I had this posted by mistake on another thread. Here it is where it belongs.
I used the example of the bow wave of a ship being "out of phase". In the news we have another example. When there is a Tsunami the initial effect is that the ocean retreats, leaving fish flopping around on what used to be the bottom of the sea.
I suspect that the "near field" sound wave polarity is what we would expect from the cone motion, but that as the distrubance propogates through the air it is modified. I wonder if anyone has done any research on the modification of sonic waveform as a function of propogation distance through air.
The effect I suggest perhaps explains the ambiguity that exists regarding the importance of "absolute polarity". And it suggests that the audibility of "absolute polarity" would best be evaluated using headphones. |
JBL are not the only ones!
I quote from the owqners manual of my Tannoy Little Red Monitor:
"Absolute Polarity
The convention on all Tannoy loudspeakers is as follows:
A positive going signal (ie =ve terminal of 1.5 volt cell) connected to the positive input terminals of the loudspeaker cause LF cone to move into cabinet. In this way, the absolute polarity of the signal from microphone to loudspeaker may be preserved."
Funny that it is JBL and Tannoy, two companies who have probably done more fundamental research into acoustics than all the others combined, would wire their speakers that way. |
That's amazimg. I need to get out more. I have a recording of Tibetan finger bells, but I had no idea there was so much interest in the gong. |
Herman...see http://www.holistic-resonance.com/index2.html |
El, can you recommend any audiophile gong recordings so I can test your hypothesis. |
Tbg...France being screwed up needs no explanation. Australia is geographically inverted, so that explains them.
By the way one could take issue with your presumption that the initial pressure disturbance is positive. How about a gong, or less obviously a drum. (Any bipole source). The initial pressure disturbance polarity will depend on where the listener is located relative to the source.
Many years ago I traveled through the Suez canal, and I observed that the initial water disturbance (wave) due to the moving ship, as measured by water movement at the edge of the canal was down. This is counterintuitive. Although I was an Engineer for four decades I never quite figured out this phenomena that I observed when I was about eleven years old. |
Presumably since the initial pulse of music is a positive pressure wave. This has always concerned me as the microphone moves away with this pressure. If it is phased properly, it seems to me that this would be a negative signal and thus with equal stages of amplification would reach the speaker as a negative signal, causing a retraction of the voice coil. This would indeed make JBL right and the others wrong.
I have heard your explanation about England's choice versus that in the U.S. It fails to explain France and the rest of Europe and obviously fails in Australia. |
Tbg...Why do Yanks drive on the right side and Brits on the left? Here is a reasoned (non-arbitrary answer).
1...In England, when people got around on horseback, it was customary, when meeting a traveler coming the other way to draw your sword. Holding the sword up in your right hand, you would pass the other traveler to the left, so that your sword was between you and him. So the Brits drive on the left.
2...In America, with people traveling longer distances, stage coaches were common, and it was usual for a man to ride one of the front horses. When you mount a horse you do it from the left side. This goes back to when people wore swords, and the scabard was on the left so that a right-handed person could easily draw the sword. (The scabard would get in the way if you tried to mount the horse from the right, and the horse wouldn't like it either). Therefore, the stage coach horse rider was on the left front horse. Now, when you met another stage coach on a narrow road you would steer to the right so that the two guys on the horses could best judge the clearance as the coaches passed.
Can you think of any logical reason why a positive voltage should push the cone out, rather than in? (Be creative). |
It is also arbitrary which side of the road on which we drive, but it helps that there is a convention. In my experience, it often is very important to have correct absolute phase. I only wish I had an easier way to switch phase. |
Tbg..."Mismark"? JBL probably thinks they are right and the world is wrong. And this is ALL JBL drivers not just the ones I use (according to what I read). It is purely arbitrary anyway, and as Sean says you just connect wires accordingly. |
Sean, I would think it would be all woofers facing the same direction should pulse together.
It is curious why JBL would mismark the woofers. Maybe in the original design for which they were used the mid-range and treble were deliberately out of phase with the woofers. |
El: While i'm sure that you know this, all woofers should displace air in the same direction for best results, regardless of how the manufacturer has them labeled. I assume that you corrected this? Sean
> |
Tbg... JBL LF drivers are also phased backwards from the rest of the world. I actually experienced this when wiring up some 15" JBL subwoofers together with some 12" subwoofer drivers of a different brand, and thought that I had screwed up the wiring at the driver inside the enclosure. They went opposite ways. Subsequently I read about it. |
Herman, I have checked seven test records and cds that I have, and you are right, they all suggest reversing leads on one speaker. I guess this is why I have always concluded that at least for the test record or cd, that my system is in absolute phase. I must admit that I wonder why such trivial information is so often provided. I do have one, however, that plays three cuts both in absolute absolute phase and out.
There is a vagueness of the image on some recording when they are out of polarity.
I cannot be held responsible for your listening too loudly. If you had polarity right, it would have blown out the candle at your volume level. Remember this if you ever have another fire in your listen room. |
Tbg, I'm not sure what was "contrary" to your experience. I would only ask you to try it again. Play track two and see if the image is centered when he tells you it should be. If it is, reverse the cable on one speaker only and I believe it will be centered when he tells you it shouldn't be.
I tried reversing the leads on both speakers at the same time and I do believe one way was better than the other even though both were centered.
As far as using the candle. I tried that and when it sucked the flame back it caught my woofer on fire which spread to the cabinet, my carpet , and resulted in about $50,000 in damage. Please PayPal that amount to me ASAP. |
In my experience I concur with Herman. You listen for the voice being between the speakers, not outside of the two speakers. |
Opps, you are right for compression drivers, and it will be hard to see what direction they go. I guess you could put a candle in front and see the drag on the flame. |
Tbg...Unless it is a JBL driver, which goes the opposite way. |
Lvk47, I thought I had posted this, but I guess not.
Take a D cell battery and connect the bottom to one of the leads to the drivers. Then just touch the top + of the battery to the other lead. If the driver moves forward that is the red or hot end. If it move backward, the lead to the + is actually the neutral.
Herman, what you say is contrary to my experience. |
Tbg, it really is one channel out of phase (actually inverted polarity) with the other on the recording and then the two channels are in phase. If you reverse the wires on just one of your speakers the image will be centered when he says the signal is out phase and diffuse when he says they are in phase.
Your question is the tricky part. Once they are both the same polarity (image centered when he says they are in phase) the idea is to listen to the part when he says they are in phase, then reverse both speaker leads to invert the polarity and try it again. The bass should sound more natural one way or the other. As Rives pointed out, some people are more sensitive to this than others. Or perhaps some know what to listen for and others don't. I've never tried it, but I will later today to see if I can hear the difference. |
how can you tell which binding post on the speakers is + or - if they are not marked. I have altec horns and can remove the backs. Thanks |
It is probably the common confusion in the booklet as it is played "in" phase and then "out of" phase. If it is really one channel out of phase with the other, which channel would be reversed for "out of" phase? |
I never looked at the booklet before. If you only listen to the CD then my assessment is correct. Track 2 is labeled "channel phasing, not absolute phase. "Having "a clearly defined central image" as the recording tells you to listen for has only to do with the speakers being in phase with each other, not absolute phase.
Then if you read the booklet the discussion is all about absolute phase. So it appears that it can be used for both. |
The test is for absolute phase or polarity not for one speaker being out of phase with the other. Only careless installation would have that happen.
Changing the leads at the amp or at the speaker equally deal with change the absolute phase.
Remember, however, that many recording are recorded out of phase and some with some microphone in phase and others out of phase.
Some preamps provide a phase switch, although many add another stage to do so, which hurts more than it helps. |
That test shows whether your speakers are in phase with each other, it doesn't show absolute phase. That means if they are being fed the same signal the cones will be moving in and out together. If the phase is reversed then when one speaker is moving in the other is moving out.
If your speakers are in phase, and you reverse both speaker cables at the same time, they are still in phase. You just reversed the polarity.
FYI, the correct term is really polarity, but phase is used so often it has come to be accepted as being correct.
Phase has to do with time.
Polarity has to do with whether the signal is positive or negative, or in the case of a speaker, whether it is moving in or out.
If two signals reach their peaks at the same time they are said to be in phase. If one signal reaches it's peak earlier or later in time than the other then they are out of phase. If you switch the leads on your speakers you haven't changed any timing, you have reversed the polarity. |
Absolute phase is not recogonized by everyone. I used to not be very sensitive to it, and would still say that I'm less sensitive to it than some of my colleges. I do find that it's pretty apparant with solo upright bass. Piano works well too--but not quite as good as bass. Our test CD has phase testing, but goes through inverting individual channels and then both channels. |
