dB Question

[djba]

I hate to show my ignorance but here goes ....

I just bought a pair of new speakers (EV ZLX15Ps). They have a volume control on the back for each of the 2 inputs and a master volume knob.

The input volume knobs are marked with a negative infinity sign for all the way counter-clockwise (no volume) to "Max" which is all the way clockwise (max volume).

The way you are supposed to set the input volume is by watching a little indicator on the back of the speaker (level meter) and setting the volume so that the level meter shows a strong signal without going too high. Then you bring up the master volume to the level that you want. That's where my question comes in.

The master control knob will spin 20, 30, 100 revolutions ... I don't know how many. The led on the back of the speakers shows the level in dB. At a very low level the led shows something like -135dB. As you increase the volume the dB number moves closer and closer to 0dB. 0dB was pretty loud and I didn't turn the knob any further but I am curious to know what this dB number is telling me. (from the video that I found, a "clipping" sign will come on if you turn the master volume up too much)

Here is a little video found from EV that explains the setup but it doesn't really answer my question.

http://www.youtube.com/watch?v=nAEs5GqEJtc&NR=1&feature=endscreen


Thanks in advance for any help.

[Lane]

dB is a 'relative' audio reference. In your case, 0dB is considered the maximum volume. Every 3dB you subtract, you're halfing the power. dB is used by engineers to make the math easy. Instead of having to multiple and divide power levels, you can simply plus and minus dB. This gets handy because the difference in power level from your -135dB to 0dB is something like the difference between 750 watts and 0.0000007353 watts (made up, not real numbers). Speaking in dB gives us simple numbers to deal with.

So anyway, what to do with this?

I don't know this equipment at all, so I'm going to give some general guidelines I follow. If 0dB is your maximum, stay away from it. You don't want to over drive or clip etc. I typically use a -12dB below maximum as my cruising level. It allows a little headroom should something get momentarily louder. You can go closer than that of course, but I'd probably never get close than -6dB.

When dealing with analog circuits, 0dB won't be a hard maximum. You'll start to get some distortion, but not like a harsh clipping that will happening when you go over 0dB in digital. You can always test with the master volume set low and see what happens if you over drive the input. But do be gentle.

And also, a couple of things to keep in mind. dB is a logarithmic scale, not linear. Keep a couple of values in your head. 3dB increase is double the power. 10dB increase is 10 times the power. That should give you some sense of how that sort of scale works.

I hope that made sense.

[djba]

Hi Lane,

That was a great explanation.  Just what I wanted to know.

Thanks.