What is the difference in line level, mic level, tape level, consumer level, +4, and -10? Some are the same, some are louder. Clear it up for you? I didn’t think so.
Audio levels are measured in decibels. Tons of logarithmic math can explain this, and many others have explained the topic extremely well. Google “decibel” and you’ll find a wealth of information on the topic. So in this session there are few things I’ll just ask you to accept.
Decibels are measured in several ways. Each value and method provides the volume or power of a signal, but use different bases for the measurements. For instance, dBu measures power of a signal, in reference to .775 volts, whereas dBV does the same with a 1 volt reference. Also used in our industry is dB SPL (sound pressure level) this is relative to the quietest sound a human can hear. Which is about 20 micropascals. Now, at one point in my life I fully understood micropascals, not so much anymore. For most purposes how we use these numbers is much more important than where they come from.
Let’s take the infamous line levels; professional and consumer. Professional is commonly noted at +4dB. But it is actually +4dbu. This is important because consumer (tape level) is -10dBV. They are not even measured on the same scale. What this means is the true difference between the two (measured in SPL) is actually 11.79 dB, NOT 14!
Next we should tackle mic and line levels. In this discussion, we’ll be assuming line level is professional line level, +4dBu. Line level is “louder” than microphone level. Here’s an easy way to remember that. Microphones need preamplifiers to become line level. So in a mixer, the job of the mic level preamp is to create a hotter signal, as quietly as possible. Inexpensive microphone preamps are easy to produce. They make your shotgun mic louder so it is easy to mix. However, due to low specification tolerances in the electrical components, noise may be added along with the audio. “Noisy” preamps can make mixing dialog, vocal music, and anything with transient higher frequencies, very difficult. Enter in the premium preamps most common in our industry. Cooper Sound, Sound Devices, Wendt, and Sonosax, to name a few, have generally wonderful preamps. Because of this, the industry trusts these names. The added need of putting top quality components in very compact spaces, and creating relatively small quantities of products (in relation to the music industry mega-manufacturers) equals higher prices.
So what is the difference between mic and line level? There is no absolute, remember dB is logarithmic. Logarithms raise a base number to a power. An example…. (math warning!)
In a given logarithmic scale your base value is 2. In order to get to the next value of 3, you need 10 values of 2, or 210, or 2 to the tenth power. Hang with me. The value of 4 is 310. This means that a measured value of 4 is 100 times more than a 2.
A linear scale doesn’t apply to comparing volume levels. If you called Trew Audio, and wanted a simple line level to mic level pad installed in a cable, we would drop the level 40 dBu. This is not a concrete number, but for most professional gear, in most situations, it works well. I commonly get asked if we can do the opposite. “I have a mic level output and a line level input, how can I make them work together?” The answer is… buy a preamp.
Make your gear match. This is the main concept to remember with mic and professional line level. If your mixer only outputs line level, your camera, recorder, etc. need to be set to line level. The same for mic level. In the world of increasingly prosumer video products you may find yourself in a mismatch situation. Always remember line level outputs can be padded down.
If you would like to dig into the logarithmic math, history, and, nomenclature of the decibel I would suggest visiting What’s your dB IQ?