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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 19th Jun 2021, 1:15 pm   #1
Edward Huggins
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Default Curved v. Straight Sided Speaker Cones

When servicing Radios and Radiograms, my Father would always show me the curved cone drive units he came across - extolling their superiority. "Boy", he would say, "Look here, this one has got a Parabolic Cone!". I guess since then, I've always seem to show more interest in curved sones believing that they have less break up and can sound more omni-dispersant. But I do recognise that they act less like Pistons in driving the sound waves. Even so, on a blindfold test, I might not be able to tell the difference. Without going into too much Audiophoolery, I wonder what honourable Member's views are?
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Old 19th Jun 2021, 3:16 pm   #2
Leon Crampin
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Default Re: Curved v. Straight Sided Speaker Cones

The Bush VHF64 has one of each + electrostatic tweeter. I wonder what the logic was behind specifying the two differing 8" units.

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Old 19th Jun 2021, 3:52 pm   #3
duncanlowe
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Default Re: Curved v. Straight Sided Speaker Cones

I'm not an expert. But did once get a little training from Philips loudspeakers in Dendermonde. They told me that while at low frequency you are moving air as a piston, at higher frequencies you are relying on the controlled breakup to reproduce those as there is no way the full cone can move at those frequencies. Now of course as you start putting your signal into multiple drivers of different sizes, that becomes less important. They told me that this was also why they used secondary cones in applications where only a single driver was used, the 'tizzer' cone breaking up in such a awy to boost the treble frequencies. So to my understanding, and I'm prepared to be corrected, cone breakup is not per-se a bad thing.

BTW, they used straight cones, with little bumps around the cone. Has anyone seen those, and know what the bumps are for? I do (they told me) and I'll come back later with that. This was in the 1980s.
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Old 19th Jun 2021, 4:08 pm   #4
John_BS
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Default Re: Curved v. Straight Sided Speaker Cones

I'm not a loudspeaker designer, but I've been on the periphery of the design process. As Duncan says, straight-sided cones remain rigid to a higher frequency, other things being equal, but the break-up (= colouration) , when it does happen, is generally not nice.
The pseudo-parabolic profile results in increased rigidity at the point where the voice-coil attaches, and this is "good" in general for reducing unwanted modes, and the flare of the cone results in more controlled break-up at higher frequencies. There will also be less energy arriving at the cone surround, where the aim is to produce a reflection-free absorption.
I do recall that the BBC, when designing the drive unit for the LS5/9 monitor, spent months refining (by trial and error) the profile at the point of voice-coil attachment. This was a Polyprop cone which was vacuum-formed on an aluminium former produced in-house by the model shop.
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Old 19th Jun 2021, 4:58 pm   #5
Craig Sawyers
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Default Re: Curved v. Straight Sided Speaker Cones

Back in the early 90's at Wharfedale we used laser speckle velicometry to visualise what was going on in a loudspeaker design. This was early use of techniques that most driver manufacturers now use to optimise their designs.

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Old 20th Jun 2021, 2:14 am   #6
joebog1
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Default Re: Curved v. Straight Sided Speaker Cones

I have Tannoy speakers and I would have thought they were inverse parabola's. After all the woofer AND the tweeter cone is "horn" shaped rather than beam forming as in a radio telescope.
I mean a parabola is a cup, a horn disperses sound. Doesnt it/they?

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Old 20th Jun 2021, 10:30 am   #7
Robert Gribnau
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Default Re: Curved v. Straight Sided Speaker Cones

Some Philips research from 1955 which supports what Duncan wrote in post #3 (see page 241): https://worldradiohistory.com/Archiv...eview-1954.pdf
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Old 20th Jun 2021, 11:55 am   #8
mark_in_manc
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Default Re: Curved v. Straight Sided Speaker Cones

I used to work in this world - I think Duncan's post #3 is good.

Circular geometries are a bit of a PITA to analyse, regarding modal behaviour - you end up dealing with the roots of Bessel functions. In general terms the modeshapes (or eigenvectors, if you're fancy) work in two ways, producing nodal lines which are either radial from the centre to the rim, or concentric - or combinations of both, for higher-order modes. Radial modes shouldn't happen for devices which are axisymmetric, but they do happen on speaker diaphragms! If you imagine populating the surface of an imaginary speaker with a load of teeny tiny hemispherical radiators - 'monopoles' - you can add up their radiated pressure contribution at some listening position, taking into consideration the phase put in by the different path lengths to the receiver, and the magnitude / phase differences put into each tiny source velocity given by what the modes are doing at that frequency. So what is going on varies considerably as you move off-axis, and this effect gets more extreme the closer you are to the loudspeaker (the so-called 'near-field' - though this is a term which gets abused in all sorts of ways).

Changes in geometry shift the modal frequencies (eigenvalues) and the mode-shapes a little, but they still happen. The other thing you can do is mess around with their Q (by playing with material damping in the cone, and at the surround as already mentioned) and also by messing around with anisotropic cone materials, which is where B&W's interest in woven kevlar came from. That square-weave encourages square modeshapes (at least at low orders), which tend to cancel for on-axis radiation. You can also control the Q with whatever you dope it with.

Perhaps I need to go and lie down. I don't revisit this stuff all that often
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Old 20th Jun 2021, 6:03 pm   #9
John_BS
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Default Re: Curved v. Straight Sided Speaker Cones

Interesting use of rice!

https://youtu.be/nO0bSSXmr1A
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Old 20th Jun 2021, 10:38 pm   #10
mark_in_manc
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Default Re: Curved v. Straight Sided Speaker Cones

Quote:
Originally Posted by John_BS View Post
Interesting use of rice!

https://youtu.be/nO0bSSXmr1A
I sort of knew what to expect there! We too got into that when DML (distributed mode loudspeakers) were kind-of fashionable, a while back - cue lots of jokes in the lab about seeing the face of Mary (or name of Allah) in the rice shapes. Don't get those two the wrong way around. That free-edge condition is one of the few for which an analytical solution exists, along with clamped, and simply- (pin-) supported. Even though FEA is common now, I don't think it aids understanding un-lumped (!) behaviour as much as an old-fashioned modal decomposition. At least, that's how it was for me.

In the end we bought a scanning laser-doppler interferometer - which eventually were things one could buy off the shelf, though I saw earlier devices at B&W and Celestion which I think were more homebrew (I never went to Wharfedale Craig, until it was a small import-operation in Huntingdon, which I think was probably after you were there?). Our interferometer was expensive and I'm not sure the EPSRC ever saw full value out of it, but it was very useful and interesting to use. One of the more unusual applications I saw those things put to was at Ferodo, looking at modal behaviour of brake disks under 'squealing' conditions.
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Old 21st Jun 2021, 2:11 am   #11
Synchrodyne
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Default Re: Curved v. Straight Sided Speaker Cones

I suspect that the original enquiry might have referred to equipment of the 1950s and perhaps earlier 1960s. If so, then it might be worth looking at viewpoints of the time, before really good speaker analysis tools were available.

Gilbert Briggs, in his book “Loudspeakers” devoted a couple of pages to cone shape, as follows:

Briggs LS Cone Shape.pdf


The comments therein tend to support Edward’s position.

Also, pertinent to Leon’s comment about the Bush VHF64, Briggs also provided some comments on the benefits of parallel operation of close-mounted speakers:

Briggs LS Parallel Operation.pdf


It could be that Bush used a pair of dissimilar but same-sized speakers to increase the cone area without using a much larger diameter unit, to obtain the benefit of smoother bass performance from the mutual coupling, and, through different cone shapes, to obtain the desired mid-range balance.

When all is said and done, I imagine that final performance is more dependent upon well executed is the chosen pathway rather than the actual choice of pathway. (Although many hi-fi makers would try to persuade us otherwise.)


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