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Topic: Phase invert on woofers (Read 7745 times) previous topic - next topic
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Phase invert on woofers

Can anyone direct me to a page where i can find a nice and simple (or at least simple enough) explenation on what Phase inverting by subwoofers is, how it wokrs and what are the pros and cons?
Tried to find in google - no luck.

Phase invert on woofers

Reply #1 –
If you are asking what happens when woofers are "out of phase", the result is that the frequencies from the woofers are all but gone.  The reason is that when one woofer is"pushing" the other is "pulling" effectively cancelling each other out.  If this is not the answer you were looking for, sorry.   
Nov schmoz kapop.

Phase invert on woofers

Reply #2 –
Inverting the phase of a single subwoofer, compared to the rest of the speakers, will not delete the bass because there is only a single source.

The background of this idiocy is people doing measurement without understanding them. If you do a DIRAC impulse measurement on a normal pair of speakers, and lets assume for a while they have a perfect phase response, the result of the impulse measurement will look a lot different that the original impulse. Reason being, speakers cant do DC

Now, if you invert the phase of the bass speakers, the impulse looks GRAFICALLY[/i] a lot more similar to the source impulse, for reasons i cant explain anymore ( i once could ).

Again, these people have NO idea what they are doing, as our ears are not liking the result at all, even if the impulse response LOOKS better  .....

Christian
matroska project admin
http://www.matroska.org

Phase invert on woofers

Reply #3 –
You do want your sub to be in phase with your main speakers in the crossover region.
Othewise there will be dip in the frequency response.

If you look at the phase vs frequency response of common smallish ported sat speakers down at the low end of their range (where you would set the crossover frequency), often reversing the phase of the sub gives a better match.

Phase invert on woofers

Reply #4 –
Lots of garbage. Don't trust anything you've read above and keep on googling.

Or, shortcut here: http://www.lenardaudio.com/education/01_eduintro.html and read it all.
It really really did sound different. Not in a placebo way.

Phase invert on woofers

Reply #5 –
Quote
Lots of garbage. Don't trust anything you've read above and keep on googling.
[a href="index.php?act=findpost&pid=280369"][{POST_SNAPBACK}][/a]


Wrong attitude. Examine what you've read above and ask questions. On Google, you can find everything and anything, like websites telling you that CDs have to be de-magnitized with a special device. Unless the source has a good reputation, be very careful with what you read, no matter how professional the website looks. Here on HA, you have at least some better-informed people on some topics, like ChristianHJW who built speakers for quite some time.

Phase invert on woofers

Reply #6 –
Quote
Wrong attitude.
If anyone feels insulted, I apologize. That was not my intention at all.

Simply, the topic went south at 100mph, and it takes few lectures to clear it all up. I'm not in the mood nor have time to engage. So I just issued a warning to the unsuspecting. And, I offered a good source for background aswell. Have you checked it?

With all due respect, there was only one fact unquestionably true in this thread: speakers cant do DC.
It really really did sound different. Not in a placebo way.

Phase invert on woofers

Reply #7 –
Quote
http://www.lenardaudio.com/education/01_eduintro.html[a href="index.php?act=findpost&pid=280369"][{POST_SNAPBACK}][/a]

Thanks for the link, looks promising:
Quote
Audiophiles'   became one of the most bizarre cults of the previous century.   Claiming to be gifted with golden ears, audiophiles can hear electrons traveling in wires, and have the ability to channel thought into crystals to ward off evil harmonics.   Cloaking themselves in mystical terms unrelated to music and physics, they can be heard chanting model numbers and superlatives.   This delusional mix of paranoia and marketing hype, where descriptions attain to greater meaning than the described, exists in all areas of society.

Phase invert on woofers

Reply #8 –
Phase inversion between sub and satellites to avoid problems in the crossover region can only be a problem with 12 dB Filters, and you wont ever have an active sub using such. 12 dB filters are mainly being used in cheap analog crossovers, and analog crossovers are basically crap for subs.

Or to say it in other words :

Somebody buying a subwoofer system using an analog crossover should be stoned to death  ... at this price level, a normal pair of speakers for the same price is always better. In addition to that, 12 dB / Octave  damping is not enough to make the sub unhearable, your stereo image will fully go down the drain.

For a standard active crossover with 24 dB / Octave Linkwitz characteristic, the necessary precondition for a real sub, you dont need phase inversion, not at all. The only sensible reason i could think of are situations, when the sub has to be placed on the opposite side of the wall than the satellites ...... but then you are probably putting out fire with gasoline anyhow  .....

Christian
matroska project admin
http://www.matroska.org

Phase invert on woofers

Reply #9 –
That link does look nice, but it's a bit a little complicated for me. I will check it though.
What i meant by phase invert? Well, in the description for a Creative set of speakers i read that the subwoofer is phase inverted and wondered what it means. The hi-fi at home has also a subwoofer which has a "Phase" option. It produces different bass when it's on and off. I wanted to know what exactly this is? If it improves quality of bass or something?

Phase invert on woofers

Reply #10 –
First off, no offense to him, but just ignore ChristianHJW, he's either trying to prove something or is not reading your question which includes the words "nice and simple".

(now I may oversimplify and offend people)
In a simple two speaker system (only a left and right speaker), if you flip the + and - wires on one of the speakers, the two speakers will be out of phase. 

By out of phase, I mean, if you played the same sound wave through both speakers, one speaker would be pushing out while the other was pulling in and vice versa.

Being out of phase can effect sound quality and usually lower frequencies are effected more, but to what degree depends on many things including what you're listening to and speaker placement. Regardless you should always wire + to + and - to -.

Now if you take that first example and add a subwoofer to it you now have a left speaker, a right speaker, and a subwoofer.  A subwoofer only handles lower frequencies.  In this system, the lower frequencies need to be directed to the subwoofer.  This is done with a crossover.

Let's say the crossover directs the frequencies from 40Hz to 100Hz to the subwoofer and 100Hz to 20kHz to the left and right speakers.  You want the stuff from 100Hz to 20KHz to be in phase with the stuff from 40Hz to 100Hz. 

So if your subwoofer is placed such that it is not in phase with your left and right speakers, flipping the switch may get the phases closer in alignment, thus improving the sound quality. As you have observed one setting will sound better than the other, pick the one that sounds better.  As far as proper placement, that's a topic for someone more knowledgable than myself.

[span style='font-size:8pt;line-height:100%']edit: pushes and pulls mixed up[/span]

Phase invert on woofers

Reply #11 –
Just a few notes further to idioteque's excellent post.

It is crucial to realise that it is the phase of the signals at your listening position that is important. This will be a function of the wavelength of the sound (ie depends on the particular frequency) and of the distance between the speaker and the listener. So, as has been alluded to in some of the posts above, if you move your speakers around, or move around the room yourself, the sound reproduction will have different characteristics (there are in fact many other causes of this, but in terms of this discussion, phase is important around the subwoofer crossover point).

So why is phase so important for subwoofers that even cheap subwoofers will have a phase polarity switch and expensive ones will have 360 degree continuously variable phase settings?

This is because, as ChristianHJW says, crossovers are not perfect. In a perfect world, my crossover would send frequencies 40 - 100Hz to the subwoofer, and only those frequencies. Not a single speck of sound above 100Hz would be produced by the subwoofer, and likewise, nothing below 100Hz would go the the main speaker. But this is not the case. Around the crossover frequency, part of the signal goes to the subwoofer, and part to the main speaker. So at these frequencies, what you hear comes in part from the subwoofer and part from the main speaker. If these two sources are out of phase, there will be partial cancellation, and instead of the signals adding to get the desired loudness level, you will instead end up with a dip in the frequency response curve.

Remember that the phase will vary as the frequency changes, so it will not be possible to get all frequencies in phase between the subwoofer and the main speaker - but this is not really necessary - you only need frequencies where part is going to the subwoofer, and part going to the main speaker to be in phase - ie, those frequencies around the crossover point. And again, as ChristianHJW states, a crossover with a steeper rolloff will be better; this is because there will be a narrower range of frequencies being produced by both sources, and so this out-of-phase-problem will be less noticeable.

Furthermore, you want the subwoofer to be in phase with both left and right main speakers. This is quite difficult to achieve unless the path difference between subwoofer and main speaker is the same for both left and right. As your main speakers should be placed symmetrically in front of you, your subwoofer would ideally be placed midline. (If it is not midline, then it should be placed such that the difference between the two path differences is some whole multiple of the wavelength of the crossover frequency).

So, you may find that a simple phase inversion switch is not adequate to properly align your subwoofer with your main speakers. For example, if you're  90 degrees out of phase at one setting, you will be 270 degrees out (or equivalently -90 degrees out) at the other setting, which is no better. In that case, you will need to move your subwoofer backwards or forwards a bit (or move your listening position, or change the separation distance of your main speakers).

Finally, to get some idea of what all this phase nonsense means to the actual sound, grab some good stereo music that you know well, and can get a feel as to where each instrument is on the soundstage. Open this in a reasonable wave editing program and invert the phase on just the left channel. Save this in stereo and compare it to the origianal recording - try to pinpoint the instruments now. Incidently, if your edited version sounds better, you've mixed up the wiring to one of your main speakers - you've just created a very cheap phase tester for your main stereo speakers.

I know this is a long post, but I've tried to keep it as simple possible, while exploring some of the deeper issues. Hope this helps.

Matt

Phase invert on woofers

Reply #12 –
Sorry, in the above post, the phase inverting trick is most appropriate in mono. Take a stereo source: downmix to mono as one sample, go back to the original stereo, invert on channel, and downmix to mono again as the 2nd sample. You lose all the stereo information, but the phase inverted sample will also have a lumpy frequency response.

Phase invert on woofers

Reply #13 –
Finally some simpler explanation for me.
Thank you.
:-)

Phase invert on woofers

Reply #14 –
Quote
First off, no offense to him, but just ignore ChristianHJW, he's either trying to prove something or is not reading your question which includes the words "nice and simple".




Quote
(now I may oversimplify and offend people)
In a simple two speaker system (only a left and right speaker), if you flip the + and - wires on one of the speakers, the two speakers will be out of phase. 
By out of phase, I mean, if you played the same sound wave through both speakers, one speaker would be pushing out while the other was pulling in and vice versa.
... in normal rooms, this is only valid for deep frequencies ( = bass ). If wavelength of the frequency is < 1x distance of the speaker ( 100 Hz = 3,4 m ), you get attenuation or amplification depending of the angle/distance to the speakers. For example, if distance to speaker 1 is 1/2x wavelength, and to speaker 2 is 1x wavelength, you get full amplification if they are phase inverted. At 100 Hz, this means 1,7 m to sp 1 and 3,4 m to sp2. On listening axis, means equal distance to both, you should get almost perfect attenuation of the signal. As soon as wavelength is bigger than 1x speaker distance, you can go anywhere and the signal will simply be gone, means no bass  ....

Quote
Being out of phase can effect sound quality and usually lower frequencies are effected more, but to what degree depends on many things including what you're listening to and speaker placement. Regardless you should always wire + to + and - to -.
.. agreed. Note that your stereo image goes nuts if you invert one speaker, instead of a singer you will have a HUGE, undefined mouth in front of you, being anywhere and nowhere .....

Quote
Now if you take that first example and add a subwoofer to it you now have a left speaker, a right speaker, and a subwoofer.  A subwoofer only handles lower frequencies.  In this system, the lower frequencies need to be directed to the subwoofer.  This is done with a crossover.
Let's say the crossover directs the frequencies from 40Hz to 100Hz to the subwoofer and 100Hz to 20kHz to the left and right speakers.  You want the stuff from 100Hz to 20KHz to be in phase with the stuff from 40Hz to 100Hz.
 

What you are missing is, that crossovers arent perfect, and this is specially valid for analog crossovers. They wont simply 'cut' frequencies above or below, but in a certain range they will direct equal power to both the sub and the satellite. NOw, corssovers with a 12 db/Oct. damping will cause a phase shift of 90° in this area, -90° to the low and +90° to the high pass. This will result in a 180° phase shift between sub and satellite, and this means no sound at all. Thats why crappy 12 dB analog crossovers will REQIRE a inverse phase between the 2, to work at all.

Quote
So if your subwoofer is placed such that it is not in phase with your left and right speakers, flipping the switch may get the phases closer in alignment, thus improving the sound quality. As you have observed one setting will sound better than the other, pick the one that sounds better.


Wavelenght

25 Hz : 13 m
50 Hz : 6,8 m
100 Hz : 3,4 m

To be 'out of phase' between sub and your satellites, in a normal room, there is usually only one possibility and that's placing them on the opposite wall. Exactly what i said above. But again, if your room forces you to do this kind of shit, you better try to move somewhere else if sound is important for you anyhow  ....


Quote
As far as proper placement, that's a topic for someone more knowledgable than myself.



Again :

A phase inversion switch is a gimmick for a serious sub/satellite system, not worth a penny, only oure marketing stuff. It can only help in some obscure situations, like

- the sub has to be placed on the opposite wall to the satellites ( god may help you to get out of there asap )
- the 'creator' of the sub system had no idea what he is doing, and was 'tuning' it visually, by looking at the graph from the impulse response

For cheap charlie analog sub systems, maybe even with 12 dB corssovers, get rid of them a buy a nice pair of speakers  ....

Christian

Phase invert on woofers

Reply #15 –
I stand by my previous statement.