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LE, not MMS determines woofer "speed"!

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asaa00 View Drop Down
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    Posted: 15 September 2016 at 3:09am
Read this a while ago, but just recently found it again:

Quote There's a common misconception out there that heavy woofers must be "slow", and light woofers must be "fast". If a woofer A's moving mass is higher than woofer B's, then woofer A is probably going to be sloppy, or slow and inaccurate. Can't keep up with the bass line. Woofer A simply can't respond as fast as woofer B. 
There's also this concept that the "acceleration factor" (BL/Mms) is an indicator of woofer speed/transient response. High BL, combined with a low Mms, should give great transient response, right? Well, on surface these might sound like logical assumptions. However, they are in fact incorrect! More to the point, moving mass has precious little to do with woofer speed or signal response! And we'll prove it...

In this paper, their tests are showing that Inductace (Le) has more of an effect on a drivers transient response than cone weight/motor speed. This is contrary to what I've heard, but I am no expert, what do you all think? 
Thoughts?Big smileQuestion


Edited by asaa00 - 15 September 2016 at 3:14am
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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 15 September 2016 at 4:39am

The Le is the inductance. Using the words “Slow” and “Fast” leads me to believe that the author stems from the home audio market. The majority of woofers on the market can achieve frequencies within the midrange region. I have no idea why the author feels Le is going to affect the transient response a woofer when, electronic crossovers are tuned below the Le of a woofer.

 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote midas Quote  Post ReplyReply Direct Link To This Post Posted: 15 September 2016 at 8:32am
This is knowledge gained from electric motors. 

I used to upgrade Airsoft guns and there are a few things you can do to improve the speed of an electric gun. Now I know it is not directly the same, but I can see there are some pretty close resemblances. However if I am wrong, someone please explain.

The basic motor had relatively low power with thinner, fewer windings.

I used to buy motors that had thicker wire with less coils on the armature and much stronger neo magnets. It was easy to tell the difference in strength in the motors if you held the motor in one hand and spin the armature between thumb and forefinger of the other. The standard motor would continue to turn for maybe a couple of seconds, where as the better motors aimed at high torque it could actually be an effort to turn and as the armature is wound making it click as the sections confront the neo magnets. 

So using this logic as it is very similar. 

A good bass driver will have very strong magnets, with the strength per weight being better with neo magnets.

The length of windings and diameter of the wire will be specifically designed to the weight and properties of the cone. Combined with the diameter of the voice coil which will give more or less power in the piston.

It then follows, the higher the magnet strength combined with a specifically designed coil size/ wire size will determine the torque in the piston. It is this torque that will give you the 'speed' of the cone. With the higher ratio magnet to voice coil the higher the torque. At a given voltage. Obviously the higher the voltage the more power goes through the piston. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 15 September 2016 at 11:25pm
Originally posted by asaa00 asaa00 wrote:

Read this a while ago, but just recently found it again:

Quote There's a common misconception out there that heavy woofers must be "slow", and light woofers must be "fast". If a woofer A's moving mass is higher than woofer B's, then woofer A is probably going to be sloppy, or slow and inaccurate. Can't keep up with the bass line. Woofer A simply can't respond as fast as woofer B. 
There's also this concept that the "acceleration factor" (BL/Mms) is an indicator of woofer speed/transient response. High BL, combined with a low Mms, should give great transient response, right? Well, on surface these might sound like logical assumptions. However, they are in fact incorrect! More to the point, moving mass has precious little to do with woofer speed or signal response! And we'll prove it...

In this paper, their tests are showing that Inductace (Le) has more of an effect on a drivers transient response than cone weight/motor speed. This is contrary to what I've heard, but I am no expert, what do you all think? 
Thoughts?Big smileQuestion

As I understand it, the inductance ie the given amount of magnetic field , is dependant on various factors, if you take a simple large gauge wire and use that for a coil then the inductance will be much lower than if you use a smaller gauge with multiple windings. The windings in terms of length and gauge have to be selected upon the basis of their ability to handle the current and create a magnetic field. Inductance is directly related to force within the coils flux acting against the fixed magnet,the more windings, the more inductance, common sense would say that a lighter cone  would be more responsive given the same coil assembly otherwise any other view would appear to defy the laws of physics.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RoadRunnersDust Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 7:25am
Acceleration = Force/Mass

So if the motor assembly (VC & Magnet) stays the same and the amount of power being put into the speaker stays the same, a driver with lighter cone will accelerate faster (will have better transient response and better response at higher frequencies)?

EV for a long time used a single magnet design from 18" through to 10" (with two different 2.5" VC options, Edgewound Aluminium or a Conventional Copper VC) and most of the difference between the "bass" drivers and the "mid" drivers was just a lighter thinner cone as the "mid" driver wasnt expected to be asked to push as much air and so, didnt need to be as strong
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 8:44am
Yep +1 I am not sure what the OP means, what is being said is that a high mass can be accelerated with the same force as a lower mass? Seems a bit odd unless there is a difference in the power used.
 




Edited by 70,s hero - 16 September 2016 at 9:33am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 9:59am
The author of that paper seems to be confusing frequency response with transient response a little in some places, or if they are not then it isn't very clearly written. Getting my science hat on the paper is written enthusiastically and it is clear the author knows a few things, but could be tidied up a lot with a few redrafts. No offense intended, was just the way it read to me. The conclusions are probably valid, but a lot more testing and proper analysis of data is needed for a really watertight conclusion. (I.e. more than just visual graph comparison). No strong evidence is presented for why Le is more significant than added mass, a single data point of comparison is presented for each. How do you relate the effectiveness of each change such that the test is fair?

Anyways from the top of my head I would hazard that increasing Le does two basic things. It is an electronic change after all. Larger inductances give increasing impedance for a given frequency, which means your frequency response will fall off quicker because current reduces and so does the ability of the voicecoil to generate magnetic force which is linearly proprtional to current.

The second is that when you have a series RL circuit, current no longer leads voltage by 90 degrees as it does in a pure inductor circuit. The phase shift will be spread out from 0-90 degrees as the inductor moves from being a short (0 phase) at DC to infinite resistance relative to R (90 degree phase) at high frequencies.

I strongly suspect that it is this variable phase shift (depending on the bandwidth of the driver and additional effects on phase of the cabinet design) that most causes poor time alignment and 'time smearing' of an input signal that was originally time aligned.

With added mass, I would expect a phase shift to beconstant over a driver bandwidth, not so with an Le change. Damping would also change as a result of added mass.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 10:13am
Originally posted by asaa00 asaa00 wrote:

Read this a while ago, but just recently found it again:


thank you for the link. Their reasoning about BL and Le was interesting and makes sense to my simple mind....

In their example and measurements it could be clearly seen that adding inductance inhibits current flow and thus acceleration and transient response. But I think their conclusion is a bit flawed. In the end you can't look at one parameter in isolation and draw conclusions about driver performance.

One thing I think they should have addressed in the paper is the overshoot of the driver with modified mass. It seems to act pretty much complementary to the increased induction case.

So the right ratio of BL, inductance, and mms is required to get clean impulse response and signal tracking ability for a transducer.....

Food for thought.....




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Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 10:20am
Originally posted by odc04r odc04r wrote:


The second is that when you have a series RL circuit, current no longer leads voltage by 90 degrees as it does in a pure inductor circuit. The phase shift will be spread out from 0-90 degrees as the inductor moves from being a short (0 phase) at DC to infinite resistance relative to R (90 degree phase) at high frequencies.

I strongly suspect that it is this variable phase shift (depending on the bandwidth of the driver and additional effects on phase of the cabinet design) that most causes poor time alignment and 'time smearing' of an input signal that was originally time aligned.


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so does this mean that they should have rather built a second driver with identical specs except for a longer voicecoil with higher Le to make the research valid?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevinmcdonough Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 10:25am
hey

yeah I don't think it takes very much more than basic high school physics to understand that both parts will effect the result and the "speed" of the driver. 

A stronger magnet will push and pull harder, and so be able to move quicker given the same cone. 

A lighter cone will be easier to move so will move faster given the same magnet. 

And given stronger inductance in the coil, it will also react faster to the changes.

So a combination of all will decide ultimately how quick it sounds.

(Though obviously it becomes far more complicated than this as there are many factors to consider when deciding on your driver design. No point in having a super light cone that can move quickly if it tears up with the pressure of the speaker design. An no point in having a huge heavy magnet if it then affects the other t/s paramiters of the driver and takes it away from it's design goals. )



Now, my problem with the article is that he suddenly discounts a load of parameters that can't just be ignored for no reason. 

I haven't had time to really read the article properly and drill down into his graphs etc at the end, but from my quick skim h<span style="line-height: 1.4;">e seems to be saying that BL and MMS will have no effect on "speed", and doesn't really qualify it so we're left to assume he means between any driver. </span>
<span style="line-height: 1.4;">
</span>
<span style="line-height: 1.4;">He starts off well describing how having stronger motor force will give stronger acceleration of the driver etc. </span><span style="line-height: 1.4;">However then goes on to say because we're dealing with a "time" that we can just discount all of these things. The time is only part of the "speed".</span>

Lets compare it to his car analogy, and assume we've raced our quarter mile and want to slow down again. Now the drivers reaction time can be thought of as his "time" thing that the article speaks of. When the driver receives the signal to slow down, there will be a little delay in his reaction time and then he starts to push the breaks. and so yes one driver will likely react quicker than the other. But this isn't the only measure of how the car will slow down, that's just a measure of when it STARTS to slow down.

Imagine the driver that was slightly slower to react had a set of breaks on his car that were 3 times as good. Yes he may take a slight few milliseconds to react, but will still come to a stop much quicker overall because of his much better breaks. 

Or if they have the same breaks, but one car is a steel body and one carbon and aluminium and so much lighter. Again, the lighter one will stop quicker, even if the reaction time was slightly longer at the start.

So with out speaker, we can't just discount BL and MMS because it's not just how quickly it starts to change, its how quickly it makes the change overall based on the signal. 

If the driver is in the process of an inward movement and gets a signal to change to outward. Yes the reaction time it takes to realise this will have an effect, of course. And this is what the author of the article seems to argue. But then how quickly it decelerates and then accelerates in the other direction to complete this movement. It may start sightly quicker, but if it's a sloopy loose motor and takes ages to then reverse and go in the other direction, it'll sound slow. And it may start a fraction of a second later, but if it has a super strong motor and then completes the change quickly, it'll still sound fast. 

k








Edited by kevinmcdonough - 16 September 2016 at 10:41am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 10:30am
Originally posted by kevinmcdonough kevinmcdonough wrote:

And given stronger inductance in the coil, it will also react faster to the changes.


isn't that exactly the opposite of what he is stating?

Originally posted by kevinmcdonough kevinmcdonough wrote:


Or if they have the same breaks, but one car is a steel body and one carbon and aluminium and so much lighter. Again, the lighter one will stop quicker, even if the reaction time was slightly longer at the start.


That's exactly what I was referring to with my comment about overshoot.....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevinmcdonough Quote  Post ReplyReply Direct Link To This Post Posted: 16 September 2016 at 10:40am
Originally posted by bob4 bob4 wrote:

Originally posted by kevinmcdonough kevinmcdonough wrote:

And given stronger inductance in the coil, it will also react faster to the changes.


isn't that exactly the opposite of what he is stating?




Sorry yes, what I meant lol. Was trying to type quickly
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