It seems like everyone is talking about decibels these days: “5 dB of this” and “3 dB of that.” As acoustical consultants, we hear people using these metrics and often wonder to ourselves, “Do they really know what a decibel is?” Occasionally, someone is brave enough to ask the more important question, “Just how much is a decibel?” The answer to that question is, of course: well, it depends. Are we talking about physical sound levels or perceived sound levels? There is a big difference!
Let’s lay some ground work. A decibel is a ratio of how much acoustical energy we hear compared to some “reference level”. For humans, the reference level is the “threshold of hearing”. This means that we always talk about decibels in the positive sense, such as 50dB or 60dB; we don’t say that some noise is -20dB. To futher confuse matters, humans hear “weighted” noise levels. It is “weighted” because our ears do not treat all frequencies the same. We “hear” some frequencies as being louder than others; our ears do not have a very “flat” response. To accommodate this, there are several different algorithms of decibel weightings that we call “scales.” Most commonly, you hear about the dBA scale (this is the one that applies to humans) and the dBC scale (but that’s for another blog). Just remember that the most commonly used scale is dBA, and that’s what most people are referring to when they talk about decibels (whether they know it or not). At any rate, the dBA scale gives us a good starting point from which to measure sound. It yields a single-numbered result and gives us a good idea of the overall sound level (when, in reality, the sound is at many different noise levels all across the frequency spectrum).
Physics tells us that for every doubling of acoustical energy, there is a 3dB increase. Conversely, a 3dB decrease means the sound is cut in half. So, 3 is the magic number right? Well, not so fast. This is where we see a conflict between scientific calculations and perceived sound levels. “Perceived” sound levels report how our ears and brain interpret the sound. In other words, perception answers the question of “What sounds ‘twice as loud’?”
Sound studies tell us time and again that a 3dBA increase in sound level is barely noticeable to the human ear. In fact, you have to raise a sound level by 5dBA before most listeners report a noticeable or significant change. Further, it takes a 10dBA increase before the average listener hears “double the sound.” That’s a far cry from 3dB.
So is perception the ultimate reality? What really constitutes twice the sound or half the sound? And why can’t our ears hear what is proven to be scientific fact? The answer to each of these questions is, of course: well, it depends!