Audio "Dead Zone"

[Altman364]

Hello everyone,

In our auditorium we normally have a mono system (mounted dead center above the apron) but due to its age the amp is acting up rendering the system useless (a local professional will service the system soon).

In the mean time we are using our 2 portable speakers (Mackie SRM 450v2), one on either side of the proscenium. Since our system was designed to be mono the two outputs from the mixer are bridged together to one patch. I have then split this signal to the two speakers with a simple "Y". Each speaker then recieves the same signal.

This set up works great except for one fatal flaw, right between the two speakers there is a "dead zone" line that runs right up the middle of the house and happens to find itself right to where our mixer is positioned. When standing in this "dead zone" you hear a very low lacking muffled version of whatever you're listening to (quite annoying).

I know this has to do with phasing somehow, but I have been unable to find a simple explanation as to why this phenomena occurs. I know how to fix the problem (simply wait for the permanent system to be repaired), I just want to know why it occurs.

Thank you!

 

[Chris15]

It sounds to me like somewhere in one of the signal runs, there's a polarity reversal happening.

When everything is in phase, at the centre, you should have a doubling of audio power, ie. a 3dB increase in volume.
If you have one out of phase, then in the centre the 2 signals will cancel each other out...

I'd be going and checking cables if I were you, or as a cheat, stick a phase reverser on either left or right to get out of trouble for now...

 

[Altman364]

It sounds to me like somewhere in one of the signal runs, there's a polarity reversal happening.

When everything is in phase, at the centre, you should have a doubling of audio power, ie. a 3dB increase in volume.
If you have one out of phase, then in the centre the 2 signals will cancel each other out...

I'd be going and checking cables if I were you, or as a cheat, stick a phase reverser on either left or right to get out of trouble for now...

How exactly would the signal get out of phase in the first place? In theory I am sending the exact same signal from the mixer to the two mic jacks on either side of the stage (I know that the "Y" multiplier in the patch bay is wired correctly). Does that mean that on one of the XLRs connecting the speakers to the jacks that a pin is flipped? Or is this something that happens inside the speaker some how?

 

[Chris15]

The normal culprit is a cable, installed or portable, where pins 2&3 have been reversed at one end of the connection.
The person who made it is after all, only human...
It is much less common, although not unheard of, for the boxes to be the problem, but most modern equipment is wired for pin 2 hot.

 

[damjamkato]

When everything is in phase, at the centre, you should have a doubling of audio power, ie. a 3dB increase in volume.

Just to be pedantic, in a perfect world with two speakers combining completely in phase, you would have a 6dB gain.

 

[museav]

Just to be pedantic, in a perfect world with two speakers combining completely in phase, you would have a 6dB gain.

Provided the speakers are reproducing the same content and it is arriving at the same level and response, then that may be true for SPL, but it would still be +3dB in power and much less than a doubling in perceived volume.

You could try inverting the polarity at one speaker, that should quickly verify if that is indeed the problem, than you can track down where it might be occurring and which side is correct in terms of absolute polarity.

 

[dbrown]

Hello everyone,

In our auditorium we normally have a mono system (mounted dead center above the apron) but due to its age the amp is acting up rendering the system useless (a local professional will service the system soon).

In the mean time we are using our 2 portable speakers (Mackie SRM 450v2), one on either side of the proscenium. Since our system was designed to be mono the two outputs from the mixer are bridged together to one patch. I have then split this signal to the two speakers with a simple "Y". Each speaker then recieves the same signal.

This set up works great except for one fatal flaw, right between the two speakers there is a "dead zone" line that runs right up the middle of the house and happens to find itself right to where our mixer is positioned. When standing in this "dead zone" you hear a very low lacking muffled version of whatever you're listening to (quite annoying).

I know this has to do with phasing somehow, but I have been unable to find a simple explanation as to why this phenomena occurs. I know how to fix the problem (simply wait for the permanent system to be repaired), I just want to know why it occurs.

Thank you!

you are having phase cancellation. i is an old problem with mono singles and two speakers in a stereo separation. if you walk the line so to speak it will get louder and softer as you move away from the speakers

 

[0HZ]

If you have and android device I know there is an application that sends the clicks and you can easily tell which speaker is out of phase from others.

 

[WooferHound]

If a speaker has been replaced in the cabinet, the new speaker could have been connected out of phase

 

[museav]

Like I said, just temporarily invert the polarity at one speaker. If the problem is resolved or the dead spots and loud spots switch then you have pretty much confirmed that as the cause and can work on correcting it.

As far as why it occurs, that relates to the time arrival of the signals from the two speakers and their relative phase. If they arrive at the same time the two signals will be 'in phase' and will sum with one another (+6dB). If they are 180 degrees out of phase they will cancel one another (infinite loss). Between those extremes the result will be somewhere between summation and cancellation.

When you have mirror imaged left and right speakers then down the center of the room you inherently have a line that equidistant from both speakers. That means the path length for the sound from the speakers should be the same for both speakers and the sound from both should arrive in phase, thus summing. If one of the speakers is out of polarity then that makes the two signals on the centerline instead received 180 degrees out of phase and thus summation instead becomes cancellation.