Reactive sound fields

Since sound intensity meters can differentiate between the active and reactive sound fields, measurements of intensity taken close to noise somces can faithftiUy indicate the radiated sotmd energy. However, using a conventional sound level meter near to a noise source may indicate higher sotmd power levels than occur in the far field because these instruments cannot differentiate between active and reactive fields. 

Just as it is effective in the other fields of inorganic descriptive chemistry, the Periodic Table is an essential reference point in intermetallic chemistry too. The general alloying characteristics of the different metals, their reactivity towards the other metals, the variety of their intermetallic derivatives usually are very complex and cannot be easily explained and rationalized on the basis of a few concepts and data. Nevertheless a sound first criterion for a description and classification of the intermetallic behaviour of the various metals lies in their position in the Periodic Table. 

In a free acoustic field such as that in an open air or anechoic chamber, the pressure and intensity levels in the direction of propagation are numerically the same. In a diffuse field in which sound is reflected so many times that it travels in all directions with equal magnitude and probability (reverberation chamber), the pressure and intensity levels are different and this difference is known as the pressure-intensity index (phase index or reactivity index). 

Tlie relationship between sound pressure level and sound intensity level. When the sound intensity level is measured in a free field in air, then the sound presstme level and sotmd intensity level in the direction of propagation are numerically the same. In practice most measurements of the sound intensity are not carried out in a free field, in which case there will be a difference between the sotmd pressure and intensity levels. This difference is an important quantity and is known by several terms, such as reactivity index, pressure-intensity index, P-7 index, phase index, or LK value. This index is used as a field indicator to assess the integrity of a measurement in terms of grades of accuracy or confidence limits. This will be considered in more detail later in this section. 

In our experience, on the final approach to the steady state in an Eulerian type of time-dependent reactive flow calculation (see Section 4), when the properties at a point in the flow field vary slowly with time and fresh calculations of the transport coefficients are only required at infrequent intervals, use of the detailed transport model presents little problem. A sound basis then exists for studies of the effect of variations in basic transport parameters on the steady-state flow. Unsteady flows, on the other hand, are most economically treated by less demanding, though more approximate, methods. Some approaches along these lines are discussed below and in Section 3.4. 

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