Help Me Understand Phase Reversal

[MT Spaces]

It's Rookie Bob again . I thought I would start a new thread here. In the other mondo thread about AI Series III amps, I think we all agreed that HP and notch filters and phase reversal are desirable features to have in an amp but that we would like to see them independent of each other.

OK here's my new query about Phase Reversal (PR):

Doc Upright said: "As for phase reversal, that's something you want to employ based on the room, the distance between the speaker and the bass, the phase of the signal coming from the pickup, the phase of the signal from the amp (which, by the way, will vary as a function of filter settings) and other factors. The idea is that one usually does not want the output of the speaker canceling the acoustic output of the bass itself." Papa Toad, Tornado, and others also said that they use PR quite a bit, especially at high volumes.

I guess my questions are: How do you know when you need PR? Is it related to feedback somehow? What are the symptoms of the "speaker canceling the acoustic output of the bass itself." How do I hear that happening when I'm standing right next to my DB? Do I hafta have a trained ear out in the crowd / room to give me a clue? Is there some experiment I can try at home with the bass and amp to get a gut feel for this? I really don't know how to get a handle on this .

Alright DrURB, here's your chance to write another chapter in your book . I promise I will still buy a copy when you finish the whole shebang. Thanks All.

[drurb]

Originally Posted by Bob Knebel

Alright DrURB, here's your chance to write another chapter in your book . I promise I will still buy a copy when you finish the whole shebang. Thanks All.

Well, this is a pretty straightforward one. I hope others will chime in with their experience concerning the utility of a phase-reversal switch and how they use it. You don't need much of a trained ear at all. Flip the phase-reversal switch one way or the other and judge the amplified sound. In addition, the phase-reversal switch can help tame feedback. Again, it is a matter of listening. I suggest that you click here and download the manual. There is a fine discussion within it of the use of the phase-reversal switch.

[tornadobass]

The Fishman manual that the Dr mentioned is the first real detailed discussion I've seen. In comparison, the manual from my original ProEQ simply says to flip the switch and decide which sounds better.

I've used a phase switch in two ways. First, if there's uncontrollable low feedback before adequate volume has been reached, that will clean things up. Second, if there isn't low feedback, but the sound is sort of "ringy," the phase switch will get that to stop.

Usually, reversing phase will cut the bottom end somewhat...how much depends on the room, distance from the rig, and probably, the ratio of string-to-body that the pickup reproduces. You can take advantage of that when there's too much bottom in the mix and you want to clean up how your bass fits in.

I'm used to playing in crummy acoustic settings that encourage bottom end mush. Whatever trick helps is worth exploring. Although I haven't done this before, my next gig on upright will probably have the cab up off the floor. I used to actually carry a heavy plastic crate (or two for bigger cabs), and might be turning back to that. One restaurant used to keep both a crate and a little black tablecloth available for this purpose.

[mje]

There are some many complex factors at work here- the distance from the player to the bass and to the amp, distances to the audience, to room boundaries- that this would be very complex to calculate. As DRURB says, flip the switch, and if it sounds better, well, there you are!

[Bob Gollihur]

Make and carry an extra speaker cable that is wired differently on one end than the other: + to - and - to +.

You've changed the phase by 180 degrees and didn't even have to buy a new amp or preamp to do it. If you carry two cabinets, make one up for each or daisy-chain them so their phase is identical.

[drurb]

Originally Posted by Bob Gollihur

Make and carry an extra speaker cable that is wired differently on one end than the other: + to - and - to +.

You've changed the phase by 180 degrees and didn't even have to buy a new amp or preamp to do it. If you carry two cabinets, make one up for each or daisy-chain them so their phase is identical.

EXCELLENT suggestion!

[Uncletoad]

Originally Posted by Bob Gollihur

Make and carry an extra speaker cable that is wired differently on one end than the other: + to - and - to +.

This is a great solution if you are only using the amp these speakers are hooked to to amplify your instrument. If you are going through any other sound reinforcement you won't have the benefits of the phase reversal. In fact you could have some weird phase cancellations that would make you sound pretty puny or at least odd out front if the phase reversed stage sound leaks off the stage and mixes with the non reversed front of house.

I use phase reversal on the Full Circle mostly because it sounds tighter and more focused out of phase with the bass than in phase. I don't know why.

I think phase reversal really helps in higher volume situations where the sound from the bass is not as loud (or important) as the sound from the reinforcement. It reduces feedback and can clean up mud from the bottom.

Conversely 'out of phase' sounds from the amp when you are trying to blend the actual bass acoustic sound with the phase reversed amp sound can lead to the weirdness of phase cancellation (see above). In this case the movement of the top and the movement of the speaker need to be in phase with each other for the amp to be supportive.

This is easier than it sounds. Just flip the switch back and forth and use the sound that doesn't suck.

As everyone else has said flip it and use what works best.

[ToneRanger]

A couple of non-bass phase reversal applications that might help clarify the issue:
First time I came across it, I was running the sound for a concert with a solo singer/pianist. We had a nice condensor mic on the piano, and a vocal mic. The vocal sound was very thin & nasty. Part way through the show I thought I'd try that mysterious phase reversal switch on the mixer input - instantly all the missing low frequencies came back. The piano mic (with lots of gain on it) had been amplifying the voice at about the same level as the vocal mic, but from about a metre away, so all the lows had been cancelled out.

I regularly use phase reversal when snare drums are miked top and bottom - you need to reverse phase on one of the mics to get the two signals lined up.

When recording bass with a mix of mic and pickup sound, you need to check for phase cancellation. I generally roll all the low end off the mic signal so that only the direct signal is providing the low end, and the mic is used more for ambience and colour.

[drurb]

Originally Posted by Uncletoad

This is a great solution if you are only using the amp these speakers are hooked to to amplify your instrument. If you are going through any other sound reinforcement you won't have the benefits of the phase reversal. In fact you could have some weird phase cancellations that would make you sound pretty puny or at least odd out front if the phase reversed stage sound leaks off the stage and mixes with the non reversed front of house.

This may happen anyway because there's no telling what will be the relative phase among the various sound reinforcement systems (e.g., your amp, the house system, its monitor system).

Originally Posted by Uncletoad

I use phase reversal on the Full Circle mostly because it sounds tighter and more focused out of phase with the bass than in phase. I don't know why.

How do you know that the Full Circle is out of phase with your bass when it sounds better? The phase relation that matters, of course, is between the acoustic output from the speaker cabinet and your bass. Depending upon what's in your equipment chain, the phase could be flipped over a number of times. I understand that, with your setup, one position of the phase switch sounds better than the other. My question is how you know that the position in which it sounds better is one in which the sound reinforcement system cancels (is out of phase with) the acoustic output of your bass at its position in space. There are ways to determine this. I'm just wondering what you might have done to do that.

[drurb]

Originally Posted by ToneRanger

I regularly use phase reversal when snare drums are miked top and bottom - you need to reverse phase on one of the mics to get the two signals lined up.

It is worth pointing out that you are flipping the phase switch in order to line them up as closely as possible by choosing one orientation or its opposite. I mention this because microphones and the equipment that they feed certainly can produce phase-shifts anywhere between 0 and 360 degrees. To really do the job, you'd want to use time-alignment and/or an actual phase-shifter but, to my knowledge, this is rarely, if ever, done.

[flatback]

I've just been trying Uncles method of reversing the faze on the FC into the Focus. Because the Faze button is also the notch filter, if you want it out of faze you gotta give up the HP...but I was just wondering about what Drub is talking about, for example how do I determine if the faze(the hazy faze) going into the Avalon U5 preamp I am using isn't flipped in there somehow?
I end up going by my ear and what is useful. The HP filter SOUNDS different when I twiddle the Knob. The Notch, not so much. So I use the HP, and I use the Notch on the mic, not because I think it sounds better( I cant hear any difference) but because I learned here that it SHOULD be that way (to avoid the fazing fuss)...

[ToneRanger]

Originally Posted by Bob Gollihur

Make and carry an extra speaker cable that is wired differently on one end than the other: + to - and - to +.

In addition to this excellent suggestion, in my leads box I keep a (well labelled) short mic lead with pin 2 & 3 connections swapped at one end for reversing the phase of signals going to PA if the desk doesn't have a switch.

[Chris Fitzgerald]

Originally Posted by drurb

How do you know that the Full Circle is out of phase with your bass when it sounds better? The phase relation that matters, of course, is between the acoustic output from the speaker cabinet and your bass. Depending upon what's in your equipment chain, the phase could be flipped over a number of times. I understand that, with your setup, one position of the phase switch sounds better than the other. My question is how you know that the position in which it sounds better is one in which the sound reinforcement system cancels (is out of phase with) the acoustic output of your bass at its position in space. There are ways to determine this. I'm just wondering what you might have done to do that.

I've switched the phase on the Full Circle numerous time with a phase reversal switch on a preamp, and I can tell you it makes a big difference in sound not only in an acoustic environment, but also as a direct signal. In phase, the FC tends to have an overly fat, "gooey" bottom (sorry for the mental image this will bring up to some); with the sound like this, the whole signal is out of whack because of the ratio of lows to mids and highs.

Live, I always use the Focus' HP filter on the FC channel, which *almost* accomplishes the same thing. I've A/B'd the HP vs. Phase thing by running the FC through a preamp first in the chain. The HP filter is almost as good at taming the gooey lows as the Phase reversal, but I like the phase reversal slightly better. However, since the HP filter is built into my amp and the phase reverse isn't (at least, not in the most basic sense), I use the HP filter instead and save the extra schlep.

[drurb]

Originally Posted by flatback

but I was just wondering about what Drub is talking about, for example how do I determine if the faze(the hazy faze) going into the Avalon U5 preamp I am using isn't flipped in there somehow?
I end up going by my ear and what is useful.

Absolute phase does not matter. What matters is the phase of the system relative to that of the bass itself. Go by what sounds better to you.

[drurb]

Originally Posted by Chris Fitzgerald

I've switched the phase on the Full Circle numerous time with a phase reversal switch on a preamp, and I can tell you it makes a big difference in sound not only in an acoustic environment, but also as a direct signal. In phase, the FC tends to have an overly fat, "gooey" bottom (sorry for the mental image this will bring up to some); with the sound like this, the whole signal is out of whack because of the ratio of lows to mids and highs.

Chris-- my points was that, while one position of the switch will produce a better sound than the other, you do not really know (unless one does the analysis) if that is actually "in-phase" or "out-of-phase" with the bass. For example, suppose we take a FC pickup and run it through an amplifying system and a speaker system that we KNOW does not reverse or substantially alter the absolute phase of the signal. Will the output of the speaker cone be in phase with the bass? It might not be at all! The FC, itself, may flip or alter phase. When the switch on your pre-amp indicates "in-phase" that means, if we take the labeling at face value, that the pre-amp is not inverting phase. Again, the FC may be doing that, the power-amp may be doing that, and the speaker may be doing that. So, when you set your pre-amp switch so that it is inverting phase, the net effect may be that the reproducing system, taken as a whole, is in phase with the bass.

[jsbarber]

Also, the feedback effect involes the sound wave created by your amplifier/cabinet propagating to the bass and interacting with the instrument. Even if the speaker cone is "completely in phase with the bass", when the pressure wave reaches the bass a phase shift will be present between the bass and the arriving wave. This shift depends on the frequency of the wave, the distance from the speaker cone to the instrument and the speed of sound through the air.

So, as drurb says, it's not a binary thing. There is a frequency/distance dependent phase shift between the sound arriving at the instrument. If you are using a microphone, it will react to the interfering sound waves produced by the bass and the cabinet. But if you use a pickup, then the pressure wave provided by the cabinet will drive the body of the bass and cause it to respond, this will be transimited to the pickup...

That's why folks are just saying try the two phase switch positions and see what sounds best.

Jim

[drurb]

Originally Posted by jsbarber

Also, the feedback effect involes the sound wave created by your amplifier/cabinet propagating to the bass and interacting with the instrument. Even if the speaker cone is "completely in phase with the bass", when the pressure wave reaches the bass a phase shift will be present between the bass and the arriving wave. This shift depends on the frequency of the wave, the distance from the speaker cone to the instrument and the speed of sound through the air.

So, as drurb says, it's not a binary thing. There is a frequency/distance dependent phase shift between the sound arriving at the instrument. If you are using a microphone, it will react to the interfering sound waves produced by the bass and the cabinet. But if you use a pickup, then the pressure wave provided by the cabinet will drive the body of the bass and cause it to respond, this will be transimited to the pickup...

That's why folks are just saying try the two phase switch positions and see what sounds best.

Jim

Perfecto! Thanks for the addition.

[fdeck]

The phase reversal switch is for in case you are playing in the southern hemisphere.

[MT Spaces]

Originally Posted by fdeck

The phase reversal switch is for in case you are playing in the southern hemisphere.

Thanks fdeck ...... finally a straight answer to my original question. Seems like all the other guys were skirting the question with their usual smoke and mirrors !

OK, really, thanks to all you other folks, I think I may be understanding phase reversal a bit more now.

[Bobby King]

Originally Posted by ToneRanger

When recording bass with a mix of mic and pickup sound, you need to check for phase cancellation.

This is very important! When using two signals for bass, if they are out of phase, you get a substantial loss of bass and other tonal qualities. If anyone records on ProTools or a similar system, try recording a DI track and mic track simultaneously. Then "expand" the tracks on the screen so you can see the individual crests of the waveforms. Side by side, you'll see that the DI track most always is slightly earlier in time (by milliseconds) than the mic track. Depending on how out of sync they are, one track's crest can line up directly with the "valley" of the other track. This results in the "out of phase" cancellation of sound. To correct this, you can either flip the phase on one track, or on the computer you can "nudge" one track until the crests line up and violà, your tone is restored! Likewise, if you use two mics you should also check for phase cancellation.