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Fine tuning and phase alignment
The null method
Local walls and floors (known as boundaries) will tend to reflect sound and cause phase shifts at low
frequencies where systems tend to be less directional. Multiple boundary effects can make mid-high-to-
bass enclosure alignment quite complex but there is a simple phase alignment procedure that requires
very little test equipment apart from a graphic equaliser and some kind of pink noise or bass-rich music
source. This “null method” makes use of the fact that two signals with opposite polarities create a very
audible dip in output when perfectly aligned.
You’ll need to set up a narrow band signal at the crossover frequency to make this alignment method
easier. Music or noise via a 1/3
rd
octave Graphic EQ is great for this, with all faders pulled down except
for one at the required frequency. If a graphic equaliser is not available use a boosted, narrow band PEQ
in the input stage of the Loudspeaker Management System. Don’t use a sine wave tone in case you’re in
a room null.
1) To avoid aligning a whole cycle out, make sure you follow the physical or time alignment
procedures first using the appropriate Funktion One settings as the starting point. See earlier
2) Listen to the bass enclosure and the mid-high enclosure one at a time at your chosen reference
position and adjust each one by ear to give the same approximate level with the narrow-band
signal
3) Once level balanced, switch the bass system temporarily to reverse polarity
4) Now run both sections together and fine trim the mid-high enclosure delay for a dip in level at
the reference position.
Note: If the mid-high enclosure is near a large local boundary (a ceiling or wall, for example) it
may be necessary to put the mid-high delay setting back where it was and fine adjust the bass
enclosure delay instead. A little experimentation may be required
5) Now switch the bass system back to the correct polarity – this should give you maximum
alignment and impact through crossover
If no obvious dip is heard – even with delay adjustments of several milliseconds – the room
characteristics may be too complex and confusing to deal with easily. If this is the case, it’s probably best
to reinstate the standard recommended delay settings and fine tune by ear.
Experience teaches us that the “ideal” setting is not always as predicted because the real-world
acoustical environment is so complex. Calculations, if not absolutely correct, are usually pretty close
though,
To confirm that the above adjustments have been effective, the system can be rechecked using a
variety of known, well recorded tracks from different genres. If necessary, try varying the timing
slightly until the most satisfying result are heard and felt. This method can also help to affirm that
the above methods have been applied correctly.
Important note on bass enclosure placement
Most bass enclosures are fairly omnidirectional because of their small size relative to bass wavelengths.
They therefore tend to be affected by a nearby wall and care has to be taken to ensure that direct and
reflected bass do not destructively interfere with each other.
Wherever possible, position bass enclosures hard against a wall; so that the time difference between
direct and reflected sound is minimised. The further the bass enclosure is placed from the wall, the
more the reflected sound will be out of time with the direct sound. This can cause cancellation
(reduction in level) due to the phase difference.
www.funktion-one.com © Funktion One Research Limited 2011
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