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Old 3rd Oct 2017, 12:55 pm   #1
mole42uk
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Default Why are speakers elliptical?

Another thread set me musing; why are some loudspeakers made elliptical?
What is the advantage to making something that isn't quite square and isn't quite round? The one that fits the dash aperture on my car is elliptical but it doesn't need to be, a round one would fit just as well. Can an elliptical speaker move enough more air to make it worthwhile?
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Old 3rd Oct 2017, 2:13 pm   #2
Dave Moll
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Default Re: Why are speakers elliptical?

My understanding has always been that the elliptical shape is a compromise on circular speakers of the larger diameter when space is limited, but there may well be further considerations known to other members.
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Old 3rd Oct 2017, 2:15 pm   #3
merlinmaxwell
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Default Re: Why are speakers elliptical?

It reduces the resonances compared to a circular one, modern materials have knocked that on the head. They also fit the available space giving the biggest radiator for loudest sound.
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Old 3rd Oct 2017, 2:25 pm   #4
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Default Re: Why are speakers elliptical?

Also Purchase Tax implications at one time, I believe.
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Old 3rd Oct 2017, 4:49 pm   #5
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Default Re: Why are speakers elliptical?

I believe that it was Dr G F Dutton of EMI Research Labs who was an influential proponent of the theory that an elliptical shape spreads out the 'bell' resonances of a speaker cone so that the reproduction has less of the characteristic 'speaker sound'.

Certainly, you'll usually find an elliptical speaker in any post-war HMV kit and their 13" x 8" speakers have a good reputation. Dutton's theory is plausible, though I'm not convinced it was ever borne out in measurements. Perhaps the idea got seized and propagated by HMV marketing.

As merlinmaxwell points out, the preference today is to use more heavily damped cone materials to reduce the effect of the majority of cone bending resonances.

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Old 3rd Oct 2017, 6:31 pm   #6
kalee20
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Default Re: Why are speakers elliptical?

I have seen mention of a rectangular speaker (though not seen one in practice). Similar to elliptical, in that it had a circular voice coil, and the cone smoothly morphed from the circle into the outer frame shape (in this case, rectangular).

Never caught on it seems...
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Old 3rd Oct 2017, 6:39 pm   #7
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Default Re: Why are speakers elliptical?

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Originally Posted by merlinmaxwell View Post
It reduces the resonances compared to a circular one, modern materials have knocked that on the head. They also fit the available space giving the biggest radiator for loudest sound.
Indeed, much the same reasons that speaker cabinets aren't usually cubes.
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Old 3rd Oct 2017, 6:46 pm   #8
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Default Re: Why are speakers elliptical?

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Originally Posted by kalee20 View Post
I have seen mention of a rectangular speaker (though not seen one in practice). Similar to elliptical, in that it had a circular voice coil, and the cone smoothly morphed from the circle into the outer frame shape (in this case, rectangular).
I've got some square speakers! Look up Sony APM speakers, flat aluminium diaphragm in place of a circular cone, but if you look at it from the back you'll see a small round cone attached to the back of the diaphragm. Not bad speakers, I have some here, part of the FH-7 mini hi-fi made in the mid 80's.

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Old 3rd Oct 2017, 9:22 pm   #9
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Default Re: Why are speakers elliptical?

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As merlinmaxwell points out, the preference today is to use more heavily damped cone materials to reduce the effect of the majority of cone bending resonances.
Amplifier power is much cheaper nowadays.
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Old 3rd Oct 2017, 10:37 pm   #10
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Default Re: Why are speakers elliptical?

If you can locate a copy of "Loudspeaker and Headphone Handbook" by John Borwick, he covers ovoid cone driver characteristics in section 2.3.8


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Old 3rd Oct 2017, 11:05 pm   #11
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Default Re: Why are speakers elliptical?

See this thread from >3 years ago: https://www.vintage-radio.net/forum/...=104217&page=2
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Old 3rd Oct 2017, 11:14 pm   #12
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Default Re: Why are speakers elliptical?

While the thing is still pistonic (so, up to 200 Hz then ), if the long axis of the ellipse is vertical then the thing will have slightly tighter directivity in the vertical plane, than in the horizontal.

A rather hopeful marketeer might claim that this means that the design offers increased radiating area (true) and hence better LF response (true) whilst pushing the frequency at which horizontal directivity narrows, up to a higher value than would be the case if that area were to be achieved using a circular cone.

In practice, by the time you get up there, the thing is probably starting to vibrate modally and so directivity gets a lot more complicated anyway. Hey ho.

(Btw - anyone looking at Borwick would do well to find ed. 2 in my view - quite a lot of useful math detail was lost in ed. 3, including chap 1 by the great R.D. Ford).
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Old 4th Oct 2017, 8:45 am   #13
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Default Re: Why are speakers elliptical?

I've always wondered why speakers are traditionally conical rather than flat! Surely the difference in distance travelled by sounds from the centre vs sounds from the edge must introduce some degree of 'colouration' particularly at higher frequencies?

A solid, flat plate would seem instinctively more-sensible: that's pretty much what electrostatic speakers used.
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Old 4th Oct 2017, 9:18 am   #14
Peter.N.
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Default Re: Why are speakers elliptical?

I first noticed them in sets like the Pye V4, due to the shape of the cabinet only elliptical ones would fit and give a decent cone area.

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Old 4th Oct 2017, 9:46 am   #15
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Default Re: Why are speakers elliptical?

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Originally Posted by G6Tanuki View Post
I've always wondered why speakers are traditionally conical rather than flat! Surely the difference in distance travelled by sounds from the centre vs sounds from the edge must introduce some degree of 'colouration' particularly at higher frequencies?

A solid, flat plate would seem instinctively more-sensible: that's pretty much what electrostatic speakers used.
Interesting, but the advantage (if only slight) is a cone shaped diaphragm will have the advantage of more surface area to move air and being a good deal more rigid and stable structure.
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Old 4th Oct 2017, 9:57 am   #16
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Default Re: Why are speakers elliptical?

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Originally Posted by G6Tanuki View Post
I've always wondered why speakers are traditionally conical rather than flat! Surely the difference in distance travelled by sounds from the centre vs sounds from the edge must introduce some degree of 'colouration' particularly at higher frequencies?

A solid, flat plate would seem instinctively more-sensible: that's pretty much what electrostatic speakers used.
I guess the cone is a rather clever means of creating a diaphragm stiff enough to move as a whole (at low frequencies) when driven only from the centre by the voice coil. Otherwise it would flap around. I wonder who first thought of using a cone. It was certainly used in the balanced armature speakers of the 1920s before the moving coil came along, and I think in some telephone receivers.

The flat plate is feasible in an electrostatic speaker because the drive force is distributed right across the diaphragm, so there is very little bending moment involved. This tight drive control across the diaphragm helps the electrostatic speaker to avoid much of the characteristic 'loudspeakeriness' of most moving coil units.

Speakers such as the KEF B139 and Leak Sandwich increase flat diaphragm stiffness using an expanded polystyrene sandwich. Again, I wonder who did it first. Their performance suggests that the idea has proved successful.

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Old 5th Oct 2017, 7:25 am   #17
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Default Re: Why are speakers elliptical?

I have always found that an elliptical speaker has a nicer tone/sound than an equivalent sized round one, is this because of the shape or other factors?
Maybe it is because elliptical speakers usually have a more pronounced parabola shape to the diaphragm profile, could this help.
Also the varying effective size of the diaphragm in say a 6 x 4 speaker could maybe give the speaker a response pattern similar to 2 separate speakers, like a 6 inch speaker and a 4 inch speaker.
Only a guess, what do you think.

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Old 16th Oct 2017, 11:07 pm   #18
Craig Sawyers
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Default Re: Why are speakers elliptical?

Oh dear, I'm going to disrupt another thread.

I was talking to Laurie Fincham of KEF a rather long time ago, (um - 30 years or so) and I asked what the thinking was behind the B139 - since it was the iconic low frequency driver of its day.

"Purchase Tax"; "er, pardon?"; "Yes - purchase tax applied to speakers which were round or elliptical. So we took a circular speaker, cut it in two and put two straight sections in. Since this was neither circular or elliptical it evaded Purchase Tax"
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Old 17th Oct 2017, 4:38 am   #19
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Default Re: Why are speakers elliptical?

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"Purchase Tax"; "er, pardon?"; "Yes - purchase tax applied to speakers which were round or elliptical. So we took a circular speaker, cut it in two and put two straight sections in. Since this was neither circular or elliptical it evaded Purchase Tax"
That doesn’t surprise me at all.
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Old 17th Oct 2017, 8:30 am   #20
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Default Re: Why are speakers elliptical?

I suspect that was why is used expanded polystyrene (cone-like shaped at the back to match the voice coil former, and flat at the front). Because of the B139 shape it would have been difficult to form using materials of the day - KEF at that point used Bextrene (an acetate) for conical circular drivers like the B110 and B200.

As far as I know, the B139 was the only KEF driver to use expanded polystyrene.
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