Audio Workshop Announcement

Audio workshop in Trivandrum, Kerala, India on the 25th June 2016

How to Avoid Feedback

Rock guitarists are known to use feedback as an effect but for most of us, it’s almost always considered undesirable.

What is feedback?

Also known as Audio Feedback, Acoustic Feedback or Larsen Effect. It is a positive feedback loop that exists between audio input and audio output of a PA system.

By now, we have all experienced that the easiest way to cause feedback is to point a microphone (audio input) directly at a loudspeaker (audio output). This causes the preamp and amplifier to repeatedly amplify the sound of the loudspeaker picked by the microphone causing a ‘loop’ which we hear as feedback. Feedback occurs at one frequency at a time, but there could be multiple instances of feedback taking place at the same time at various frequencies.

 

A feedback loop

Factors affecting feedback

Now that we know a mic and speaker can cause feedback, let’s learn how to avoid it. There are four factors that come into play when we have feedback:

1. Distance
2. Directivity
3. Frequency response
4. Gain

Let’s talk about each in detail.

1. Distance
   The sound pressure level drops with an increase in distance. The closer the input (microphone) and output (speaker), the higher are your chances of feedback. To maximise gain before feedback, the mic and speaker must be as far apart as possible. Keeping the large main PA far from the stage will help a long way.
2. Directivity
   The tighter the directivity of speaker and polar patter of mic, the lesser are your chances of feedback. Your chances of feedback are lowest when the speaker points to the back of the microphone. Speakers are more directive in the higher frequency regions and lose directivity in the lower end. With the help of directional subwoofer setups (viz – Cardiod), we will be able to achieve a higher gain before feedback even in the lower frequencies of audio.Use microphones with tight polar patters like Cardiod and Hyper-Cardiod instead of omni-directional or figure-of-eight to avoid feedback from stage monitors firing directly at the back of the microphone. Choose your mics wisely.
   Tip: Don’t cover the mic with your hand. By covering a mic you are making it an omni-directional mic, which will increase your chances of feedback.
3. Frequency Response
   Almost all PA systems are not ‘true’ flat response. They have bumps of energy which combine with room modes and ringing (resonance) to trigger feedback at the slightest excuse. Now add a mic that has bumps of energy in those same frequency regions and you’ll have the perfect recipe for disaster. Start with ‘ringing out’ the system. More on this a little later.*
4. Gain
   Gain before feedback is the maximum gain you can achieve before causing feedback on a given system. Here, the system includes the microphone, amplifier, speaker and everything else in between. Change any one component and the system changes. You could achieve more gain before feedback by replacing a microphone for one with a tighter polar pattern or by replacing the stage monitor loudspeaker with a better one. Always keep your ears and eyes open for changes you can make to achieve the maximum ‘gain before feedback’.

Ringing out the system*

This is one of the most effective ways to maximise gain before feedback. Any good live sound engineer would start by doing this first. As I mentioned earlier, feedback occurs at single frequencies. There may be multiple instances of feedback if you’re seeing more than one on your frequency RTA. These bumps where feedback happens is usually a by-product of the resonant frequencies of the ‘room’ and system.

Start with the right monitor-mic placement. As shown in the illustration below, it is best if the monitors face the back of the microphone.

Point monitors at the back of a microphone to minimise chances of feedback

1. Open a mic on stage one monitor at a time and slowly gain up till you begin to hear feedback, identify that frequency by ear (this takes some practice), or you could use a portable RTA like Phonic PAA3 or buy AudioTools by StudioSixDigital for iOS. RTA apps on android phones are not accurate but work fairly well only in quiet environments.

2. Reduce the identified ‘feedback frequency’ or the frequency closest to it by half a dB at a time on the graphic EQ for that monitor.

3. Repeat this process for all the monitors. Beware of cutting out too much or too many frequencies while you reach the limit of your system’s gain before feedback. By cutting out too much, you are essentially reducing the output level of the system.

Two instances of feedback occurs in this at around 45Hz and 250Hz. On a 31 band graphic EQ I would cut 40Hz and 250Hz as necessary

 

 

How to make the perfect tech rider for your band

Being a live sound engineer has taught me many life lessons; one of those is ‘not having a tech rider is better than a bad tech rider’. If you came looking for this post, I don’t have to tell you what a tech rider is. It’s your band’s technical profile/requirement for the organizers, sound vendor and engineer to be prepared with stuff you need on stage.

Do you need one?

If you want to put a gig together without too many words flying around during sound check, yes. Tech riders are great, they not only bring the sound vendor and sound engineer up to speed, but also work as a brilliant check list for the band.

It’s your insurance policy in case the equiment is a mess. I am using some screen grabs from a tech rider I use for Swarathma, a band I frequently travel with.

What goes into a tech rider?

1. The line up
This gives a simple but precise overview of the band.

Audio-Technica ATH M50 User Review (Anniversary Edition) (Updated)

I got the Audio-Technica ATH-M50s/LE 50th Anniversary Edition about a year ago when it was time retire my old Sennheiser cans which I reviewed here. The Audio-Technica M50’s have been around for a long time and people swear by them. This is a straight cable 50th Anniversary Edition which is in a metallic silver-blue finish instead of the regular black-grey. In all other ways, it is exactly like the regular ATH-M50’s and has the same specifications.

Audio-Technica ATH-M50s/LE Anniversary Edition

It was a big leap from the budget friendly Sennheiser’s that served me for many years. I have used various headphones on the road including Beyerdynamic DT770 Pro, Shure SRH840 and more in the past few years, however I never tried an Audio-Technica ATH M50 which had numerous good reviews. Based on this, I had short listed the DT770 Pro and the Audio-Technica ATH-M50 as final options.

Make the most of Sound Check for a Chorale or Acapella

A reader asked me about sound check for a choir and acapella performances after seeing my blog post on sound check for bands. Because sound reinforcement for choral music works very differently, I realized the topic requires a separate blog post. I have worked with numerous gospel choirs and acapella groups and here are my observations on what usually goes wrong.

A few pointers before I dive in to the technicals:

1. Most musicians from choirs and acapella groups are also a part of rock/pop bands. They spend more time with the bands and assume things work the same way for a chorus on stage, but it doesn’t.

2. The reason you have a choir/acapella is because you intend to use ‘voices’ as an ‘instrument’. If you have an army of other instruments ‘backing’ your group, you better have voices that can be heard over them, of course without sounding like they are straining.

Audio Workshop for Musicians – A series starting in Bangalore

In all these years of gigging and traveling, I met many great musicians who were either partially or totally handicapped by technology. I tried to help many on stage, at the last minute, tweaking their tones and helping them with all the stuff that is “pro-audio”. I thank all those wonderful musicians who inspired me to come up with this special “Audio Workshop for Musicians” series aimed at enabling musicians to conquer technology and concentrate on music.

Rahul Samuel’s Audio Workshop for Musicians