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

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.

Standing Waves, Credits Sengpielaudio

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.

Different Axial Modes, Credit Sengpielaudio

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.

Room modes, Credits cs4fn

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.