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Author Topic: Unused subwoofer shorting  (Read 3125 times)

Paul Miller

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Unused subwoofer shorting
« on: March 06, 2020, 02:07:48 am »

My understanding is that having an unused, disconnected passive sub next to a normal working sub is a bad idea, as the drivers in the unused sub will act to absorb some of the sound waves. This makes sense. Is it enough to simply short the + and - terminals of the sub at the connector, say a Speakon with a jumper between 1+ and 1-? Is that sufficient? Or does the sub actually need to be connected to an amp, powered on but with the level down? What actually needs to happen to make the driver inert?

And how about an unused, self-powered sub? If I set up two powered KW181s, and only turn one on, is the unused one absorbing sound? Or is it internally shorted via the amp, even when it's not powered up?
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Taylor Hall

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Re: Unused subwoofer shorting
« Reply #1 on: March 06, 2020, 11:18:00 am »

Yes, shorting the terminals on the cabinet is sufficient. I would wager the amp idea would work, too, but I'm not an expert in that field so i'll let someone else chime in there.

For the powered sub, if the above is true for leaving a cabinet plugged into the amp while powered on, I would think that simply leaving the other cabinet powered but with no signal would accomplish the same thing?

What kind of conditions are you trying to satisfy by doing this? Are you trying to save electricity by running fewer amps? Or reducing overall output by using fewer cabinets?
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Steve-White

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Re: Unused subwoofer shorting
« Reply #2 on: March 06, 2020, 12:20:50 pm »

My understanding is that having an unused, disconnected passive sub next to a normal working sub is a bad idea, as the drivers in the unused sub will act to absorb some of the sound waves. This makes sense. Is it enough to simply short the + and - terminals of the sub at the connector, say a Speakon with a jumper between 1+ and 1-? Is that sufficient? Or does the sub actually need to be connected to an amp, powered on but with the level down? What actually needs to happen to make the driver inert?

And how about an unused, self-powered sub? If I set up two powered KW181s, and only turn one on, is the unused one absorbing sound? Or is it internally shorted via the amp, even when it's not powered up?

Not to bash you brother.  But why would you ever haul and setup two subs and only use 1?  Backup in case of failure or something?  Even if doing that, I'd run both, then if one dies it's simply a level change or do nothing and get by.

There's no simple answer to this question you pose - many many variables.  Some enclosures are designed with passive radiators and blah blah blah.  Taylor is right, just short it out.  For active or any subs in a separate enclosure, probably best to turn the active on with no signal or gain to 0, passive with shorting jumper across it.

A second sub could actually act as a "bass trap" and cancel out frequencies.  Again, depending upon the scenario there could be many variables in this equation.
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Magnus Högkvist

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Re: Unused subwoofer shorting
« Reply #3 on: March 06, 2020, 12:22:34 pm »

What kind of conditions are you trying to satisfy by doing this? Are you trying to save electricity by running fewer amps? Or reducing overall output by using fewer cabinets?

I know I have had situations where I needed 2 subs to get the rest of the speakers stacked on but not enough amps to run more than one.
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Miguel Dahl

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Re: Unused subwoofer shorting
« Reply #4 on: March 06, 2020, 01:45:43 pm »

Will just turning an active sub on prevent it from moving its cone? Why? I can see that with a passive speaker that afaik a positive pressure on the cone would zero out at the terminals as the cone would then want to push and pull equally at the same time since the same voltage would be applied to both + and - ?
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Paul Miller

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Re: Unused subwoofer shorting
« Reply #5 on: March 06, 2020, 01:56:43 pm »

What kind of conditions are you trying to satisfy by doing this? Are you trying to save electricity by running fewer amps? Or reducing overall output by using fewer cabinets?

Not to bash you brother.  But why would you ever haul and setup two subs and only use 1?

Fair questions. I work in several different clubs and venues. In one club, the small back DJ room is also being used to store two old 2x18 subs that aren't being used. The only practical space available is right next to the TH118. Sort of a worst case scenario, those four 18s just sitting there about a quarter wavelength away.

In another club, there's a row of seven VRX918SPs hard packed directly underneath the stage. Three out of the seven don't make any sound, and since it's not a club I manage the sound at, I haven't had the opportunity to figure out why. I can't even tell if they're powered on. Anyway I want to explain to the owner that they need to be either repaired or removed, otherwise they're compromising the rest of the subs' performance.

The KW181 question was only theoretical, just trying to illustrate my question in the most straightforward way possible. I'm definitely not hauling and setting up two subs and just using one, nor trying to save electricity or short of amp channels.
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Paul Miller

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Re: Unused subwoofer shorting
« Reply #6 on: March 06, 2020, 02:11:34 pm »

For active or any subs in a separate enclosure, probably best to turn the active on with no signal or gain to 0

Will just turning an active sub on prevent it from moving its cone? Why?

I'd also like to get a definitive answer and explanation on this. I want to be able to explain it to others, not just offer a vague, "well that's what they say on PSW".
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Art Welter

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Re: Unused subwoofer shorting
« Reply #7 on: March 06, 2020, 02:49:51 pm »

I'd also like to get a definitive answer and explanation on this. I want to be able to explain it to others, not just offer a vague, "well that's what they say on PSW".
Paul,

The output impedance of an amp is usually less than 0.1 Ω, effectively the same as shorting out the wire that connects it to the driver.
Most decent amplifiers have output relays that disconnect the speaker from the amplifier when power is shut off, or a fault develops, so the amp needs to be "on" and powered to dampen driver movement.

Art


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Paul Miller

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Re: Unused subwoofer shorting
« Reply #8 on: March 06, 2020, 03:31:18 pm »

The output impedance of an amp is usually less than 0.1 Ω, effectively the same as shorting out the wire that connects it to the driver.
Most decent amplifiers have output relays that disconnect the speaker from the amplifier when power is shut off, or a fault develops, so the amp needs to be "on" and powered to dampen driver movement.

The man himself, thanks Art!

So now I can explain why, if a sound company brings in an outside system, and our club's system isn't being used, I still need to have our subs' amps turned on to prevent them from compromising their subs' output. Probably the low/mid amps too.
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Steve-White

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Re: Unused subwoofer shorting
« Reply #9 on: March 06, 2020, 03:40:10 pm »

The man himself, thanks Art!

So now I can explain why, if a sound company brings in an outside system, and our club's system isn't being used, I still need to have our subs' amps turned on to prevent them from compromising their subs' output. Probably the low/mid amps too.

Maybe.

It could deaden, enhance, change, or essentially be null.  Way too complex to answer here - you'd have to setup measurement equipment to know for sure.  Use your own discretion, however the "safest" overall condition is to dampen your system, the easiest way being power it up and 0 signal applied, amps at idle.  The house system in a "dampened" state is probably the best condition.

Very good question sir.
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ProSoundWeb Community

Re: Unused subwoofer shorting
« Reply #9 on: March 06, 2020, 03:40:10 pm »


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