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Old 12th May 2005, 08:19 AM   #1
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Default Magnet size basics

Can anyone point me to some basic information on the effects of magnet size on driver performance and application?

Some things I noticed are...[list=1][*]Older console systems and electronic organs with open back enclosure universally use small magnet drivers.[*]Large magnets are suggested for horn enclosures.[*]Have heard that small magnets require large enclosures.[*]People equate small magnet with cheap.[/list=1]

So I assume that large magnet = powerful motor (more force applied to the cone). How does this affect enclosure choices? Could a powerful motor also be designed using smaller magnet and larger coice coil? Are powerful motors required for horns due to the large acoustic load (resistance to cone movement)? When are small magnets preferred? How is magnet size related to efficiency?

I have also heard it said that large magnets improve midrange response rather than bass response. Is this true? If so it seems that a small magnet woofer could produce good results if crossed over at a low enough frequency.

I am interested in this topic as I have access to some older drivers of the old fashioned small magnet variety that I would like to use for some experiments in low power applications (infinite baffle, horn etc.) but I am not sufficiently familiar with the basics of these drivers to know where to start.

Thank you for your patience.

mike
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Old 12th May 2005, 08:30 AM   #2
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Magnet size is a bit of a red herring. There are many different qualities of magnetic materials and it's possible for a monster cheap magnet to be totally outperformed by a high yield compact magnet. The magnet size is also influenced heavily by the voice coil as it has to accomodate the coil and former.

The magnetic flux and the number of turns on the voice coil will be one of the influences on sensitivity.

But, as a very broad brush stroke, a small magnet means a cheap speaker means high Qes and consequently high Qts.
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Old 12th May 2005, 03:59 PM   #3
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Thanks for the reply. Given the interaction of other parameters then I take it that in older speakers just looking at the size relative to modern designs doesn't tell us much.

So I guess my best bet is to try to measure the T-S parameters and proceed accordingly.

Would it be correct to assume that high Qts dictates infinite baffle and rather lower Qts indicates sealed or horn loaded enclosures in general?

mike
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Old 12th May 2005, 04:15 PM   #4
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Qts ... - 0.4 ish = OK for vented and horn and TL
Qts 0.3 ish - 0.5 ish = OK for sealed
Qts 0.5 ish - 0.8 ish = OK for infinite baffle

However with sealed or IB a Linkwitz Transform can be used to bring Q to a usable value.
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Old 12th May 2005, 09:33 PM   #5
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Quote:
Originally posted by richie00boy
Qts ... - 0.4 ish = OK for vented and horn and TL
Qts 0.3 ish - 0.5 ish = OK for sealed
Qts 0.5 ish - 0.8 ish = OK for infinite baffle

However with sealed or IB a Linkwitz Transform can be used to bring Q to a usable value.
Those confines are somewhat narrow... i've had many a vintage driver with Qs in excess of 1 that work just fine on an OB.

In a TL i'm usually looking for a driver with a Q between 0.35 & 0.7, in a horn less than 0.3, but higher Q drivers can be used to get a rising (anechoic) response in the bottom that helps obviate the need for BSC.

dave
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Old 12th May 2005, 09:35 PM   #6
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Quote:
Originally posted by richie00boy
But, as a very broad brush stroke, a small magnet means a cheap speaker means high Qes and consequently high Qts.
Don't let the size of the magnet deter you. Listen. I have been very surprised by old vintage speakers with diminuative magnets. They can really shine on an OB, or in a big IB or aperiodic box.

dave
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Old 12th May 2005, 10:42 PM   #7
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Default More salvaged speakers

I just began looking into an electronic organ that I am salvaging and I found what looks like some really nice speakers. Since the organ has an open back (with grill cloth) I suppose there is no hope that the speakers would be useful for a horn sub but they do appear to be of fairly modern design with large magnet structures. The 12" would probably be good for an IB sub and the 10" and 6x9" could be used in a utility amp for my Korg Poly-six (once I finish repairing the NiCad leak ). There is even a Leslie in there that I could probably find some use for.

As a bonus there is a good sized two channel power amp in there. There is a lot of noise when the unit is powered on so the amp may need some repair but even if I can't repair it that big long heatsink will make a nice platform for a new amp.

Later I will go write down the numbers on the drivers and see if anyone has any information on their specifications.

The big speaker: Looks like 12".
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Old 12th May 2005, 10:48 PM   #8
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Default The Smaller speakers.

Looks like 10" and 6x9".

mike
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Old 12th May 2005, 10:51 PM   #9
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Default Amplifier

About 18" long.

mike
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Old 12th May 2005, 11:02 PM   #10
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pictures aren't big enuff for me to pick off the numbers... if you can tell me what they are i can probably tell you more... organ drivers are invariably open baffle class, but unlike most MI are quite neutral so also suitable for hifi.

dave
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