[jules]

I refer you all to the following pdf recently quoted in Audio Central for its suggestion that mass is not an important factor in driver responsiveness.

http://www.audiocircle.com/index.php?topic=27190

Adire is a well known company which has used the promotional line "superior sound through science" so it's views probably carry weight.

I would ask others who might have a physics background to evaluate this paper as it seems to me to be close to nonsense.

The following logic seems weak ...

  * Newtons law of motion F=ma is re-written as BLi=ma. [I've got a lot of problems with this but it takes a fair bit of explanation and since Adire have not properly described what they mean by their terms, it's hard to explore all the possibilities]

  * the author argues that BL is time invariant. [BL is very time related. BL is measured in gm, cm, second to the minus two, amp (current) to the minus one!]

  *  the author re-writes BLi=ma on the basis that it's ok to replace "time-invariants" with a constant "C". This gives an equation that states Ci=Ca. Literally, since the equation has the same "C" on both sides it states that i=a. The author conveniently re-states this as i is proportional to a and concludes that ...  "this says that the change in acceleration of a driver, how fast it can change position-is strictly a function of the current through the driver. In fact, if you could make the current change infinitely fast then the driver would accelerate infinitely fast and we'd have infinite transients - zero time to change between states. Infinite frequency response." [this last bit even challenges Einstein's laws].

I invite others to comment on the impulse response graphs in the latter part of this paper but I wouldn't like to think that a "myth" that cone mass is important would be replaced by a "myth" that it isn't.

jules

[kfr01]

I refer you all to the following pdf recently quoted in Audio Central for its suggestion that mass is not an important factor in driver responsiveness.

http://www.audiocircle.com/index.php?topic=27190

Adire is a well known company which has used the promotional line "superior sound through science" so it's views probably carry weight.

I would ask others who might have a physics background to evaluate this paper as it seems to me to be close to nonsense.

The following logic seems weak .. ...

Dan Wiggins, Adire's ceo is a member of this forum.  Perhaps he'll post.

[jules]

So what's the verdict here, the Adire case is sound?

Well if so this has big ramifications. For a start, using the same logic on Newtons 2nd law F=ma, the mechanical form of the law in question (BLi=ma) we can simply remove m [mass] from the equation. This gives us F=a. Hmmm, doesn't look quite acceptable lets call it "F" is proportional to "a" ... neat, but whatever you do, don't mention mass.

This is great and has some fabulous applications. If mass doesn't matter I can simply remove the motor from my motor bike, drop it into a Kenworth and wow ... a rig that can do 0 to 100mph in 10 sec! Or maybe, a rocket that can take a payload into space using only gunpowder wrapped in paper.

Perhaps I've gone too far here and the best application would be for a 12" cast solid gold woofer ...  rich warm tones free from the hinderance of mass and able to do everything right up to negating the need for a ribbon tweeter!

I suggest that it would be a great idea for Adire to review their tech. paper and either back it up or withdraw it.

jules

[gitarretyp]

I have not read the adire paper, so all comments are based upon jules' post. I am, however, a physicist in training (working on PhD).

Arguing that BL i = m a is fine, if you ignore a few quantities whose level of effect i am not sure (I am wondering what BL really is, however). Cancelling BL and m is not correct, however. The author's intent may have been poorly worded. Since BL and m are constants, the acceleration of the cone is indeed proportional to i, ie one could re-write BL i = m a as a = C i, where C := BL/m.

So, yes the author is somewhat correct in claiming that "the change in acceleration of a driver, how fast it can change position-is strictly a function of the current through the driver." This requires neglecting some terms in your momentum equation (BL i = m a) that i think are likely relevant, such as the volume of air moved by the cone, friction, mutual inductance, heat losses,...

Regarding the infinite acceleration claim: this would obviously violate the basic tenants of special relativity. I think the author was trying to sensationalize his claims or does not understand relativity.

[kfr01]

I suggest that it would be a great idea for Adire to review their tech. paper and either back it up or withdraw it.

Jules:  

Perhaps you should e-mail dan and civilly ask him for more explanation rather than sensationalizing the article.

His point wasn't that mass never matters.  With all due respect, I think you're slightly taking that out of context.  His argument is that inductance matters more in some limited situations only regarding speaker transient response.  

dan at adireaudio com

gitarretyp:  

Please read the article, I'd hate for some language essential to Dan's argument to be missed in a summary.  

That said, I am interested to see where this discussion goes and thank Jules for questioning it.  The thirst for more knowledge and answers is a noble cause.

Cheers,

Karl

[jules]

giarretyp,

please read the paper!

BL is a measurement of the motor strength of a speaker. The unit of measurement is Newton amp to the minus one. If you are a physicist you might like to delve into some serious Thiele-Small analysis on this site

http://www.silcom.com/~aludwig/Sysdes/Thiel_small_analysis.htm


m is a constant for a given driver but the paper tries to demonstrate that if drivers have different values of m it doesn't matter. It does this by simply removing m witout justification. I have no problem with the suggestion that "I" is proportional to "a" BUT it is also proportional to m and inversley proportional  to the BL factor for the particular driver.

PLEASE read the paper!

jules

[jules]

sure kfr01, I did stir things a bit with my post but while Dan's conclusion might be pragmatically ok it would appear to be based more on experience than the logic of the maths and physics in his paper.

jules

[kfr01]

sure kfr01, I did stir things a bit with my post but while Dan's conclusion might be pragmatically ok it would appear to be based more on experience than the logic of the maths and physics in his paper.

jules

Jules:  I'm not faulting you.  Rather, I applaud your effort to get to the bottom of the article.  The article uses strong language, so your use of strong language to challenge the article is 100% appropriate!  

I really wish you would e-mail dan so the author can fairly respond.

Cheers,

Karl

[jules]

thanks Karl  

I did think twice before posting this topic in the light of some shocking fracas that have developed here in recent times. Still, a forum is nothing if we can't debate topics with vigour.

I'm happy to talk to Dan by email or pm but I suppose that since his papers are public and this post is also public, there's a good case for someone to totally demolish my case here on the forum  

My post is in now way intended as a slur on Adire products. So far as I'm concerned their drivers are totally awesome.

jules

[Mudjock]

Jules,

You are right on the money with your observations regarding Dan's white paper.  Carrying through the simple algebra to get a=(BL/m)*i tells you that you can get greater acceleration through more current, more motor strength, or less mass.  

It is pretty clear to me that the impulse graphs show that the Adire cone is not in good control - yes it responds quickly from a standstill, but it does not stop particularly well.  Obviously, the high frequency response rolls off when you put an inductor in series with the woofer, but the overall response is perturbed much more when you add mass to the cone.  

FWIW, I do hold a Ph.D. in Solid State Physics, but it really doesn't require an overly technical discussion to see the issues with the analysis presented in the paper.

[jneutron]

B is the strength of the magnetic field within the voice coil gap.

L is the length of wire of the voice coil that is within the gap.

F = BLI  is the equation of force which is imposed on the moving structure when a current "I" is going through the voice coil.

F=MA should require no explanation.

The acceleration of a voice coil is proportional to the current flowing within it.

The JERK of the voice coil (the time derivative of the acceleration) is proportional to the rate of change of the current.

gitarretyp:  ""So, yes the author is somewhat correct in claiming that "the change in acceleration of a driver, how fast it can change position-is strictly a function of the current through the driver." This requires neglecting some terms in your momentum equation (BL i = m a) that i think are likely relevant, such as the volume of air moved by the cone, friction, mutual inductance, heat losses,... ""

The current is not dependent on the heat loss.  Volume of air moved only comes into play if the acceleration of the cone is sufficient to cause non linear coupling with the air mass.  Friction is simply neglected.  Mutual inductance to what?

The excitation current, if of sufficient magnitude, could alter the BL product, either by simple strength, or through eddy coupling to the magnetic circuit.  The shorting ring is in fact, intended to reduce time varying motor flux via Lenz's law.

Jules:  ""In fact, if you could make the current change infinitely fast then the driver would accelerate infinitely fast ""

No, changing the current infinitely fast does not cause infinite acceleration.  It causes an (infinite) step change in the acceleration.  A significant difference.

Cheers, John
PS..now to read that paper..my interest has been tweaked.

[jneutron]

Wow.  Read the paper.

Quite a few misconceptions.  It is in need of many changes, I would have sent it back with quite a few changes, had I reviewed it prior to publication.

Unfortunately, white papers are not subject to peer review.

Cheers, John

[Mudjock]

It may be worth noting that the "Jerk" (da/dt) is still related to the mass and BL:

da/dt = (BL/m)*(di/dt) with the relatively safe assumption that BL and m do not change with respect to time.

I'm not sure I follow the assertion that current is not dependent on heat loss.  Voice coil heating causes the electrical resistivity of the conductor to increase.  This can become significant at higher current levels.  Or are you talking about something else?

[jneutron]

It may be worth noting that the "Jerk" (da/dt) is still related to the mass and BL:

da/dt = (BL/m)*(di/dt) with the relatively safe assumption that BL and m do not change with respect to time.

Yes.

I'm not sure I follow the assertion that current is not dependent on heat loss.  Voice coil heating causes the electrical resistivity of the conductor to increase.  This can become significant at higher current levels.  Or are you talking about something else?

Heat loss in itself, is meaningless with respect to current drive.  You are correct in that the vc temp and the vc's temco of resistance is a factor, as the increase causes power compression, which of course affects drive current.

But the fact that dissipation occurs is not in itself the issue.

I had to chuckle at the white paper mass/inductance thing..putting more mass, noting very little change..Weren't they interested in the affect the mass had on the drive current??

They bandy back and forth on this inductance vs mass thing, but the whole paper runs without a clear direction nor a clear understanding of what's going on.

While they do not really understand the physics well enough to generate a concise and accurate paper, I for one, tend not to judge the merits of a product on the ability of someone to write a tech paper.

It's better to listen to the darn things and judge for one's self.

Cheers, John

[Mudjock]

It's better to listen to the darn things and judge for one's self.

 

I couldn't agree more.  I own a pair of Extremis woofers, which I have run in a two-way crossed at about 2 kHz and generally like them.  They suffer some loss of detail, having a poly cone and have some roughness in response above 1 kHz, which I presume may be an issue with trying to pound out such amazing bass and still cover higher frequencies.  

Overall, I really appreciate people who challenge convention and give us real choices (Bob Carver is a classic example), but I really think the author got in over his head with this white paper.

[gitarretyp]

gitarretyp: ""So, yes the author is somewhat correct in claiming that "the change in acceleration of a driver, how fast it can change position-is strictly a function of the current through the driver." This requires neglecting some terms in your momentum equation (BL i = m a) that i think are likely relevant, such as the volume of air moved by the cone, friction, mutual inductance, heat losses,... ""

The current is not dependent on the heat loss. Volume ...

I was just proposing possibly neglected terms. I've never studied in any depth the physics behind loudspeaker design.

I have now read the paper and agree with jneutron's observations. The paper does not present a clear understanding or conclusion regarding the effect of mass. The author doesn't understand some basic physical principles and never really tested what they sought out to measure.

I also agree that this should not necessarily be viewed as a condemnation of adire's products. I would imagine many (most?) speaker designers don't have a good understanding of physics.

[DanWiggins]

Hi all, perhaps I can clarify a few things?  I designed the Extremis 6.8 (among a few hundred other drivers), and wrote that white paper.  I think there may be some confusion about it, perhaps I need to clarify it a bit more...

The paper dealt with transient response - is it a mass issue (as many audiophiles believe), or is it an inductance issue (I as contend).  The paper is pretty clear in its derivation - mass affects efficiency, but does NOT affect transient response.  The proof is in the frequency response measurement (frequency response and transient response being related via the Fourier transform).

Now, mass will definitely affect the amount of current required to reach a given excursion (and hence SPL) level; no question there!  However, that is says nothing about transient response (frequency response); it simply says that if the cone is heavy, you need more current to get a given acceleration (and hence, SPL) out of it.

Transient response - frequency response - is really limited by how fast you can change the current in the voice coil (change the SPL).  That is really dominated by the inductance of the voice coil.  That is why, from a frequency response standpoint, mass (and, if your BL is constant, BL) is really just a constant.  It scales the relationship of force to current, but does not alter how current changes over time.

Anyway, if there are questions about the paper, I'd love to try to answer them, and if the paper needs to be amended I'll be grateful for the input!

Dan Wiggins
Adire Audio®

[jules]

jneutron,

it might be construed that you attribute this quote to me where it is actually an extract from the paper.

""In fact, if you could make the current change infinitely fast then the driver would accelerate infinitely fast.""

I have limited my comment to the assertion of the Adire paper that "there is a common misconception that heavy woofers must be slow and light woofers must be fast", followed by the claim that the paper will prove it.

There is also a second assertion that "BL/Mms .... has precious little to do with woofer speed or signal response". The proof of this seems to be based on the graphs and not the physics /maths.

Without even delving into the maths and physics there is a very fundamental oversight here. The arguement deals with a single driver and not a comparison between drivers with different BL factors and different mass.

Sure, for one driver on a work bench it might be ok to say that current is proportional to acceleration x [a nearly constant] [/i]but if we increase the mass of a driver we will also change the BL factor. So if Adire want to mount a case against the "common misconception" this is not it. In fact this arguement does not address the case in any way.

jules

[jules]

Hi Dan,

thanks for joining in  

I hadn't seen your post at the time I posted my last comment.

jules

[kfr01]

Anyway, if there are questions about the paper, I'd love to try to answer them, and if the paper needs to be amended I'll be grateful for the input!

Dan - thanks for chiming in; the discussion has a better chance of being fruitful with you being here to explain your meaning.

Others, especially those with strong physics backgrounds - please take Dan up on his request for constructive input.  

{kfr quickly shuffles to the side as his physics background is not particularly strong}