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Sound Advice

Graphics and Noise: Hiss & Hum

Hiss (the steady ‘tssss’ noise – like escaping steam – made by the graphic itself) is not much of a problem on the more professional units. It’s most likely to be an audible problem on bad or low-budget designs, and older or damaged units. With a decent unit you shouldn’t hear any hiss at the FOH mix position at all, over the FOH PA, when the bypass switch is flicked in and out, and while the sliders are all at 0dB.

Of course the prevailing hiss level will be made more audible if you have to boost frequencies above 5 kHz, or if you’re in a small venue where some of the audience are unavoidably seated close to the FOH PA cabs. Hopefully, they won’t notice it over the music.

Lower-frequency noise (buzz and hum) may be induced if graphic EQs are placed (for any reason) near to ‘leaky’ AC transformers – meaning their magnetic fields leak outside their casing and affect other equipment.

Graphics are more likely to pick up hum than other units in the drive rack, so before finalizing a drive rack set-up, it’s a good idea to spend a little time experimenting with the relative locations and spacing of the units in the rack, with the graphic placed furthest (if feasible) from any other units that induce hum.

To make good and bad positions easier to locate, you can temporarily exaggerate the problem by fully boosting the sliders at 50, 150, 250 and 350 Hz (or in the US and wherever the local AC power frequency is 60 Hz, use the sliders nearest to 60 Hz and its multiples). For this test, nothing is plugged into the graphic – though any gear around it must be powered-up and switched on – and it’s simply plugged straight into a PA amp and a bass bin, or any bass speaker made to reproduce frequencies between 50 and 400 Hz.

Note: Although you can use the graphic to exacerbate hums in this way to help positioning, never use a graphic to eliminate hums. You’ll reduce the hum, but also cut an unnecessarily wide range of bass frequencies. For a suitably selective cutting you must use a parametric EQ (PEQ).

This article is reprinted with permission from The Live Sound Manual, published by Backbeat Books, www.backbeatbooks.com. All information is copyrighted and cannot be reprinted without the permission of the publisher.

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