Acoustical Isolation vs. Acoustical Treatment – How to isolate the room? (Part 3)


Moving on from the previous article, let's now spend some time in exploring and understand the different methods available for you to isolate a room. Depending on how handy are you with tools and construction works, some of the methods below may be possible for you to do it yourself. However, you should check on any existing regulations that might require you to engage specific contractors in carrying out the works you have in mind (e.g. some office buildings have regulations for contractors to work in existing premises certified by the lease owner).

As mentioned in my previous article, every single surface of the room are as important as they might be the weakest link to leak sound in and out of the room. Thus, below are some practices that can help you improve on your room isolation index.

How much to isolate? 

When space within is acoustically sensitive, the walls take on additional importance. They must work as sound barriers to isolate the interior space from exterior noise, and to separate the exterior from the interior sound. Before deciding on the design and construction of the wall, there are two factors for us to consider, how loud is the surrounding ambient noise and how quiet do you want your room to be? To clarify the two consideration even further, the first refers to the level of exterior noise the room must reject under your specifications, and the later would refer to the lowest sound pressure level (SPL) your room operates in when all equipment are turned on. The difference in the two measurements would also give you a good idea on what would the sound transmission class (STC) rating be for the wall (part 1).

Airborne vs. Structureborne sound

Sound can propagate through any medium, for example, it can pass through air and solids. When it comes to room construction, the solid medium would be more of the interest to us as that involved structure-borne transmission. A simple analogy would be the low thumping sound you felt and heard while you are outside, distance away from a night club. The low frequencies are being radiated structurally (in this case, floor and walls); hence you can hear it outside even though the subwoofers are inside the night club. Airborne sound is higher in frequency, and the structure-borne noise is present only as a vibration that is felt. Therefore, any barriers (walls) must be designed to minimise both airborne and structure-borne transmission.

Airborne transmission is minimised by sealing any air leaks in a partition, if the barriers are not sealed properly, the acoustical performance of these walls will exponentially decrease. Structureborne transmission can be reduced by utilising decoupling elements, breaking the transmission path. Also, structure-borne sound can also be reduced by eliminating any resonant conditions in the transmission frequency range.

Example of a wall structure, credits Johnsayers

The walls

A high transmission loss characterises a wall that is useful as a noise barrier. Simply put, the sound energy (SPL) is significantly decreased by passing the wall. Although transmission loss varies with frequencies, the STC rating can give you a practical understanding of how efficient the wall is acting as a sound barrier. In general, for every doubling of mass (density) material used, the wall would be able to reduce about 5dB of sound.

Variations of wall constructions, credits Homeminimalis

To further improve on the transmission loss (TL), an interior airspace can be introduced between the wall materials. The insulating effectiveness of this airspace will improve when the gap distance increases. However, such procedures may not be applicable enough to help achieve a high STC rating as a 0.1m airspace sandwiched between a 0.02m and 0.01m drywalls (total of 0.13m thickness) can only provide an STC-38 rating approximately. Of course, by increasing either the density of walls or airspace gap can improve transmission loss, but there should be a practical limit to this as it will eat up your precious space in the room (especially in Singapore). Therefore, it is also common for such barriers to be stuffed with absorbent materials such as glass fibre to help improve transmission loss while not compromising too much on the thickness. To illustrate, a wall with an interior airspace of 0.06m (stuffed with glass fibre) sandwiched between a 0.01m drywall on each side (total of 0.08m thickness) can provide an STC-45 rating. By now, you probably get the picture that although mass plays a huge role in absorbing sound but to make things more efficient, a wall that employs different insulating medium can yield a better result. As for the structure-borne transmission, the walls can be decoupled by building a staggered-stud wall to break the transmission path.

Walls with insulating materials, credits Dxcasters

Building a room inside a room

Side note, it is still possible for you to improve on the isolation index of an existing room. Existing walls are rarely built for high transmission loss. One way to improve an acoustically weak existing wall is to construct another partition (Gypsum board is a good choice) on either side of it. The design of the walls can be adopted from the paragraphs above. In cases where an outlet (e.g. patch bay or electrical outlet) is needed to be fabricated into the wall, it is advised that the outlets do not face directly opposite each other from both sides of the walls to help prevent any sound leaks and flanking. The acoustic sealant should be used to help improve on transmission loss.

Construction of an outlet box

Floor and ceiling

Floor and ceiling construction deals with many of the same issues as wall construction. The floor and ceiling must provide sufficient isolation between the rooms above and below. Clearly, noise intrusion can move in either direction, and a good floor and ceiling design can help isolate the room from its environment. As noted, floor and ceiling are particularly prone to impact noise such as the dropping sound of glass marbles (non-paranormal of course) or the moving of furniture. Due to the nature of these noises, an effective way to counter such problems would be to decouple these surfaces from the source. A floating floor or a suspended ceiling would have excellent results in these cases as they can break the transmission path. Additionally, impact noise can radiate outward through structural elements (e.g. adjacent rooms). Thus, another effective way to treat these noises would be to treat them at the source. A simple implementation such as the use of soft carpet can help reduce the structure-borne transmission. Additional rubber tiles can be added to improve transmission loss further.

An example of floating floor., credits InstaCoustic

If you are engaging someone to fabricate these surfaces for you, it will be good if you can read up on Impact Insulation Class (IIC). IIC is a single number rating of the impact sound performance of the floor and ceiling constructions over a standard frequency range. The IIC is compared to the STC standard as mentioned previously.

In the next post, we will take a look at how to improve the isolation index for the various entry points in a room.