Acoustical Isolation vs Acoustical Treatment – How to deal with entry points? (Part 4)

December 30, 2016 by Fu Yongwei in audio

Isolating the entry points of a room can be one of the trickiest parts when constructing an acoustically sensitive room. For this article, the following features door, window, ducking and outlet bays will be referred as entry points. As these entry points require some level of manoeuvrability (e.g. opening of doors), the design of these features demand it to be acoustically sealed when it's put in place. If the features are not done properly, they will be the weakest link of the room acoustically as air provides almost zero transmission loss (TL) (Part 3).

The importance of a door

Essentially, a room cannot function without a door. While most of the features mentioned above are designed to be permanently sealed, doors are designed to be open. This presents a challenge where the doors need to maintain at least the same level of transmission loss to the wall attached to it. Thus, a door design, in this case, would require sufficient mass (part 3) and also a closure system that reliably seals the entire perimeter around the door. Some doors are specially engineered for this very purpose, and it will provide excellent results in this application. However, if you are on a budget, here are some pointers for you to look out for some alternatives with the appropriate mindset.

In general, a hollow door can only provide an STC rating of around 20s and of course a door with a solid inner core (more mass) will have a higher rating (around STC-25, higher but still ineffective). In contrast, metal door performs better than wood doors, but that poses another challenge for reflection (covered in later parts of the blog series). Similar combinations between interior airspace stuffed with absorbent material and partition board can be employed for the fabrication of the door (part 3). As for isolation when the door is closed, we need to ensure that the door is gasketed and airtight. To seal the gap between the door and frame, several materials have been used to provide pleasing results, namely, rubber strips or neoprene foams. In some cases, magnets have been used to DIY between the frame and door to help improve the compression of the door to the insulating materials.

Neoprene tubing, possible material to stick between frames, credits 1001blocks

Windows are important too!

If you have the luxury to have separate rooms for your recording needs, then a window might be essential. Although in today’s technology standard, it might be cheaper to install a set of camera and screen in both rooms as it can provide higher insulation when done properly, it does not compensate for the human interaction component for artistes to help them calm their nerves down. A window can contribute to making space feels and looks less claustrophobic; it also provides the visual stimulant for the artiste and engineer to communicate. In some cases where an artiste does not wish to be looked when recording, curtains can be hooked up to provide the privacy he or she needed.

Despite that, most of the suggestion I’ve mentioned so far require construction works, and I will not be surprised that some of you have superb craftsmanship to do it yourself. However, building an acoustically sound window is challenging (in my opinion) but here are a few recommendations to help you look out for or build (seriously? If you do please share it with me!) a window. Firstly, plastic can be used in the construction of a window, although it does not provide as much transmission loss rating as glass does. The principle difference is that plastic would require double the thickness to provide the same amount of transmission loss on glass. Hence, the glass may be a better material to start on. Secondly, which is better single-pane or double-pane window?

Single-pane window vs. Double-pane window

The single-pane window is commonly used in household and does not provide good sound insulation. Thus, you might have observed that if you are living near a bus interchange, your flat will probably come with a double-pane window to help insulation the outside noises. The ideology follows similarly to the construction of a wall (part 3), a logical thought process would conclude that more panes (higher density) will offer greater loss. Beside mass law, it is shown that a double-pane window separated by an air gap can provide approximately 30dB of transmission loss. As the window provides a visual cue to both parties on either side of the room, it will not be advisable for you to stuff absorbent materials such as fibre glass to help improve the insulation. Hence, it would be even more important for you to find a good ratio between the partitions thickness (e.g. glass) and the interior airspace. For a simple guideline, a double-pane window with an interior airspace of 0.1m sandwiched between a 0.01m and 0.006m glass panels (total thickness of 0.116m) can provide an STC rating of 42.

Cross-section of a double-pane window, credits Glassdoctor

What about introducing a third pane?

If you’re wondering whether does introducing the third pane further improve the STC rating, the short answer is yes but not significant. Through extensive research in 1983 from Quirt, J. D. measuring the difference made between windows (double glazing and triple glazing), the result in the differences are subtle. Although some might argue that a triple-pane window does still increase the insulation, I feel this is a diminished effect, and you can spend it better in other areas. Otherwise, do indulge yourself with it if you can afford.

In the next article, we will look further into treating other entry points such as heating, ventilating and cooling system.

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

December 16, 2016 by Fu Yongwei in audio

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.

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.

Acoustical Isolation vs Acoustical Treatment - How to evaluate a room? (Part 2)

December 09, 2016 by Fu Yongwei in audio

We have spent the time in the previous post understanding the distinction between sound isolation and treatment. Now, let's consider some of the common approaches that are implemented to help improve the acoustic isolation of the room.

Before we start to attempt any form of reconstruction, it will be wise for us to do a survey of the room. Depending on your budget, an extensive review can very much help you in spotting potential problems of the room and also improve the effectiveness on your proposed implementations. However, staying on a tight budget does not necessarily obstruct you in conducting any evaluation with a reasonable cost yourself.

Evaluating a room - noise survey

A simple noise survey will be beneficial for you to get an impression on how does your space interacts with its environment throughout the day. Such tests can be conducted by simply staying in the venue for the whole of your "proposed hours" (the time and duration you probably on working) and observed how noisy/quiet the space is at different time. To illustrate, if you're hoping to convert a warehouse space into something that is recording friendly, you need to ensure that you have taken note of all the noises that are evident during your time of stay (e.g. low rumbles of lorries passing by or noises from heavy machinery). In a densely populated country like Singapore, it will be almost impossible for you to find a suitable place with no intrusive noises surrounding it. Even if you can find a place that is secluded enough from "civilisation", you would also need to take into account the inconvenience caused by the effort you and your client needed to put into travel. However, this paragraph is not meant to discourage you in searching for the "perfect" space or rather it is intended to help point out what are things you can do and look out for with little to no money at all.

The shape of the room - room modes

Next up the list would be the geometry of the room. A room can be considered as a resonating chamber. In reality, sound will travel forward and be reflected between walls, hence resulting in different standing waves. The first mode of this standing wave, at its fundamental frequency, will have the highest level of energy (amplitude), causing the room to emphasise on this particular frequency (bias reproduction). Furthermore, multiples of the fundamental frequency, e.g. second or third mode, will also have some level of emphasis in the room, though it will have as much energy as compared to the fundamental frequency. Hence, different geometry of the rooms will result in various room modes, as these variables are proportional to one another. To get you going, I'll be introducing three modes, namely Axial, Tangential and Oblique, which are commonly accounted for when surveying a space.

Axial Modes

Axial modes exist in each of a rectangular room's three-dimensional space. All six surfaces of the room play a role, but each axial mode reflects between opposite and parallel walls. There is one set of axial modes created by reflection from the near and far end walls of the room. The second set of axial modes exists between the left and right side walls of the room. The third set of axial modes exists between the floor and ceiling. Depending on the distance between these walls, each set of axial modes will produce a fundamental frequency based on twice the distance between the wall pairs, as well as the multiples of the fundamental frequency.

Tangential and oblique modes

Tangential modes occur for a wave that strikes four walls and comes back to its starting place. As with axial modes, tangential modes exist in each of the room three-dimensional space and appear as a series of fundamental multiple frequencies. Oblique modes strike all six surfaces of a room each round trip. As with axial modes, oblique modes use all three of the room's dimensions and have a series of fundamental frequencies.

Axial, tangential and oblique modes have different energy levels. Axial modes frequencies will have the highest level of amplitude as they incur fewer boundary reflections. Hence, axial modes are also more potent, and an appraisal of axial modes alone might give a good estimate of the performance of space under considerations.

It is common that any given space would have some problems outlined during the survey and evaluation phase. However, this does not necessarily deem that room is not suitable for any acoustic works as there is rarely any "perfect" room in the market. In the next article, we will take a look at some of the options to help eliminate these problems.