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Old 17th Oct 2017, 8:26 am   #21
mark_in_manc
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Default Re: Why are speakers elliptical?

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Originally Posted by Hartley118 View Post
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.
I agree with Martin. I also thought I'd add that acoustically the cone shape behaves as a flat piston - both in terms of its radiation impedance, and in terms of the agreement between near- and far-field radiation models based on a circular piston assumption, and what you get when you measure in practice. That is, until the cone starts to 'break up' (vibrate in its higher resonant modes) - which is (as Martin notes) what the cone shape is helping to push to higher frequencies than would otherwise the the case, by virtue of increasing the geometric stiffness of the structure (the material stiffness and inherent damping is, of course, what it is).

The radiation impedance of circular pistons has been appearing in acoustics books almost since Rice and Kellog did their thing (20s?), so these are ideas which go a long way back. A book from the 50s, Hunt 'Electroacoustics' Chap 1 gives a nice readable historical review (starting with Benjamin Franklin!) with no maths, if anyone is interested.

Sorry -
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Old 22nd Oct 2017, 6:50 pm   #22
stevehertz
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Default Re: Why are speakers elliptical?

I always thought the narrow width provided the higher frequency, 'tweeter' effect, and the wide width provided the bass notes, the 'woofer'. Sort of combining two speakers of different diameters, and to some extent offering the frequency response benefits of both in a single speaker. But I never read that, I just believed it!
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Old 5th Dec 2017, 5:04 am   #23
Refugee
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Default Re: Why are speakers elliptical?

Here is a photo of some stretched round speakers that have been pulled from a dead flat screen TV.
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Old 5th Dec 2017, 8:30 am   #24
flyingtech55
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Default Re: Why are speakers elliptical?

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Originally Posted by Craig Sawyers View Post
"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.
I remember my father telling me purchase tax was the main reason for the development of multi-section valves and why very few valved radios in this country had push-pull output stages. Apparently at one time, purchase tax was based on the number of valve envelopes fitted the radio. Hence, if the manufacturers could effectively get several valves into one envelope it saved on purchase tax. I believe that if a radio had a push-pull output stage (and therefore an extra valve) it was considered to be a luxury item and attracted higher taxation. This could have driven the development of the likes of the ECLL800.

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