2-layer vs 4-layer voice coil

StigErik

Member
2008-02-21 12:30 pm
What are the pros and cons of a 4-layver voice coil vs a 2-layer?

Obviously, the technical data changes. Most significant is the inductance, resulting in a rolled-off HF response. Anything else?

I have compared Seas L26ROY in 2-layer and 4-layer versions here:

[IMGDEAD]http://www.tangenavdesign.com/_stuff_/L26ROY-2.png[/IMGDEAD]

Frequency response - blue line is the 4-layer version.

[IMGDEAD]http://www.tangenavdesign.com/_stuff_/L26ROY-1.png[/IMGDEAD]
 
Thanks for posting the data.
Basically, the 4 layer version is horrible compared to the 2 layer one, loses a lot at about any frequency except at the lowest ones.
On average it's 3 dB less efficient, while between 300/600 Hz loss is around 5 dB and above some 1200 Hz it worsens to 10 dB or more.

Then why on Earth is this speaker built?
Only justification I find is that from, say, 40 to 100Hz efficiency is almost the same, but VAS is halved, which roughly means you can have a box half as large with basically same output and efficiency.
Which is very low anyway, in the low 80's.

Only place I find this an advantage is in Car Audio Subwoofers, where you are severely limited in box size but the "listening room" is also very small.

EDIT: shame on these cheaters!!!!!
Efficiency is even *worse* than stated, on both models.
The standard reference point is 1W [email protected] Meter distance, which for 8 ohms amounts to 2.83V RMS
For 4 ohms speakers, the proper value is 2V RMS ... but they use the 8 ohms value with a 4 ohms speaker, doubling the power fed to it.
So all SPL values must be corrected down by 3 dB.
Oh well :mad:
 
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Bl is higher at the cost of higher inductance. Moving mass is also higher, which will result in a lower Fc, although in this case the suspension has also been stiffened to support the higher Mms. But in most (smaller) enclosures the compliance of the air volume will dominate and the compliance of the speaker is only a minor factor.

Higher Mms and Bl will make it more suitable for small subwoofers. The higher inductance also limits the driver to subwoofer applications. I'd choose the 4-layer version in most cases.
 
EDIT: shame on these cheaters!!!!!
Efficiency is even *worse* than stated, on both models.
The standard reference point is 1W [email protected] Meter distance, which for 8 ohms amounts to 2.83V RMS
For 4 ohms speakers, the proper value is 2V RMS ... but they use the 8 ohms value with a 4 ohms speaker, doubling the power fed to it.
So all SPL values must be corrected down by 3 dB.
Oh well :mad:

They are not cheating, since they are using the correct term: (voltage) sensitivity (@2,83 V). This is not the same as efficiency @ 1W.

Yes, efficiency is low. But small closed box subwoofers inherently have low efficiency if you want a low Fc.
 
Hi,

Whilst the 4 layer version will work in a smaller box
with lower efficiency for the same bass extension,
I cannot see the point of this driver in the LX521.

In an open baffle it just seems less efficient than
the 2 layer version for no real purpose I can fathom.

rgds, sreten.
There's even more unfathomable technology there.
Didn't know what "LX521" meant (for me it matches Italian "Elettronica" magazine project codes) so I found Mr Linkwitz page.
I was aghast at the choice of a small open baffle design for a woofer, (might do if he had used a 4 x 6 meter open baffle or something), even more
Response -3 dB at 30 Hz (Q < 0.5) on ground plane, free-field
- 3dB maybe ... but relative to *what*?
 
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Relative to 0 dB ! :p

I guess relative to the overall level above 30 Hz....

I know the "math" definition :p:p

What I'd love to see is a speaker full frequency response graph.
Can you link to it?

Mr Linkwitz speaks about "my outdoor measurements." , which is acceptable if no anechoic chamber is available , even more if he does not *want* anechoic measurements.
OK, but please show something.
Like the old saying: "an image is worth 1000 words" :)
 
Well obviously, it flattened the rising response which was tilted in the opposite direction that is required for baffle step compensation but why not just use a real inductor with the 2 layer version? It would be interesting to simulate the 2 layer driver with increased inductance to compare.

The higher VC inductance will be subject to inductance modulation at high excursion.

It is odd that they were able to obtain the same DC resistance since I usually think of a 4 layer as being in the same gap and therefore requiring smaller wire. Smaller wire with more layers and turns has to result in higher DCR so they must have opened up the gap so that they could use heavier wire, or they might have wound down and up twice and put them in parallel. This would be like a dual voice coil driver but with the coils internally in parallel.
I don't think such a large rise in inductance would be seen if this was the case - very odd.

I find it hard to believe that the moving mass increase is due to the voice coil, and I don't know why SL would want increased moving mass in this application. However, if they did open up the gap and use heavier wire then perhaps that is the case.

Interesting question.
 
Indeed, the 4-layer version performs disappointingly... and for dipole use the higher sensitivity of the two-layer version is an advantage. Strange.

I think it comes down to the same reason why the Millenium tweeter is used and not the cheap 27TDFC which measure the same.
It's about long term reliability, heat management and such I believe, dipoles are a very harsh environnement for small drivers, especially when they are Eq'ed that low!
 
It is odd that they were able to obtain the same DC resistance since I usually think of a 4 layer as being in the same gap and therefore requiring smaller wire. Smaller wire with more layers and turns has to result in higher DCR so they must have opened up the gap so that they could use heavier wire, or they might have wound down and up twice and put them in parallel. This would be like a dual voice coil driver but with the coils internally in parallel.
I don't think such a large rise in inductance would be seen if this was the case - very odd.

I find it hard to believe that the moving mass increase is due to the voice coil, and I don't know why SL would want increased moving mass in this application. However, if they did open up the gap and use heavier wire then perhaps that is the case.

Interesting question.

I design and build my own speakers and instantly thought the same as you.
They *must* have enlarged the gap , which is partially confirmed because the 4 layer one has less gap flux density.

By the way, using a same winding length coil, for twice the layers (which is not twice the turns) to keep the original impedance, you only need 25% thicker wire, which is not *that* dramatic.

Or use your idea of dual (thinner wire) 8 ohms coils in parallel .... which anyway means the same amount (mass and space) of copper wire.

As a side note, whan I asked my custom VC winder for 4 layer coils "to put more copper in the gap" (I was designing a small, portable Bass amplifier) he answered: "why bother? I can make you a "flatwound" (really rectangular wire) VC with the turns and resistance you wish"
He did have the machines to make rectangular copper wire any reasonable width x height size I wanted.