An Issue With Phase....

[zozoe]

Greetings~ A strange thing happened to me on the way to the gig.... I had my Fender Blues Jr. & my Pro Jr. side by side, slightly angled away from each other, planning to come out of any of my true stereo effects for some tonal bliss... But before ever engaging said effect, I noticed that when the amps were of equal volume, the entire bottom half dropped out.. OOP!! So, simple, I just reverse the leads to one of the speakers,,,

But my assumption is, that one of the amps had been tampered with internally, at some point, & ONE of the 1/4" chassis mounted outputs got reversed??... I mean, for all of us, i always trusted that you could depend on the +/-- coming off the jack to be correct....

All good now, & have a great holiday all!!
ॐ

 

[jeremy]

each tube gain stage flips phase, id guess the blues jr has an extra stage compared to the pro jr. same thing is true for the normal channel and vibrato channel of bf/sf fender reverb amps, they are out of phase due to the reverb channel having three gain stages vs two on the normal channel

 

[zozoe]

wow... did not know that Jeremy!! Thnx, & I assume that my remedy is logical, painless & safe, & that there is NO other solution...?

 

[jeremy]

what you did is perfectly fine. as far as other solutions, you could get an aby box that has a phase switch (i use a radial twin city) but if you are using effects in stereo, its easiest to do just what you did

 

[Ewizard]

The acoustic phase is different from the electrical phase. The acoustic phase is what you hear in the physical space relative to the acoustic source, while the electrical phase is simply the signal electrically that is sent to the speaker. I.E. it is possible to have two devices in the same electrical phase that acoustically don't align.

Reversing the polarity of a speaker, is one way to solve the electrical and physical phase issue. The amp does not care whether it is pushing or pulling a speaker; in fact, it doesn't know what phase it is.

The physical aspect is where the issues are MOST garbled up. The electrical phase is easy to nail down, you can see it on a scope, or use a pulse signal to see which way a speaker moves when the pulse hits. This doesn't do much of anything for the physical location in space relative to another sound source though.

If you have the same signal coming from two physical sources that are electrically in phase, the issue is that the two sources are not able to be close enough together that the acoustic phase will be perfect from any given listening position. If speaker one is closer than speaker two, you hear speaker one first, then you hear speaker two, which WILL be out of phase to some degree.

Anytime the acoustic center of two drivers are more than 1/4 of a wavelength apart, they will start to destructively interfere with each other. To give you an idea, a 100hz frequency has a wavelength of roughly 10', this means that the two acoustic centers can't be more than 2.5' apart, or destructive interference will start to occur. If the acoustic centers are 5' apart at the relative listening position, they will completely cancel out because the signal from one is 180* out of phase with the other. Acoustic phase is a PITA because it is all relative and based entirely on the listener's superposition. This means every single listening position will be different. This means swapping the electrical phase can fix one problem area, but create another.

 

[zozoe]

yeah,, I get it, & whatever electrical stuff may have been done, upset my output +/--,, All's good now.... My testing was with amps side by side & no effects... It was definitely an acoustic? Phase issue,, but I would like to know that if the cone is pulsed to move forward, it does so....

 

[Ewizard]

Well, does it really matter? What phase are your pickups in? Don't know, and can't tell. It's all relative. The easiest way to tell if your speakers are in the same relative phase would be to place a microphone on each one that is at the same exact distance from the speaker. Then place a full bandwidth signal into the amps and get them to the same level ( a song from your phone or something ). Then record the two mics in a DAW, you only need a few moments. If both the speakers are in phase the output will be strong when both recordings are played together. Swap the phase of one of the recording tracks, and if the sound gets weaker, or is obviously not right, you will again confirm, they are in phase. If you have to swap the phase of the recording to get the sound to be right, then one of the speakers is out of phase. Which one is going forward with a + pulse is not going to be answered. But at least you can know that the speakers are in electrical phase.

The only way to know is to repeat the test with a known test unit. Unless you do the test with a very specific signal that you can use to confirm phase, it is very difficult to determine.

Luckily humans are phase blind for the most part. What you hear when you know it is wrong is because you are presented with a comparative result. You can hear the shift or result of phase. BUTTTT, if you only hear one device at a time, your mind wouldn't know or care.

 

[NegativeEase]

Did you change your listening position to other places to ensure you werent just sitting in the bottom (most phase canceling part) of your stereo amp positions and phasing producing a comb filter?

 

[zozoe]

no,,, perhaps I'll try,, but I did 1st try them side by side, & then angled apart,,,, both results were similar,,, there was a massive drop-out when the volumes were matched, then as you lowered one amp ever so slightly, the other amp slowly came up, with the low end intact....
I trust my ears & my experiences, but now with the info on the added preamp tube flipping the phase, it makes sense & sounds fab!! thnx all & happy times for allॐ

 

[Demanic]

The acoustic phase is different from the electrical phase. The acoustic phase is what you hear in the physical space relative to the acoustic source, while the electrical phase is simply the signal electrically that is sent to the speaker. I.E. it is possible to have two devices in the same electrical phase that acoustically don't align.

Reversing the polarity of a speaker, is one way to solve the electrical and physical phase issue. The amp does not care whether it is pushing or pulling a speaker; in fact, it doesn't know what phase it is.

The physical aspect is where the issues are MOST garbled up. The electrical phase is easy to nail down, you can see it on a scope, or use a pulse signal to see which way a speaker moves when the pulse hits. This doesn't do much of anything for the physical location in space relative to another sound source though.

If you have the same signal coming from two physical sources that are electrically in phase, the issue is that the two sources are not able to be close enough together that the acoustic phase will be perfect from any given listening position. If speaker one is closer than speaker two, you hear speaker one first, then you hear speaker two, which WILL be out of phase to some degree.

Anytime the acoustic center of two drivers are more than 1/4 of a wavelength apart, they will start to destructively interfere with each other. To give you an idea, a 100hz frequency has a wavelength of roughly 10', this means that the two acoustic centers can't be more than 2.5' apart, or destructive interference will start to occur. If the acoustic centers are 5' apart at the relative listening position, they will completely cancel out because the signal from one is 180* out of phase with the other. Acoustic phase is a PITA because it is all relative and based entirely on the listener's superposition. This means every single listening position will be different. This means swapping the electrical phase can fix one problem area, but create another.

That must be the root of the magic of mixing different speakers.
Forgive me for stating the obvious.

Sent from my SM-A115A using Tapatalk

 

[zionstrat]

The acoustic phase is different from the electrical phase. The acoustic phase is what you hear in the physical space relative to the acoustic source, while the electrical phase is simply the signal electrically that is sent to the speaker. I.E. it is possible to have two devices in the same electrical phase that acoustically don't align.

Reversing the polarity of a speaker, is one way to solve the electrical and physical phase issue. The amp does not care whether it is pushing or pulling a speaker; in fact, it doesn't know what phase it is.

The physical aspect is where the issues are MOST garbled up. The electrical phase is easy to nail down, you can see it on a scope, or use a pulse signal to see which way a speaker moves when the pulse hits. This doesn't do much of anything for the physical location in space relative to another sound source though.

If you have the same signal coming from two physical sources that are electrically in phase, the issue is that the two sources are not able to be close enough together that the acoustic phase will be perfect from any given listening position. If speaker one is closer than speaker two, you hear speaker one first, then you hear speaker two, which WILL be out of phase to some degree.

Anytime the acoustic center of two drivers are more than 1/4 of a wavelength apart, they will start to destructively interfere with each other. To give you an idea, a 100hz frequency has a wavelength of roughly 10', this means that the two acoustic centers can't be more than 2.5' apart, or destructive interference will start to occur. If the acoustic centers are 5' apart at the relative listening position, they will completely cancel out because the signal from one is 180* out of phase with the other. Acoustic phase is a PITA because it is all relative and based entirely on the listener's superposition. This means every single listening position will be different. This means swapping the electrical phase can fix one problem area, but create another.

Well described... This is why stereo live environments are very rare. It is possible, but it takes a tremendous amount of planning to get decent stereo in somei locations without ridiculous phase cancellation than others.

 

[Ewizard]

In the sound engineering world, we call it " dual Mono " mixing. As opposed to trying to create a stereo mix per-se, instead, we mix to try and minimize the same exact sound coming from any two sound sources. If you can imagine tuning the left speaker to be optimized for its intended coverage area, and then tuning the right speaker to be optimized for its coverage area. The two sides will likely not have exactly the same tuning and ultimately, you don't want them to. Then you pan things left and right to reduce the likelihood of destructive interference. Doing it this way is not a problem for listeners that are in the far field listening zones, they can't hear the phase issues as much, or which side of the PA is affecting the sound one way or another. The near-field listeners just hear it as a stereo mix, and if the system is tuned well enough, they don't really notice the difference in sound from the left and right sides of the pa. Dual-mono mixing is more about panning the instruments to the opposite sides of the stage that they are actually on. So a stage left guitar is panned to the house left speaker. This means that the guitarist can be heard just fine anywhere in the venue, but there are two very distinct sound sources that are not the same and therefore not as easily able to have destructive interference.

There are realistically three or four major types of mixing strategies. There is complimentary mixing, where you are doing more of the traditional " sound support " method where you don't really create more volume, but instead, enhance what is missing from the stage mix (lots of vocals and generally more bass frequency addition to round out the mix). Then there is traditional mixing where you have a stereo system and make a nice studio-like mix. Then you have dual mono mixing where you try and create a panoramic mix that is perceived as mono to almost all listeners. Then you have what I will call " corporate " mixing, which is almost always in mono ( despite the number of speakers hung in the air ) and utilizes significant anti-dynamics and aural enhancement to make a very quite voice heard by thousands over a large area.

Sound engineering is a field where physics dictates your entire existence. And those same physics dictate what happens in your guitar rig. The big question is whether you can heart the problem, and if you can, are you able to understand what is causing it, and then finally being able to find a way to remedy the issue within the confines of physics.

And I will give you a HUGE hint. EQ doesn't fix the problem created by superposition, it requires tools and techniques that are not available to the general public. Moving a speaker a couple of feet, can fix more than EQ alone will.

 

[Demanic]

Years ago I used to run two half stacks in stereo out of a Quadraverb. I had a 30ms digital delay with minimal feedback on one channel and 40ms delay on the other. It sounded just fine but I think that I unconsciously was using one of the strategies that ewizard mentioned. The destructive interference created a pleasing effect.

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[JMP/HBE]

,

 

[Ewizard]

When you start introducing delay much beyond 5-10ms, you get into what is known as the Haas effect. Once you go above about 40ms you get into the critical distance realm. An average person can hear the apparent effect of 5ms difference between two sound sources, but they can't localize which sound comes first. After about 40ms or so, the delay between the two sources starts to get to a point where you can begin to localize the first sound that is heard vs. the second one. Well, trained ears can anyway.

Sound at sea level travels at roughly 1' per ms ( 1.125 at 68* if you want to be more exact, but let's round it out ). So 40ms of delay is roughly 40' ( Actually closer to 45' by that generation ), so if you had two speakers that were right next to each other and one was delayed by 40ms. you would hear that as if one speaker was behind in time by roughly 45'. You would hear one before the other. Now if you separate them by any considerable distance, you can even more easily locate which speaker is closer relatively speaking.

Now, the destructive interference at that point is up for debate. With 40ms of delay, the signal from each speaker is now so far away from each other in time, that they don't really combine anymore. You would be several phase shifts back in time with one of the speakers. So by the time the two signals mixed, they wouldn't have enough of the same media anymore to have total nulling or summation. A 100hz frequency has a wavelength of roughly 10', and a 30hz frequency ( the lowest we can practically reproduce ) has a wavelength of 37'. So with a 40 ms delay, you would only just be 360* out of phase. So it would nearly be in perfect phase with the original sound, but would be behind in time by just over one full wavelength at 30hz. All this means is that you really wouldn't have too much destructive interference that could be heard from a tonal perspective. You would be more annoyed by the obvious delay you could hear.

Generally speaking, speakers are not much more than 25' apart in the day to day world. At a large concert, they certainly are, but by the time you work out the Pythagorean theorem, for those in the intended coverage area of the main hangs, the tangent is less than you would think. When speakers are less than 25' apart ( remember the Haas effect ), people don't really associate one speaker before the other, they just hear a slight delay and are affected mostly by the destructive interference.

Using delay ( shifting of the time element of one source to the other ) we are playing mostly into the psycho-acoustic type of effects. For tuning of PA systems, we are generally working with times of less than 5-10ms for the purposes of time/phase alignment depending upon several factors. Imagine the inverse of the Hass effect to correct time-related phase issues. You delay the nearest speaker so that it meets with the sound from the further speaker. We hear that as 1 speaker if well done. Since we are mostly only delaying speakers to align with a second set or with teh subs, we often don't need much more than a few feet worth of delay to align things.