Wrapped Phase; Environmental Reflections - PROEL AXIOM - VERSION 2.0 Manual

B. Notes on system measurement
spectrum of the output as the response of the system. With this method, we are given no information
about phase.
In order to acquire data about phase response, it is necessary to use an instrument of measure more
complex that utilizes test signals known to the instrument (the pink noise to which we referred previously
is known only for its frequency content, but not for its temporal content). There are systems that use
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MLS , or swept sinusoidal signals, and others that utilize dual channel FFT, i.e. the comparison of two
signals applied at the input and output of the system
This table briefly summarizes the characteristics of some systems of measurement:
The most useful information, at least regarding a live sound installation, can be obtained from mea-
surement equipment which uses the cross-correlation method. Most importantly, as evidenced above,
with an RTA it is not possible to acquire phase information and therefore not possible to have information
about delays introduced on the various pass bands of the system.
B.3.1 Wrapped Phase
In systems of measurement, the phase is represented on a logarithmic scale; thus it will not appear as
a line, but as a curve with ever-increasing inclination in one direction.
Furthermore, these instruments display phase on a graph with and axis scaled from -180
and not absolute phase. As the instruments displays every variation in phase equal to 360
the sinusoid to the same value, it displays a peculiar "wrap-around" at the point where this occurs. This
can essentially be considered as a display artifact and has no bearing on the analysis of the system, as
when the phase graph makes a drastic leap from one extreme of the axis to the other it is not because
of some discontinuity of the system, but because the graph simply picks up from one end of the axis
where it left off on the other.
B.4 Environmental Reflections
In the previous paragraphs we have not yet introduced a factor that consistently affects real-world mea-
surements. The instrumentation used for the measurements listed above is calibrated with the as-
sumption that the assumed propagation of the system under test is in an anechoic chamber or in an
environment with absolutely no environmental reflection. In a real world measurement situation, with
the microphone on a stand 1.2 to 1.8 meters high, it is necessary to consider the nearest reflections
and, inevitably, the reflection from the ground. The measurement with the microphone on a stand is
sufficiently accurate for the higher end of the frequency spectrum, but not at all reliable for the Mid to
Low range.
When taking measurements for the time alignment of the SUB pass band, it is
essential to position the microphone in a ground-plane configuration, as close to the
This method avoids ground reflections and provides a reliable measurement in the Mid to Low range.
Contrarily, this method will not yield reliable results for the High end of the spectrum.
In summary: There is no single correct position for the measurement microphone that will give reliable
results for the entire audio spectrum. It will always be necessary to reposition the microphone according
analyzer, the reading on the Y (f ) instrument exactly corresponds to the sought H(f ) module, as the stimulus contains a constant
quantity of energy per each octave.
3
For instance, MLSSA and CLIO Systems. For further information check www.mlssa.com and www.cliowin.com.
4
For instance, SmaartLive, CLIO and SIM. Further details are available at www.siasoft.com, www.audiomatica.com and
www.meyersound.com/products/sim/sim3/.
Type Module
RTA
MLS/Chirp
Dual Chann. FFT
surface of the ground as possible.
B.4. ENVIRONMENTAL REFLECTIONS
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.
Phase Musical signal
Yes No
Yes Yes
Yes Yes
97
No
No
Yes
◦
as a return of
◦ ◦
to +180 ,