We’ve spent some time with test equipment, and that is a great thing. It removes ambiguity. It tremendously cuts down troubleshooting time. It stops Tech Support in its tracks. (Let’s see them try to continue with the script when faced with: “Your amp is spitting out 5% THD on a -10dBu signal that tests clean when connected directly to my analyzer.”) However, people too easily get caught up in the numbers and testing procedures and forget why they are taking the measurement in the first place. In this “Back to Basics,” let’s talk about what a good signal to noise ratio really means, and hopefully dispel some myths.
First, electrical signal to noise ratio is simply the difference in levels between a system signal and the electrical noise present. So, if we have a 0dBu speech level, and a noise floor of -70dBu, our electronic signal to noise ratio is simply 70dB (the difference).
Similarly, acoustic signal to noise ratio is the difference, acoustically, between signal levels and noise levels. So, if we measure the same speech level as before at 65dB-SPL in the room, and the ambient noise is 40dB-SPL, our acoustic signal to noise ratio is 25dB (the difference).
We want to make sure that our electrical signal to noise ratio is higher than the acoustic signal to noise ratio. Why? Simply because we don’t want the audience to hear a hiss when no signal is present.
Now, let’s dispel some myths:
Myth 1: “I tell my clients the noise is there so you know the system is on.”
Wrong. The noise is there because the AV specialist doesn’t understand gain structure.
Myth 2: “Isolated ground is always the way to go.”
Wrong. Isolated ground is expensive and, at times, dangerous if not properly installed. Most systems today can produce an electrical signal to noise ratio of 60dB or more without any specialty grounding. Also, most systems today only have acoustic signal to noise ratios of roughly 25 – 30dB. Why spend more time and effort when your needs are already more than met? Audio studios and theaters are different, of course, but typical conference rooms usually don’t need isolated grounds.
Myth 3: “Attenuating at the mixer is way better than at the amp. I always leave my amps turned all the up (to 11, even).”
Wrong. This is a holdover from live sound folks who want to give the operator as much control as possible at the mixer, and signal to noise is a secondary concern. For installed, conference systems, from a signal to noise standpoint, it is better to amplify the signal as close to the source as possible. The amplifier will amplify everything (signal AND noise). By leaving the amplifier all the way up, the output from the mixer will have to be brought way down so you don’t blow the listeners away (think -30 to -40dBu). By doing so, you bring the signal level (-40dBu) closer to the hiss (-60dBu). So, the amplifier will only give you a 20dB electrical signal to noise (the difference). People will definitely hear the hiss. It is much better practice to feed the amplifier 0dBu (well away from the noise floor), and just let the amplifier be lazy. The listeners will thank you for it.