Phase 0-180

[VWGUY]

I understand Phase is either 0 or 180 because it is basically swapping polarity.

by swapping (+) & (-)

A sub @ 0 would push out on the base hit

A sub set @ 180 would pull in on the base hit

How then do some subwoofers have a switch to adjust between 0-180 by degree/percentage:scratch2:

If I went out to my car and kinda connected the positive to the negative........we all know the result of that!!!!!

 

[1tumbleweed]

Phase is not just 0 or 180; it can be anywhere in between.

Here's an easy way to picture it. Imagine a stereo signal. A sound at the extreme left also is present at the extreme right, but at a reduced level and 180 degrees out of phase with the left. The signal is also present in the center, but 90 degrees out of phase.

It also helps to picture a sine wave; at the crossover point from + to -, it's in phase. At either the top or bottom it's 180 degrees out of phase with regard to the center; at any point along the wave it's somewhere in between.

http://en.wikipedia.org/wiki/Sine_Wave

Happy trails,
Larry B.

 

[JBL GUY]

Polarity and phase are not the same thing.

Changing polarity is just reversing the connections.

Phase relationship is a change in the time relationship of two signals.

The phase relationship between two signals can vary from 0 degrees to as long in the time domain the difference is and the time difference is frequency dependent.

It is common but not correct to use the terms phase and polarity interchangeably.

When the phase is adjusted on a subwoofer, per you question, it is changing the time relationship between the acoustic output of the subwoofer and the electrical input signal to the subwoofer.

Some subwoofers have a switch that simply inverts the polarity and they call it a 180 degree phase shift, which is not technically correct. The phase has not changed, only the polarity.

The first pictures shows the relationship between 2 signals with reversed polarity. Note that the signals start at the same time, that is even though the polarity is reversed, the 2 signals are in phase (they start at the same time, that is they have the same relationship in time and are in phase.)

The second picture is of 2 signals with a phase shift of 90 degrees. Note that they do not start at the same time, that is they are not in phase.


The third picture is of 2 signals with a phase shift of 180 degrees. Note that while the middle of the 2 waveforms resembles the first picture, the 2 signals do not start at the same time, that is they are not in phase.

It is this difference in the relationship in time that differentiates a reversed polarity signal and a signal with a 180 degree phase difference relationship.

When the polarity is changed by reversing the connections the polarity of the signal changes but the phase does not change.

Edit the pictures did not post. Adding pictures.

 

[whoaru99]

Phase and polarity are two different things, although on frequent occasion they're used interchangeably, albeit incorrectly at the 0 and 180-deg points.

Phase is an offset in time between two signals, polarity doesn't have anything to do with time pe se.

 

[coonmanx]

However, if you incorrectly hook up one speaker with reverse polarity you will definitely hear something that is out of phase. So they are related.

 

[JBL GUY]

However, if you incorrectly hook up one speaker with reverse polarity you will definitely hear something that is out of phase. So they are related.

Again that is not correct. The polarity will be reversed and that can lead to issues with the sound quality, but as shown in the pictures that I posted the relationship in time (phase) does not change. Although the polarity will be reversed, the signals will start and stop at the same point in time (they will be in phase).

While you may hear differences in sound quality, it is not due to a change in the timing (phase relationship) of the signals, it is due to the relationship of the polarity of the signals.

 

[sprins]

According to the pictures phase seems to apply to equal frequency waves. How does this work with actual music?

 

[JBL GUY]

That is a good question. If one is specifying the phase difference in degrees it can only apply to a single frequency.

If you specify it in terms of time, then it can apply to all the frequencies contained in the music and one can calculate the difference in phase in degrees for specific frequencies.

I used the pictures of a single frequency for the sake of simplicity.

And of course reversing the polarity affects all frequencies in the signal that has its polarity reversed equally.

 

[so_ein_pech]

In other words, mathematically 180° phase shift and -1 are the same thing. Practically speaking, you have to worry about grounding. For instance, if you reverse the leads of your RCA input to flip the polarity, you are going to have a bad time.

EDIT: JBL, is right, this only applies to one frequency at a time... So phase shift and polarity are fundamentally different operators. Ouch now my head hurts. Still... I like my Morpheus meme so I'm leaving this post here.

EDIT 2: Now, I am beginning to think I was right to begin with. If phase is measured in degrees, then no matter what frequency, a 180° phase shift will result in an inverted signal.

 

[VWGUY]

JBL GUY thank you very much I am sure I am not the only person confused on this topic.

check out this MTX audio article that talks about "wired in phase" or "wired out of Phase" this refers to mounting subs that are outside of the box pointing in as you will see in the pic.

https://www.mtx.com/t/library-phase-discussion

Is this just a example where the word phase in used improperly?

I would expect better from an audio company

 

[JBL GUY]

"In other words, mathematically 180° phase shift and -1 are the same thing."

Except at time 0 for each wave form. See the picture below. It shows 2 waveforms that have both opposite polarity and a 180 degree phase shift.

Note that the 2 signals do note start at the same time, they have a different phase relationship, even though one is polarity inverted.

In our hobby, the time relationship between the beginning and the end of a musical note(s) is important.

And yes VWGUY, from the quick scan I did of the article, the author is incorrectly using the term. This is not uncommon, but it does lead to some confusion when one gets deeper into the technical theory.

 

[coonmanx]

Here's the thing. If you reverse polarity of one speaker and then play a sine wave through it you will get cancellation of that signal by the sine wave coming from the other speaker. The same will happen if you keep the polarities the same but shift the sine wave going to one speaker by 180 degrees. So the result is exactly the same even though in the second example you did it by using a phase shift.

 

[whoaru99]

If I ask someone for a Kleenex they know I what I mean, hand me some tissue, and it gets the job done. But that doesn't change a Puffs into a Kleenex.

 

[JBL GUY]

Here's the thing. If you reverse polarity of one speaker and then play a sine wave through it you will get cancellation of that signal by the sine wave coming from the other speaker. The same will happen if you keep the polarities the same but shift the sine wave going to one speaker by 180 degrees. So the result is exactly the same even though in the second example you did it by using a phase shift.

I mean no disrespect, but this is not correct. You are ignoring the difference in the beginning and the end of the waveform. They do not start or end at the same time and that is the point.

Music is not a continuous sine wave. The musical notes exist for a finite time and it is important that the timing relationship (phase) be correct for the finite time of existence of the musical note.

For a simple example lets take a frequency of 1000 Hz. If 2 1000 Hz signals have a phase shift of 180 degrees there will be a time difference of 1 millisecond between the two signals. If this were to be a musical note of a finite length, then one signal would start and end 1 millisecond sooner that the other signal.

Your example only applies to the waveform at places other than the beginning and the end.

Keep in mind that the human auditory system, when listening to complex waveforms such as music is able to differentiate time differences of 5 microseconds or less.

 

[JBL GUY]

In other words, mathematically 180° phase shift and -1 are the same thing. Practically speaking, you have to worry about grounding. For instance, if you reverse the leads of your RCA input to flip the polarity, you are going to have a bad time.

EDIT: JBL, is right, this only applies to one frequency at a time... So phase shift and polarity are fundamentally different operators. Ouch now my head hurts. Still... I like my Morpheus meme so I'm leaving this post here.

EDIT 2: Now, I am beginning to think I was right to begin with. If phase is measured in degrees, then no matter what frequency, a 180° phase shift will result in an inverted signal.

Again this is ignoring the beginning and the end of a musical note of finite length.

Phase is not about whether a signal is inverted or not, but about the relationship in time of 2 signals. See my previous post.

 

[JBL GUY]

To be clear, for a continuous single frequency signal, the result of polarity inversion and a 180 degree phase shift will be the same as long as both tones exist simultaneously.

But the result is not the same when the tones begin and end. As in my 1000 Hz example, there will be a difference of 1 millisecond in the beginning and ending of the 2 signals.

 

[Audiovet]

Whoa guys - time out please!

The OP asked the simple question regarding the effect of reversing the connections/polarity and understand this as a phase reversal.

JBL, mathematically you are of course correct, but in this simple case the only (theoretical) difference is in the omission of the first half-cycle and the last. We are not discussing signal generator waveforms here, gated precisely to the last microsecond. Any musical instrument I know of does not instantly start at a perfect set of sine waves (a la Fourier) but takes at least a few cycles to reach pitch (analyses of this all over internet).

Now I know there are some folks who claim they can hear this initial half-cycle business and I will not enter that discussion (again the audibility of the forward or backward motion of a driver at any instant has been debated on internet). But unless Vwguy is writing a mathematical exam paper, why are we making this into a theoretical treatise? After all the maths that can ever be quoted, there is no practical (as in audible) difference between seeing a 180 deg as a reversal in polarity or a phase shift. (Unless I need to return my varsity certificates for an upgrade ....)

 

[whoaru99]

Part of it is human nature. We're pretty good at arbitrarily deciding when words mean something and when they just sorta mean something.

 

[Audiovet]

Follow,

JBL Guy,
Yes, a phase difference does denote the comparison between two signals. In this case the relation between what was before and what is now, the second signal being the other stereo channel.

.....there will be a difference of 1 millisecond in the beginning and ending of the 2 signals.

Not wishing to be pedantic, but since you seem to be bent on accuracy: That would be 0,5 mS for a 1kHz signal .... (in the case of your 180 deg out-of-phase example)

 

[JBL GUY]

Whoa guys - time out please!

The OP asked the simple question regarding the effect of reversing the connections/polarity and understand this as a phase reversal.

JBL, mathematically you are of course correct, but in this simple case the only (theoretical) difference is in the omission of the first half-cycle and the last. We are not discussing signal generator waveforms here, gated precisely to the last microsecond. Any musical instrument I know of does not instantly start at a perfect set of sine waves (a la Fourier) but takes at least a few cycles to reach pitch (analyses of this all over internet).

Now I know there are some folks who claim they can hear this initial half-cycle business and I will not enter that discussion (again the audibility of the forward or backward motion of a driver at any instant has been debated on internet). But unless Vwguy is writing a mathematical exam paper, why are we making this into a theoretical treatise? After all the maths that can ever be quoted, there is no practical (as in audible) difference between seeing a 180 deg as a reversal in polarity or a phase shift. (Unless I need to return my varsity certificates for an upgrade ....)

Every musical note has a finite beginning and ending.

Recent research shows that the human auditory system is at least an order of magnitude better at determining the beginning of a musical note than the Fourier uncertainly principle would predict.

This has a bearing on our perception of sound quality.

From the OP's original question "How then do some subwoofers have a switch to adjust between 0-180 by degree/percentage?"

With all due respect, this is what I have explained. The difference between phase and polarity.

I did not mention anything about absolute polarity. That is another can of worms.

Are you saying that as in my 1000 Hz example that it is not possible to differentiate a time difference of 0.5 milliseconds?

For a simple example, taking a conservative estimate, the minimum detectable interaural time difference is 10 microseconds or 0.000010s, corresponding to a shift in sound source location by 1 degree in azimuth relative to straight ahead (consistent with the minimum audible angle).

it is not just the omission of the data in the first and last half cycle, it is the relationship in time of the finite beginning and ending of the musical note.

This does not take into account how important the timing (phase) relationship is when it comes to the fundamental energy of a musical note and its harmonic energy.

The proper relationship in time (phase) of our musical signals is important in the arena of sound stage, imaging and timbre.

"That would be 0,5 mS for a 1kHz signal"

I stand corrected, as I have posted before, I am an old disabled man and the medications that keep what is left of me alive sometimes muddle my mind.

Again, I answered VWGUY's original question, "How then do some subwoofers have a switch to adjust between 0-180 by degree/percentage?"

It is because phase and polarity are not the same thing.