The Qualiverse - Signal to Noise Demystified, or rather, De-Hiss-tified
Is there anything more annoying than listening to a wonderful podcast with great audio where the talent sounds crisp and clear, only to be interrupted by some low-budget advertisement where the hiss in the recording is nearly as loud as the speech? I’m all for supporting artists, but if you’re going to interrupt my show, the LEAST you can do is pay attention to the audio quality of the content. I know I sound like a grumpy old man, “Back in my day, advertisements were recorded in a professional broadcast or recording studio where they knew a thing or two about signal to noise.” The point is, that hiss that creeps up every now and again is representative of an audio system that is not meeting user expectations. If users can hear that hiss, something is not right in the gain structure of the system. The system signal (the sounds we want) are not playing at levels far enough away from the system noise (the sounds we don’t want). This difference is called a signal to noise ratio, and it’s what we’ll be discussing today.
SIGNAL TO NOISE RATIO
Whenever I talk about signal to noise ratios I am always reminded of the classic vision of a middle school rock guitarist getting ready to jam out, flips the amplifier on, and everyone hears that hum and hiss before he starts to play. Clearly, his system is not optimized for great signal to noise performance. We hear the hiss before they start to play. I even had one guitarist say that the hiss is how they know the amp is “on”... so it’s not a system flaw, it’s a feature... Riiiiiight.
So, why do we care about this signal to noise stuff? Is it just about not hearing hiss? Well, yes and no. There is certainly an aspect of not wanting people to notice the hiss in a professionally installed system. More importantly, however, we want people to easily understand or enjoy the signal audio. If the system noise is too loud compared to the signal, it becomes difficult and/or fatiguing to pay attention. Think about having a conversation in a nice quiet room versus having a conversation in a crowded bar. In a nice quiet room (aka little noise, large signal to noise ratio) it is easy to pay attention and stay engaged with what is being discussed. In a crowded bar (aka lots of noise, small signal to noise ratio), people have to shout to be heard, not everything is understood, and it’s tedious trying to pay attention. My audio systems have been called many things, but if someone called one “tedious”... that would hurt. So, we certainly want a large signal to noise ratio for our audio system.
WHAT'S THE IDEAL SIZE OF THE RATIO?
But, exactly how large a signal to noise ratio (SNR) do we want? If we are saying that a large SNR is great, shouldn’t we just maximize this system performance criteria to get it as big as possible? I’ll say, not exactly. In most systems, the noise level is what it is. A room’s acoustical ambient noise maybe due to HVAC, building noise, outside noise, etc. There may be ways to improve it slightly, but typically, to get a larger SNR, we have to increase the signal level. But, if we increase the signal level too much, the system becomes uncomfortable to listen to. So, as long as I can clearly understand what is being said and I don’t notice the noise, our needs are met. We don’t need to maximize signal to noise, we just need it to be adequate for the use case. So, what’s adequate?
For intelligibility, AVIXA suggests having an acoustical signal to noise ratio of at least 24 dB. That is, the sound pressure level (SPL) of the signal audio should be at least 24 dB above any ambient noise in the space. In “well-behaved” rooms, this is easy to attain. In a well-behaved classroom or conference room, the ambient noise level should be well below 40 dB-SPL. To achieve an acoustic signal to noise ratio (SNR) in a room with an ambient noise of 40 dB-SPL, our signal level needs to be at least 64 dB-SPL. This is a very comfortable listening level for conference spaces. With a SNR of more than 24 dB, it will also be very intelligible.
The trouble comes when the ambient noise in a room is loud, like in a restaurant for example. Ambient noise may be upwards of 75 dB-SPL with people talking, music playing, glasses clinking, etc. Attaining a signal level 24 dB above that becomes problematic. It’s certainly attainable. Several loudspeakers and amplifiers can deliver audio at well over 100 dB-SPL... but that is nearing the threshold of pain for humans. A compromise is required to maintain the intelligibility of the system while also protecting the listeners. However, as long as we are paying attention to it, justifying our recommendation to the client, whatever it may be, is better than just surprising the users at the end of the project with a less than ideal audio system.
DON'T FORGET ABOUT ELECTRIC SIGNAL TO NOISE RATIO
We’ve spent a lot of time talking about acoustical signal to noise ratios, where we are comparing the listen signal levels with the ambient noise levels in terms of dB-SPL. But, how does this translate to the electrical signal to noise ratio? In the sound pressure realm, our biggest concern is intelligibility. In the electronics realm, intelligibility is still a concern, of course. But, we also want to make sure we don’t hear that pesky hiss when no one is talking. We are still taking a similar measurement: finding the difference in levels between the signal and noise. However, instead of measuring levels in sound pressures (dB-SPL), we are simply measuring voltages (dBu). The signal levels are the same. They are still the wanted audio signals when the system is in use. However, the noise level is not the acoustical ambient noise in the room. The noise level we are measuring is the electronic noise level of our system. All electronic systems have a noise level. The key here is we want that electronic noise level to be much less than our acoustical ambient noise level so no one can hear the hiss. As long as the electronic noise level is “much less” (roughly 30 dB) less than the ambient noise in the room, no one will notice it.
Let’s think about this for a second. Let’s say we adjust our listen level (our “signal level”) of the system to be 65 dB-SPL. In order to generate this listen level, our amplifier must deliver a voltage signal level of +10 dBu to the loudspeakers.
Our signal level acoustically is 65 dB-SPL. Our signal level electronically is +10 dBu.
In order to maintain the intelligibility of our system, the acoustical noise level, or ambient noise level in the room, must be 40 dB-SPL or less, providing an acoustical signal to noise ratio of more than 24 dB.
In order to assure that no one in the room can hear hiss in the system, the electronic noise level in the system must -45 dBu or less, providing an electronic signal to noise ratio of more than 55 dB.
IT'S THAT EASY
That’s really all there is to it. We want to make sure our signal level (the wanted audio) is much higher than the noise (the unwanted audio) in the system. Acoustically, we want a signal to noise ratio of more than 24 dB, to assure we can understand what is being said. Electronically, we want a signal to noise ratio of more than 55 dB, to assure that nobody hears any hiss. You’d think advertising and marketing firms would understand this. But no... they will spend all their time making sure the blue in the logo stays in the “Royal Blue” realm without bleeding into “French Blue” realm, while not even noticing the annoying, amateurish, and, dare I say it, tedious hiss in their ad. It’s much more important to make sure the clients sound good, rather than just look good. After all, “audio” comes first in the audiovisual industry for a reason.
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