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Author Topic: Spaced subwoofers.  (Read 7616 times)

Kevin Maxwell

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Spaced subwoofers.
« on: January 28, 2016, 02:56:38 pm »

Spaced sub woofers.

How far apart can you space subwoofers before you get the power ally effect? I know that indoors the room definitely impacts the way the sub frequencies will interact. But is there a rule of thumb that says as long as they are x-feet or closer together it will work due to the fact that the sub waveforms are rather large? And how much of a difference would it make if they are mounted in the front of a stage (Permanent) with an enclosed front except where the subs are mounted flush with the stage? I donít think I can get away with center clustering them I would like to not melt the people sitting in the middle.

If one were to use 3 subwoofers one in the middle and one on each side of that how far apart can you put them and have a usable pattern. A little bit of lobe-ing might not be as bad as if there were just 2 subs too far apart.

This for a church that at the moment has 2 EV dual 18Ē subs. They are under the stage and my guess is they are about 20í apart, inside edge to inside edge. They had a power ally up the middle of the room, I donít remember if I delayed one of them or just turned down one or a little of both. It decreased the power ally and shifted it so where the mix position used to be was no longer getting hit as hard.
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Stephen Kirby

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Re: Spaced subwoofers.
« Reply #1 on: January 28, 2016, 05:59:27 pm »

Typically "power alley" is where the power is.  Often the nulls are pretty much in line with the boxes if outdoors, in a room all bets are off.  Since there are sidewall reflections you may find a spot closer or further from the sidewalls that works better.  There is an EV white paper on subwoofer placement.  You can do some rudimentary steering with placement but there will be lobes.

Under the stage makes the decorators happy but it has a tendency to excite the stage and boom up there making it hard for performers to hear the timing of low frequencies like the bass or bass drum.  Also causes any microphones not high passed to howl.
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Bob Faulkner

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Re: Spaced subwoofers.
« Reply #2 on: January 28, 2016, 06:33:41 pm »

Spaced sub woofers.

How far apart can you space subwoofers before you get the power ally effect? I know that indoors the room definitely impacts the way the sub frequencies will interact. But is there a rule of thumb that says as long as they are x-feet or closer together it will work due to the fact that the sub waveforms are rather large? And how much of a difference would it make if they are mounted in the front of a stage (Permanent) with an enclosed front except where the subs are mounted flush with the stage? I donít think I can get away with center clustering them I would like to not melt the people sitting in the middle.

If one were to use 3 subwoofers one in the middle and one on each side of that how far apart can you put them and have a usable pattern. A little bit of lobe-ing might not be as bad as if there were just 2 subs too far apart.

This for a church that at the moment has 2 EV dual 18Ē subs. They are under the stage and my guess is they are about 20í apart, inside edge to inside edge. They had a power ally up the middle of the room, I donít remember if I delayed one of them or just turned down one or a little of both. It decreased the power ally and shifted it so where the mix position used to be was no longer getting hit as hard.
We've had great success with our JBL dual-18" cabinets by keeping them 12' apart on center.  But it really depends on the bass frequencies.  The bass frequencies will behave differently as the cabinets are moved closer or further apart.  In our experience, a guaranteed power-alley starts at 25' on center.
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Kevin Maxwell

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Re: Spaced subwoofers.
« Reply #3 on: January 28, 2016, 07:01:59 pm »

Typically "power alley" is where the power is.  Often the nulls are pretty much in line with the boxes if outdoors, in a room all bets are off.  Since there are sidewall reflections you may find a spot closer or further from the sidewalls that works better.  There is an EV white paper on subwoofer placement.  You can do some rudimentary steering with placement but there will be lobes.

Under the stage makes the decorators happy but it has a tendency to excite the stage and boom up there making it hard for performers to hear the timing of low frequencies like the bass or bass drum.  Also causes any microphones not high passed to howl.

Thank you for the replies.
As it is built at the moment there isnít a problem with the subs they have under the stage, its built ok not flimsy. I have been working with the former music leader that used to be there on a different project and he told me recently that the walls of the cubby holes that the subs are in are lined with sheets of lead. I think he said it was ľĒ thick. They are talking about bumping out the stage so the subject of the subs came up. And speaking of bumps the stage has a slight bump up where the subs are underneath it. They would like to get shorter subs that would allow them to not have to do that again.

I work in one room regularly that has one sub way up in a corner of the room just outside the proscenium. It works pretty well. This church seats about 2000 but the ceilings arenít very high (relatively speaking) so I canít hid a sub somewhere up high easily. It is a weird room itís wider then deep (about 170íx90í) not counting the recessed stage and at the moment the stage is IN the long wall. There are some seats off to the side that you canít see into the stage. That is why they are going to bump the stage out into the room. 

They had one of the big name Contemporary Christian artists in there one time and they brought in some subs that they really liked the sound of. I suggested that they call them to ask what they were but I guess it was long enough ago that it isnít feasible. I have sent them some pictures and links to see if the recognize any of them as looking like what they used. I actually think it was that they had someone that knew what they were doing mixing and that the church would be surprised how their system can sound in the right hands. And by that I am saying that I could do a much better job than they are but they use volunteer amateur sound people.
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Luke Geis

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Re: Spaced subwoofers.
« Reply #4 on: January 28, 2016, 08:48:25 pm »

Any time the speakers are separated by more than about 1/4 wave length you can start to notice the comb filtering, the fortunate thing is that lower frequencies have really long wave lengths. A 100 hz. tone for instance has roughly a 10' wave length. So if you have the speakers within about 2.5' of each other you won't really notice too many problems up to that frequency. Obviously if you have the speakers spread by about the length of the highest wave length your producing you will start to get the power alley. The lower frequencies will have less comb filtering, but again it all depends. The rooms walls and other hard surfaces do have a huge factor. You can still get cancellations / additions at certain spots in a room even if the speakers are placed right next to each other. Subs are a wicked beast really. They are nothing but a compromise all the way around. It is more a matter of what will work the best for that situation. It sometimes works out hat having the power alley with cancellations in the 1/3rds helps more than it hurts.
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Gert Sanner

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Re: Spaced subwoofers.
« Reply #5 on: January 29, 2016, 03:51:22 am »

OK here it goes,
The answer is frequency dependant. However the formula is pretty simple.

In phase terms you get addition ( IE more Power ) if the distance of the centre of two drivers is smaller then 90 degrees of phase. That equals 1/4 wavelength. Between 90degrees and 120 you will experience no gain and no loss.
After that you are getting into the destructive area. Which means areas in the auditorium will have less sub then other areas.

the way to calculate the wavelength is

speed of sound/frequency

so 340 (SoS) / 100 ( HZ ) = 3.40 meters  ( 1 wavelength 100Hz )
Distance between them smaller then 1/2 a wavelength means 1.7 meters at 100 Hz.
This is from speakercenter to speakercenter.
Any lower frequency will behave just as good.
Any higher frequency will start to experience Combfiltering.
You can simulate most of these effects with any of the current manufacturers Softwares.


« Last Edit: January 29, 2016, 03:54:41 am by Gert Sanner »
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Thanx for reading.

Merlijn van Veen

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Re: Spaced subwoofers.
« Reply #6 on: January 29, 2016, 05:23:12 am »

In phase terms you get addition ( IE more Power ) if the distance of the centre of two drivers is smaller then 90 degrees of phase. That equals 1/4 wavelength. Between 90degrees and 120 you will experience no gain and no loss.

I can tell you mean well but AFAIK this is not correct. First of all power is a scalar with only magnitude but no direction. Phase plays no part in power summation, contrary to sound pressure or voltage which are both vectors. Vectors have both magnitude AND direction and are susceptible to time / phase offset.

In other words 2 subwoofers, when powered accordingly, produce more acoustic power (+3 dB [10*log{2}]). But, an increase in SPL, what our ears register, remains to be seen. This is where time comes into play.

Coupling occurs up until 120į or 1/3 wavelength of phase offset. This is the milestone of no gain / no loss (illustration 3). Beyond this point losses will occur.

The first lateral spurious side-lobe occurs when the physical spacing exceeds 240į or 2/3 wavelength of phase offset with a (far-field) magnitude equal to those of the individual sources (illustration 4).

To avoid one ore more spurious side-lobes, which make up for the alternating pattern of power-alleys and power-valleys, keep the physical displacement between the subwoofers equal to or less than 2/3 wavelength of the upper frequency limit of the intended operational bandwidth.

Consult my phase calculator for more information.

Sincerely,

Merlijn
« Last Edit: January 29, 2016, 05:25:30 am by Merlijn van Veen »
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Jay Barracato

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Re: Spaced subwoofers.
« Reply #7 on: January 29, 2016, 07:49:18 am »

I can tell you mean well but AFAIK this is not correct. First of all power is a scalar with only magnitude but no direction. Phase plays no part in power summation, contrary to sound pressure or voltage which are both vectors. Vectors have both magnitude AND direction and are susceptible to time / phase offset.

In other words 2 subwoofers, when powered accordingly, produce more acoustic power (+3 dB [10*log{2}]). But, an increase in SPL, what our ears register, remains to be seen. This is where time comes into play.

Coupling occurs up until 120į or 1/3 wavelength of phase offset. This is the milestone of no gain / no loss (illustration 3). Beyond this point losses will occur.

The first lateral spurious side-lobe occurs when the physical spacing exceeds 240į or 2/3 wavelength of phase offset with a (far-field) magnitude equal to those of the individual sources (illustration 4).

To avoid one ore more spurious side-lobes, which make up for the alternating pattern of power-alleys and power-valleys, keep the physical displacement between the subwoofers equal to or less than 2/3 wavelength of the upper frequency limit of the intended operational bandwidth.

Consult my phase calculator for more information.

Sincerely,

Merlijn

I believe Gert's use of "power" was not meant in the absolute physics sense but just as " how much". I always harp on my students about using words in a general sense in a scientific setting when that word has a more specific meaning in science.

But it does bring up one of my pet peeves when I try to discuss this with other techs.

Yes power is a scalar. It is a the rate at which work is done. Or if you like, work can be defined as the path dependent integral of the power.

I like to think of what is referred to as coupling as two separate processes. The first is where the driver does work on the air. People seem to forget that sound is a compressional wave that oscillates parallel to its propagation, not a transverse wave which is how it is usually shown. So simply, in order to couple, first the drivers must be able to do work on the same mass of air. Since the driver is relatively small compared to the wavelength this relies on the cohesion of air. This is also why the sub moves air a significant distance behind it.

Once the air is disturbed with a vibration, the same cohesion allows the wave to form. The wave then propagates and is able to do work on things it encounters as it moves ( ears, gut, pant legs, walls, the wine bottle we knocked of the shelf a couple of weekends ago). While the wave propagates it is subject to wave mechanics, which Merlijn covered.

That means the work done at a location and time is due to the sum of the waves . The potential for doing work at that point does not change other positions. I.e. canceling at one point does not mean the wave stops.

In a practical sense I feel that in a room up to may be 75m by 75m ( that is a SWAG) that the primary reflections are as large or an larger effect on the soundfield then the effect of splitting the subs up to 12m or center grouping.

With that said, I recently moved 2 srx218s together at the club. They were previously about 3m apart. It did smooth out one null I had at the edge of the dancefloor but I am not sure if that is due to the spacing, moving 4 feet further from the corner (center of stage) or that fact that I totally retuned the system ( the tops were moved as well. My real reason for moving them was to make room for 2 more in the future.

Sent from my XT1254 using Tapatalk
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Jay Barracato

Greg Percy

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Re: Spaced subwoofers.
« Reply #8 on: January 29, 2016, 08:13:14 am »

OK-I was with you for a while...probably just need more cowbell though...
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Chris Hindle

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Re: Spaced subwoofers.
« Reply #9 on: January 29, 2016, 12:17:25 pm »

OK-I was with you for a while...probably just need more cowbell though...

The TRUTH ? YOU can't handle the truth.......
Seriously though, information provided (freely) by those way above my pay grade is worth it's weight in gold.
Read, and reread until you grok the concept. Ask questions if needed.
Chris.
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Ya, Whatever. Just throw a '57 on it, and get off my stage.

ProSoundWeb Community

Re: Spaced subwoofers.
¬ę Reply #9 on: January 29, 2016, 12:17:25 pm ¬Ľ


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