.

Adding an extra layer of drywall is also beneficial in reducing noise transmission. Be sure to stagger the layers of drywall, so that the seams in between boards do not line up.

Adding airspace behind the finished wall increases the Sound Transmission Coefficient rating. For each doubling of air space, the STC increases by approximately 3. Therefore, an airspace of 1 1/2" will improve the STC by approximately 3 dB. An air space of 3" will improve the STC by approximately 6 dB, and so on. Resilient channels can be used to accomplish this.

Soundproofing panels have become the de facto approach in increasing the STC between walls for home theatres, offices and bedrooms. More and more new home constructions are going above and beyond meeting minimum sound requirements and are including Enermax panels between each wall partition. Renovations are also commonplace and use this approach, since it is a low-cost method of soundproofing.

Exterior Walls

By sealing the concrete block surface in new home constructions, an STC rating of 45 to 55 is achieved. Adding sand or grout to the core of the concrete blocks further improves the soundproofing properties of the material, making the blocks perform like solid concrete. For pre-existing homes, improvements to exterior walls can be very costly. Most homeowners choose to add soundproofing panels behind the interior wall leading to the outside. When using Enermax to bolster noise reduction, a furring strip is recommended between the Enermax and the finished wall, as this air space helps to improve the STC rating.

Floor/ceiling impact noise

Impact noise is measured by the Impact Insulation Class (IIC). This refers to the insulating properties of the floor-ceiling assembly with respect to sounds such as footsteps. Commonly constructed floors using three layers of fiberglass batt insulation yield an STC rating of 51 and IIC rating of 49. Put simply, this is the minimum rating you should strive for.

An economical and effective method for reducing both airborne sounds as well impact noise is by adding a thin wood fiber panel such as Enermax. Since it is only 12mm in thickness, it can be applied underneath the floorboards. This has the effect of increasing STC to 61 and IIC to 57, thus keeping the home quiet from undesirable noises such as feet stomping or furniture dragging on the floors above.