Signal-To-Noise Ratio: When It Really Counts

AV9000 Checklist Item Under Test: 6.2.7: The electrical noise levels for all audio channels are “xx”dB (“55dB” recommended for most systems) below the normal operating level for all audio sources. “Noise” refers to hum, electrostatic noise, RF interference, etc.

Reasoning: In this year’s “The Commish,” I will discuss some potential pitfalls that integrators and designers may encounter in the market segment that Sound & Communications is highlighting that month. In this issue, the focus is on Theaters and Auditoriums.

The goal of integrating systems with good signal-to-noise ratios is to ensure that the listeners do not hear any sound from the system when they are not supposed to. If listeners hear hiss or hum when there is no signal, the system has poor signal to noise. In most conference spaces, having 55dB signal-to-noise ratio is more than adequate to assure that system noise is well below the acoustical ambient noise in the space. However, this is not the case with theaters and auditoriums. The dynamic range of these spaces, where the “louds” have to be loud and the “softs” have to be soft, requires a larger signal-to-noise ratio.

The Story: Back when I was just a wee lad starting out in AV, we were installing a large auditorium for a financial institution. I didn’t know much about gain structure back them, and neither did anyone else on the team. What we lacked in knowledge was made up with our quick wit (and modesty). As you might expect, the main program loudspeakers in the space produced a significant amount of hiss when there was no signal present. When the client asked the project manager about the hiss, without skipping a beat, he turned a system flaw into a feature and said, “That’s how you know the system is on.”

Since then, I’ve learned a bit more about gain structure, as well as setting and measuring signal-to-noise ratio. In most cases, as long as the signal-to-noise ratio in the system is greater than 55dB, no hiss or noise will be heard when there is no signal present. Why? Most business-class rooms have acoustical ambient noise measurements of 35dB SPL. The default audio levels (or “signal” level) in these rooms is typically set to about 65dB SPL. If the system has a 55dB signal-to-noise ratio, that means the electrical noise will be distributed at a measly 10dB SPL. Even if they really crank up the volume to 80dB SPL, the hiss/hum/noise level will only be distributed at 25dB SPL. This is well below the ambient noise level, so no hiss is heard and everyone is happy.

This recommended signal-to-noise ratio of 55dB makes sense for most conference spaces and classrooms. The levels never really get too high, and there is usually plenty of ambient noise (HVAC, street noise, high-efficiency floor plans, etc.) to cover any noise. Where this is no longer acceptable is in theaters and auditoriums. Why? The dynamic range in these spaces is much larger than that of a conference space. Theaters are typically acoustically treated, and have less ambient noise than an office building. It’s quiet in these spaces when there is nothing going on. Not only that, but signal levels can be much higher than those desired in an office building where you might disturb your neighbors or leak sensitive information. It can get pretty loud in theaters. So, we have a lower ambient noise and a higher signal level in theaters and auditoriums. That means that we also need a larger signal-to-noise ratio in the system to maintain the audio fidelity.

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