New to live sound and looking for tips to improve? Or simply looking to learn more about it? Well, whatever the case, if you are serious about being a live sound engineer, then you need to understand the possible problems you will face. Audio feedback is an issue that is prevalent in the world of live sound. Hence, the question that we will be asking today is – What is “Audio Feedback”?
In this article, we will cover various core topics surrounding the subject of audio feedback. These fundamental topics are vital when it comes to understanding how audio feedback happens, what it is affected by, and how it affects the audience in a live concert. You will also learn how to prevent it from occurring, and how it can be desirable in some cases. Ready to learn? Then let’s begin!
Introduction – Basic Concept
Also commonly referred to as acoustic feedback or simply feedback, audio feedback is a unique type of “positive feedback” which is initiated whenever a sound loop is created between an audio input (typically a microphone or acoustic guitar pickup) and an audio output (usually a loudspeaker). In this case, an audio signal captured by the microphone is amplified and output through a loudspeaker.
The audio signal from the loudspeaker can then be captured by the same microphone, be further amplified, and output via the same loudspeaker again. The resulting frequency of the sound is determined by factors such as resonance frequencies (in the microphone, amplifier, and loudspeaker), a room’s acoustics, the microphone and loudspeaker’s pick-up and emission patterns, and also the distance between them.
Sound engineers often consider feedback to be undesirable, especially when it happens with a public presenter’s (or singer) microphone during a live show that is using a PA system. In an effort to prevent unwanted squealing or screeching sounds from happening (as a result of feedback), various audio equipment such as graphic equalizers and feedback suppressors are used. In general, audiences of live shows do not appreciate feedback.
Possibly the only situation where feedback is desired, is in rock music. Ever since the 60s, rock electric guitar players have intentionally produced feedback, to be included as part of their music productions. This is often done through the use of loud guitar amplifiers and various distortion effect units. Jimi Hendrix was a considered a pioneer in the artistic and musical use of guitar feedback in his guitar solos.
In the following sections of this article, we will look at these few topics:
- History and principle
- Core concepts
History and Principle
The principle behind audio feedback actually follows the “Barkhausen stability criterion”. This means that if the overall gain present in the system (PA or Sound reinforcement) is high enough, a stable oscillation will usually be produced within a feedback loop, whose frequency has gain that is equal to “1”. However, if the signal gain is greater than “1” for a certain frequency, then the system’s oscillation will be at that frequency instead, due to the noise at that frequency being amplified.
Take note that we are simply talking about gain here (which is the cause of feedback), thus you will hear sound being produced (feedback) without anyone playing. The sound level keeps increasing until the output starts clipping, where the loop gain is then reduced to “unity” (0dB). Take note that all electronic oscillators are also based on the same principle, but the feedback loop is purely electronic (not audible).
Dr. C. Paul Boner produced the early academic material for acoustical feedback. He states that feedback only occurs at one specific frequency. He also mentions that feedback can be nullified by using a very narrow notch filter (inserted in a loudspeaker’s signal chain) at that frequency. Together with Gifford White (founder of White Instruments), hand crafted notch filters were produced to tackle feedback in specific rooms.
Dr. Boner was also largely responsible for establishing fundamental theories of acoustic feedback, room-ring modes, and room-sound system equalizing techniques.
In this section, we will discuss the core concepts surrounding audio feedback. You will need to understand these concepts, in order to find suitable methods that you can use to prevent feedback. Here they are:
- Frequency response
- Feedback suppressors
Distance is a crucial element when it comes to maximizing gain, before the sound system starts to feedback. Basically, you would want to have the most out of your live sound system, in terms of gain control. This is done by reducing the level of sound energy (as much as possible, within practical limits) that is fed back to the microphones. Hence, the very straightforward method used to achieve this, is by placing the microphones at a distance large enough from the speaker cabinets.
Another common way to prevent feedback is by placing the main speakers far away from the performers, and then placing smaller speakers (known as monitors) that are facing each performer (in the opposite direction of where the main speakers are facing). Take note that the microphones are also facing the performers. Sound pressure levels can then be controlled separately for the audience and the performers.
Since the monitors are facing the performers, the microphones (which are also facing the performers) should have a cardioid pickup pattern (to reject sound from the back). Super- or Hypercardioid patterns will be ideal, if the monitors are placed at a different angle (other than 180 degrees) behind of the microphone. These patterns also better cancel reverberations coming from the surroundings.
You need to understand that most sound reinforcement systems, do not have a natural frequency response that is ideally flat. Often times, this will lead to acoustical feedback at the frequency that has the highest loop gain, which may also be higher than the average gain of all the other frequencies combined (resonance). Thus, it is always ideal to use a graphic equalizer for reducing the gain of this “problem” frequency.
Feedback can be avoided by the process of “ringing out” a sound system. This is usually done by the sound engineer, some time before the performance starts. The gain level for a microphone’s channel (on the mixer), will be raised to the point of feedback. The affected frequency can then be attenuated on an equalizer, preventing feedback at that frequency and also giving more volume to other frequencies.
Professional sound engineers are generally able to accurately identify the frequency of a feedback by ear. However, many other engineers utilize a “real time analyzer” for this purpose.
There are automatic anti-feedback devices out there, that can be used to eliminate feedback (also known as “feedback destroyer” or “feedback eliminator”). Some units function by slightly shifting the frequency, resulting in a “chirp” sound, instead of a howling sound due to the feedback frequency being up-shifted. Other units work by filtering out affected frequencies with sharp “notch-filters”. Adaptive algorithms are engineered to automatically tune these notch filters.
Alright, we have come to the end of this educational journey. By now, you people should be able to at least minimize the chances of having feedback in a live concert!
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