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Let's continue our discussion of decibels.

When you look at equipment specs, you often see the term dBm. What does the "m" stand for?

The m refers to 1 milliwatt, which is a 1/1,000th of a Watt.

Now, in audio, it's assumed that there is a 600ohm load when you measure this power. Using ohm's law, 0.775Volts (rounded) applied to a resistance of 600ohms, results to about 1 milliwatt of power.

So if you buy a preamp or mixer, and it says  it's maximum output level is +20dBm, we can compute that it's maximum output level is 100 milliwatts.

Now, if the technical specs say the maximum output level is +20dBm into 600 ohms, then we know it's max. output level is 100 milliwatts, or 0.1 Watt. Now that we know the Watt, and we have the value of the resistance (600 ohm), transposing the formula P=(ExE)/R, we can compute for the value of E, and it's 7.745 volts, or approx. 7.75 Volts.

So now you know how much voltage an equipment will give when it says in it's specifications +20dBm max. output level. Cool!

TIP: Pick equipment that can give high output levels... +24dBm is better than +20dBm, which is way better than +18dBm.

Okay...? Now you understand what dBm stands for. Are we done? Nope, not yet....

There is another term, called "dBu". You can think of dBu as "u" for unloaded. i.e. without the 600 ohm load. Since dBm is referenced to 1mW (which is a POWER), dBu is used for Voltages.

0dBu is ALWAYS EQUAL to 0.775 Volts (rounded), no matter what the load is. The load can be 600ohms, or 10Kohms. It doesn't matter. dBu is always about the volts. 0dBu = 0.775Volts.

QUIZ: Let's say, you found two equipment at your favorite music store. The output of the first gear is +24dBm (since it's using dBm, it's implied to be 600ohms).  The output of the second gear you're interested in is +24dBu at 10Kohms. 

Which gear do you think has enough power and oomph?

Answer: the  +24dBm.  Why?

Here's why.... the 2nd gear, can only deliver +24dBu at a minimum load impedance of 10Kohms. If you used a lower impedance (less than 10Kohm), this piece of gear will be struggling to maintain +24dBu to the lower impedance load. In fact, if you put a 600ohm load into the 2nd gear, you'll see a reduced output voltage, and you may even possibly overheat the component, or break it.

Meanwhile, the first gear can deliver +24dBm at a load of 600ohms.  No sweat.