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Live Sound Systems and the Noise Floor

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Q: “I want to put together the best live sound system possible for my band, choosing mics, a mixer and amps with the widest possible dynamic range, highest signal-to-noise ratio, etc. But how do these specs apply in a real live club setting? How do they affect overall volume?”

A: While component specs such as self-noise, dynamic range and signal-to-noise ratio are useful in selecting gear, in a live sound environment they are usually outweighed by a major outside factor – the noise floor of the room itself. This is a slightly different definition of the term “noise floor.” Every room, whether it’s a club, a church, a huge arena or anything else, generates noise. One of the most common sources is the whoosh of air conditioning units, often combined with whine from the fan motors. In a club, the inescapable sound of conversational voices becomes part of the noise floor, as do the clinking of glasses, sounds from the bar, and (this author’s favorite) the click of billiard balls. Ultimately you can end up facing a noise floor of 40-50dBA.

So it’s normally the environment that determines both the dynamic range and signal-to-noise ratio of a sound system, as far as the audience is concerned. Since most electronic components in the system have a dynamic range on the order of 100dB or more, the sound system itself should never be the weak link when it comes to the end result. Only in a studio should the equipment noise floor become a factor in determining either the dynamic range or signal-to-noise ratio at the listener’s position.

In your sound system, the noise floor would be established by the ambient pickup of the open microphones. In a room with a 40dBA noise floor, the signal-to-noise ratio of a typical vocal microphone will be limited to the mic’s ability to exceed that noise floor. If a strong vocalist produces 120dBA into a hand-held mic (not unusual for close-miked vocalists), the signal-to-noise ratio would be a healthy 80dB, since the 120dBA – 40dBA = 80dB. Remember that each open mic increases the noise; ten open microphones could increase the noise floor by another 10dB if their sensitivity and level setting were the same as the original mic (since the 10 log (# open mics) = 10dB).

Unfortunately, the signal-to-noise ratio of the system cannot exceed the worst-case condition at the open microphone since the sound system has no choice but to amplify the room noise along with the desired signal. Distant miking (such as drum overheads) and failure to mute unneeded mics can cause audible problems with your signal. This is why good mic technique is essential for good system performance, as it ultimately establishes the signal-to-noise ratio of your system.

So what’s your goal – to just be able to compete with ambient noise or to annihilate it? Get an SPL meter (either from Sweetwater or that other musician’s stalwart, Radio Shack) and take a reading of room noise. If your output signal exceeds the noise floor by at least 25dB you’ll establish yourself nicely without too much risk of being too loud. Of course it’s easy to move well beyond this level.

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