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# Casework: bigger drivers are slower than smaller drivers

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### #61 of 66 Vaughan Odendaal

Vaughan Odendaal

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Posted March 15 2006 - 06:24 AM

So Adire's article only applies to higher frequencies ? Not really subwoofer frequencies from 80hz and down to the deep bass ?

I was wondering how the inductance would play a part in all this if SPL was a function of how quick the driver would respond to the input signal.

The reason why I thought BL was the force of accelerating the driver was because just as two permanent magnets will attract/repel each other depending upon how they are oriented, a bigger/stronger permanent magnet would mean stronger attraction/repulsion.

--Sincerely,

### #62 of 66 Vaughan Odendaal

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Posted March 15 2006 - 07:06 AM

So how would F=Ma factor into all of this ? If you wanted to explain the whole "mass issue of larger cones" with simple physics, using plain theory, how would one go about it ?

Thanks again.

--Sincerely,

### #63 of 66 Vaughan Odendaal

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Posted March 15 2006 - 08:56 AM

One last post before I have to go to sleep. If acceleration is proportional to SPL, then the less SPL the less acceleration.

Correct ?

So then drivers that are SPL limited will not accerelate as quickly as those drivers with more SPL in their own respective bandwidths.

So a 10" driver with 15mm of one-way linear excursion will not be able to accelerate as quickly as a 15" driver with 12-15 mm of linear excursion ?

At what point is the SPL going to affect the acceleration times of the drivers ability to track the input signal ?

--Sincerely,

### #64 of 66 Dennis XYZ

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Posted March 17 2006 - 06:26 AM

Quote:
 The reason why I thought BL was the force of accelerating the driver was because just as two permanent magnets will attract/repel each other depending upon how they are oriented, a bigger/stronger permanent magnet would mean stronger attraction/repulsion. So how would F=Ma factor into all of this ? If you wanted to explain the whole "mass issue of larger cones" with simple physics, using plain theory, how would one go about it ?

Vaughn, you're making this much harder than it needs to be. You've been given all the right answers but I think you're getting bogged down in the details. So back to basics.

First, these are not two permanent magnets. Rather there is an electromagnet (the voice coil) inside a permanent magnet. The F in your F=ma equation depends on how much current you pump through the voice coil. More current, more force and more acceleration. So, if the driver's efficiency is low for whatever reason (high moving mass, small permanent magnet, small voice coil), all you have to do is pump more current through it, i.e. use a bigger amp and turn up the volume.

Long story short, if you hold everything else the same, the lighter driver will play louder at a given setting on the volume knob than the heavier driver. The simple cure: turn it up. There are of course subtle differences as you add mass but they miss the big picture. Neither driver is "faster" than the other. One just needs more power to do its job and they both work equally well.

### #65 of 66 Vaughan Odendaal

Vaughan Odendaal

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Posted March 18 2006 - 06:51 PM

Dennis, thanks for the clarification. I know that the voice-coil is a temporary electromagnetic field that must work in conjunction with the fixed, permanent magnetic field.

But if we are comparing an 18" driver to a 10" driver and the 18" driver has far more excursions, then it doesn't require fast accelerations. The 10" would.

Correct ?

Because Dan at Adire in a number of threads used to say that the "fastest" sub is the sub that can play the lowest, the loudest.

But if the 18" driver with a small magnet does not have the necessary current flowing through the voice-coil compared to the 10" driver, then what would happen ? Would it then be slower ?

If SPL is a function of acceleration, then the 18" driver should be quicker to reach the required velocity compared to the 10" driver.

Correct ?

But if the 18" driver has pathetic excursion levels (like 4-5 mm linear excursion), hypothetically, compared to the 10" (with 1" linear), then what do you think the outcome would be ?

BTW, I am read through the Vance Dickason book and have learn't a lot. Admittedly, I am going to read it again. And then again.

Would the "Master Handbook on Acoustics" also help me to understand whats going on here or is there another book that is more advanced in theory compared to the Louderspeaker cookbook ?

Thank you for your help. That goes to everyone who offered there advice.

I really appreciate it

--Sincerely,

### #66 of 66 Dennis XYZ

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Posted March 20 2006 - 05:46 AM

Quote:
 But if the 18" driver with a small magnet does not have the necessary current flowing through the voice-coil compared to the 10" driver, then what would happen ? Would it then be slower ?

No it would be quieter. That's what volume controls are for; turn it up. "Faster" and "slower" are not terms that have any objective meaning in speaker design. They are subjective words with no accepted definition (everybody's is different) and no accepted way to measure it. The quicker you can get over using those words the better. Think of fast and slow as 4-letter words, the F word and the S word.

I think what you are getting at is an intuitive sense that the driver is not tracking the signal that's fed to it. The correct term for that kind of problem is "distortion" which is something that can be easily measured. Low distortion is good, however the designer accomplishes it. We don't really need to worry about how he does it, just measure the result. There are good and bad 10" drivers as well as good and bad 18" drivers.