Do these formulas work for setting limters?

[Dace]

I found this on the DBX Forum ( http://www.dbxpro.com/forum/viewtopic.php?f=60&t=2242 )
and wonder if these formulas are correct?



You can calculate the maximum power available to the drivers and set your limiters accordingly.

First you need to convert the power rating of your driver to Volts rather than watts.

Lets assume you have a driver rated for 600 watts at 8 ohms nominal. The formula would be....

Sqrt(watts*ohms) so sqrt(600*8) = 69.28 volts

Next you need to convert the volts to dBu using the formula 20 Log(volts/.775)

so 20 Log(69.28/.775) = 39.03 dBu

Now lets assume you have an amplifier capable of 800 watts at 8 ohms. We now need to figure out mow much gain the amplifier has using the first formula.

sqrt(800*8) = 80 volts. then we convert that to dBu also.
20 Log (80/.775) = 40.28 dBu

Now to find the limiter setting you subtract the amplifier gain from the dBu required from the driver.

39.03-40.28 = -1.25 dBu

So you would set your limiter for 1.25dBu below full output of your amplifier.

So if your amplifier has full output at +4dBu then you would set your limiter for +2.75dBu.

Hope that helps.

Now If your amplifier has a constant gain such as 26 dB or 32 dB gain then you would subtract the gain of the amp from the driver dBu

So for 26 dB gain the math would be
39.03-26 = 13.03 dBu for the limiter

Or for 32 dBu then it would be
39.03-32 = 7.03 dBu for the limiter.

All of these answers would give you ~600 watts to the driver depending on your particular amps settings.
There you go, now your are a little safer.

[sine143]

this is using the thermal rated power though. here in bfm world we have to limit based on excursion, and those numbers are givin in the plans

[Bruce Weldy]

The theory is fine.....but, a volt meter is reality.

Don't trust your settings to math.

[byacey]

That's putting too much trust in published specs for the power amp sensitivity. Far better to just take some measurements and be sure.