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jdm56
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dipoles / monopoles

OK, help me out here someone: I stuck my neck out on another audiophile forum and now I need backup. Some yayhoo was saying dipole speakers output falls off with distance more than monopoles. He even trotted out some impressive formulas. But I, on the other hand, inserted that in fact the opposite was true; i.e. that dipoles output falls off LESS than monopoles. I just know I've read as much many times in speaker reviews over the years, but of course, I can't seem to find any such backup when I need it, so...am I right, or have I again stuck my neck out only to have it lopped off by some smarmy know-it-all with a danged algebra degree?

Jan Vigne
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Re: dipoles / monopoles

I don't got no stinkin'graphs but I would guess the answer is somewhere in between. One issue with dipoles/bipoles is how you obtain reimforcement in the low frequencies which is achieved by adding the reflected wave off a rear radiating speaker. Since the wavelength of the higher frequencies increases their rate of declination a bit more than low frequencies, you aren't going to get additive information anywhere except the lowest few octaves. If those reflected waves are added in properly by way of speaker placement, then the rate of declination would be slightly less in the lowest frequencies. The roll out would approach the same rate as a monopole speaker above the lowest octaves. One problem there is placement to gain additive effect in the lowest octaves is limited to a very small group of frequencies before the additive wave is once again out of phase with the direct signal wave. Your answer would also depend on whether the measurement was taken in a "typical" listening room which would provide boundary reinforcement or in an anechoic chamber. If pressed, this could be your way out of this argument. Suggest the other fellow just doesn't grasp the difference of one vs. the other and claim victory. Then skeedaddle off the forum and don't come back for about two weeks.

How long has this discussion been going on over at the other forum?

jdm56
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Re: dipoles / monopoles


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How long has this discussion been going on over at the other forum?

Not long. This person is the process of getting some speakers designed by Sigfried Linkwitz (sp?) of crossover design fame. I'm sure those of you familiar with his work are aware he is a champion of dipole designs using cone drivers. Anyway, this prospective buyer was stating some of the wonderful advantages of said loudspeakers when I had the audacity (stupidity?) to take issue with some of his statements; mainly, that dipole radiator's output falls off more with distance than monopoles, and also, his seeming interchangeable use of the terms "non-directional" and monopolar. Oh shoot, here's the quote:

I've heard two sets of Orions, both with the most recent revision (a rear-facing tweeter). They're pretty close to perfect dipoles throughout the frequency range. One characteristic of dipoles is that the power in the radiation falls off faster with distance than for monopoles. As a result I expect that they excite room modes less than standard box speakers (which are close to monopoles at low frequencies), but that in the higher frequencies they produce a higher ratio of reflected sound to direct sound (since tweeters are pretty directional). It's interesting to walk behind a pair of Orions - the sound changes, but much less so than for conventional speakers.

cyclebrain
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Re: dipoles / monopoles

Yea, I pretty much have to agree with Jan on this.
Wavelength and speaker distance from the reflecting wall will reinforce or cancel certain frequencies.
A.K.A. "It's always something".

Ergonaut
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Re: dipoles / monopoles


Quote:
OK, help me out here someone: I stuck my neck out on another audiophile forum and now I need backup. Some yayhoo was saying dipole speakers output falls off with distance more than monopoles. He even trotted out some impressive formulas. But I, on the other hand, inserted that in fact the opposite was true; i.e. that dipoles output falls off LESS than monopoles. I just know I've read as much many times in speaker reviews over the years, but of course, I can't seem to find any such backup when I need it, so...am I right, or have I again stuck my neck out only to have it lopped off by some smarmy know-it-all with a danged algebra degree?

Can I just ask

Is this an audible effect perceived by human hearing that they are discussing?

I.e. the fact that dipole can have cancellation effects as it has a polar response at both front and rear, and someone is supporting that with a few mathematical expressions.

Or is it something that has been measured with instrumentation?

They cannot get away from two laws that bind our lives.

Inverse square law

and the maximum permissivity of free air.

basically - "you canna change the laws of physics, Cap'n"

Jan Vigne
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Re: dipoles / monopoles

I'd have to agree his post doesn't seem to make a lot of sense.


Quote:
They're pretty close to perfect dipoles throughout the frequency range.

This would be all but impossible unless the radiating diameter of the tweeter is sufficiently small to maintain 180 degree dispersion up to its frequency limit. Once the wavelength's ptp measurement falls beneath the dimension of the driver, frequency response becomes more and more directional as the signal decreases in size. This is one reason we typically toe in speakers. If the driver is 1" in diameter, at roughly 13kHz the signal becomes increasingly directional. http://www.mcsquared.com/wavelength.htm
While this wouldn't necessarily be a problem with a ribbon or true omni-directional driver, the Orion appears to use a conventional dome type tweeter. (Sorry, I don't care to go through all the literature on the Linkwitz site just to fight your battle for you, you'll have to find out if this is true or not.) Whether you wish to make an issue of this flaw in logic as applied to all or even this particular dipole system is probably dependent on just how pedantic the poster becomes.


Quote:
One characteristic of dipoles is that the power in the radiation falls off faster with distance than for monopoles. As a result I expect that they excite room modes less than standard box speakers (which are close to monopoles at low frequencies) ...

Once again I think the poster is mostly just very sloppy with his language and too lazy (read as, he doesn't know what he's talking about and he what he "expects" just ain't gonna happen) in his reasoning. There is no proof offered for the statement, "One characteristic of dipoles is that the power in the radiation falls off faster with distance than for monopoles." He "expects" you to believe that.

For most frequencies this statement would be entirely untrue. For that narrow band of low frequencies where the front and rear/reflected waves are additive, this could be considered quite incorrect. If you do not use these reflected low frequency wavelengths to your benefit, you will have wasted much of the value of a dipole design and low frequency response will suffer. For those low frequencies where the front and rear/reflected wave are increasingly out of phase, this would be true as the signals increasingly cancel each other. Since low frequencies excite room nodes and affect room nulls, there is some very weak logic to the argument presented. However, for the most part, this guy's talking out of his ass. (For one thing; am I incorrect that the Orion has a sealed low frequency enclosure which would mean it is not a true "full range" dipole?) As an example, since low frequencies are increasingly omni-directional as the wavelength increases in size even standard box speakers are not close to monopoles at low frequencies and become even less so as the frequency descends. Standing in front and then behind any low frequency driver in any enclosure should demonstrate the error of his statement. A benefit of dipoles is their lack of radiation patterns to the sides of the speakers where wavelengths canel each other. This primarily causes less room problems due to lowered first reflection points at higher frequencies where broad dispersion monopoles have limitations. Depending on room dimensions, this could be used to your benefit at lower frequencies also, but with every advantage here you also face a disadvantage in reduced bass extension and power. The poster seems to have his high and low frequencies confused. The room remains the same no matter what speaker is in that room and, if the wavelengths are long enough and the room short enough, low frequency problems will occur no matter the type of enclosure. Even infinite baffles where no back wave is present in the listening room can excite room problems.


Quote:
... in the higher frequencies they produce a higher ratio of reflected sound to direct sound (since tweeters are pretty directional).

Again this is gobbledeegook. At various frequencies where the dipole acts to null the dispersion to the sides, the speaker has less reflected energy at high frequencies. This is an advantage of a dipole. As the dispersion characteristics of the tweeter narrow the signal pattern (it becomes more directional), the amount of reflected energy at high frequencies will also decrease, not increase. Certainly, there will be less reflected energy at very high frequencies than at the more broadly dispered, long wavelength lower frequencies. But, if he admits the speakers become more directional at higher frequencies, this would be at odds with his idea the speakers are near perfect dipoles throughout their frequency range. They cannot be both directional and non-directional at the same time and to be "perfect" dipoles they must be capable of broad and consistent dispersion at all frequencies. His statement inferring an open baffle mounted tweeter will result in more reflected energy than a single front monopole unit is obvious and doesn't need to be stated as long as the rear wave of the tweeter is allowed entry to the room or a second tweeter is mounted on the rear of the cabinet. Many tweeters are sealed systems which do not allow for such rear wave dispersion. But his statement has nothing to do with the basic dispersion characteristics of a tweeter and everything to do with the addition of another driver facing toward the front wall of the listening room or allowing the high frequency driver to radiate energy from the back of its diaphragm. The same could be said for any driver aimed away from the listener and toward a reflective surface. This is, in the most basic terms, the benefit of rear facing drivers/radiation patterns and his logic for why this happens would not pass the test of a high school term paper.

I suppose none of this really answers your question about the die off of energy to which I assume you would have preferred a quick link to an article which proved this guy a dunce. Sorry. To me, this guy is making stuff up as he goes along and one statement is no more untrue than any other.

Jan Vigne
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Re: dipoles / monopoles

Possibly, if we saw some of the supporting evidence the poster provided, we might be in a better position to help. Is it possible his statement was meant to infer a fast roll out of low frequency energy only? Open baffle low frequency systems do loose energy at a rate similar to any other vented/ported/reflex system, which would be a fourth order acoustic filter of -24dB per octave. This would make his statement true if he were making a comparison to a sealed enclosure or true infinite baffle with their second order action. But, compared to the majority of speakers being sold on the American market today, the dipole's low frequnecy response rolls out at the same rate as a $250 pair of Polks with a hole in the box. There are additional arguments to be made which could be accounted for by the various methods of taking measurements. A rear ported enclosure will measure as having far less bass extension if the test is performed in an anechoic chamber and compensation for the effects of the room are not added back into the calculations. The same would be true for a dipole system. Your poster deosn't seem smart enough to know these things but give us more information and we might get you to the point where you can convince him to buy those Polks instead.

jdm56
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Re: dipoles / monopoles


Quote:
Possibly, if we saw some of the supporting evidence the poster provided, we might be in a better position to help.

There has been no supporting evidence.


Quote:
Is it possible his statement was meant to infer a fast roll out of low frequency energy only?

No. He was using the assumption of increased spl drop-off at all frequencies to support the claim of less room interaction.

Actually, I have found working definitions of the term monopolar, that would support his argument there, but those same working definitions run contrary to what I believe is the definition within the audiophile world, ie, that a monopole radiates sound in all directions, while the audiophile definition is that a monopole radiates sound in one direction. Perplexing, to say the least!

And try as I might, I have still found no "official" verification of my argument that dipoles output falls off at a LOWER rate, in room, than monopoles. This would be a good time for JA to jump in here and bring some clarity to the proceedings!

Jan Vigne
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Re: dipoles / monopoles


Quote:
Quote:
--------------------------------------------------------------------------------

Possibly, if we saw some of the supporting evidence the poster provided, we might be in a better position to help.

--------------------------------------------------------------------------------

There has been no supporting evidence.

I thought you posted that he had "trotted out some impressive formulas."


Quote:
This would be a good time for JA to jump in here and bring some clarity to the proceedings!

You mean, to hopefully make your stated position "official"?

My guess is, you would be better off taking this route, "Suggest the other fellow just doesn't grasp the difference of one vs. the other and claim victory. Then skeedaddle off the forum and don't come back for about two weeks."

Good luck.

Jan Vigne
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Re: dipoles / monopoles

I don't know if this helps your argument "that dipoles output falls off at a LOWER rate, in room, than monopoles."

http://homepage.mac.com/tzagar/dipoles.html

Note: "In the example, the dipole radiator (as well as the monopole radiator) has a toe-in angle of 32 degrees in order to steer the dipole null so it corresponds with the early reflection at 3.2 milliseconds after the direct sound. The result is that this early reflection drops below audibility with a corresponding improvement in clarity and left-right imaging. With an omni radiator, this same early reflection would be at -3.3 dB, about 3.7 dB above the image shift threshold, resulting in a degraded left-right soundstage presentation. In addition, the reflection from the right hand corner of the room is also reduced to below audibility with the dipole radiator as compared to the omni radiator's reflection which is above the audibility threshold. Thus the dipole radiator contributes to increased sideness as compared to the omni radiator, suggesting improved depth of imaging and improved soundstage detail. The monopole does even better than the dipole in this regard, since it has greatly reduced rearward energy. The cardioid radiator contributes slight higher left-right energy, but reduced rearward energy compared to a dipole.

IN-ROOM DIFFERENCES ... MULTI-REFLECTION INTERACTIONS
Note in the table above that three reflections arrive 8.5 milliseconds after the direct sound. In the case of the omni radiator, the result is 3 reflections of equal level, -6.7 dB. If we combine these together, the resultant level at 8.5 milliseconds at the listener's ears is -1.9 dB, about 1.1 dB above the image shift threshold. In the case of the dipole, the result is three reflections of different levels. If we combine these, noting that the reflection from the front wall is 180 degrees out of phase with the reflections off the rear and right walls, the result is -6.4dB.. This is 3.4 dB below the image shift threshold. Again, the omni radiator tends to distort the left-right soundstage presentation as compared to the dipole radiator.

Here the monopole radiator is at a disadvantage compared to the dipole radiator. In this case, the monopole does not have a compensating out-of-phase component, resulting in a combined level of -4.6dB at the listener's ears at 8.5 milliseconds after the direct sound. Fortunately, this is still slightly below the image shift threshold, and about 2.7 dB better than the omni radiator. The cardioid's combined level at 8.5 milliseconds after the direct sound at the listener's ears is -4.0 dB, a worse performance than the dipole or monopole, but better by 2.1 dB compared with the omni radiator. The cardioid's combined output, however, is still below the image shift threshold unlike the omni radiator's which is above this threshold."

The problem seems to me to be one of a very complex situation being described in very simple terms by both you and the other poster.

jdm56
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Re: dipoles / monopoles

Thanks for the help, Jan. You've certainly gone over and above what I could have hoped for. Everyone wants to "win", but really, after getting past that aspect of these encounters, I am more interested in getting at the truth and maybe learning something.

Yes, there was support for his statements offered in the form of a couple of mathematical formulas expressing the "inverse square law", that honestly, didn't mean squat to me. (math was definitely not my thang!) So I did some research, and that is when I discovered the (apparently) official, academic definition of monopole, which to my understanding, runs contrary to the audiophile definition I have accepted for years. The academic (his) version, is that a monopole radiates equally in all directions, what we audiophiles would term an omni-directional radiator, while I have always understood a monopole to be, at least when referring to loudspeakers, a speaker or speaker system that radiates primarily in one direction.

Oh well, to be honest I'm weary of the whole thing at this point.

jayg
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Re: dipoles / monopoles

May I suggest you read the information I have posted at my Web site...
http://www.planotspeaker.com

This will I hope extend this discussion in a new direction. I have invented a new semi-omnidirectional full range dynamic driver which breaks some previous conceptions about the radiation pattern of acoustic transducers. I would hope that the members here will be able to help elucidate this phenomena. BTW it has been tested in a 10 meter semi-anechoic chamber.

John

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