Test and Measurement 1619(low frequencies) the direct field may include the
effects of boundaries near the loudspeaker and micro-
phone. As frequency increases, the sound from the loud-
speaker becomes less affected by boundary effects (due
in part to increased directivity) and can be measured
independently of them. Proper loudspeaker placement
produces a time gap between the sound energy arrivals
from the loudspeaker and the later arriving room
response. We can use this time gap to aid in selecting a
time window to separate the loudspeaker response from
the room response and diagnosing system problems.46.3.5.5 Acoustic WavelengthsSound travels in waves. The sound waves that we are
interested in characterizing have a physical size. There
will be a minimum time span required to observe the
spectral response of a waveform. The minimum
required length of time to view an acoustical event is
determined by the longest wavelength (lowest fre-
quency) present in the event. At the upper limits of
human hearing, the wavelengths are only a few millime-
ters in length, but as frequency decreases the waves
become increasingly larger. At the lowest frequencies
that humans hear, the wavelengths are many meters
long, and can actually be larger than the listening (or
measurement) space. This makes it difficult to measure
low frequencies from a loudspeaker independently of
the listening space, since low frequencies radiated from
a loudspeaker interact (couple) with the surfaces around
them. In an ideally positioned loudspeaker, the first
energy arrival from the loudspeaker at mid- and high
frequencies has already dissipated prior to the arrival of
reflections and can therefore often be measured inde-
pendently of them. The human hearing system tends to
fuse the direct sound from the loudspeaker with the
early reflections from nearby surfaces with regard to
level (loudness) and frequency (tone). It is usually use-
ful to consider them as separate events, especially since
the time offset between the direct sound and first reflec-
tions will be unique for each listening position. This
precludes any type of frequency domain correction (i.e.,
equalization) of the room/loudspeaker response other
than at frequencies where coupling occurs due to close
proximity to nearby surfaces. While it is possible to
compensate to some extent for room reflections at a
point in space (acoustic echo cancellers used for confer-
ence systems), this correction cannot be extended to
include an area. This inability to compensate for the
reflected energy at mid/high frequencies suggests that
their effects be removed from the loudspeaker’s direct
field response prior to meaningful equalization work by
use of an appropriate time window.46.3.5.6 Microphone PlacementA microphone is needed to acquire the sound radiated
into the space from the loudspeaker at a discrete posi-
tion. Proper microphone placement is determined by the
type of test being performed. If one were interested in
measuring the decay time of the room, it is usually best
to place the microphone well beyond critical distance.
This allows the build-up of the reverberant field to be
observed as well as providing good resolution of the
decaying tail. Critical distance is the distance from the
loudspeaker at which the direct field level and reverber-
ant field level are equal. It is described further in Section
46.3.5.7. If it’s the loudspeaker’s response that needs to
be measured, then a microphone placement inside of
critical distance will provide better data on some types
of analyzers, since the direct sound field is stronger rela-
tive to the later energy returning from the room. If the
microphone is placed too close to the loudspeaker, the
measured sound levels will be accurate for that position,
but may not accurately extrapolate to greater distances
with the inverse-square law. As the sound travels far-
ther, the response at a remote listening position may
bear little resemblance to the response at the near field
microphone position. For this reason, it is usually desir-
able to place the microphone in the far free field of the
loudspeaker—not too close and not too far away. The
approximate extent of the near field can be determined
by considering that the path length difference from the
measurement position (assumed axial) and the edge of
the sound radiator should be less than^1 / 4 wavelength at
the frequency of interest. This condition is easily met for
a small loudspeaker that is radiating low frequencies.
Such devices closely approximate an ideal point source.
As the frequency increases the condition becomes more
difficult to satisfy, especially if the size of the radiator
also increases. Large radiators (or groups of radiators)
emitting high frequencies can extend the near field to
very long distances. Line arrays make use of this princi-
ple to overcome the inverse-square law. In practice,
small bookshelf loudspeakers can be accurately mea-
sured at a few meters. About 10 m is a common mea-
surement distance for moderate-sized, full-range
loudspeakers in a large space. Even greater distances are
required for large devices radiating high frequencies. A
general guideline is to not put the mic closer than three
times the loudspeaker’s longest dimension.