1622 Chapter 46
number of these time windows in succession, each sepa-
rated by a time offset, can form a 3D plot known as a
waterfall.
46.3.5.9 Data Windows
There are some conditions that must be observed when
placing cursors to define the time window. Ideally, we
would like to place the cursor at a point on the time
record where the energy is zero. A cursor placement
that cuts off an energy arrival will produce a sharp rise
or fall time that produces artifacts in the resultant calcu-
lated spectral response. Discontinuities in the time
domain have broad spectral content in the frequency
domain. A good example is a scratch on a phonograph
record. The discontinuity formed by the scratch mani-
fests itself as a broadband click during playback. If an
otherwise smooth wheel has a discontinuity at one
point, it would thump annoyingly when it was rolled on
a smooth surface. Our measurement systems treat the
data within the selected window as a continuously
repeating event. The end of the event must line up with
the beginning or a discontinuity occurs resulting in the
generation of high-frequency artifacts called spectral
leakage. In the same manner that a physical discontinu-
ity in a phonograph record or wheel can be corrected by
polishing, a discontinuity in a sampled time measure-Figure 46-19. A room response showing the various sound fields that can exist in an enclosed space, SIA-SMAART.
Figure 46-20. A time window can be used to isolate the loudspeaker’s response from the room reflections.