Knowledgebase: Mixers
Having Problems with Feedback & Hiss?
Posted by Rick Shao on 13 November 2015 01:36 AM

Our sound systems can be friend, or seem like foe. As worship leaders, praise musicians, or pastors, we are usually entirely dependent on acoustic technology, and the faithful and courageous folks who do their best to manage sound every week. One of the recurring frustrations can be unintended noise such as speaker hiss or feedback (the f word in sound management circles). If you never have any howling acoustic feedback loops, then immediately fall to your knees and give thanks. For the rest of us, there are a few general precautions we may take, before we kneel and give our daily thanks for the sound ministry team. If you have a multi-band frequency equalizer, pay for an inexpensive consult to have your room tuned. This process de-emphasizes certain natural culprit bands peculiar to your sanctuary. It can add considerably to your safety margin of gain before feedback. This simply is the amount of amplification your space can handle before feedback occurs - the more the better.

If feedback is recurring, get a helper and simulate the condition (force it to occur). Learn which speaker(s) and microphone combinations cause the loop. You may find more than one. Common causes are a praise singer holding a unidirectional mic so that it points at their monitor. Singers also need to understand basic mic and sound mechanics. Keeping the mic on its stand for praise presentation is one way to fix this. Another common source is a loop between the preachers lapel (almost always an omnidirectional mic) and the main speaker cluster. Omnidirectional mics are the most prone to feedback, as they pick up sound equally from all directions. Reduce the gain on the mic, have the user speak louder, move the mic closer to the mouth, or some combination of these. Learn where the hottest spot is in the sanctuary. This is often near the first few pews, as theyre closest to the speaker cluster. Then avoid that spot during the sermon. A common mistake is that sound people dont realize the number of live mics factor. As the number of live mics doubles, the gain before feedback margin is reduced by three decibels. So, when music is finished, either mute those channels, or bring the faders down for the unused mics. Better boards let you assign each channel to a sub bus. You could put all singers on one sub, and all instruments on another, for example. By working only two faders, you could then regulate the music quickly and effectively.

If you hear significant hiss from your systems main speakers, then you probably have a maladjusted gain situation. Usually the main power amplifier is cranked up too high because the mixer and any special effects units or equalizers are not sending a sufficiently strong signal. A little prudent adjustment can easily fix this. Start by cranking the main power amp down to an acceptable signal to noise level (youll know when the hiss no longer bothers you). Then modestly bring the feeder components up until the overall volume level is sufficient. No component should have its output level turned up much above two-thirds of its capacity. That only begs for noise or distortion in the system. If you cant get enough volume without noise, its a sign you need a more powerful amplifier.

By Tom LeFevre

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