Acoustic Properties of Rectangular Rooms

Contents

Introduction 
Room Modes 
          The Issues Created by Room Modes and Common Solutions 
          The Schroeder Frequency 
          Axial, Tangential and Oblique Modes 
          Tackling Room Modes for the End User 
Room Size and Ratio 
          The Issues Created by Room Ratios and Common Solutions
          Old and New Methods for Determining Optimum Ratios
          Large vs. Small Rectangular Rooms 
Building Materials 
          How A Sound Wave Will Interact with a Material
          Room Resonances with Regards to Building Materials
          Reflectors, Absorbers and Diffusers 
          Absorption Coefficient 
          Porous vs. Nonporous Absorbers 
Conclusion 
References 

Introduction

In the modern day, significant importance has been placed on the acoustic quality of critical listening spaces. Offsetting acoustic abnormalities is useful for professional critical listening spaces so that they may fulfil their purpose of allowing a person to listen to a source appropriately. These abnormalities affect a room’s acoustic quality in the context of modes and can include, but are not limited to size, ratios, and building materials. Discussing and suggesting ways to reduce the effects of these variables is the purpose of this report. Relevant information on these sub-topics is explored to inform and educate the reader on this subject so that they may treat their space accordingly.

Room Modes

The Issues Created by Room Modes and Common Solutions
Room modes refer to an irregular perceived frequency response of a room due to specific frequencies reflecting on themselves between parallel walls, this can also be called “colouration.” Colouration can be particularly apparent with lower frequencies as room modes are less densely packed at this range. This effect can be alleviated by altering the position of the source and listener in the room, but this on its own will not completely negate the issue. [1] M. Louden outlines this phenomenon, where he notes that a particularly bad case of an irregular frequency response is when eigentones group at lower ranges which can leave large gaps in other areas of the frequency spectrum. This results in a room which has an ununiform acoustic response. [2] This becomes an issue because, for the listener, the acoustic quality of the room is altered, as the frequency content of small critical listening spaces can be over or underrepresented in specific ranges. [3] In the context of a recording studio, this creates weak frequencies which, if they represent specific musical notes, can be under-represented and therefore can be mistreated in a mix by the engineer. [4] (1) Variable frequency responses can be addressed by controlling sound reflections with passive acoustic treatment such as bass traps, membrane absorbers and Helmholtz resonators. [3]
The Schroeder Frequency
The Schroeder frequency refers to the transitional region of the frequency response of a room that distinguishes the lower frequencies which consist of separate modes from the higher frequencies which consist of densely overlapping modes. [5] This phenomenon was investigated by Manfred Schroeder and in 1962 he came to an equation that estimated this region of overlap. [6] Estimating this frequency is useful in situations where an acceptable degree of homogeneity in the sound field is required for study or design, such as the design of a reverberation room. It is important for the Schroeder frequency to be considered in rooms with smaller volumes in particular, as larger spaces do not share the same diffusion properties as smaller spaces. [7]
Axial, Tangential and Oblique Modes
Axial, tangential, and oblique modes can be described as modes that exist in different planes. Being mindful of these different modes can help recognise the redistribution of energy these modes can create. [8] These distinct types of modes are also known as natural frequencies, standing waves and normal modes, respectively. [4] (1)