How can an overhung voice coil produce good sound? - diyAudio
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Old 30th March 2005, 02:42 PM   #1
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Unhappy How can an overhung voice coil produce good sound?

Hi,

An overhung voice coil is in a non linear magnetic field. How does it have any chance of producing good sound?

Thanks in advance,
Goldy
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Old 30th March 2005, 03:47 PM   #2
Wizard of Kelts
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I think the idea is that part of the voice coil is in the magnetic gap, and part of the voice coil is out of the magnetic gap compeletely-not being driven.

In other words, as long as a 6 mm wide magnetic gap has a full 6 mm worth of voice coil inside it, it should be linear.

The non-linearities occur when a excursion has carried part of the voice coil out of the gap completely. So 6 mm of magnetic gap is only reacting to 3 mm of voice coil.

But as long as a 17mm, 25 mm, or even 40 mm voice coil has 6 mm of it's length inside the 6 mm magnetic gap, it should be relatively linear.

That is why the linear excursion formula is:
(length of voice coil - magnetic gap length) / 2.

So a 17 mm voice coil with a 6 mm gap has a linear excursion of
(17mm - 6mm) / 2 = 5.5 mm.

Whether the overhung voice coil ever has quite the linearity of an underhung voice coil-one where the gap is 10 mm and the voice coil length is 6 mm-might be another question.
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Old 30th March 2005, 03:51 PM   #3
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hi

there are 2 seperate things

as -

1) the entire motor structure being - non linear

2) only the magnet field at the magnetic gap being non linear

for no 2 - there are 3 simple appraches that will make the magnetic field symmetrical - hence jbl term - sfg , symmetrical field gap

1) an overhung sysytem in a symmetrical magnet field is never non - linear - as the driver will be designed to have a certain number of turns in the magnet field at all times

on which condition the driver specs will be measured and mentioned

- there-fore the bl

any conditions beyond which the bl will reduce - or the number of turns in the magnetic gap will reduce - is generally beyond which a driver is designed and therefore not recommended for usage

- x max

the overhang voice - coil system is linear ( other than when crossing the x- max - beyond the drivers design ) on all instances when the magnetc gap is of a - sfg design

suranjan das gupta

transducer design engineer
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Old 30th March 2005, 04:57 PM   #4
Ron E is offline Ron E  United States
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Neither conventionally-gapped overhung nor underhung coils have a linear magnetic field - therefore by your reasoning neither should sound good.

Truth is, overhung coil is a much better compromise than underhung in terms of economy and distortion performance in a long excursion woofer.

Underhung coils with large gaps need HUGE magnets and many (most?) do not run the iron near the gap saturated - which means that the magnetic field of the magnet system can be modulated by the coil - creating further nonlinearity.

Overhung coils also benefit from the fringe field more and their Bl falloff is shallower than an underhung design. While Underhung may make intuitive sense, the pitfalls in practice make it impractical for large excursion designs. TC Sounds (once famous for their underhung designs) 20+mm drivers are all overhung AFAIK.
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Old 31st March 2005, 03:09 AM   #5
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There's also XBL^2 where multiple gaps are used, and JBL has differential drive.

I had an idea once for a commutated voice coil where it would have many coils and change between them through the excursion.... well tom danley thought of it first and had a patent on it, so i was out of luck. I've never seen it in use, but I think it has potential.

Of course you know why you never see it in use, those servo motors keep mr danley occupied!
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Old 1st April 2005, 08:53 AM   #6
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Default overhung vs underhung

In standard overhung motors, there is more steel below the top-plate than above the top-plate making 2 problems.
a) Induction differences as the coil goes in and out of the pole.
b) Non linear magnet field from the top to the bottom of the coil.

There are 2 ways to make good overhung voice-coils in standard magnet sandwiches.

1) If the pole-piece is under-cut so the area close to the voice-coil is the same "height" as the top-plate, there is good linear motor strength and also pretty good induction linearity.

2) If the pole-piece is extended to 2x the Xmax, there is good linearity. This is great for sub-woofers because the L is high and there is a lot of extra steel to help cool the coil.

Under-hung voice-coils are naturally very linear inside the Xmax and have low induction. These are usually low Xmax drivers though and distort a LOT when overdriven. This is because the voice-coils are real short. If a voice-coil that is only 4mm long goes out of the gap by 1mm, 25% is out of the gap, if a 22mm long voice-coil goes 1mm out of the gap, it is only 4.5% outside its normal gap.

Under-hung coils are low in induction and very light because the coils are short with less turns of wire than overhung coils. This is good for high frequency extension, but not great for long term power.

Under-hung voice-coil drivers also tend to be higher Qts drivers because of a low Bl. This is not so much because of a low B, but more because of a low L.

I design drivers with both types. They all have a place. It depends on the application.

Frank
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Old 1st April 2005, 12:45 PM   #7
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Default Re: overhung vs underhung

Quote:
Originally posted by spkrguru
In standard overhung motors, there is more steel below the top-plate than above the top-plate making 2 problems.
a) Induction differences as the coil goes in and out of the pole.
b) Non linear magnet field from the top to the bottom of the coil.

There are 2 ways to make good overhung voice-coils in standard magnet sandwiches.

Frank
Spkrguru:

Isn't there a third as well-put a magnetic shorting ring, (Faraday ring), in the magnetic circuit, such as Peerless and ScanSpeak?
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Old 1st April 2005, 07:12 PM   #8
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Some overhung speakers (especially for subwoofers) have high inductance due to the fact that they have a much longer coil than underhung designs. I think underhung is truely the best if cost is of no factor, but it takes massive magnets and massive amounts of steel to get massive excursion. XBL^2 offer a good comprimise
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Old 28th December 2007, 07:59 AM   #9
el`Ol is offline el`Ol  Germany
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Jim Thiel claims in the interview at his site to have 1/10 of the distortion of an overhung coil with the underhung construction of his flat diaphragm midrange.
http://www.thielaudio.com/THIEL_Site...midrangeB.html
I ask myself:
Is it better to have "worse" iron in such a case to get closer to saturation?
Or does the copper ring help to overcome this problem?
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Old 28th December 2007, 08:26 AM   #10
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IMO underhung makes most sense where it 1.) can be economically applied and 2.) the ear is most sensitive to nonlinearities - i.e. midranges and tweeters !

Regards

Charles
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