Double Bass Out of Phase: how can you tell?

MingusAmongUs said:

I might be opening myself up to ridicule here, and this subject applies to other instruments as well, but how do you know if your line is "out of phase" and you need to reverse polarity or "phase shift" ?
Is it always obvious by the sound alone?
What causes this?
In what kind of DB->pickup or mic->amp scenario might this occur?

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No cause for ridicule at all. First, it's important to recognize that the human auditory system is essentially "phase-deaf." That means it essentially does not care about absolute phase. For example, if you are listening to music via a single loudspeaker, there will be no discernible difference if you simply invert the phase. (Humans are exquisitely sensitive to phase/timing differences across ears but that is another matter).

With musical instrument amplification, the phase or polarity of the amplified signal matters only to the extent that it interacts with the instrument itself and other sources of sound. The DB, as we all know is a resonant cavity that creates its own acoustic energy. When you amplify it, the wavefront emanating from the speaker cabinet can relatively reinforce or cancel the acoustic output of the bass. Reinforce it too much and you could end up with feedback. Cancel it too much and you can impede the vibration of the bass itself. So, you change the polarity of the amplified signal to achieve the best compromise. Ideally, you would want a continuously variable phase shifter so that the best result could be achieved but usually one or the other phase orientation is sufficient.

If you have a device that allows you to invert the polarity of the signal, you simply choose the one that gives you the better result in terms of how it sounds.

Ideally, you would want a continuously variable phase shifter so that the best result could be achieved

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And these devices DO exist (I own two for recording purposes)...

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Two ways to tell if your polarity is out:

1- Record direct to recording software. Look at the wave on screen. The first 'swoop' of the wave should go up.

2- Look carefully at the cone of the speaker on a bass amp. Thump a string to make that 'thump' sound. You should see the cone move 'out' first. If 'out of polarity' it will move 'in' first.

VERY difficult (if not impossible) to hear this on a single source/single speaker listen.

It's when there are TWO sources (or more) that it becomes a little bit of a mess.

BEEF said:

Two ways to tell if your polarity is out:

1- Record direct to recording software. Look at the wave on screen. The first 'swoop' of the wave should go up.

2- Look carefully at the cone of the speaker on a bass amp. Thump a string to make that 'thump' sound. You should see the cone move 'out' first. If 'out of polarity' it will move 'in' first.

VERY difficult (if not impossible) to hear this on a single source/single speaker listen.

It's when there are TWO sources (or more) that it becomes a little bit of a mess.

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Well, I disagree. There is no physical reason at all why the first bit of speaker cone motion should be a compression (outward movement) rather than a rarefaction (inward movement). Neither should the first displacement in a waveform necessarily be "positive." There is nothing more "correct" about either and, as a listener, it is of no consequence at all. By the way, when considering complex physical resonators and complex signal chains, phase often shifts with frequency. So, while the speaker cone or waveform might move in or out at the onset of a sound within a particular frequency, it could likely do the opposite at another.

As for phase-shifters, I sure do know that such devices exist. In fact, decades ago, I used in my research rack-mount vacuum-tube, Grason-Stadler broadband continuous phase shifters complete with OC3 gas voltage regulators. I still have them. They are now 35-40 years old. By the way, I went digital a long time ago.

Hmmm

If you wanted other people's opinion...why are you arguing?

Current feel, in both the academic and field arenas, is that it should be positive/upswing in the metering that you are looking at. If you knew different then why hammer someone that is trying to give you advise?

Not a good way to go about getting information.

Stumpy said:

Hmmm

If you wanted other people's opinion...why are you arguing?

Current feel, in both the academic and field arenas, is that it should be positive/upswing in the metering that you are looking at. If you knew different then why hammer someone that is trying to give you advise?

Not a good way to go about getting information.

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I'm not the one who asked for advice. I was answering the question. By the way, I am certainly in the academic arena. The "feel" that there should be an upswing is little more than convention. It has little, if any, practical basis. What you are referring to is the notion that one should preserve absolute phase through a recording and reproducing chain. That is, if the microphone was hit with a compression, the speaker that ultimately is the reproducer should also produce one. The truth is there is no reason that this needs to be the case and it is often a crap-shoot anyway, considering all of the phase-altering devices that are employed in a typical recording chain. In addition, anecdotal claims to the contrary, it is well-understood (at least by academics) that altering the absolute phase of a running, complex waveform (such as music) is of no perceptual consequence. The only credible studies that have found any effect of absolute phase were those in which highly asymmetric, temporally-restricted signals were employed.

This is not to say that phases should not be aligned across devices. This is the impetus for time-alignment in multiple-driver loudspeakers and across microphones in recording setups.

As far as musical instrument amplification goes, there certainly is no "right" or "wrong" orientation of phase. As I mentioned above, the issue is important only insofar as it affects acoustic interactions between devices (e.g., speaker output and double-bass, two speaker outputs, etc.)

Chris Fitzgerald said:

[KY resident translation]

Try 'er both ways, then use the one that sounds better, y'hear?

[/KY translation]

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Precisely!!!!!!!!!!!!

I stand corrected.

Cut a speaker cord and take both contacts of a 9v battery to a bare end while the other is connected to the input jack of the cab.Just watch the speakers carefully its only one quick jolt that theyll make but youll find the one out of phase

stefandisgust said:

Cut a speaker cord and take both contacts of a 9v battery to a bare end while the other is connected to the input jack of the cab.Just watch the speakers carefully its only one quick jolt that theyll make but youll find the one out of phase

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...or put the two cabs face-to-face while they're playing anything with low-frequency content. If you hear the bass "suck out," they're out of phase.

how do you know if your line is "out of phase" and you need to reverse polarity

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I wuz just answering the above question.

The only thing 'correct' about polarity is the design intention of the piece of gear.

If it's polarity is one way on input and the opposite on output, something is wired wrong (unless it's purpose is to reverse polarity).

The 'convention' of positive polarity from a source (a speaker compression/outward movement as opposed to the opposite) is just a standard. All microphones (as well as ears) hear things by rarefection. The opposite of the source.

You can't tell from a distance. It only matters up close (where you hear/feel the 'sucking' vs outward pressure. And it's only a problem with MULTIPLE SOURCES.

I can think of a GREAT example where BOTH of these things occur at the same time and cause trouble.

When recording/listening to any instrument with a direct input and a mic at the same time. The mic hears only ONE polarity (The capsule never goes 'out' first, just 'in'. Like an eardrum.) shortly AFTER the sound is made (it takes time for sound to travel). The DI signal is heard 'instantly' (no time delay between the 'pickup' and the source) and either has positive or negative polarity.

Well, I disagree.

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I understand that we are arguing semantics here. I apologize if I inferred a 'correct' polarity when I should have stated 'conventional' polarity.






Everyone go wire their stereo's the opposite way now.

Please accept that I am offering the following in a friendly spirit. My intention is not to be argumentative. Rather, I'd like to clear up a few things.

BEEF said:

I wuz just answering the above question.

The only thing 'correct' about polarity is the design intention of the piece of gear.

If it's polarity is one way on input and the opposite on output, something is wired wrong (unless it's purpose is to reverse polarity).

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This is not true in general. For example, many op-amp devices will invert phase. The output of such devices has polarity opposite to the input yet they are not wired wrong and there is no specific intent to invert phase.

BEEF said:

The 'convention' of positive polarity from a source (a speaker compression/outward movement as opposed to the opposite) is just a standard.

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I agree. It has become convention that when a positive voltage is applied to the "hot" side of a loudspeaker connection, the cone should move outward. When you play some source of sound through it, the first voltage change need not drive the cone outward or inward.

BEEF said:

All microphones (as well as ears) hear things by rarefection. The opposite of the source.

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Something is amiss here. Sound waves are longitudinal waves consisting of compressions and rarefactions. Neither microphones (be they piezo, moving-magnet, ribbon, electret, condensor) nor ears "hear" by only rarefactions. It takes both in order for anything but a DC pulse to be transduced. That is, you gotta wiggle it back and forth (that's technical talk ).

BEEF said:

You can't tell from a distance. It only matters up close (where you hear/feel the 'sucking' vs outward pressure. And it's only a problem with MULTIPLE SOURCES.

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Such "feeling" has nothing to do with hearing. If you are considering audio frequencies, you cannot discern the individual "pushes" and "pulls" of the transducer (the speaker) re what you are hearing.

BEEF said:

When recording/listening to any instrument with a direct input and a mic at the same time. The mic hears only ONE polarity (The capsule never goes 'out' first, just 'in'. Like an eardrum.) shortly AFTER the sound is made (it takes time for sound to travel). The DI signal is heard 'instantly' (no time delay between the 'pickup' and the source) and either has positive or negative polarity.

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With all due respect, this is completely incorrect. The microphone capsule absolutely moves in and out and so does the eardrum. In the case of a microphone, that's how you get positive and negative voltages with respect to the neutral or resting position of the diaphragm. Delays that occur via a microphone are not a result of the time it takes to move the diaphragm. Rather, they are a result of the group delay that stems from their filtering properties.

Chris Fitzgerald said:

While we're on the subject, what about those cabs that use banana plug connections on the back. Can you switch the phase by turning the banana plug the other way? If so, is that the same basic effect as hitting the "phase" switch on a preamp?

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Yes, indeed. In fact Bob Gollihur once posted here that he carries inverting and non-inverting speaker cables so that he can do just that (in his case, he's using Speakon or phone plugs so there is no dual-banana to just flip).

[So long as the ground on the speaker cabinet is not in common with the ground in any other part of the circuit which, 99.999999999999999999999999% of the time, it is not. If it were, when you flipped the dual-banana, it would short out the amp.]

Chris Fitzgerald said:

Since purchasing the Focus series III amp head, I 've just been using the HP filter to achieve the same basic effect, and I've been happy enough with that. But now that FLIGHTDECK has started making an affordable HP filter with a phase switch, it looks like a no-brainer to get one for on the road and see what it does with the Full Circle and the Focus. If I do, I'll be sure to report the results in one thread or another.

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BTW, Chris, the switch that turns on the HP filter on the Focus 2r III reverses the phase as well.

BG

bribass said:

BTW, Chris, the switch that turns on the HP filter on the Focus 2r III reverses the phase as well.

BG

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Brian - Actually the notch filter reverses the phase. The high pass filter reverses it again, effectively putting it back where it was.

From the last AI newsletter:

When the notch filter is engaged, there is a 180-degree phase shift in the amp output. ...

When the low cut filter in engaged, the phase shifts another 180 degrees which brings you back to the original phase relationship.​