Index Plots - Martin Audio MLA MINI Advanced User's Manual

MLA MINI
DISPLAY 2.2
manually adjust several thousand DSP parameters is obviously not going to be realistic however
good a system technician believes his ears might be.

INDEX PLOTS

The other concept which Martin Audio developed as part of the MLA system is the index plot.
We needed a method to display the results of the system optimisation that was clear and easy to
interpret. Because the whole basis of the system is that the radiated signal is coherent where it
arrives at all parts of the audience as opposed to when it leaves the array we needed a method
of displaying the frequency response at dozens of positions simultaneously. There are a few
methods in existence for achieving this, 3D graphs for example but they don't really meet the
brief of being quick and easy to interpret. The Index plot solves this problem.
As we will see when we look at the application in more detail, we draw a two dimensional slice
of the venue in which we are deploying our array. It then gets presented in Display like so:
The colours represent the three types of plane; the audience area are green, non-audience, areas
around the venue which we are not concerned about are red, and finally areas where we would
like the system to actively steer sound away from known as hard avoid are blue. By default the
hard avoid is the stage but this could also be reflective back walls or balcony fronts, or noise
sensitive areas beyond an arena at an outdoor event. The planes are represented by a series of
dots. These are spaced closer together in the audience region where we need the most detail.
Ideally we would like to be able to view the frequency response at each of these dots so each
becomes a sort of virtual measurement mic.
The normal way to present a graph of frequency response is of course for the horizontal 'x' axis
to be frequency, usually from 20Hz to 20KHz, and for the vertical 'y' axis to display magnitude,
usually in dB. If however we display magnitude with colours we can easily show many response
plots simultaneously. If we make red the highest magnitude- the loudest- and blue the quietest,
by running through the colours in the spectrum we can very easily identify the peaks and troughs
in a response. This frees up the vertical axis which we can use for all of our "virtual mics":
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