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Author Topic: Speaker cable db / power loss  (Read 1281 times)

David Allred

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Speaker cable db / power loss
« on: June 01, 2018, 02:46:21 pm »

Here is a good one for the really smart ones here.
Past threads have made reference to "Q: Why do we need such large cables (10 or 12 awg) for long runs when the wire inside most speaker cabinets is 16 or even 18 awg?  A: Because it is such a short length.  Capacitance or inductance, or some such."
The image shows the typical scenario above and a quasi-reverse scenario.  Are the losses the same for both?  The question arises from awg requirements in an amp rack between amps and patch panels, or even wire size (if wire used) in an amp itself

thanks.
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Chris Grimshaw

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Re: Speaker cable db / power loss
« Reply #1 on: June 01, 2018, 03:20:26 pm »

The resistance of a cable is proportional to its length, and inversely proportional to its conductor area.

What that means is 1' of 1mm^2 copper has the same resistance as 10' of 10mm^2 copper.

From that, we can see that a cable that's 10x longer than the patch cables in the rack/cabinet will completely dominate the total cable resistance.

Chris
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Art Welter

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Re: Speaker cable db / power loss
« Reply #2 on: June 01, 2018, 03:21:53 pm »

The image shows the typical scenario above and a quasi-reverse scenario.  Are the losses the same for both? 
The losses are the same for both, the series wire resistance is the same, assuming the connectors have no resistance. Since the connectors do have resistance, the lower example with presumably an extra connector in the rack would have more series resistance.

The loss of power (a tiny fraction of a dB) is not nearly as noticeable as the loss of damping factor (DF).
With an amp with a DF of 500, 100' of 12AWG at 8 ohms drops the DF to 24.88, with a 4 ohm load the DF would drop to just 12.44.

DF below about 20 will tend to sound "floppy".
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David Allred

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Re: Speaker cable db / power loss
« Reply #3 on: June 01, 2018, 04:39:45 pm »

Thanks
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Len Zenith Jr

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Re: Speaker cable db / power loss
« Reply #4 on: June 01, 2018, 04:48:33 pm »

At the bottom of this link is a handy calculator for wire length:  http://www.bcae1.com/dampfact.htm
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Helge A Bentsen

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Re: Speaker cable db / power loss
« Reply #5 on: June 02, 2018, 03:51:16 am »

One more reason to use Powersoft amps on subs/lows, they can compensate for cable losses below 400hz.
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David Allred

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Re: Speaker cable db / power loss
« Reply #6 on: June 02, 2018, 11:42:11 am »


DF below about 20 will tend to sound "floppy".

My subs are 8 ohm, though my tops on some setups are doubled to 4 ohms for coverage. Can I assume floppiness is only a concern for the subs?
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Art Welter

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Re: Speaker cable db / power loss
« Reply #7 on: June 02, 2018, 01:28:52 pm »

My subs are 8 ohm, though my tops on some setups are doubled to 4 ohms for coverage. Can I assume floppiness is only a concern for the subs?
"Floppiness" could also be considered "lack of definition", it applies to the upper cabinets as well as the subs.

One day on a corporate gig with plenty of extra time we experimented with a pair of 8 ohm center fill cabinets on a 40' wide stage, one had a 25' and the other a 75' 14AWG cord. We had noticed the longer cord cabinet seemed less loud (to be balanced, had to stand about  5' closer to it) so to confirm what the problem was, moved the cabinets next to each other to A/B cords and cabinets and amp sides.

The cabinets/cords/amp sides all read the same level with a dB meter, but to "sound the same", the longer cord cabinet needed to be 3dB louder, as verified by blind A/B tests with the three audio techs on the show.




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Weogo Reed

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Re: Speaker cable db / power loss
« Reply #8 on: June 02, 2018, 01:38:41 pm »

Hi David,

My subs are 8 ohm, though my tops on some setups are doubled to 4 ohms for coverage. Can I assume floppiness is only a concern for the subs?

The 4 ohm load of the two nominal 8 ohm top boxes will need twice as much copper as the single 8 ohm sub. 

Damping Factor is a measure of the amplifier/cable/speaker system to
accurately control the movement of a cone or diaphragm at low, mid or high frequencies.
For instance, with a really low DF, a cone that is supposed to move exactly in/out while tracking an
audio signal may 'wobble' a little bit at the farthest points of travel in and out.  This is heard as distortion.

You could try an experiment with a short, heavy cable feeding a box, and then a long, skinny one.
Note that really good speakers will more easily show DF differences than low quality boxes.
This will probably be most obvious when running the speakers pretty loud.

I generally design for a DF of 40+, but really, it's ok to go with what you hear.
(The Copper industry thanks me! )


Art, I've seen some pretty high dollar boxes that needed a db or two more than a
'perfectly' matched box from the same production run...

Thanks and good health,  Weogo
 
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Art Welter

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Re: Speaker cable db / power loss
« Reply #9 on: June 02, 2018, 01:53:09 pm »


Art, I've seen some pretty high dollar boxes that needed a db or two more than a
'perfectly' matched box from the same production run...

Thanks and good health,  Weogo
Yes, my first thought was we were hearing cabinet differences, but the A/B testing confirmed the differences we heard were due to damping factor.
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Luis_Marquez

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Re: Speaker cable db / power loss
« Reply #10 on: June 02, 2018, 02:28:39 pm »

Can we maintain DF at a certain level by running multiple conductors to speaker for longer runs ? Say for subs on wider stages,you would normally run 12awg/2c at 75feet. Would 12awg/4c at 75feet maintain DF? The down side is adapters and additional weight.
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Chris Grimshaw

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Re: Speaker cable db / power loss
« Reply #11 on: June 02, 2018, 02:36:54 pm »

Can we maintain DF at a certain level by running multiple conductors to speaker for longer runs ? Say for subs on wider stages,you would normally run 12awg/2c at 75feet. Would 12awg/4c at 75feet maintain DF? The down side is adapters and additional weight.

If you need to power multiple cabinets, you could just run cables to individual cabinets - that effectively puts a load of copper in parallel. You'll need a splitter panel at the racks.

Remember, double the length and you've to double the area.

Chris
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Luke Geis

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Re: Speaker cable db / power loss
« Reply #12 on: June 02, 2018, 08:14:07 pm »

If you have 50' of 10awg and then the last 1' is 22awg, the total losses will be the series total of the two. The question might be, what if the 1' of 22awg was what was in the rack? The wires specific resistance would be factored into the whole of the calculation. The 2' of 22awg with a 4 ohm load would reduce an amp with a DF of 500 down to 49 and would reduce output by .1db with a 2% loss in energy. After adding another 50' of 16awg you would loose another .8db of output and another 9% of energy. The Df of the amp would be reduced to essentially nil as the 50' 16awg cable would reduce the DF to 9 ( this is regardless of the 2' of 22awg ). In total the db loss would be .9 db and the total energy loss would be 11%. In other words the 2' of 22awg is not really hurting things too much. The wattage loss in the 50' 16awg cable into a 4 ohm load alone, assuming a power amp of 1000 watts, would be reduced by 174 watts for a total of 826 watts actually dissipated. The 2' of 22awg would loose less than 10 watts.   

The best calc for this I have come across: https://www.electrovoice.com/cableloss.php?x=34.647102&y=-120.4645547
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Re: Speaker cable db / power loss
« Reply #12 on: June 02, 2018, 08:14:07 pm »


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