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In the Forum: Horn-Loaded Speakers
In the Thread: Ultimate MF compression driver?
Post Subject: Larger diaphragm area vs. inductance increasePosted by haralanov on: 3/4/2012
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 Romy the Cat wrote:
So, if the Ultimate MF Compression Driver is used from 1000-1500Hz then why shall it be able to play 500Hz? The answer is: he harmonic reserve – the driver that will be suited for 500Hz will have larger cone and softer sustention and consequentially it will play 3khz with more harmonic richness.

Sure, I’m also supporter of the big diaphragms in general, and they really provide more harmonic richness in the midrange, just as you said, but I also see one very big disadvantage with them in context of compression drivers. The problem is that the larger the diaphragm becomes, the larger its driving voice coil has to be. For the same length of wire in the gap, the inductance raises proportionally with raising the VC diameter. So 4” VC will have twice more inductance compared to 2” VC. And since the VC inductance acts as electrical low-pass filter for the input signal, the more the inductance, the lower the frequency it will start to rotate the electrical phase of the incoming signal within the usable bandwidth of the driver. Everybody knows what happens when the phase shift enters the game – it starts to kill the time (phase) relation between the higher and lower overtones of a given instrument and this instrument starts to lose its tone identity and sense of realism…
It is practically very difficult for a big diaphragm compression driver to have low inductance and to be efficient at the same time. One could perform the following experiment. He has to disassemble a given CD, and then to remove the second layer of wire out of the VC. He will loose 3dB of efficiency, but when he hear the effect of the more than halved inductance (it is not mistake – inductance is lowered more than 2 times, because with just one layer there is no additional mutual inductance between the layers) and the effect of the extended with a whole octave phase linear response, he will quickly understand the amount of inductance is essential for the harmonic relation and expression of the upper midrange and the lower HFs in context of the midrange. So as we see, the "ultimate" compression driver should have big cone (in order to reproduce the fullness of the midrange correctly) but at the same time it should have very low inductance to avoid the huge phase rotations within its working range. So the only solution remains to sacrifice some efficiency by lowering the amount of voice coil turns...
 
Best regards,
P. Haralanov

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