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.

Acoustical Isolation vs Acoustical Treatment - What is soundproofing? (Part 1)

December 04, 2016 by Fu Yongwei in audio

In the month of December, let's explore some fundamental differences between acoustical isolation and treatments. I'll start off by answering one of the most commonly asked questions, "does sticking acoustic foams on the wall soundproof your room"? The short answer would be no, and we will take the chance to understand why in this blog series.

Acoustic foams (egg crates) on the wall, Credits Aksa Akustik

What is acoustical isolation?

Acoustical Isolation, commonly referred as “sound proofing” by many, is the work needed to help the acoustical space to isolation itself from its environment, usually with the use of insulating barriers. Simply put, acoustical isolation adjusts the effectiveness of keeping any sounds in and out of the room. When evaluating a place, even if the room has excellent acoustical characteristics (e.g. even low-frequency radiation), if external sounds can intrude the space easily, the room will not worth much. Imagine that you're recording a soft passage in your room, but it is always being interrupted with the barking of a neighbour’s dog (a typical scenario in the heartland of Singapore). The dog now governs the whole working process as you must work around it, possibly resulting in a less efficient process.

Difference between insulation vs. isolation

The distinction between insulation and isolation should also be clarified. A given wall construction, for example, will offer a particular transmission loss to sound travelling through it. This absorptive sound transmission will be referred as insulation. On the other hand, isolation is a general term applying to structures. For instance, the difference between the sound pressure level (SPL) taken inside and outside a studio would be a measure of the isolation of the studio itself. Generally speaking, these acoustically sensitive spaces would desire a high isolation index and we would look into that at the later part of the blog.

In the topic of creating a long-term effective and efficient space for any acoustical work, it is frequently being agreed upon that room with a high rejection of intruding / escaping sounds will be preferred. As sound isolation works require some level of structural modifications to the room, it will be impossible (at least for most of us) to execute these structural changes once your system is up. So, if you have the luxury to work on a room from scratch, sound isolation would be your priority.

Good separation across the full audio spectrum would demand heavy walls, decoupled noise sources and other architectural features. Sadly speaking, there isn’t any "one fits all" solution when it comes to acoustic and most calculations would have some level of tolerances between theory and application. Several factors cause these and just to name a few, material differences, craftsmanship and air humidity can affect the accuracy of calculations.

Sound Transmission Class (STC)

The Sound Transmission Class (STC) Standard rates the sound-blocking ability of a material or structure. The STC is a single-number rating of airborne transmission loss measured at sixteen 1/3 octave band (16 divisions) with centre frequencies ranging from 125Hz to 4000Hz. In summary, the higher the STC value, the greater the sound insulation provided. STC is a reasonable simplified (convenient) measurement standard. Although there are differences between iterations (mainly pre 1999) and it account of transmission loss and not frequency, the STC rating is adequate for one to evaluate and make an informed decision on the sound isolation ability in the room. In short, when comparing a wall rated at STC-30 to STC-42, it is quickly evaluated that the STC-42 wall offers 12 dB more transmission loss than the other.

It is without a doubt that every single wall surfaces (e.g. ceiling) can affect the isolation index of the room and how "soundproof" your room will be determined by the weakest link, commonly a "sound leak". To achieve a low noise level in a room, every aspect of the design and construction must be scrutinised. Common leak sources range widely from an unsealed door to window. Simply put, as long as sound can be displaced easily by air, this will exponentially decrease the overall isolation quality even though the rest of the surfaces are treated properly. Hence, it is important to ensure that attention is put on every single surface of the room (don't put all your eggs in one basket) to make sure that the room is properly sealed.

In the next article, we will look at how to survey and evaluate a prospective room.