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Old 22nd October 2011, 08:53 PM   #1
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Default Why are boxes stiff but cones not?

Why are boxes extremely stiff but cones not at all?

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Old 22nd October 2011, 10:41 PM   #2
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Boxes because one does not want losses due to the exchange of sound energy for heat or re-radiation... so stiff and damped is often best... also one does not want the speaker wiggling in reaction to the motion of the cone...

Cones are made extremely stiff. Beryllium domes for example. Metal cones. Sandwich honeycomb, sandwich foil/foam... but with cones one also needs to watch things like breakup modes and traveling waves. Stiff cones tend to often be hard and of materials that have few losses, also higher frequency resonances (often undesirable).

So the usual thing is to include some losses in the cone itself, as well as in the surround...

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Old 23rd October 2011, 12:17 AM   #3
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Quote:
Originally Posted by bentoronto View Post
Why are boxes extremely stiff but cones not at all?

Ben
Actually good cones are made just about as stiff as is humanly possible - given that they also have to be very light. The light weight is essential so that the cone can vibrate rapidly to produce sound.

So why do cones feel floppy when you push them with a fingertip? The key is that matters is not the stiffness by itself, but rather the RATIO of stiffness divided by mass. A heavy object like the wooden panels of the box have to also be very stiff; something a thousand times lighter, like a speaker cone, can also be a thousand times less stiff, and still work just as well. I'll come back to this idea in a minute.

Every stiff mechanical object is rigid (moves as one solid body) if you shake it at very low frequencies. If you keep increasing the frequency, at some point the object no longer remains rigid; different parts of it move by different amounts, in different directions. It acts "floppy" instead of rigid. As you go even further up in frequency, the object acts more and more floppy and less and less rigid.

I've avoided technical terms so far, but if you're up for one, here goes: when you reach this frequency where the object first begins to act floppy, we say you have reached the objects lowest mechanical resonance frequency.

The object with loudspeaker cones - and loudspeaker enclosures - is to try to raise this resonant frequency (also called cone breakup in this case) high enough so it doesn't affect the sound coming from the cone. If you're dealing with a woofer that crosses over to a tweeter at, say, 2 kHz, then you want the woofer cone to have its first mechanical resonance (cone breakup) well above 2 kHz. That way, the woofer cone acts like one rigid object over the entire band of frequencies that the woofer handles.

At very high frequencies the woofer cone is floppy, but it doesn't matter, because the tweeter has taken over the job of creating sound by then - the woofer is no longer being asked to move at these high frequencies.

And here's where we go back to the ratio of stiffness to mass: that ratio is the thing that determines the resonant frequency. To be exact, the square root of the ratio of stiffness to mass is the thing that determines the lowest resonance frequency. So a good cone is made of material that is both stiff and light.

Surprisingly enough, most metals are actually terrible choices for loudspeaker cones. Aluminum - quite popular with some speaker designers these days - is incredibly bad. This is because aluminum is quite heavy, and not very stiff for its weight. The ratio of stiffness to mass is lousy, so aluminum speaker cones tend to suffer cone breakup at rather low frequencies, limiting the range of frequencies over which they can work.

Equally surprisingly, paper is a really good speaker cone material. It's not terribly stiff, but it's extremely light, so the ratio of stiffness to mass is much better than any metal I could find data for! Unfortunately, paper does not age very well (gets brittle with age), and it doesn't cope well with high humidity, or very low humidity, or very high temperatures (like the ones inside a parked car in summer in Arizona).

Other materials fall in between. Most plastics are worse than paper - they weigh too much for their stiffness. But some plastics have a smooth and gentle cone breakup, which eases the speaker designers job.

Similarly, carbon fibre is pretty good - light and stiff - but when it does break up, it tends to happen rather violently.

Some loudspeaker designers blend plastic and carbon fibre to try to get the best of both worlds in a loudspeaker cone. Alesis M1 and M1 Active woofers use this sort of cone, with a mix of plastic and carbon fibre.

-Flieslikeabeagle
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Old 23rd October 2011, 01:58 AM   #4
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Sorry for being so succinct. I suppose after 56 years I just might be unaware of the T/S parameters and asking why a cone moves in and out! Sorry, I think I know why cones move in and out and about resonance.

What I am puzzling over what is the difference between box walls which can't be floppy and mid-cone areas which have to floppy*? Shouldn't what degradation that is sounded by insufficiently rigid enclosure walls also come through the thin cardboard cone? Even if we consider the phase of the driver and the phase of the interior pressures, the only place where the cone is controlled is the place where the VC is glued to the cone. As we move from that circle, we are looking at thin cardboard, often big expanses of cardboard as with 15-inch drivers.

Ben
*Funny thing, my AR-1 woofer has a cone made of seriously stiff cardboard... but then it has a resonance of 12 Hz.
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Last edited by bentoronto; 23rd October 2011 at 02:03 AM.
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Old 23rd October 2011, 04:21 AM   #5
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Essentially you do not want the box to add its own vibrations to the sound coming from the speaker. The panels of the enclosure make a much larger radiating area than the speaker diaphram, so it does not take much movement to be audible. A stiff enclosure will push any resonances higher in frequency. This is mostly a good thing, since music has less energy at higher frequencies and damping material inside the enclosure will absorb higher frequencies more effectively than low frequencies.

This is not to be confused with the resonance of the woofer/box system, which is due to the mechanical resonance of the woofer combined with the air trapped in the box. Ported systems and passive radiator systems also use the air within the box to create a controlled resonance to enhance low frequency response.
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Old 23rd October 2011, 07:45 AM   #6
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Quote:
Originally Posted by bentoronto View Post
Sorry for being so succinct. I suppose after 56 years I just might be unaware of the T/S parameters and asking why a cone moves in and out! Sorry, I think I know why cones move in and out and about resonance.
Good, you have some technical background. But the resonance I described has nothing to do with Thiele-Small paramaters. The TS parameters have to do with the movement of the entire cone as one piece. Cone breakup has to do with the "floppiness" and mass of the cone itself.
Quote:
Originally Posted by bentoronto View Post
What I am puzzling over what is the difference between box walls which can't be floppy and mid-cone areas which have to floppy*?
I explained this already in my post above. What matters is not the rigidity by itself, but the ratio of rigidity to mass. Since the cone is much less heavy than the box walls, it can be much less stiff and still achieve an equally high resonant frequency.

In fact, a good woofer achieves higher cone breakup frequencies than the panels in a typical loudspeaker enclosure do. In other words, as far as the sound is concerned, those "stiff" box walls are actually less stiff than the thin paper cone! (Remember, it's the ratio of stiffness to mass that matters.)
Quote:
Originally Posted by bentoronto View Post
As we move from that circle, we are looking at thin cardboard, often big expanses of cardboard as with 15-inch drivers.
That's one of the reason why a cone shape is used (and not a flat disc of paper). Cones are pretty stiff, at least to axial forces.

Again, I explained the logic of the thin paper cone in my first post - a 15" driver will indeed go into cone breakup at a lower frequency than, say, a 6" one. A good loudspeaker design will crossover from that big 15" driver to a smaller driver before the frequencies get high enough to cause cone breakup.

This is why when you use a big woofer, you need to have a 3-way or 4-way speaker system. The big woofer suffers cone breakup at a relatively low frequency, and there is no way it can cleanly reproduce sound high enough in frequency to smoothly cross over to a small 1" tweeter. You have to cross over at a lower frequency, and therefore to a (much bigger than 1") midrange or mid-bass driver.

If you use a smaller woofer (like the 6.5" one in the near-field monitor in front of me right now), you can cross over to a small 1" tweeter without needing a midrange or mid-bass in between. The smaller cone can be stiff enough to get all the way up to 2 or 3 kHz, at which point a little 1" tweeter can take over.
Quote:
Originally Posted by bentoronto View Post
*Funny thing, my AR-1 woofer has a cone made of seriously stiff cardboard... but then it has a resonance of 12 Hz.
No - the 12 Hz is the resonance caused by the mass of the voice coil and speaker cone, hanging from the springiness of the spider and cone surround. It is the Thiele-Small resonance (fs). Fs has nothing to do with the stiffness of the cone itself, or with cone breakup.

The cone stiffness (along with its own weight) will cause cone breakup at a much higher frequency than 12 Hz. For reference, one 8" driver I measured started cone breakup at around 1 Khz. Your 15" driver will start to break up well below that - but a long, long way above 12 Hz.

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Old 23rd October 2011, 07:52 AM   #7
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Old 23rd October 2011, 10:47 AM   #8
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I think I understand the question which is being nicely answered by flieslikeabeagl and 454Casull. Cones are wonderful creations designed to achieve a bunch of conflicting demands put on drivers (although there has not been vast improvement in cone design for many years). Thank you. But my question is different.

Think of it this way. If you cut a 5 x 5 inch hole in the wall an enclosure and "repaired" it with a 5 x 5 inch piece of speaker cone cardboard, people would think you are nuts (assuming of course, you didn't have a kind of passive radiator port in mind).

How is that piece of wall hole different from the "piece of wall" which is filled by the driver itself and composed of cone cardboard? Isn't the cone, dust cap (for magnets with holes bored through), and surround part of the enclosure perimeter?

Just because our first impulse is to think of the cone as an active part of the system shouldn't we also recognize that it is part of the box perimeter? And that is my question.

Ben
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Last edited by bentoronto; 23rd October 2011 at 10:58 AM.
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Old 23rd October 2011, 11:03 AM   #9
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The "patch" in your analogy isn't being driven. Stick a motor in the middle of it that works in phase with the signal.

At higher frequencies, cones can indeed be acoustically transparent; that's why you'll often see slant panels behind drivers to prevent a notch at a wavelength of twice the distance from the cone to the back wall.
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Old 23rd October 2011, 11:38 AM   #10
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stiff relative to what?

The box is stiff for two reasons.
1.) to reduce the change in volume due to pressure changes resulting from the signal.
2.) to reduce panel deformation due to pressure changes resulting from the signal.

The cone is stiff for at least three reasons.
3.) to reduce the change in shape due to accelerations of the mass of the cone.
4.) to reduce deformations that are the result of pressure from the air load, both inside and out.
5.) to reduce deformations in general that could cause spurious sound output as a result of the two differing loads in 3 & 4.

I suggest that the box is designed to be stiff enough to meet conditions 1 & 2
I further suggest that the cone is stiff enough to meet conditions 3, 4 & 5.

What I am stating is that both the box and the cone are stiff relative to the loadings they each are designed to handle.
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