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HOW TO DEAL WITH ACOUSTIC FEEDBACK
Simply put, the annoying shrilling howl of growing and untamed feedback is the result of "in-phase"
frequencies getting together and being reinforced by the sound system and environment.
Phase Switch – since the purpose of the Phase Switch is to reverse the polarity, the first line of defense
is to change the phase, which can sometimes be a cure all by itself. This has the effect of changing the
phase relationship between the amplified signal and live sound, which can break up some "in-phase"
reinforcing components. On the other hand, it's preferred if we can leave the phase set to provide the
most natural and balanced sound.
Notch Filter – Another way to prevent "in-phase" reinforcement is to simply find and "notch" out the
offending frequency. The approach is straight forward; with the Notch set to full "cut" (counter clock-
wise), slowly rotate the Notch Frequency to align with and cancel the offensive tone.
Phase and Notch – probably the most widely used approach is to start without the notch filter engaged,
and to bring up the gain and or volume to encourage feedback. Changing the Phase will change the
frequency of the feedback. Leave the Phase set to whichever setting produces the higher pitched note.
Next, use the aforementioned Notch technique to hone in on and prevent the feedback. Keep in mind,
fighting feedback in this manner may or may not appeal to your taste in overall tone. It's certainly fair
to experiment and try it the other way around, i.e., Phase switch to stop HF tone and Notch to cutout
the LF tone. After all, Live Sound is an Art and when it's all said and done tone is King.
ABOUT EQ AND PHASE SETTINGS
High – used to add sibilance and crispness to your sound, with the knob at 12:00, there is no boost or
cut to the high frequency content.
Mid – Helps control the clarity of your overall mix and should be adjusted to suit lower or higher
volume performance levels. Boosting the mids helps speech clarity and intelligibility at lower volumes
while cutting the mids produces a more natural sound at higher volumes. The Mid frequency selection
knob allows for a range extending from high LF to low HF. When set to 12:00 the Mid frequency peak
is around 1.2k Hz, with +/- 12dB of boost or cut depending on the setting of the Mid knob. Here again,
with Mid set to 12:00 the mid control is effectively out of the circuit.
Low – In general, lower volumes can benefit from LF boost to help bring out the bass at these lower
settings, but it is usually left flat or even slightly cut for high playback levels.
Notch – When it comes to the mechanics, Notch filtering is very much like the adjustable mid frequen-
cy described above. However, the frequency range is much lower, and the filter itself is much tighter,
hence the term "notch. " When used in "cut" mode, this is very effective in managing feedback, which is
explained in some detail in our Feedback section. However, the notch can be set to cut or boost to allow
for the full range of possibilities. Many times feedback is not an issue, and the artist can take advantage
of a particular notch setting to effectively tweak a final mix to accommodate specific room or instru-
mentation considerations.
Phase – Use the phase setting that best supports your overall sound and system integration consider-
ations. For example, Phase can be set to facilitate the best bass response at low volumes, and it can also
be effective as a feedback deterrent, which will be reviewed in our Feedback section.