Old 07-09-2019, 08:52 AM   #1
Peterk312
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Default Checking if monitor speakers are out-of-phase?

I discovered in an odd way thru interest in the JS Stereo Width plugin that I may have been working with monitors (Alesis Prolinear DSP) and was unaware they were not in phase with each other. I attributed some of my problems with poor mixes to my skills but may have actually been dealing with inaccurate reference monitors for the audio. Ten years or so ago I never thought that monitors costing $700 for the pair could have a defect like this.

These connect with balanced cables. I use TRS jacks. The cables were checked over and have no polarity issues or poor solder joints. The pin assignments for both the monitors as well as my interface (1616m Microdock made by E-mu, now discontinued) are the same: +tip/-ring/sleeve.

I pulled out the drivers to check the wiring to the drivers inside the cabinets. It looked correct, but these are powered monitors and I couldn't access the amplifier section to check those connections.

There are a couple things I know that are different between the two monitors but I'm not sure why they would cause phase issues. For example, when these power on there's an LCD screen in the front that indicates firmware version. One says V 1.10 and the other says V 1.20. The one that says V 1.20 has a sticker on the back that says "Positive Phase."



The other monitor does not have this sticker.

Also, you can assign these monitors to a Channel using a Windows application. The monitors connect to a PC with a COM port serial cable. When I check to see they're both on Channel 1, I get a different display. The speaker that has the "Positive Phase" sticker on the back says +Ph in addition to Channel 1. Click below for larger image. I know they are both on the same Channel, but why the +Ph only on one?



Then there's my listening tests. Out-of-phase speakers have a few symptoms compared to in-phase: thin sound, unfocused stereo imaging (especially panning center), frequency cancellation in certain circumstances that produce silence.

I had been working in Reaper on an imported Sonar project and could not get the vocal (mono track) to pan center, so I knew something was very wrong.

One of the first tests I did was go to a website: https://www.richardfarrar.com/are-yo...red-correctly/ Downloaded the file called "out-of-phase." When the announcer says "You should now be hearing my voice in phase," it sounds like a dual mono with the voice equally coming out of both speakers way to the side but definitely NOT centered in the stereo field. When the announcer says, "You should now be hearing my voice out of phase," the voice is dead center in the stereo image. That is indicative of the speakers being out of phase, yes?

I inserted a 100 Hz Sine Wave into two tracks in Reaper, panned one hard left and the other hard right. When I inverted polarity on one track there was much more bass and the sound was louder. Changed the audio clips in the two tracks to a Charlie Hunter tune. Inverting phase in this case moved all audio to the center of the mix and made the bass much louder. Sounds like the speakers were out of phase?

One fix for out-of-phase monitors is to reverse polarity on one of the speaker cables (one jack only). So, I did another set of tests with one speaker cable normal and then reversed polarity on one side:

1. Speaker cable polarity normal (+tip/-ring/sleeve)

Two mono tracks, one panned hard left the other hard right. Both tracks in phase sounded very thin but did not cancel out. It almost sounded like the sound was behind and to the sides of the monitors. Inverted the phase on one track and bass came back full, sound was in the center right in front of the monitors.

Note, inverting phase on one track and panning one side over to the other the sound cancelled out and there was silence, as you'd expect. Same with panning both tracks center, one track out of phase cancelled the sound. Put the tracks back in phase sound came back, and if I panned one side over to the other the sound was louder with more bass than hard right/hard left.


2. Speaker cable with one jack inverted polarity (-tip/+ring/sleeve)

Again, two mono tracks, one panned hard left the other hard right. Both tracks in phase sounded like more bass and louder compared to when you invert phase on one track. That's the opposite of what I got above with speaker cable having normal polarity, but here in this test I believe this is what you'd expect: Thin sound out of phase and more bass/louder when both tracks are in phase. It suggests reversing polarity on one of the TRS speaker cables has fixed a problem, yes?

If I inverted phase on one track and panned it over to the other side the sound cancelled out, as in the previous test condition. Same with panning both tracks center, one track out of phase cancelled the sound. Put the two tracks in phase and sound came back. Panned one side over to the other and the bass was basically the same, the overall sound was slightly louder.

So I got cancellation in both test conditions, but condition 1 still suggests that's when the monitors are out of phase with each other. Right?

Also for test condition 2 (reversed speaker cable polarity) I created two tracks with white noise, one panned hard left the other hard right. Moved the speakers back to their normal arrangement about 3 feet apart for good nearfield stereo imaging. With the two tracks in phase the white noise was dead centered, as you'd expect. Inverted phase on one track, and the sound was way to the side and almost behind the monitors and thinner. Panned tracks to the center, inverted phase on one track, and sound cancelled out. Again, the testing here with the white noise suggests the speakers are in phase with each other, but this is when I've swapped polarity on one of the cables, and that suggests the two monitors have some factor causing them to be out of phase with each other. I just don't know what's causing it.

Sorry for the long post, but if anybody can tell me if I'm troubleshooting this right, or perhaps another test to check phase, I'd appreciate it. I also sent Alesis an email on this days ago, but they have not yet responded.


.

Last edited by Peterk312; 07-10-2019 at 07:04 AM.
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Old 07-09-2019, 09:19 AM   #2
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You could run a 1Hz sine wave at just enough level to see the woofers move and confirm if they are moving in sync. Some sound cards however filter out before that so depending on the card this will/won't work but something like that really is the most definitive as far as woofers go anyway. You don't even really need to see it well, I've been known to do this test and just place my fingers on the woofers and be able to tell if they are in phase.

For example my RME will go all the way down to DC so the test would work but all sound cards don't do that.
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Old 07-09-2019, 02:04 PM   #3
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Quote:
Originally Posted by karbomusic View Post
You could run a 1Hz sine wave at just enough level to see the woofers move and confirm if they are moving in sync. Some sound cards however filter out before that so depending on the card this will/won't work but something like that really is the most definitive as far as woofers go anyway. You don't even really need to see it well, I've been known to do this test and just place my fingers on the woofers and be able to tell if they are in phase.
That's funny you mention this. I actually tried lightly touching the woofer cone at the surround to see if I could feel a thump outwards on a kick drum. I really am not sure I'm feeling an excursion outward. Maybe I'll try what you suggest with the monitors side by side.
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Old 07-09-2019, 02:26 PM   #4
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I can't see any speaker excursion with a 1Hz sine wave.

But I did put on a Joe Locke Quintet tune called Cecil B. DeBop, not in Reaper just using VLC Player. This track has very heavy bass and drum hits. Light touch on the woofer surround and with every thump of the drum both drivers are definitely moving outward.

But this is with one of my speaker cables having reverse polarity, so it suggests something is up with the monitors and they are out of phase with each other somehow.
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Old 07-09-2019, 02:36 PM   #5
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The simplest way to check polarity, is using a 1,5V AA battery as a "sound" source...
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Old 07-09-2019, 03:28 PM   #6
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Originally Posted by cyrano View Post
The simplest way to check polarity, is using a 1,5V AA battery as a "sound" source...
Active speakers. I'd do it for sure on passive.
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Old 07-09-2019, 05:55 PM   #7
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Quote:
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The simplest way to check polarity, is using a 1,5V AA battery as a "sound" source...
Yeah, I've already been advised not to do that with active speakers...
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Old 07-10-2019, 06:42 AM   #8
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Originally Posted by Peterk312 View Post
Yeah, I've already been advised not to do that with active speakers...
If an active speaker fails due to 1,5V DC on it's hot leg, it was either dying anyways, or it is a VERY bad design...

I use that test at least once a month. Never ruined anything and it showed me a few leaky input caps on occasion.

Using a drum kit can be very confusing. I'd sooner adapt Karbo's solution, putting a very low tone on bothe speakers and comparing woofer displacement while the speakers are next to one another.
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Old 07-09-2019, 03:29 PM   #9
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Originally Posted by Peterk312 View Post
That's funny you mention this. I actually tried lightly touching the woofer cone at the surround to see if I could feel a thump outwards on a kick drum. I really am not sure I'm feeling an excursion outward. Maybe I'll try what you suggest with the monitors side by side.
Right, they need to be both touched at the same time to really get a "feel" for it.
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Old 07-10-2019, 04:01 PM   #10
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There are a couple things I know that are different between the two monitors but I'm not sure why they would cause phase issues...One says V 1.10 and the other says V 1.20....
If they were originally bought as a pair shouldn't they be identical? So what's their history? It seems that they were manufactured at different times and a deliberate change was made for some obscure reason.

Did you or a previous owner buy them separately? Did the retailer decide it was OK to provide two different versions as a pair? Last one of V1.10 and first of V1.20?

If you can open one up just swap the two wires at the speaker. If they are spade connectors it'll only take a second.
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Old 07-10-2019, 04:12 PM   #11
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Originally Posted by alanofoz View Post
If you can open one up just swap the two wires at the speaker. If they are spade connectors it'll only take a second.
I was able to get the speakers out and observe the wiring, which looks to be correct polarity to each driver. What you suggest is interesting to try, but these are active monitors and I don't know how they connect at their respective power amps. Are you saying to swap connections at both tweeter and woofer?

If you really want to know the history of these. I purchased them some time in 2004. I got them from Sweetwater. What they did was send me two (they didn't come in "matched pairs"), but one of them had obviously been returned and had cosmetic damage. I didn't like that they tried to sell something used at a price like it was new, so I requested a return. They sent me back a single monitor. I don't believe at the time I checked the firmware versions, and like I said above I always assumed they were not defective, despite having had problems with mixes.
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Old 07-11-2019, 08:02 AM   #12
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Yes, the woofer and tweeter should both be swapped. Photograph the connections first, then assuming different colour wires you can confidently return them to their starting point. You would then no longer need your reversed lead - a situation that could be forgotten at some future date.

However your question about the tweeters raises another question - what exactly went wrong here? Was one of the monitors incorrectly wired somehow (just the woofer and not the tweeter)? If so which one? If the woofers and tweeters are incorrect relative to each other you'll get frequency response problems around the crossover frequency, and that might already be the case with one of the monitors. If you fix the right one (by swapping only the woofer), fine. If you fix the wrong one then both monitors could have a lumpy response. If you do nothing at all, you might still have one with a lumpy response.

That second paragraph is all just speculation, sorry if I've opened a can of worms!
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Old 07-11-2019, 08:27 AM   #13
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Hold on...
Put down the screwdrivers just yet.

Grab a mic (a shit 57 will do in a pinch). You're going to record each speaker.

JS Tone Generator plugin.
Record 3 tones from each speaker separately. A high, mid, and low frequency. Pick something that's in the range of each driver in the speaker. (Two tones if these are only two way speakers.)
Then record a frequency sweep on each one. This will let you look at the cross-over points for any phase issues.

Mic in front of left speaker. Record. Beep Boop Bum Sweep. Repeat for the right.

Zoom in on the waves you recorded. You'll see if anything is out of phase and be able to evaluate each driver. There's no reason for any of this to be a mystery or difficult to evaluate. But real quick, out of phase speakers in stereo will throw everything to one side of the room.
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Old 07-11-2019, 11:23 AM   #14
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Quote:
Originally Posted by serr View Post
Hold on...
Put down the screwdrivers just yet.

Grab a mic (a shit 57 will do in a pinch). You're going to record each speaker.

JS Tone Generator plugin.
Record 3 tones from each speaker separately. A high, mid, and low frequency. Pick something that's in the range of each driver in the speaker. (Two tones if these are only two way speakers.)
Then record a frequency sweep on each one. This will let you look at the cross-over points for any phase issues.

Mic in front of left speaker. Record. Beep Boop Bum Sweep. Repeat for the right.

Zoom in on the waves you recorded. You'll see if anything is out of phase and be able to evaluate each driver. There's no reason for any of this to be a mystery or difficult to evaluate. But real quick, out of phase speakers in stereo will throw everything to one side of the room.
Yes. What you describe about sound thrown to the side (almost sounded like behind the monitors) is exactly what was happening. One of the first tests I did was go to a website: https://www.richardfarrar.com/are-yo...red-correctly/ Downloaded the file called "out-of-phase." When the announcer says "You should now be hearing my voice in phase," it sounded like a dual mono with the voice equally coming out of both speakers way to the side but definitely NOT centered in the stereo field. When the announcer says, "You should now be hearing my voice out of phase," the voice was dead center in the stereo image. That's supposed to be indicative of the speakers being out of phase. Only when I reversed polarity on one of the speaker cables, I got the expected results with the test.

The test you describe is interesting. I will try that later.
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Old 07-11-2019, 11:12 AM   #15
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Quote:
Originally Posted by alanofoz View Post
- what exactly went wrong here? Was one of the monitors incorrectly wired somehow (just the woofer and not the tweeter)? If so which one? If the woofers and tweeters are incorrect relative to each other you'll get frequency response problems around the crossover frequency, and that might already be the case with one of the monitors. If you fix the right one (by swapping only the woofer), fine. If you fix the wrong one then both monitors could have a lumpy response. If you do nothing at all, you might still have one with a lumpy response.
Like I said above, as marked with a red dot on the drivers for + polarity, the drivers are wired correctly. I cannot see how they are connecting at their power amp outputs because I can't get the back plate off the cabinet and don't want to damage it.



Click on the above for a larger image. The connections appear to be correct polarity for both monitors. Red + for woofer. Tweeter has Blue + which you can't see in the photo (that's not a white wire coming from the + connection at the tweeter). I was going to check with a battery only because it's possible, though unlikely, that the woofer polarity may have been marked incorrectly.
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