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Two cabs 180 out of phase - which one is "right"?

Phenometron

Phenometron

Jun 11, 2011
12
0
Atlanta, GA
#1
First, wanted to say that this is my first post on TalkBass. I've lurked around here for years and have gleaned much useful info. One of the most useful forums on the net for me.

Question: I've have two cabs 180 degrees out of phase verified via the 9volt/speaker cable test. I've played through both of the cabs individually for years but only recently matched their impedance to start messing around with all the simultaneous configurations that my swr sm-900 allows.

My first thought was obviously just to choose one and rewire to flip the phase. But then I started wondering. One of them has to be out of phase on an absolute level. In other words, out of phase with the strings on the bass as well as a DI signal sent to the house speakers. I'm thinking that this could potentially be killing tone (sustain and frequency response) as the sound emanates from my cab and interferes with acoustic vibration of the strings as well as the sound from the house mix interfering with the cab on stage.

Does anyone know of a test to determine which cab is in true phase and which is not?
 
#2
With the positive terminal of the battery on the positive input of the speaker, the cone should move out/forward.

As far as being in-phase with the strings and house... good luck. Are you going to determine where the strings are in-phase with your cabinet(s) output and stand in that spot all night? Besides that, not everything imparts a pure 0 or 180 degree phase shift across the full spectrum so you might be in phase at one frequency and not at any other frequency.

- John
 
#3
Thanks John,

Assuming the tip of a TS speaker cable is the positive side and I apply positive battery terminal to it, whichever cabinet's speakers move in is the one I'll rewire.

I know what you mean and yes if I could find that spot on stage I would stand there all night. :)

I'm just obsessive like that I guess.
 
#5
> the energy that the strings share with the environment is negligible compared with the
> energy coming out of your cab and FOH

Certainly seems to have some effect... That's why guitarists find just the right position and place to hold the guitar to get a certain note to feedback. I forget who now (Ted Nugent?), but one famous guitar player would actually put labeled pieces of tape on the stage during sound check for where he could get certain notes to feedback.

I would agree it is probably more the interaction with the body and neck of the guitar than the strings themselves; however, in that case, phase does still have something to do with it.

- John
 
#6
Well, I'm thinking of the way that the sound coming out of the cab can potentially reinforce the strings causing feedback/sustain (more of a guitar thing but also occurs at high volume or with a fuzz pedal or something on bass). It seems like if the cab is wired backwards, the speaks are oscillating exactly opposite of the strings which would have the opposite effect of the feedback/sustain thing.
 
#9
The fact is that phase is frequency dependent like John noted, and the frequencies that seems to be affected sometimes are the lows :(

Especially the lowest 5 notes or so on a four string. Sometimes it's the mids. I just can't be sure if it's a backwards wired cab that I'm hearing or just different room nodes in the different places I'm playing....or the body/neck. (I am playing a Squier after my G&L got stolen:crying:) Part of the reason I ordered a Warmoth maple/maple lacquer finished neck that will get here in 5 -12 weeks depending on how humid it is in Seattle.
 
#10
JKos said:
With the positive terminal of the battery on the positive input of the speaker, the cone should move out/forward.
Click to expand...
+1.
As far as being in-phase with the strings and house... good luck. Are you going to determine where the strings are in-phase with your cabinet(s) output and stand in that spot all night? Besides that, not everything imparts a pure 0 or 180 degree phase shift across the full spectrum so you might be in phase at one frequency and not at any other frequency.

- John
Click to expand...
+1. Those who adhere to the notion of absolute phase or polarity are delusional. At the low end of the spectrum, 40Hz, move away from the speaker 14 feet and you are 180 degrees out of phase. At the upper end, 5kHz, move 1.4 inches and you're 180 degrees out of phase; move 1.4 inches more and you're back to 0 degrees, move 1.4 inches again and you're back to 180 degrees. :eek:
A phase chart of a typical speaker looks like this:
phase.jpg


If phase was audible no speaker would sound good. Luckily, it isn't. Phase only matters when you have two out of phase sources playing the same material.
 
#12
The Diaper Geni said:
Here's one for you smarty-pants types: Is it possible for your backline to be "out of phase" with the FOH/PA/monitors?
Click to expand...
Of course. And it's worst in the low end, so if you put your bass into the monitors it should be high-passed, otherwise the backline and monitors will alternately augment and cancel each other depending on the frequency and the listeners position. For the same reason multiple bass cabs should never be split on the stage.
It's also not only possible but guaranteed that two different size speakers in the same cab running the same bandwidth will be out of phase with each other in the midrange, not to mention two cabs loaded with different drivers.

Oh, yes, I have already mentioned it, in the FAQ, where of course it was argued to death by non-smarty pants types whose knowledge base of how speakers work consists of 'you plug them in'. :hmm:
 
#13
Search for articles about "Time Alignment." ("Time Alignment" refers to positioning multiple speakers so that the sound waves arrive in phase.) You may not have to actually move any wires to use both cabinets together next to each other.

If you simply move one cabinet backwards or forwards slightly in relation to the other cabinet the sound waves will be in phase. Start with the front of both cabinets together and then slide one cabinet backward or forward a little bit until you find the point where the notes you play are loudest - that will be a point where the sound waves are in phase. (Have a buddy help. One plays and the other moves the cabinet.) Once you have them lined up mark them in some way so you can repeat your setup quickly for later gigs.

;)
 
#15
MusicBear said:
If you simply move one cabinet backwards or forwards slightly in relation to the other cabinet the sound waves will be in phase. Start with the front of both cabinets together and then slide one cabinet backward or forward a little bit until you find the point where the notes you play are loudest - that will be a point where the sound waves are in phase. (Have a buddy help. One plays and the other moves the cabinet.) Once you have them lined up mark them in some way so you can repeat your setup quickly for later gigs.

;)
Click to expand...


Seems like too much work, and the possibility of making the stack unstable by having the cabs not seated correctly upon each other.

It is so much easier to switch the wiring ONCE, and be done with it. Then it will always work properly when used with other cabs.
 
#19
MusicBear said:
Search for articles about "Time Alignment." ("Time Alignment" refers to positioning multiple speakers so that the sound waves arrive in phase.) You may not have to actually move any wires to use both cabinets together next to each other.

If you simply move one cabinet backwards or forwards slightly in relation to the other cabinet the sound waves will be in phase. Start with the front of both cabinets together and then slide one cabinet backward or forward a little bit until you find the point where the notes you play are loudest - that will be a point where the sound waves are in phase. (Have a buddy help. One plays and the other moves the cabinet.) Once you have them lined up mark them in some way so you can repeat your setup quickly for later gigs.

;)
Click to expand...
Except that different freqs have different wavelengths, so the distance will change depending on what note you're checking.

And depending on where you stand, based on the distance from both speakers, you will be in phase at one spot, and out of phase if you move, and that point will also change with frequency.

Randy
 

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