Driving The Ls From The Aux Send; Recommended Installation Tools And Equipment - Nexo PSR2 Series System Manual

7.2

Driving the LS from the AUX send

It is quite common to use the AUX send of a mixing desk to drive the Sub section of a PA system. This gives the mixing
engineer more flexibility to set the level of the subbass relative to the main PA, apply special effects, or to use a different EQ
on the Sub. However, it also raises some serious issues for the performance & safety of the system (mostly time alignment).
At NEXO, great care is taken to design optimum phase alignment from one octave above to one octave below the crossover
frequency point. By doing so, drivers are working perfectly together and providing the best efficiency possible. It is then up
to the user to adjust the delay on the NEXO TD Controllers to match the physical path difference of the different systems. It
is thus possible to get a well-adjusted system, even without measuring instruments.
If LS are driven form an AUX output, NXAMPs are fed with two signals coming from different sources. If those two sources
(MAIN output & AUX send) are not exactly in phase, delay is introduced into the crossover between the PS and the LS. It is
then mandatory to use proper measurement tool to optimize phase response.
Why is it unlikely that AUX and MAIN outputs have the same phase?
•
Signal paths are likely to be different; any filter modifying the bandwidth and EQ of the signal is also affecting
the phase.
Example: a 24dB/oct high pass filter set at 15Hz is affecting amplitude of the signal by only 0.6dB at 30Hz, but
the phase shift is 90°!! At 100Hz we can still measure 25° of phase shift.
•
Limiting bandwidth with a low pass filter can introduce a phase difference of up to 180° (completely out of
phase) at the cross over point.
•
If the signal is passing through any digital equipment, between 1.4ms and 2.2ms is being added (around 70°
phase shift at 100Hz) due solely to the converter delay! The additional delay due to the processing itself (look
ahead compressor, delay...) can be quite important as well.
If both outputs are not measured in the actual configuration, it is very likely that phase alignment will not be correct.
Consequences of badly aligned systems
Mis-aligned systems have lower efficiency: i. e. for the same SPL the system will have to be driven harder, activating the
displacement & temperature protection at lower output levels. Both sound quality and reliability will decrease as the system
is stressed.
Precautions & Checks
Before using the AUX of a mixing desk, ensure that MAIN and AUX outputs are in phase;
Always apply identical EQ or processing on both channels, so that the phase relationship will not be altered;
Never add additional low pass filtering on the SUB or high pass filtering on the main system;
Inverting polarity on one channel should always result in a massive difference near the crossover point. If that is not the
case, the system is no longer aligned.
7.3

Recommended installation tools and equipment

Tape measure – should be 30m/100ft in length and be of durable fibre material. Have one per array available to speed up
the installation process.
Laser Inclinometer – For measuring vertical and horizontal angles in the venue.
Spirit level – used to ascertain the trueness of the surface from which the angle measurements originate.
Rangefinder measuring device – either a Disto type laser measure or an optical laser rangefinder can be used. Devices
such as the Bushnell 'Yardage Pro' sports rangefinders provide sufficiently accuracy and are easy to use. They have the
additional advantage of working very well in bright sunlight.
Electronic calculator with trigonometric functions to calculate the height from ground level to points in the room. The formula
to calculate height of a point from measured angle and distance is:
Height of point = Sin (vertical angle in degrees) x distance to point
NB: Take care when using spreadsheets as they calculate using radians by default. To convert degrees to radians, use the
formula:
Angle (in radians) =3.142 x Angle (in degrees)/180
Computer – Laptop or Desktop PC running Windows 8 with the current version of NEXO NS-1 installed. Note that, when
NS-1 designs are prepared prior to arrival at the venue, it is often necessary to modify or update the design to accommodate
special circumstances. A PC is absolutely essential to make such changes.
Audio Analysis Software – recommended but not absolutely essential, programs such as Systune
and detailed analysis of the installation. Consider taking a training course in using one of these tools if you are not already
competent with them – it will pay dividends in increased performance of the system.
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SYSTEM CHECK ALIGNEMENT GUIDELINES
System Manual PSR2 Series
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