Deang Posted July 10, 2002 Share Posted July 10, 2002 ...polarity. I don't know why, but I can't for the life of me remember what the difference. Isn't one A/C related and the other - + relatedf>------------------ Deanf>s> Klipsch RF7s - AE-25 DJH - AE-3 DJH - Sony 9000ES f>s> Quote Link to comment Share on other sites More sharing options...
lynnm Posted July 10, 2002 Share Posted July 10, 2002 You have it right Dean. In terms of speakers the speaker cones should,(when fed with an identical signal) move forward and backward in unison. If the speakers are not "in phase" the signals will tend to cancel each other. In theory if the speakers were "perfectly" out of phase the result would be silence. In practice however the result is that the lower frequency response goes to hello and the upper midrange and high end sounds flat and lifeless because noone - not even PWK - has ever succeeded in building speakers that are absolutely perfect in their performance and mated them to an amplifier that is capable of absolutely perfectly and identically reproducing the waveforms produced in an original performance. One may test the phasing of a pair of speakers by putting a battery across the leads of a speaker and noting whether the cone of the woofer moves outward or inward when the +'ve terminal of the battery is connected to the +,(almost universally the red post) terminal and then repeating the test on the speaker's mate. If both move outward or inward the speakers are wired in phase - on the other hand if one goes out and the other in then the speakers are not in phase and should be connected accordingly to the amp or the internal connections of one of the speakers should be corrected. The midrange and tweeter cones will also respond to the battery test but typically the resulting cone movement will not be easily detectible to the observer. A properly configured speaker will be set up so that the drivers will always respond in the same way to a positive or negative signal. Quote Link to comment Share on other sites More sharing options...
Mallette Posted July 11, 2002 Share Posted July 11, 2002 Gee, thanks Lynn. Great tip! Since you didn't specify what battery I figured more is better. Took my 12 volt booster battery and perform the experiment. Yep, out of phase. Right woofer is now in the pool out back. Upside is the left woofer is embedded in the sofa and marks the sweet spot. Love these tips! Keep'em coming. :-) Dave ------------------ David A. Mallett Come taste muh' Klipsch! This message has been edited by Mallett on 07-11-2002 at 09:48 AM Quote Link to comment Share on other sites More sharing options...
lynnm Posted July 11, 2002 Share Posted July 11, 2002 LOL Good point though!! The battery used for this test should be a 1.5v battery - preferably a AAA or AA size. In addition the battery power should be applied only long enough to establish the phase of the speaker. ------------------ It is meet to recall that the Great Green Heron rarely flies upside down in the moonlight - (Foo Ling ca.1900) Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted July 11, 2002 Share Posted July 11, 2002 Deang, One of those questions whose answer is easy to visualize, but difficult to state simply... hummm... Okay, here's my shot at a simple answer. Let me define an arbitrary term. I'm going to define a "positive" motion of a speaker cone as one which moves the speaker cone towards the listener, and a "negative" motion of the speaker cone as one which moves the speaker cone away from the listener. Obviously, music causes the speaker cone to go through cycles of positive / negative / positive / negative movements. Now, let's record a drum. *BANG* I just hit the drum. Now, let's play the recording on our speakers. *BANG* go the speakers. Question for you - when the first bit of the signal from the amp hits the speaker, should the motion of the speaker cone be *TOWARD* you ot *AWAY* from you? Yes, after that first initial transient motion, the speaker cone will move back and forth at whatever frequency the drum is tuned to, but that first motion has to be one or the other. It might seem intuitively obvious that the first motion of the cone ought to be in a "positive" direction, but I really have no idea. Seems to me when you hit a drum, you are moving the drumhead *DOWN* with the impact, which would result in a *NEGATIVE* motion relative to the drummer, and thus a *NEGATIVE* motion of the speaker cone might be the more "accurate", but in any case it doesn't matter to this discussion. What does matter is that if the speaker cone moves in a positive direction when you play that recording, then you reverse the speaker cables (switch red and black at the speaker end without changing the amplifier end) you will reverse the direction of the signal, and the speaker cone will now move in a negative direction. This is called "reversing the polarity" of the speakers. Polarity simply refers to which way the current flow, and resulting driver motion, are oriented. Phase, a word which is somewhat sloppily used in High End Audio , involves the change in the magnitude of a signal over time relative to some reference point. If you take two signals, starting from the same reference point, and measure them at different times, and the signals (magnitude and polarity) are the same, then those signals are "in phase" with each other. If you measure them, and their magnitude is different, than the signals are not in phase with each other. If you graph two signals, use time as the x-axis and voltage as the y-axis, if the two signals are in phase their curves will overlap each other. If they are not in phase, the curves will not overlap. Signals can be out of phase because they have different frequencies. Signals can also be out of phase even though they have the same frequency if one of them is "shifted" foward or backward with respect to the other. Of example, going back to high school math, a sine wave and a cosine wave can have the same frequency, but they are out of phase. The sine wave starts at the origin (0,0) with a magnitude of 0, but the cosine at the origin has a magnitude of 1. The degree of the phase difference between the signals is usually expressed in angular units with one full cycle being 360 degrees - in this case, the sine and cosine are 90 degrees out of phase (one forth of the cycle.) If you take two speakers, feed them the same signal, but have them at different distances from the listener, the speakers will not be "phase aligned" with the listener because the signal from one will lead the other, and this results in distortion. See following diagram. This shows a sine and cosine wave, and the summed signal you would perceive listening to them. Note that the out of phase condition of the two waves creates a summed signal which is neither a sine nor a cosine, but something else. (The red line is a sine curve, the green line is a cosine curve, and the black line is the sum) This diagram shows an extreme case. The two signals are *COMPLETELY* out of phse with each other, or 180 degrees out of phase. Their signal would cancel. If you play these two signals simultaneously over two speakers, you won't hear a sound - it will cancel (uh, this is a *VERY* simplistic case, please don't jump all over me, guys.). Note that you could accomplish this 180 degree phase shift a couple of different ways. You could move one speaker relative to the other so that it is exactly one half wave cycle further away from you - or, you could REVERSE THE WIRES at one of the speakers, thus reversing the polarity of one of the speakers. Your speakers are now playing in opposite polarities, or they are playing 180 degrees out of phase with each other, both descriptions of this condition are accurate. Most people refer to this condition by saying "The speakers are out of phase with each other", which is not really the best description. This is the worst possible mistake to make with setting up a stereo system - wire the speakers so that their polarities are different. It'll sound AWFUL. Note that going back to my first question - should the drum make the speaker driver move in or out - is a different question. This is called the "absolute polarity" of the system. If you reverse the speaker wire leads on BOTH sets of speakers, you reverse the absolute polarity. Some folks think that detecting whether the absolute polarity of a system is "right" or "wrong" is obvious, easy, and has a significant effect on the sound. Other folks don't think it makes any difference at all. Ray ------------------ Music is art Audio is engineering Ray's Music System This message has been edited by Ray Garrison on 07-11-2002 at 12:52 PM Quote Link to comment Share on other sites More sharing options...
pzannucci Posted July 11, 2002 Share Posted July 11, 2002 One thing that Ray I think missed is the fact of the battery thing. When connecting the battery to each of the drivers separately, they DO NOT need to go in the same direction to be in phase when hooked to an amplifier. The crossover introduces phase shift which may imply, such as in a 12db per octave crossover, that the two drivers actually need to be 180 degrees out of phase (polarity reversed) to be in proper phase when a signal is sent to it. Of course this again doesn't take into account driver alignment and spacing which throws other kinks into the equation when going between drivers. Guess this does become a real sticky issue when real life multiway speakers are involved. Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted July 11, 2002 Share Posted July 11, 2002 I think pzannucci's comments are referring to a *SINGLE* speaker with multiple drivers in the *SAME* cabinet. I was talking more about two separate speakers, and the woofer cone in those speakers. If you look at the drivers in a multi-driver speaker that uses a crossover to split the frequency into different bands, then the phase relationship between the different drivers becomes *VERY* complex, involving phase distortions caused by the crossover(s), the mechanical construction of the drivers, and the positioning of the drivers on the cabinet. To keep things simple, I was thinking about two speakers, one full range driver per speaker. ------------------ Music is art Audio is engineering Ray's Music System Quote Link to comment Share on other sites More sharing options...
pzannucci Posted July 11, 2002 Share Posted July 11, 2002 Thanks Ray, You are correct. I was a little more worried about Lynnm's comment with respect to "The midrange and tweeter cones will also respond to the battery test but typically the resulting cone movement will not be easily detectible to the observer. A properly configured speaker will be set up so that the drivers will always respond in the same way to a positive or negative signal." Those remarks would lead to midranges wired backwards in some speakers which should be wired in reverse polarity. See, your wording helped me state polarity instead of phase - didn't think of it that way. Quote Link to comment Share on other sites More sharing options...
WMcD Posted July 11, 2002 Share Posted July 11, 2002 I'll weigh in here. The term phase is always used to compare two sinusoids. They are typically of the same frequency. So they go through the 0 to 1 to 0 to -1 to 0 cycle which describes a complete wavelenght in the same time. They may be of different magnitudes at the peaks. The issue is, how do we decribe the fact that they don't begin at the same point. As the diagram cleverly points out, the extent to which they are "in phase" or not describes how the two add. Now, that is important because we see that two signals may or may not add to a double value. And they may add to zero. Therefore, knowning that the two signals have an equal frequency and known magnitudes fails to tell us how they add. We must know the phase relation. As we see, if they are 180 degrees out of phase, and they're of equal magnitude, they cancel. Another important issue is that just describing "phase" doesn't tell us WHY one waveform if different than the other. The phenomenon can arise from different causes. And they have different results. Let's take the 180 degree phase difference as an example. 1) That can be caused by a delay of time (of one signal) equal to half a wavelength, at a given frequency. Note that it is a special case. The time delay of one, from the referenced other, must correspond to half a wavelength. If the frequency of the waves or time delay change, there is different phase relation. The most common way this comes into play in our application is that one speaker, signal source, is more distant from the ear than the other, speaker, signal source. The difference in time of sound path length causes the delay. So even if the speakers are in phase sending out the signal, the path delay can cause them to arrive out of phase. (As we'll see below, even if they send out of phase, they might be received in phase, if the distances are of certain values.) Please note that if the path length difference is equal to 1, 2, 3, etc wavelenghts, they add. If the path length difference is 0.5, 1.5, 2.5, etc wavelengths, they cancel. So if you move around the room while playing a single frequency, you can achieve all of those easily at midrange frequencies. 2) One signal may be 180 degrees out of phase from the other because of electical polarity reversal. One speaker wired backwards. We see that the polarity reversal looks to be the same as a half wavelength delay. There is cancellation. However, this is not a matter of time delay. And it is not dependent on freqency/wavelenght. If the frequency (of both) is changed, the effect is still the same. The waveform is still inverted by polarity reversal. So it is safe to say that polarity reveral leads to cancellation at all frequencies. 3) One signal may go through a "calculus machine". I'm talking about a capacitor or inductor. These are the components of our crossovers. You may have heard that the current through a capacitor is the rate of change of the voltage. "Rate of change" is the slope of the sine wave. If you look closely at the diagram, you'll see that the cosine wave describes the the slope of the sine wave. They appear to be the same sinusoid, except that they are 90 degrees out of phase. This leads to the statement that that current and voltage "lead" or "lag". Cross overs are more complicated than that simple priciple. And indeed there are issues of differnt "group delays" at different frequecies. There is a time issue. - - - - The most important concept is that knowing phase is necessary to know how two sinusoids add. It describes an essential quality. Equally important is that when the term "phase" is used, one can't jump to conclusions about what might be true over a range of frequencies. - - - - The exception to the frequency/wavelength issue is speaker polarity. Here all the signals going out of one speaker is inverted or 180 degrees out of phase compared to the signal out of the other. Now . . . does that mean that they arrive at the ear 180 degrees out of phase? Yes, if the path lengths to one's ear are exactly equal. Maybe that is not so easy to achieve. And perhaps we can see why speakers wired out of phase lack bass, but only bass. Again we have to consider the wavelengths and path length differences. Sort of an exercise in what we understand about phase, delay, and polarity. We'll assume the two speakers units each have a bass, midrange, and tweeter component drivers. Say it is a Heresy. One Heresy (and thus its component drivers) is wired out of phase, to create polarity reversal out of its three drivers. We'll feed the two speakers with three signals which get sent to the internal component speakers. The first signal will be 100 Hz (which has a wavelenght of about 10 feet) and is sent to the bass radiators. The second signal is 1000 Hz (which has a wavelength of about 1 foot) and is sent to the midranges. The third signal is 10,000 Hz (which a has a wavelenght of about 1/10 foot or an inch) and is sent to the tweeters. You can see an issue coming. If the drivers output are out of phase by 180 degrees because of polarity, the signals can still arrive at our ear "in phase" if the path length difference to your ear is 1/2 wavelength. But what are we talking about in path length difference? In this experiment all you have to do is imagine tape measures extending from you speakers to your ears. Easy. When is one longer than the other, in the legs of a triangle by some distance. - - - Looking at the bass issue. The wavelengh is 10 feet. At or around you listening position, can you get half a wavelenght (5 feet) farther away from one speaker than the other. Nope, not unless you walk around the room. The bass frequencies arrive out of phase, just like they started. Looking at the midrange. The wavelength is 1 foot. At or around you listening position, can you get 1/2 foot more or less distant from one speaker compared to the other. Yes, it is pretty much a matter of leaning one way or the other. Looking at the tweeter issue. The wavelenght is 1 inch. So you'll move to an in phase condition with a path length difference of 0.5 inches. It is hard to avoid this at all. In this please remember that if the path length difference is 1.5 wavelenghts, etc, it is the same as 0.5 wavelengths. - - - - Consider that the immediate above shows something about speaker groups which are in phase to begin with. Remember that if the path length difference is 1/2 wavelenght, there is cancellation. You can't move enough to cancel the bass. Yet moving short distances can achieve cancellation in the mid and treble. - - - Sorry if this got complicated. Gil Quote Link to comment Share on other sites More sharing options...
maxg Posted July 12, 2002 Share Posted July 12, 2002 Seriously confused now. I have several records that are mono from the 1950's. How do they play through the stereo? The exact same signal is being sent to each speaker - resulting in an image that appears to be exactly in the centre of the 2 speakers (fortunately) as opposed to the perfect silence I should be getting if I have understood all your posts. Another question: If I have 2 subwoofers, mounted for arguments sake under the speakers playing say 50 Hz and below the wavelength should be around 16 feet. Assume the speakers are 10 feet apart and I sit less than that distance from their centre line (say 8 feet). If I understand it correctly there is no way I can experience cancellation as the waves should meet be before they meet each other, whilst, a listener positioned 4 feet further behind my can have cancellation. Am I missing something or have I found a way to stop the bass waves travelling out of my appartment and bothering the neighbours? Told you I was confused... ------------------ My System: http://aca.gr/pop_maxg.htm Quote Link to comment Share on other sites More sharing options...
pzannucci Posted July 12, 2002 Share Posted July 12, 2002 maxg, The reason stereo works is that you should be at equal distances from each speaker. This will allow the frequencies to arrive at your ears at the same time without causing phase cancellation that you are thinking about. You will notice two things, as you move around the room, the sound will change. Part of that has to do with this cancellation. Secondly, the easiest test to show phase relationships is a stand up - sit down test. When you are directly in front of the speakers on their direct plane the sound should be more smooth. If you stand up allowing the distance of the drivers from one another to be altered, you will notice a difference in the sound. This is because of the time introduced phase relationships between the individual drivers. They will typically develop notches in the overall speaker response due to the drivers not arriving "in phase" with each other when standing as they do when you are in the direct path of the speaker. This is easiest heard with something like pink noise or FM hiss. That is why you see magazines sometines state frequency response above and below the tweeter. The notch deepens depending on how many degrees off axis you are. This type of test shows the relationship of driver phase and is very noticable with center speakers with a center tweeter going right or left off axis, thus typical reviewers comments about a "notch developing in the response". This is all the same theory (actually reality). With your question of the subwoofers, cancellation isn't exact unless the wavelengths are exactly 180 degrees out of phase. What you will get is variations in frequency response at those frequencies. I can wire my woofers out of phase so as I enter the room - from the left of both my speakers I will hear bass response because they are in phase due to their distances apart in the room from where I am entering from. As soon as I get equal distances from each speaker, my bass disappears. I have done this by mistake and when I entered the room to sit down and listen I thought my woofers had gone bad. Peter Z. This message has been edited by pzannucci on 07-12-2002 at 11:45 AM Quote Link to comment Share on other sites More sharing options...
maxg Posted July 12, 2002 Share Posted July 12, 2002 Thanks Pete, I had never noticed variations in my mono recordings - I will check it out tonght (I tend to sit in the sweet spot to listen and dont move from there except in case of an earthquake - happened once - I thought my sub was playing brilliantly till I realised what was going on). Quote Link to comment Share on other sites More sharing options...
pzannucci Posted July 12, 2002 Share Posted July 12, 2002 maxg, The mono test and moving will not be as noticable as the stand up-sit down because with the mono the movement you make vs the frequency you will be most readily noticing might not correlate - but hey, check out some of the variations. Time alignment and phase issues made me just redo my home built speakers from a 12db per octave to a 24 db per octave crossover (could lower crossover freq.). They are an MTM type design and crossover frequency and distance between driver centers make ALL THE DIFFERENCE IN THE WORLD because of phase distortions and frequency response dips. Peter Z. Quote Link to comment Share on other sites More sharing options...
avman Posted July 12, 2002 Share Posted July 12, 2002 mallet-LMAO!!! i use remote control batteries when my tone generator isn't handy...used a 9v transistor battery-POP!!boy it was easy to hear!(do NOT try this at home) also have found some speakers that 'charge' themselves after a test, and touching the speaker +/- after the battery test caused another 'pop' any of you guys know why? still lol! avman. KLIPSCH-So Good It Hz! This message has been edited by avman on 07-12-2002 at 12:46 PM Quote Link to comment Share on other sites More sharing options...
pzannucci Posted July 13, 2002 Share Posted July 13, 2002 avman, Think about using a car battery . Don't forget to charge up those big capacitors in the crossover network. The bigger they are the bigger the jolt . Peter Z. Quote Link to comment Share on other sites More sharing options...
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