Martin Audio MLA User Manual page 175

MLA System
USER GUIDE
This shows a system that has been set up with the reference point in the centre of the audience region and a start and finish
delta of + and – 2dB respectively. Because the reference point is central, the SPL drops evenly from start to finish, down 4dB
from the loudest point at the start of the coverage.
The next diagram shows a system where the front of house position is around three-quarters of the way back from the front but
the start and stop deltas are still at +/-2dB;-
As you can see, the level drops gradually over three quarters of the venue back to front of house and then drops rapidly back to
the audience finish. Decidedly strange and in most (but not necessarily all) circumstance, unwanted. To get the same smooth
4dB drop you should enter the start delta as +3dB and the finish at -1dB. But that only works if front of house is exactly three
quarters of the way back! In the majority of cases you are far better of leaving the reference at the halfway point to make it
easier to easily calculate how the loudness contour will flow from start to finish.
Of course they are many applications when it may be desirable to have the spl rapidly drop off behind the reference point, at
outdoor events where noise pollution is an issue, the ability to move the reference point to dictate the contour in front and
behind it individually is another very useful tool at the system technicians disposal to create the ideal coverage for difficult
environmental conditions.
The default setting is +/-3dB with the reference point in the centre, this is unquestionably a very good place to start and works
very well, unless you have a very large array or need to try something slightly unusual it is advisable to stock with these settings.
As mentioned, setting the start and stop delta is simply a case of entering the required figure in dB. As with entering hard avoid
regions, it is prudent to be mindful of how hard your DSP is going to have to work to achieve the results you have requested.
With a huge array of 22 MLA enclosures, it was possible to achieve a 0dB start/stop delta in the Tokyo Dome, the furthest seat
being around 550 feet from the array. It would however be very difficult to get such an astonishing result from say an eight box
MLA Mini array. The height of the array also has a significant effect, a ground stacked system will not be able to achieve low
figures for start and stop as a flown array. In actual fact, a 0dB start/stop sounds very unnatural and because you are used to
hearing an array get louder as you get closer, you actually get the distinct impression that it is getting quieter as you get closer
to the array. The other issue is that multicellular optimisation is only effective down to around 300Hz, low and sub frequencies
will still behave as you would expect reducing in SPL by 6dB for every doubling of distance. If the optimisation from front to back
is set to a target of as little as a few dB this will obviously differ drastically from the behaviour of low frequencies meaning that
the will be a very noticeable difference in tonal balance across the audience coverage with the loss of low frequencies being very
apparent as you go towards the back as the mid and high frequencies hardly drop in level. It is recommended therefore that
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