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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Subwoofer Forum => Topic started by: Paul Miller on March 06, 2020, 02:07:48 AM
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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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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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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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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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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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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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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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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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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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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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snip
That makes sense, short away~
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Art is correct.
Whenever I go into a facility that has an existing system, I ask the sound guy to turn the system on (powered or unpowered cabinets), but not to run a signal into it.
Most places would frown on you disconnecting their cabinets to put a shorting plug on them.
And once you "touch" the system, YOU now OWN it. If something doesn't work afterwards, YOU will be blamed because you touched it.
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Ivan it was something you posted about years ago that led to me having an "a-ha" moment. As I recall, at a subwoofer shootout with two TH115s, you did the testing, then later discovered that one of them had actually been unplugged. So not only was the setup running at half its potential, but that unused sub's disconnected driver was partially counter-acting the working sub.
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Having never experienced this, how much impact does it REALLY have? Yeah, anything that vibrates can absorb sound. Is the second sub actually absorbing enough to make a real difference?
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Having never experienced this, how much impact does it REALLY have? Yeah, anything that vibrates can absorb sound. Is the second sub actually absorbing enough to make a real difference?
It is not an "across the board" type absorption, but rather freq dependent. You will end up with notches at different freq.
There can be a number of factors that affect this, the actual tuning freq and or resonances of the non driven speaker, the location of the driven loudspeaker in relation to the non driven one, reflective surfaces around either one, the ratio of driven to non driven loudspeakers and so forth.
The actual answer will vary a good bit. It is easy to get an outside measurement, but indoors, all sorts of other factors become involved.
As a general rule, it is best not to do it, just in case
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Having never experienced this, how much impact does it REALLY have? Yeah, anything that vibrates can absorb sound. Is the second sub actually absorbing enough to make a real difference?
The results of one test under controlled outdoor settings using a Keystone tapped horn are posted below.
With the adjacent "dummy" cabinet shorted (green trace), frequency response was basically unchanged from the the single cabinet response.
With the "dummy" cabinet not shorted (purple trace), four ranges were affected, two with +1 to 2dB gain, two with -3 to 4dB less. With "smoothing" applied, the differences only appear to be 1dB ;).
I'd expect distance from the powered to "drone" cabinet as well as differing Fb (Frequency of Box tuning) to affect response with different, unpredictable frequency response shapes. Probably worse than the effect on frequency response is the drum-like response of the "drone" cabinet, as it will continue to ring after excitation, as the driver has no electrical damping.
Art
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here's a goof i did for a laugh...
as i was indoors replacing a driver on one sub, listening to some tunes with another sub cranking, i'm like wow...feel how much this driver is vibrating in my hands.
So I really crank the music and hook a voltmeter to the loose driver....get a full 1 volt out.
Then decide to make a transfer using it as a microphone..beautiful ain't it ? :)
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The results of one test under controlled outdoor settings using a Keystone tapped horn are posted below.
With the adjacent "dummy" cabinet shorted (green trace), frequency response was basically unchanged from the the single cabinet response.
With the "dummy" cabinet not shorted (purple trace), four ranges were affected, two with +1 to 2dB gain, two with -3 to 4dB less. With "smoothing" applied, the differences only appear to be 1dB ;).
I'd expect distance from the powered to "drone" cabinet as well as differing Fb (Frequency of Box tuning) to affect response with different, unpredictable frequency response shapes. Probably worse than the effect on frequency response is the drum-like response of the "drone" cabinet, as it will continue to ring after excitation, as the driver has no electrical damping.
Art
Where was the test mic in relation to the live sub? ...out where the "would-be" audience is seated?
I am thinking that we might hear less of a difference at 50 feet away than we would at 3 feet away.
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here's a goof i did for a laugh...
as i was indoors replacing a driver on one sub, listening to some tunes with another sub cranking, i'm like wow...feel how much this driver is vibrating in my hands.
So I really crank the music and hook a voltmeter to the loose driver....get a full 1 volt out.
Then decide to make a transfer using it as a microphone..beautiful ain't it ? :)
Years ago Community made some cheap speakers with a built in level indicator (LEDs).
We had one in the warehouse just sitting there-nothing plugged in.
We were playing with another sub and noticed that the meters on the Community were flashing, due to the back EMF of the driver that was not being played. That driver was actually moving a good bit with no input, which was producing that voltage.
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Where was the test mic in relation to the live sub? ...out where the "would-be" audience is seated?
I am thinking that we might hear less of a difference at 50 feet away than we would at 3 feet away.
The outdoor test was done with the test mic laying on a piece of plywood on the ground.
Moving out to 50 feet, the same dips and peaks (and drone...) would be heard about 24dB more quietly ;D.
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The outdoor test was done with the test mic laying on a piece of plywood on the ground.
Moving out to 50 feet, the same dips and peaks (and drone...) would be heard about 24dB more quietly ;D.
I was considering the possibility that early reflections could cause a null in the near field, but not a null at 50 feet away.
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I was considering the possibility that early reflections could cause a null in the near field, but not a null at 50 feet away.
Outdoors you should not have reflections (if doing it correctly-and I know Art would make sure of that).
And if there were reflections, the freq of the notch would change as you move further away. It will go lower and lower as the distance between the direct and the reflected sound gets larger and larger
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Outdoors you should not have reflections (if doing it correctly-and I know Art would make sure of that).
And if there were reflections, the freq of the notch would change as you move further away. It will go lower and lower as the distance between the direct and the reflected sound gets larger and larger
I meant reflections off of the adjacent dead sub. But okay. Never mind.
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I meant reflections off of the adjacent dead sub. But okay. Never mind.
I would "assume" that Art put them side by side.
So they would probably be within 1/4 wavelength of each other, so the "reflection" would be a non issue, because the phase difference would be small.
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I would "assume" that Art put them side by side.
Correct, photo descriptor included. I miss being able to test speakers in the road!
Having moved near noisy I-95, the ambient noise could be enough to power LED lighting if we lined the roads with discarded speakers used as generators connected via full wave rectifiers, "Freeway" energy!