The Larsen effect is the name given to the resonance of an environment [Environmental acoustics ] at certain frequencies that have an ever-increasing amplitude which triggers in the microphone-mixer-monitor chain. When a frequency enters a microphone, it gets amplified and reaches a monitor. If the frequency has an amplitude which is greater than a certain limit, this triggers a recursive process by which the frequency itself is amplified every time it goes round the chain. Usually the distance of the microphones from the monitors and their directional characteristics ensure that the sounds coming from the monitor are heavily attenuated when picked up by the microphone. The following is an example of a typical stage positioning which should minimize the Larsen effect:
As we can see, the cardioid diagram of the microphone [Polar pattern of a microphone ] directs the microphone's sensitivity towards the singer's voice, whereas it minimizes it in the direction of the monitor. If the environmental conditions were to bring about the Larsen effect anyway, we'd have to recur to using equalizers. In this case we can act on the graphic equalizers [Graphic equalizer ] which modify the monitors' responses, as well as the equalizer present on the channel of the mixer to which the microphone that is causing the resonance is connected. This manipulation consists in attenuating the amplitude of the frequency which caused the resonance and bringing it to an amplitude level where the effect doesn't trigger. The technician's ability in this case consists in immediately spotting the source of the effect and the frequency that is being excited. Once the source has been found, for example the singer's microphone, you can choose to modify the response of the singer's monitor by using the graphic equalizer or by intervening directly on the sound coming from the microphone. At this stage it is very important to know exactly which frequency has been excited; let's not forget that in these circumstances we have literally a few seconds to resolve the problem, before a deafening sound will force everybody to block their (precious) ears.
It is a good idea to train oneself to recognize the frequencies present in the audible spectrum, in order to act with self-confidence and skill when confronted with difficult situations. The following are the sounds of certain frequencies which can be taken as reference within the audible spectrum [Audible frequency spectrum ] . We advise you to listen to them lots of times in order to learn to recognise them with your eyes closed; that way you will have the necessary mastery needed to act instinctively yet correctly in situations where quick action is required.
The frequencies used in the examples are typical frequencies from a 20-band graphic equalizer (the last 20 KHz frequency may not be audible if your computer's audio system, or in fact your hearing apparatus, can't pick up such frequencies. It must be noted that even a human ear in perfect conditions isn't always capable of perceiving such high frequencies).
Table 13.1. Reference frequencies within the audible spectrum