Audio Engineering

112 Chapter 3

source continues to put energy into the room at a reasonably regular rate. This gives rise
to a number of basic sound fi elds ( Figure 3.17 ):

1. The near fi eld.


2. The far free fi eld.


3. The far reverberant fi eld.

The near fi eld does not behave predictably in terms of LP versus distance because
the particle velocity is not necessarily in the direction of travel of the wave, and
an appreciable tangential velocity component may exist at any point. This is why
measurements are usually not made closer than twice the largest dimension of the
sound source. In the far free fi eld, the inverse-square-law level change prevails. In
the far reverberant fi eld, or diffuse fi eld, the sound-energy density is very nearly
uniform. Measuring low-frequency loudspeakers is an exception to the rule, and such
measurements are often made in the pressure response zone of the device.

3.14 Conclusion ............................................................................................................

The study of acoustics for sound system engineers divides into outdoors and indoors with
indoor acoustics again divided into large room acoustics and small room acoustics. Classical
Sabinian acoustics are rapidly being refi ned where applicable, discarded where misapplied,
and reexamined where the “ fi ne structure of reverberation ” is the meaningful parameter.
The digital computer has fueled basic research into the mathematics of enclosed spaces, and
modern analyzers have served to verify or deny the validity of the theories put forward.

logr

SPL

Near

field

Far

field

Free

field

Reverberant

field

Figure 3.17 : Graphic representation of near fi eld, free fi eld, and reverberant fi eld.