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Author Topic: THD Reduction Question.  (Read 3478 times)

Antone Atmarama Bajor

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THD Reduction Question.
« on: June 20, 2006, 02:49:33 am »

     Hmmm I posted this question and it disappeared for some reason.

    Now I know this isn't specifically Pro Audio Related.  But I have some questions about THD reduction by Copper Shorting Rings.

    I have a little project I'm working on for a mid near field Home 5.1 Setup.  I've chosen to make a 3 Way set up using a bunch of European drivers.  Peerless 8", SEAS 5.5" and a Vifa Tweet.

    Originally I was going to run my woofers Push Pull trying to time align them the best I could so the Even order Harmonic distortion reduction would extend up to xover around 300Hz.  The Original Drivers I Planned on using didn't have copper shorting/Flux stabilization rings.  The peerless Drivers do.

    So heres the Question:

    With the copper shorting rings.  Will I reach a point of Diminishing Returns as far as THD reduction if I go push pull, or will I still have significant THD reduction on top of the THD reduction of the Copper shorting rings????

    I'm not sure where else to ask, Since most audiophiles don't actually know anything about speaker technology, and they seem even more mentally deranged than most of us gear heads.

    Anyhow hope someone here has some experience/knowledge on the topic.

    Thanks.

Antone-
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Tom Danley

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Re: THD Reduction Question.
« Reply #1 on: June 20, 2006, 11:58:25 am »

Hi

Iron conducts magnetic flux conceptually like how copper conducts electricity.
In reality while air or a vacuum is an insulator for electricity, magnetic flux will pass though it.
The ease which something conducts magnetic flux is the permeability, specified with a number that is in comparison to air or vacuum ( permeability of 1).
Iron for example is typically several thousand times better than air conducting flux, hence it is used to direct the flux to the air gap where the VC is.

As hard as the VC pushes on the cone, it is also pushing against the lines of flux.
These are not anchored in place but rather find the path of least resistance so if the coil pushes hard, they can move around.
If you glued a tiny coil to the magnet,  in the gap (which had enough room for the VC still), you would see a voltage on this coil produced by the movement of the lines of flux (which cut through the coil) within the iron.  The signal is induced as in a transformer.

When you push the lines of flux around, you are also changing the condition within the magnet as well.
It is in effect a battery, you are changing the load on it and its operating point moves up and down a little.
If one looks at the changes that take place, one finds that the magnet and iron are not exactly linear and often the change “pushing” the VC is a little different than when “pulling” on it.
The idea of the shorted turns is that if one substituted a solid copper sleeve for the tiny coil, the “same thing but different” happens.  
Now, when the VC pushes on the lines of flux and they begin to move, they induce the Voltage signal in the shorted turn.   As it is a “shorted turn” made of solid metal, it has a VERY low resistance, an induced voltage across a low resistance then produces a large current.  That large current produces a magnetic field which is in opposition to the induction (as it is anchored).
As a result the lines of flux become much more difficult to move with an AC signal due to the transformer induction to the anchored stationary coil.

No free lunch.
As one is dealing with induction, one finds the lower the frequency is, the more difficult it is to make a practical transformer. It is usually not possible to put enough shorted turn, close enough to the VC gap to make much difference in a subwoofer driver.
The higher you go, the easier it gets, the less you ask from a given motor, the less problem there is in the first place.
Hope that helps some.

Tom Danley

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Antone Atmarama Bajor

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Re: THD Reduction Question.
« Reply #2 on: June 20, 2006, 03:04:03 pm »

     Cool so it sounds like the Shorting ring, basically a closed loop/shorted transformer,  reduces THD in the higher frequency range will be reduced more from the shorted coil.  But as one goes lower in frequency the coil becomes ineffectual without taking extraordinary measures.

    So it sounds like doing the push pull configuration will still be beneficial in reducing even order THD in the Lower regions.

    Thanks Tom!!

Antone-
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Johan Rademakers

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Re: THD Reduction Question.
« Reply #3 on: June 20, 2006, 07:31:16 pm »

Does this make the extensive use of demodulating rings and multiple demodulating rings in today's sub woofers more beneficial to marketing than for practical use?
It seems to lower Le which could lead to better transient response?

With kind regards Johan
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Antone Atmarama Bajor

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Re: THD Reduction Question.
« Reply #4 on: June 21, 2006, 02:38:17 am »

     I think the characteristics that make a driver a good sub woofer, are going to limit Transient response.

     I don't know if its possible to have a true sub woofer with good transient response as it takes a driver with a lot of mass capable of very high excursions.  And why would a device that is intended to reproduce sub bass energy need a great extended HF response AKA transient response.

    Having a driver in a sealed box will yield the least group delay without any EQ correction,  I'm not sure exactly how dual integrators or linkwitz transform circuits effect group delay.  But I know that Parametric EQ boosts cause significant group delay.  Which translates into Late arrival of energy.

    Vented boxes with "Sub" tuning seem to have extremely bad group delay around tuning, Same for Passive Radiators.

    It looks like Toms Tapped Horn designs have some of the lowest Group Delay of any Sub woofers I have ever seen.

    Just Random Synaptic firings, pay no mind.

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Wayne Parham

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Push-pull verses shorting rings
« Reply #5 on: June 27, 2006, 07:02:14 pm »


You might be interested in the post called "Push-pull verses shorting rings", which has links to several documents about the subject.

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Phil Pope

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Re: Push-pull verses shorting rings
« Reply #6 on: June 28, 2006, 02:08:46 pm »

This cancellation of motor non-linearity will be gained if two drivers are mounted in a compound configuration face to face. what happens to the composite driver parameters in compound configuration. Is it just the same as two drivers in parallel except that sd stays the same? BL same; cms halved; rms doubled; mmd doubled; le halved; re halved; power handling doubled. ?

cheers
Phil
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Wayne Parham

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Re: Push-pull verses shorting rings
« Reply #7 on: June 28, 2006, 03:07:39 pm »


You may be talking about the isobaric configuration, where two drivers are driven out of phase, such that the pressure differential between them is zero.  This has the effect of creating a single driver having twice the strength and twice the moving mass.

The push-pull configuration has both drivers operated in-phase, so that a pressure differential is created between them.  The drivers are mounted so they are physically opposite - one facing out and the other facing in - but they are wired so that each cone moves in the same direction.

These two arrangements are similar with respect to symmetry, but not exactly the same.  The difference between them is the acoustic or pneumatic load.  In the case of the push-pull configuration, both drivers have the same pneumatic load, so there is pneumatic symmetry.  But the isobaric has one driver exposed to the listening environment, which is a large space, and the other driver faces the cabinet interior, a small space.  The only exception is an open baffle isobaric, which has equal pneumatic loads on both sides, and so can be symmetrical.

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Wayne Parham
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Phil Pope

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Re: THD Reduction Question.
« Reply #8 on: June 29, 2006, 05:37:09 pm »

wayne

yes it was isobaric I was talking about. that article you link to was very good. seems isobaric is not nearly as good as I had thought. to get decent harmonic cancellation the diaphragms need to be physically joined to make a symmetrical driver. could try filling the gap with expanding foam  Very Happy

phil
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Too Tall (Curtis H. List)

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Re: THD Reduction Question.
« Reply #9 on: June 30, 2006, 09:00:17 am »

Phil Pope wrote on Thu, 29 June 2006 17:37

wayne

yes it was isobaric I was talking about. that article you link to was very good. seems isobaric is not nearly as good as I had thought. to get decent harmonic cancellation the diaphragms need to be physically joined to make a symmetrical driver. could try filling the gap with expanding foam  Very Happy

phil


Hi Phil,
I do not believe that would be a good idea.
Iso subs use a small sealed air volume that couples the drivers like the fluid in an automatic transmission.
You can flip the direction the drivers are facing from a “clam” with the fronts of the drivers facing each other to “nested” where the front of one faces the back of the other, but in all case the drivers both move the same direction.

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