owner
’
s manual |
Earth118
18
Important
PHASE
In any sound reinforcement system the correct phase is of fundamental
importance. Two speakers that are carrying the same musical program
interact with one another and this can cause destructive interference.
This means that the sound waves, emitted by two speakers, reaching
a listener's position (or a point in the auditorium) with different phase
(two different points of the sound wave) may combine in the 'wrong
way' and, instead of summing themselves and generating a sound wave
of larger amplitude (a sound pressure 'louder' than that produced by a
single speaker), will generate a lower sound pressure.
In other words, the two sound waves will subtract instead of summing.
The worst condition is when the phase of the two signal is opposite
(a phase shift of 180°: out of phase): this results in a mutual cancellation
of the sound waves. In fact, if the first wave reaches the listener with the
maximum value (positive pressure ) and the second with the minimum
(negative pressure), the resulting sound wave sum will be of zero
value (or at lest a very low value, lower than that produced by a single
speaker).
Among the possible causes of these phase shifts are the listener's
position in respect to the speakers, the type of acoustic loading used
in the speakers, the acoustics of the location, the characteristics of the
cross-over and the electronic processors.
The phase reversal button located in the control panel (6, page 13) is
meant to avoid the most extreme condition (out of phase: 180° phase
shift), and to limit, in most cases, the phase cancellations described
above. This solution is used primarily in simple systems that do not
include a separate PA speaker processor (which, in more complex and
sophisticated systems, will take care of the phase alignment by adjusting
the time delay between the speakers used in different audio ranges).
The most 'classical' application is in the relative phase correction of a
sub woofer and a satellite speaker combination.
In the cross-over region (100/120 Hz in Montarbo
®
systems) the physical
location of subwoofer and satellites (and other factors described above)
may cause a phase shift between the sound waves generated by the
speakers, and this will result in a 'hole' in acoustic response.
In this case, the phase inversion of the subwoofer (by pushing button 8,
page 14) will minimize this effect.
It is mandatory to check that the phase is set at the same value on every
subwoofer installed.
A phase inversion in one or more of them (due to the improper setting
of the control), relative to the others, will result in the quasi-complete
cancellation of the sound produced by them.
In standard installations all the subwoofer must have the same phase
setting (all set to 0° or all set to 180°).
INPUT SENSITIVITy AND CLIPPING
Every amplified speaker system is characterized by a value of input sensitivity.
The sensitivity is defined as the value of the amplifier's input signal that will
result in maximum power output.
An increase in input signal over that threshold will result, not in increased
power, but in a distortion phenomenon called 'clipping' (output stage
saturation).
In this condition, the speaker will operate improperly. The diaphragm will
exceed it's excursion limits, and the voice coil will overheat beyond it's
thermal limits, resulting in overheating and premature failure.
The active processors will help in avoiding clipping, by reducing the amplifier
gain and thus the input sensitivity, but these protections may be overridden
in very extreme conditions. What the active processor cannot modify is a
signal that is distorted before getting to the active speaker's input.
The effects of this type of signal are the same as described above.
How to avoid clipping
The simplest way to avoid clipping is to check each level in the signal's
chain. Start from each input channel of the mixer and adjust the gain
control and the equalizer controls so that the PFL meter will never (or only
occasionally) indicate more than 0dB.
In simpler mixers, check that the 'clip' or 'peak' indicator is always off,
or blinks only occasionally. If these levels are exceeded, reduce the mixer
channel's input gain. Once the desired mix is obtained, adjust the output
level so that it never exceeds the active speaker's or the power amplifier's
input sensitivity, as displayed on the master output VU-meter.
The input sensitivity of Earth118 is 0dBu.