ABSTRACT
This opening chapter looks at aspects of sound fields that are particularly relevantto sound insulation; the reader will also find that it has general applications to roomacoustics. The audible frequency range for human hearing is typically 20 to 20 000Hz, but we generally consider the building acoustics frequency range to be defined by one-third-octave-bands from 50 to 5000Hz. Airborne sound insulation tends to be lowest in the low-frequency range and highest in the high-frequency range. Hence significant transmission of airborne sound above 5000Hz is not usually an issue. However, low-frequency airborne sound insulation is of particular importance because domestic audio equipment is often capable of generating high levels below 100Hz. In addition, there are issues with low-frequency impact sound insulation from footsteps and other impacts on floors. Low frequencies are also relevant to façade sound insulation because road traffic is often the dominant external noise source in the urban environment. Despite the importance of sound insulation in the low-frequency range it is harder to achieve the desired measurement repeatability and reproducibility. In addition, the statistical assumptions used in some measurements and prediction models are no longer valid. There are some situations such as in recording studios or industrial buildings where it is necessary to consider frequencies below 50Hz and/or above 5000Hz. In most cases it should be clear from the text what will need to be considered at frequencies outside the building acoustics frequency range.
Sound fields in rooms are of primary importance in the study of sound insulation. This section starts with the basic principles needed to discuss the more detailed aspects of sound fields that are relevant to measurement and prediction. In the laboratory there is some degree of control over the sound field in rooms due to the validation procedures that are used to commission them. Hence for at least part of the building acoustics frequency range, the sound field in laboratories can often be considered as a diffuse sound field; a very useful idealized model. Outside of the laboratory there are a wide variety of rooms with different sound fields. These can usually be interpreted with reference to two idealized models: the modal sound field and the diffuse sound field.
