[MUSIC] Welcome back. In our previous lesson, we discussed how beauty is in the nose of the beholder. In this lesson, we will discuss threshold levels. Throughout this discussion, I'll probably mention the word threshold quite a few times. You know the definition of threshold, it's the level at which something is perceived. We all have thresholds for pain, thresholds for annoyance, thresholds for when we can hear something. So the threshold level is the level at which something is beginning to be able to be perceived. If something is sub-threshold, that means that we can't smell it or sense it all. If something is supra-threshold, that indicates that it is so high in concentration that everybody can absolutely smell it. However, often times faults or idiosyncrasies in wine are just right around the threshold level for most of us. We talk about detection threshold and recognition threshold. Detection threshold is when you say to yourself, I think I can smell something there. This wine smells different from another glass of the same type. It smells different than I remember it from the last time that I tasted it. But I can't quite say what it is, there's not quite enough there for me to put my finger on it. Recognition threshold, however, is when the concentration of that component is a little bit higher and you say, I think I can detect something there. Yes, I absolutely can detect something there, and I recognize what it is. It's a banana, or it's vinegar, or some such thing. So odors that fall in that range between detection and recognition threshold, are the odors that populate wine. The odors that make wine interesting. We just hope that notable, noticeable odors that rise up to our recognition threshold level are pleasant and appropriate for the wine and not a sign of faults. Speaking of thresholds and threshold concentrations, I thought I might give you a little terminology that might come up from time to time just to make it more understandable. We hear people talk about the level at which you can smell something or the threshold for some compound in terms of parts per million. A part per million is equivalent to a milligram per liter. And another way to think about it is it's 1 inch in 16 miles. That's a very, very small amount, isn't it? Think about a place that's 16 miles away from where you're sitting. And think about one inch in that whole distance, that's very, very tiny. Many of the things we smell in wine are smellable when they reach levels of parts per million. However, many compounds in wine, and many faults in wine, are smellable at a lot lower levels than that. Sometimes, we refer to the threshold level as being a part per billion. A part per billion is equivalent to a microgram per liter. And that's equivalent to 1 inch in about 16,000 miles. That is really tiny, 16,000 miles is two-thirds of the way around the circumference of the Earth. So, extremely tiny. And yet, [LAUGH] there are still other faults in wine that we can smell at a very low level of a part per trillion. Which is equivalent to saying a nanogram per liter. Well, get ready for this, a nanogram per liter is corresponding to 1 inch in 16 million miles, or in other words, 1 inch in about one-sixth of the distance between Earth and Sun. That is extremely tiny, extremely tiny. And yet, two or three of the smells we're going to talk about are definitely smellable by most of us humans at a level of nanograms per liter. In this lesson, we discussed threshold levels. In our next lesson, we will turn our attention to oxidation faults.