I need to lowering Qms of the driver, using Resistive braking - one coil shorted - Page 2 - diyAudio
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Old 27th January 2007, 07:33 AM   #11
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Old 27th January 2007, 07:52 AM   #12
bjorno is offline bjorno  Sweden
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christophorus,

Why not use an acoustic method to bring down Qms?

Post#30

Enclosure for high Q driver

b
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Old 28th January 2007, 12:58 AM   #13
Ron E is offline Ron E  United States
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Why are you so fixated on reducing Qms?

THere is nothing wrong with a Qms of 4. There is nothing better in any way about a lower Qms. It does not need reducing for any reason other than trying to jam a woofer with a high Q into a smaller box. You can stretch a tightly woven material over the back of the driver to reduce Qms for this purpose. The typical way is to use 2" fiberglass batting stapled to the baffle. Several layers of cheesecloth may also work...

Blocking the screens will likely increase Qms and possibly cause noises during operation, not to mention reduce VC cooling, which will cause Re and Qes to rise .

Based on your initial comments you should not be confident enough in your measurements to even know what the actual value of Qms is. Please remeasure the voice coil in both series and parallel, if you do not get very nearly the same result in each configuration for Fs, Qts, Qes and Qms there is either something wrong with your measurements, or your driver does not have two identical coils.

The measurement technique I use (or used before using speaker workshop) consists of a smaller series resistor (5-10 ohm) along with measurements of the voltage across the resistor, the driver, and the combination. It is more work to find the parameters this way, but the technique allows extraction of impedance phase as well as magnitude, and it automatically corrects for frequency response issues in the meter, which are very common in today's digital meters.
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Old 28th January 2007, 02:32 AM   #14
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Hi Bjorno,
I will try your method, it looks easy to do, thank's.

Hi Ron,
originally I want to build TL sub with this driver, but the Qts is too high even for sealed box, it needs a big box. So, I think reducing Qms is the easies way to lowering Qts. I read several time this paragraph, and now Bjorno bring it here:

"You want to use a woofer with a fairly low Qms (as compared with the Qes); for instance, you'd rather have a Qms value less than 10 times the Qes value. A low Qms means that the suspension is providing a great deal of the restoring (damping) force for the woofer, while a high Qms means that the woofer is relying on the amplifier to provide the damping electrically - which your amps will not do a good job of. If you're investigating kits or DIY, take a look at the Qms and Qes values, rather than just using the Qts."

or this:
I think the high Q will dominate the sound.
The effect will probably sound like a boom box with all bass notes sounding like they were of the same frequency.

I tend to concur with the other posters, High Q speakers were never intended for use with a cabinet.

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About blocking the screen, actually it doesn't sealed off completely, because at the middle-back of the magnet there is a hole 1" diameter, so I hope it doesn't effect VC cooling too much.

The important thing is, I should do the measurement once again to check wether it is wrong as you said.

chris.
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Old 28th January 2007, 05:04 PM   #15
bjorno is offline bjorno  Sweden
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christophorus,

Quote:
About blocking the screen, actually it doesn't sealed off completely, because at the middle-back of the magnet there is a hole 1" diameter, so I hope it doesn't effect VC cooling too much
Youíre right it doesnít as long as you do not cover the rear of the magnet that should be of a vented type.

Se my simulation with a driver T/S you submitted in another post the effect when lower Qms from 8.36 to 1.

As long as the cone of the driver is off a stiffer type and the applied damping material is even distributed and secured not to move at all, then this method will work very well with drivers with high compliance.

If you already have an enclosure for your driver, why not practice both methods, i.e. simply first apply Qm lowering damping and if not sufficient; short one coil with resistors between 0 to about the Re value for achieving different Qm-values.

Test with 6.8, 3.9 2.2 and 1 to =0 Ohms while just listening to music on your speaker, if you donít have measurement equipment, and of course, post your result and opinion of the results here.

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Old 28th January 2007, 05:41 PM   #16
Ron E is offline Ron E  United States
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Quote:
Originally posted by christophorus

"You want to use a woofer with a fairly low Qms (as compared with the Qes); for instance, you'd rather have a Qms value less than 10 times the Qes value. A low Qms means that the suspension is providing a great deal of the restoring (damping) force for the woofer, while a high Qms means that the woofer is relying on the amplifier to provide the damping electrically - which your amps will not do a good job of. If you're investigating kits or DIY, take a look at the Qms and Qes values, rather than just using the Qts."
The entire quoted argument above is spurious. Nearly all drivers have a Qes that dominates, even when an amp has high output impedance. Furthermore, Qms doesn't describe a "restoring" force as stated in the quote above - it is purely a dissipative element. Qms has NO place in box design, IMO. In your case, shorting a coil will never reduce Qts below the level it had when series or parallel. Basically when you short one coil, Qes doubles and Qms ~= Qes original and Qts stays relatively unchanged.

If you want to modify response, a linkwitz transform would be more appropriate, you can use a LT in a TL You'd want to add an infrasonic filter, though. You could also just use a notch filter at resonance...

Quote:
Originally posted by christophorus
or this:
I think the high Q will dominate the sound.
The effect will probably sound like a boom box with all bass notes sounding like they were of the same frequency.

About blocking the screen, actually it doesn't sealed off completely, because at the middle-back of the magnet there is a hole 1" diameter, so I hope it doesn't effect VC cooling too much.
The screen is an added cost, they wouldn't likely use it unless it were necessary. VC cooling is not the only issue, there can be air rushing noises caused by the narrow gap. Whatever you do to block the screens, make sure it is reversible

All this silly gymnastics could be avoided if one just used a suitable driver in the first place... Sell it to some dumb kid who doesn't know any better.
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Old 28th January 2007, 06:21 PM   #17
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hi christophorus,

In all this I want to question your driver and specifically the manufacturer.
Has the manufacturer published any specs, especially TS parameters for your woofer, if so what specs?
If sold in retail and no specs avalible there really is a "nasty" driver.
Often we can find drivers that are really cheep. Usually the manufacturer slab the same motor assembly on every size in a series of woofers resulting in increasing Qts as size goes up. The small ones may be useful but the larger ones has Q values totally out of range.
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Old 29th January 2007, 03:58 AM   #18
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Hi Bjorno,
thank's for your very helpfull simulation, it's look promising, but with your acoustic damper, could it really reduce Qms from 8.36 to 1? because when I sealed off the screen, Qms only drop to 7.83 (~ -0.5), but Qes goes up! around +0,4 , so overal Qts didn't change too much...
Does sealed of the screen not enough?

Would you mind giving me a drawing/sketch of your acoustic damper, maybe 1 cross section of the driver can explain better.

Hi Ron,
Quote:
shorting a coil will never reduce Qts below the level it had when series or parallel. Basically when you short one coil, Qes doubles and Qms ~= Qes original and Qts stays relatively unchanged.
now, I confuse with your last statement above, it's opposite with what Dan Wiggins said as on my first post.

Really? Linkwitz transform can use for TL? Doesn't it for sealed box only?

Maybe you're right, maybe it's better for garage door stopper (quite heavy) , but before it goes there, I want to know how far this thing can do with some treatment to achieve what I want.

Hi Anders,
there is no T/S spec, only freq. range, magnet weight and bla bla blaa...
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Old 30th January 2007, 03:45 PM   #19
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Hi christophorus,

I agree with Ron E here regarding Q values.

I have myself never tried shorting one coil, anyway I did a quick test setup to verify.
Clio as measurement tool using internal amp.
Object: Sinus 16W10.3 6.5" dual VC 8Ohm midbass in freespace.

Connection Qms Qes Qts
Series 2.78 0.38 0.34
Parallell 2.79 0.38 0.34
One open 2.82 0.80 0.62
One shorted 0.87 1.36 0.53
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Old 30th January 2007, 05:04 PM   #20
AKN is offline AKN  Sweden
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Hi,

One more comment.

In your quote from Dan Wiggins:

"When you run a single voice coil, it has the nominal single-coil Qes of 0.8.
So, with the second coil open, you'd have a Qes of 0.8, and a Qms (unchanged) of 6.5, for a Qts of ~0.7.
Now, when you short the second (unused) coil, it essentially acts as an electric brake.
This will manifest itself by lowering the Qms to the Qes of a single coil: 0.8.
So, you'll have a driver Qes (of the coil you're driving) of 0.8, and a Qms (of the shorted coil) of 0.8, for a combined Qts of ~0.4.
One coil driven, other open, Qts = ~0.7. One coil driven, other shorted, Qts = ~0.4."

He never compares both coils connected to amp with one connected and one shorted.
Intead he states what will happen when you connect one coil and short the other coil. So you will not lower Qts with one shorted coil beyond what you will have with both coils connected.
Correct though that Qms will be lower, but for what use when Qes goes up?
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