The topic for today’s post came out of a very real conversation I had with my new audio volunteers last night. We got to talking about various signal levels that we deal with in the world of audio, and it became very clear that they had not yet been exposed to any of this nomenclature. I figured it’s entirely possible that some of you are unclear on what all these terms may mean as well.
So this will be an introductory course on basic signal level. To keep it simple, I’m not going to go into all the background math that gets us here, or define every term; I’m going to stick with the practical implications of signal level. With those two caveats, simple and practical, let’s begin.
Level and Resistance
I’m going to use two terms today that need defining, level and resistance. For the purposes of this discussion, level refers to voltage. Voltage is analogous to water pressure. More pressure, more flow. Voltage is a measurement of the amount of force behind the signal traveling down the wires. The voltage we’re dealing with is pretty small, so to make it easier for us sound guys to deal with, we don’t talk about it in terms of volts at all. Rather, we use the decibel, or dB. A dB is a unitless scale used to compare like values; in this case voltage. We have a reference value, in this case 1 Volt = 0 dBV (the V signifies Voltage), and we reference everything else to that voltage. Thus, -60 dBV corresponds to 1 millivolt. Someday I’ll go into the math on how we get there, for now, take my word for it. I encourage you to do some research on your own to learn more about voltage. For our purposes today, that’s enough.
The other term we need to know is resistance, which we measure in Ohms. Resistance (often expressed as impedance in our world—and yes I know that impedance is actually DC resistance plus capacitance; I’m trying to keep this simple, OK?) is exactly what it sounds like, the resistance, or impediment to signal flow. Going back to our water analogy, think of impedance as the size of the pipe you’re pushing water through. Obviously it’s a lot harder to push a large volume of water through a straw than it is a 4” diameter pipe. In audio land, we have two basic values of impedance, low (roughly 250-600 Ohms) and high (roughly 1,000-10,000 Ohms). At this time, don’t worry too much about the exact values, just get the concepts. Impedance matching is a whole ‘nother post.
Right, so we have that down? Signal level (dBV) and Impedance (low & high). With that as our backdrop, let’s consider the three most common types of signals we face in audio; mic level, line level and speaker level.
Think of a mic level signal as a low level, low impedance signal. Mic level is nominally around -60 dBV, so we’re looking at 1 mV (mV=millivolt, or 1/1000 of a volt, or .001 V), give or take. The impedance is also low, in the area of 250-600 Ohms. Now, even though the voltage is low, we can send mic level signals a reasonably long distance because the impedance is fairly low. When sent over a good balanced cable, mic levels will travel hundreds of feet and arrive pretty much intact. We see mic level coming from mics (obviously) as well as DIs (direct injection) boxes. A DI turns unbalanced, high impedance signals into a balanced, low impedance mic level signal so it can be sent a longer distance. I’ll deal with balanced/unbalanced signals in another post; for now think of them this way—balanced = 3 wires = better for longer runs, unbalanced = 2 wires = good only for short (2-15’) runs. Again, I’m simplifying to get concepts across.
Most professional audio gear using line level signals runs at +4 dBV, which corresponds to a little over 1.5 V. Whereas mic level signals are almost always balanced, line level can be either balanced or unbalanced. Line level is typically high impedance, on the order of 1,000 Ohms (1KOhm) or so, but because the signal level is so much higher than mic level, we can send it long distances (at least if it’s balanced).
I want to pause here for a moment to consider some practical implications. Let’s say you plug a line level signal into an input that’s designed for mic level. What would happen? Going back to our numbers, you have an input that’s looking for 1 mV and you shove 1V into it. That’s about 1,000 times as much signal as it’s expecting (see why we use dB instead of volts? We can say 64 dB instead of 1,000 Volts). You don’t have to be an electrical engineer to guess that the result will not be pleasant. While the input it not likely to be destroyed, the audio signal will be. Gross distortion will be the audible result.
On the other hand, if we plug a mic level signal into an input that is expecting a line level signal, what might happen? Again, we’re feeding a signal that’s roughly 1,000 times lower than expected; so the result will be low signal level and high noise. Starting to make sense?
The third common type of signal we deal with in audio is speaker level. Speaker level is very high level (+22-33 or more dBV and can range from 11-89 volts) and very low impedance (4-16 Ohms typically). With that kind of signal level on hand, it’s pretty clear why we don’t want to plug a speaker level signal into a mic level input. That might actually blow something up. And it’s also why we can’t drive a speaker with the output of a microphone, at least not directly.
Now we could talk about the intricacies of these signaling levels for the next two weeks, but I’m trying to keep the post length manageable. I’ll delve into some more of this stuff in the coming months. In the meantime, do some research on your own. You’ll be amazed at how much more of audio makes sense when you have a firm grasp of these concepts.
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