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Fast vs Accurate
Fast vs Accurate
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Old 23rd January 2006, 03:42 PM   #1
MaVo is offline MaVo  Germany
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Default Fast vs Accurate

I thought about what makes a driver an accurate driver, that can play very detailed musical signals without loosing details, especially in the low to mid frequency range.

The driver itself is a device that moves air in order to reproduce the waveform that is stored in some kind of container like a cd or mp3. If, on given time x, the driver has the same amount of excursion as the waveform on the time x dictates, then its an ideal reproduction. Since there are forces that work against this, an ideal reproduction wont happen. No driver has an infinite acceleration. Every driver somehow tries to follow the waveform, the better the acceleration, the more accurate is the position of the cone in comparison to the waveform it has to reproduce.

Now, what T/S parameters are there to evaluate the ability of the driver to move accurate to the signal? I've read the Adire .pdf about "fast woofers". They say, that a woofer with a low Le is a woofer with a fast acceleration and good transient response. They also state that BL and Mms have nothing to do with a fast acceleration.

I looked at some T/S parameters, the Le value is small with midrange units, a rough trend seems to be "the higher the frequency a driver can reproduce, the lower the Le". So a midrange would be a great woofer, given it could move alot of air? Is this the principle behind those line arrays of really much really small drivers?

Or could i be comparing apples and oranges and being accurate at low frequencies has nothing to do with the ability to recreate high frequencies? My intuition still thinks towards high BL, low Qts, low Moving mass. Looking at the waves: A low frequency has very few cycles per second (hz), but the excursion or length of each cycle is high. A high frequency has high hz, but each cycle has a really short way to go. So a midrange is a unit that cycles fast but each cycle is short. And a woofer clycles slow and each cycle is long. So it seems that a woover doesnt need to be as fast accelerating as a midrange since its cycles are slower and the low Le (like the extremis with its 0.13) seems to indicate the drivers capability of doing mid/high frequencies. A woofer should rather be able to be accurate on a slow and long way. A stronger magnet should be able to keep the cone at an accurate position because it can cancel all the move-opposing forces like air stiffnes and so on.
What about Qts or bl/mms as a drivers capability to be accurate in the low to mid frequencies?

Do you see where i stumble? I dont think a low Le is helpfull when the driver shouldnt recreate high frequencies but i cant prove this thought. The Qts and bl/mms seem to be a good choices. Can you help me here?
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Old 24th January 2006, 03:13 AM   #2
myhrrhleine is offline myhrrhleine  Belize
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A stronger magnet does indeed help with regard to the accuracy.
Unfortunately, there are negative side-effects.
(simplified) After a certain point, the greater the magnet strength, the less bass you'll get.

Also, Your speaker will normally be put into some sort of cabinet in order to separate the front from the back (sound pressure being opposite from front and back).
The characteristics of the cabinet will greatly effect the speaker's performance.
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Old 24th January 2006, 06:12 AM   #3
gpapag is offline gpapag  Greece
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Fast vs Accurate
I dont think a low Le is helpfull when the driver shouldnt recreate high frequencies but i cant prove this thought.
Just a picture from the dynamics (physics-wise) of the motion (cone motion as an arbitrary but realistic figure):

Speaker Freq. (Hz) Cone motion (mm) Resulting Acceleration (m/sec^2)
Woofer 100 10 100
Mid 1K 1 1000
Tweeter 10K 0.1 10000

These escalating huge (remember g=9.81m/sec^2) accelerations (A) are to be imposed on the moving mass (M) of each Speaker. The resulting force (F) of the moving assembly is: F=MxA
Again arbitrary chosen masses give the following results:

Speaker Freq. (Hz) Moving mass (grams) Resulting Force (Kg force)
Woofer 100 100 10
Mid 1K 10 10
Tweeter 10K 1 10

“Fast” cone is either a reduced mass assembly controlled by a weak motor, or a heavier mass assembly controlled by a stronger motor. Motor strength BL is affected both by coil inductance Le and strength of magnet. Increased coil inductance increases motor strength. It also increases moving mass M (thus the need for aluminum coil).
As this in not enough, it seems that "fast" is not adequate for accuracy of reproduction. It is an important part of it though

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Old 25th January 2006, 12:54 PM   #4
MaVo is offline MaVo  Germany
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I bought a Fostex FE208EZ and i am really pleased with the sound. Now i am in search of a fitting low frequency driver, since the fostex cant go very low. (i dont want a backloaded horn behind it.)

Right now it seems that strong magnet, low excursion & big cone, small mms are the right direction, since the FE208ez also has this characteristics. Those values rather push my selection into the realms of Pro drivers, even though i never listen loud music.

In my search for good drivers i dont want to rely on other peoples opinions, i want to understand whats going on physically to be able to select a driver that fits me.

As this seems to be a rather complicated subject, do you know something i could read? I am a bit tired of searching my way through the internet since there are so many different (and most of the time really limited) viewpoints on this matter.

....and thanks for both answers.
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