Thursday 29 September 2016

Technical: Practical Tips on Minimising Feedback and Maximising Gain. Part One.



Practical tips for taming 'the howl' and maximising the music. 

Part One: What Is Acoustic Feedback?  

Acoustic feedback is the great enemy of sound people and musicians alike. Minimising and controlling it can be the difference between a great gig and a disastrous one. In this blog post, I'll look at the various ways you can minimise feedback both with and without the deployment of additional equipment. We'll look at loudspeaker and monitor positioning, equipment selection and some basic practical room EQ.

We've divided this blog post up into three sections. In this, part one, we'll actually look at what feedback is and what causes it. In part two, we'll take a look at some of the physical things we can do to minimise the problem such as speaker and monitor positioning , and in part three we'll take a look at some of the electronic and digital countermeasures we can employ to combat the problem. 

Know Your Enemy: What Exactly Is Feedback? 

Whether you're a musician, a sound engineer, someone who goes to gigs regularly, or someone who at some point has been asked to setup some sound equipment, chances are, you will have experienced the phenomenon known as 'feedback'. Sometimes known as 'acoustic feedback', 'howl', 'howl-round', or 'ringing'. The beast has several monikers. 
And sound guys and musicians will all tell you what a huge problem it is. We've all experienced it. The lead vocals aren't loud enough, so we need to turn them up. But we can't because the moment we move the vocal channel fader in a northerly direction, we get a horrible, piercing scream.
So what is feedback? What causes this ringing noise?

Consider a basic sound system such as a system outlined in the image below: 




A performer sings or speaks into a microphone. We advance the gain of the mixer and amplifier and we reach a point where the system starts to 'ring'. This ringing noise is known as feedback. What's happening is that some of the sound from the loudspeaker is being picked up by the microphone, fed back into the system, to the loudspeaker again, and then picked up again by the microphone, etc, etc., forming a continuous loop.


Feedback and Frequency   

Anyone who has been involved in live sound as a musician or engineer will have noticed that feedback can occur at different frequencies. It can be a piercing high frequency 'squeal', but it can also be a low frequency 'rumble' or a mid-range 'howl'. The reason certain frequencies may feedback before others can be attributed to many factors. Frequency response of the items used in the system (i.e. the mic, mixer, amp, etc.), the directivity of the items and also the environment they are being used in. Rooms have a frequency response as well. The fact that feedback occurs at different frequencies is in many ways the key to minimising it. More on this in parts two and three. 


Maximising Gain, Minimising Feedback. 

All sound systems that have microphones and loudspeakers in proximity to one another will feedback at some point. What we need to do is raise the point at which the onset of feedback occurs. To maximise the available gain before feedback. So you can turn the lead vocalist mic up if you need to and not have it feedback. 

By increasing the available gain in a sound system we make the creative process of mixing a band much easier and more satisfying for the engineer, the band, and for the audience. As well as meeting the basic requirement of ensuring the gig is loud enough so the audience can hear it clearly. 
I kind of think of it as a painter painting on a broader canvas. The mountains can be taller, the horizons wider, and the vistas deeper! By increasing a system's available gain before feedback, we give ourselves a larger audio canvas on which to work. And you'll be able to see the painting from further away!

I think maximising a PA system's gain is really the key to successful and great sounding live music events. We may know all the intricacies of mic placement and know our reverb processors inside and out, but unless we have enough system gain to work with, it really won't matter. 

Now that we have a clear picture of what feedback is and what causes it, we can start dealing with it. 


Physical and Electronic Solutions

The battle to minimise feedback and maximise system gain can be fought on two fronts. We can employ physical factors to de-couple the microphones from the loudspeakers and prevent the sound from the loudspeakers leaking back into the mics. These solutions generally cost nothing or are at least relatively cheap. We'll look at these 'counter-measures' in part two. 
There are also 'electronic' solutions that we can employ. These take the form of processors and equalisers that we can add to the sound system. These can cost a bit more. But with the advent and growing popularity of digital mixing consoles that are stuffed full of digital processing power, you may already have these tools available. You just need to know how to deploy them. We'll deal with processing feedback counter-measures in Part 3. 

Of course, all this information can be applied to any sound system. Not just to music and band gigs. Anyone who has tried to get enough sound from a wireless lapel mic with a live sound system will know that feedback effects all types of systems from large music rigs to small AV speech/presentation systems.

So there we are. We now know what feedback is, and we know the benefits of controlling it. Time to go to war on it........





This blog post has been written by Simon Thompson, the founder and MD of The Noizeworks. Live sound equipment and accessory sales and hire. He's worked as a sound man, salesman, loudspeaker designer, and DJ, and is a not very accomplished musician. 


As with all of The Noizeworks blog posts, it is presented as an informal guide to assist musicians and sound people get the most from their gigs and/or presentations. We accept no liability for circumstances arising from the use of this equipment or the inability to interpret this information correctly.