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Old 21st December 2011, 01:43 PM   #11
forr is offline forr  France
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The Force Factor (in N/A, force in Newton obtained for 1 Ampere) is equal to the Bl product (in T.m, induction in the gap B in Tesla multiplied by the "effective" length of the coil wire submitted to the magnetic field).

The Bl product multiplied by the current value (i) in the coil gives the force applied to the moving mass Mms. This force is then equal to :
Bl* i

The acceleration in m/s of the moving mass is equal to the force divived by the moving mass :
(Bl) * i / Mms

The so-called Acceleration Factor, AF, is the Bl product divided by the moving mass Mms. Many people think of it as a quality criterion for the drivers, it is not a good conception.

The moving mass has no way to "know" the origin of the force applied to it.
It will be accelerated with the same manner, be it from a driving force issued of a given Bl and a given current or from a driving force issued of half this Bl and twice this current.

Of course, the efficiency of the first driver is better but as the movements are identical, it is not per se an indicator of better sound reproduction.

All this can be demonstrated using a dual-voice-coil driver.
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Old 22nd December 2011, 11:13 AM   #12
Scott L is offline Scott L  United States
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Default Specifics

In a nutshell: I have a 4-way system. All but the mid-bass "section"
perform to my satisfaction. Subs are mono infinite baffle and operate fine below
50 Hz, but could raised to as high as 80Hz I suppose. Trying a 60Hz x-over frq
next is very possible. Mid range is a large horn, but covering much below 500Hz
would be a real push. Above 2500 is the Stage Accompany SA8535.
For mid-bass, I have three sets of possible drivers:
Dayton Audio PRO 15" version (PA-380); the same Dayton Audio PRO, but the
12 inch version, or, the Eminence Definimax HO4012's which are also 12"

At first I was leaning towards the 15's just based on the larger available cone area, plus that really nice BL value of 23+ but since learning more through reading here on this forum, there's much more to it than just the BL or force factor. The Eminence have the lowest Le of the bunch.
Also, I intend to use 2 drivers per channel. All opinions and insights welcome.
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Old 22nd December 2011, 10:54 PM   #13
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The posted Le for most drivers is mesured at 1kHz and changes with frequency. generally it goes down as frequency goes down untill you near resonance, common stuff. The thing I look at is how fast it goes up from it's minimum. The Impedance generally mirrors the inductance, so you can look at the Impedance chart and get a good idea about how "fast" the coil will change direction at any given frequency.

For these three drivers with no other considerations. (we have not considered enclosure, application, or anything else) The driver with the most gradual impedance rollup is the Dayton 12". Charts attached. Although the Dayton 15" is a close second and if more SPL is needed it would work almost as well from 50-500Hz
Attached Images
File Type: jpg dayton12.jpg (239.2 KB, 109 views)
File Type: jpg dayton15.jpg (236.3 KB, 105 views)
File Type: jpg emenence.jpg (74.7 KB, 105 views)

Last edited by revboden; 22nd December 2011 at 10:59 PM.
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Old 22nd December 2011, 11:12 PM   #14
Djim is offline Djim  Netherlands
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Old 23rd December 2011, 10:33 AM   #15
AndrewT is online now AndrewT  Scotland
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You don't specify what your SPL target is for any of your drivers.

I doubt that you need 15" nor dual 15" for a lower Mid/upper bass driver.

Look up the data that shows Litres of displacement for desired SPL at various frequencies.
You should be able to identify a medium excursion, medium diameter driver than can theoretically achieve your desired SPL. From there you consult the driver database and determine which individual driver (or combination) can meet your SPL requirement.
Simply guessing at a driver size and displacement capability seems a waste of time to me.
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Old 23rd December 2011, 11:14 AM   #16
Scott L is offline Scott L  United States
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Default Educated guess

Quote:
Originally Posted by AndrewT View Post
You don't specify what your SPL target is for any of your drivers.

I doubt that you need 15" nor dual 15" for a lower Mid/upper bass driver.

Look up the data that shows Litres of displacement for desired SPL at various frequencies.
You should be able to identify a medium excursion, medium diameter driver than can theoretically achieve your desired SPL. From there you consult the driver database and determine which individual driver (or combination) can meet your SPL requirement.
Simply guessing at a driver size and displacement capability seems a waste of time to me.
I'm leaning towards using the Dayton 12 inchers for this application. Although it my seem like I am a novice- due to the simplistic and mysterious nature of my posting, I actually very experienced in speaker building. It's just that during my formative years, all the sims and calcs that you guys do second nature like, were simply not available years ago. I strictly remember the requirement for one acoustical watt at 50Hz requires .53 of the capabilities of one 15 inch driver. Two twelves equals a bit more than one fifteen, so, that-plus my intended loading technique should work fine. Having noted my hands-on-only experience, I also note I do not have any formal engineering back ground, which was why I was asking for help. How far back do I go? Well, to give you an idea: I walked into Lafayette Radio electronics asking for 2.5 mH air core inductor, and they looked at me like I was from Mars. Five years after that episode
Thiele Small Theory was introduced.
For the most part I find this forum and it's contributors very interesting and helpful, and I am greatful for that.

Last edited by Scott L; 23rd December 2011 at 11:37 AM. Reason: I previously posted a blank
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Old 23rd December 2011, 12:43 PM   #17
Xoc1 is offline Xoc1  United Kingdom
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We still need to acertain how loud your system is to give you a meaningful answer.
There is no point in specifying a PA style monster cab if you dont need the SPL.
More info on your current speaker enclosures, drivers, and the amplifiers you are using would help to estimate what sort of speaker would be in balance with the rest of your system.
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Old 23rd December 2011, 01:04 PM   #18
Scott L is offline Scott L  United States
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Default Certainly !!

Quote:
Originally Posted by Xoc1 View Post
We still need to acertain how loud your system is to give you a meaningful answer.
There is no point in specifying a PA style monster cab if you dont need the SPL.
More info on your current speaker enclosures, drivers, and the amplifiers you are using would help to estimate what sort of speaker would be in balance with the rest of your system.
In order to match the rest of the system, the mid bass section should have at least a maximum SPL requirement of 110 db @ 1 meter, that's per channel.

The devoted mid-bass amplifier is at maximum 400 watts. I have a strong feeling I could run out of x-max long before i hit the max power rating.

I admit I'll need help figuring the max excursion for this driver at 50 Hz vs a 60 Hz x-over frq, as it's listed as a 5mm x-max. Thanks again to all.
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Old 23rd December 2011, 03:57 PM   #19
AndrewT is online now AndrewT  Scotland
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what is the displacement of a 12" driver with an Xmax of 5mm.

Volume Displacement For SPL Chart

shows the inch units (imperial) for displacement for 50Hz and 110dB to be about 15 to 20 cub inches.
the next post shows the centimetre units (SI).

I would expect a single 12" driver to get pretty close to your SPL target.

I told you not to guess.
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Old 23rd December 2011, 06:06 PM   #20
mdocod is offline mdocod  United States
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Hi Scott,

You should download WinISD and spend some time tinkering.

There isn't a law or rule that states when you reach Xmax based on frequency and Xmax ratings alone. The driver characteristics, and box loading all come into play. Entering the driver characteristics into winISD, then simulating the performance in a box will give you a graph (amongst many others) showing maximum power handling at various frequencies as limited by either Pe or Xmax. It is an incredibly useful tool and simming a handful of different drivers side by side will often reveal which driver is most well suited to your application. When doing a handful of comparisons, you'll find that higher quality, more expensive drivers, tend to provide better use of available Pe and Xmax. A lot of people here have been doing this so long, that they can spot a set of characteristics that will work better for an application without the need of the sim at all.

Eric
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