Not at all, I meant that ONLY for waves where input and output have a same shape, like a phase-shifted sine, it LOOKS LIKE but IS NOT delay.
Look at it this way: a 90 degrees phase shifted sine is in fact a cosine, which looks the same as a time-shifted sine. But it isn't, as proven by the FACT that when you make a change in the input waveform, the output waveform IMMEDIATELY reacts, NOT 90 degrees later.
Jan
Will a bending wave and a compressional wave (or s and p wave) have the same velocity?
Not at all. Bending waves' propagation depends on geometry or shape of the structure that is bending, as well as the distributed mass, elasticity, and loss of the material. Compression waves generally rely on material properties only, and are far far faster than most transverse or bending waves. Eg a steel wire stretched loosely between two posts, deflected suddenly laterally at one end. A wave propagates transversely along the wire at a velocity far far slower than the longitudinal speed of sound in steel.......
Last edited:
No, if two non-sinusoidal waveforms are identical, except in time, that is always a delay rather than a phase shift.
If you suddenly change a sinusoidal waveform, it is immediately no longer a sinusoid and the outcome immediately depends on the nature of the filter involved beyond that behaviour described by phase for the sinusoid.jan.didden said:
Lucky,
We are not talking about drivers such as a soft dome tweeter where you are spot on about the motion.
If we take a 15" woofer and drive it with a 1 hertz signal we can clearly watch the cone move in and out. If we drive it at 1000 hertz, we can't see the motion and it is pretty much assured it is not behaving as a piston. So it seems there is some frequency at which the dominant cone behavior changes.
What in the trade is known as the mass break frequency is where (Duh!) the mass is high enough and the mechanical "stiffness" can no longer move that mass at the acceleration asked of it. This results in buckling. (That word should raise the hair on the back of George's neck!)
For a 15" driver the mass break frequency is around 350 hertz.
Now Klippel makes some nifty analyzers that show when and where the cone behavior is no longer pistonic. (Klippel is also a real marksman with an RPG launcher.) The references cited show both the pistonic motion with the unwanted deviations.
So the dominant mode of motion in a properly designed cone transducer should be pistonic.
Which brings up the Queen patent. If I get the point, the claim is that a normal cone transducer has the pattern narrow with increasing frequency. Or in other words as the wavelength approaches the piston diameter the coverage angle narrows. A well known condition. (The basis for a line array.)
Cerwin Vega used to make a loudspeaker with a 15" driver and a piezo tweeter. The on axis frequency response was quite flat. The power response was as expected, tilted down until the tweeter kicked in then it popped up.
The patent seems to be to tapper the thickness of the cone so that as frequency increases the cone buckles then the radiating area is smaller allowing for a uniform coverage angle. Now why such a patent where you increased distortion to get more uniform coverage never became popular ????
ES
I didn't say that so that's irrelevant
Didn't say that either, but we're now at the point where you have to resort to shifting goal posts, so probably not useful to continue.
Jan
Does anyone truly believe that either of these shapes can move as a purely rigid structure giving the force vector of the voice-coil former and the shape of either cone shape? Both of these shapes bend when force is applied, even in the so called pistonic range, you can not imagine that either shape is infinitely rigid in any way. That posting by Scott shows you that. Not only that many still want to believe that the surround moves in the same direction as the cone, very far from true, it is in anti-phase relationship, the wider the surround the larger the problem with surface area moving in opposite directions. This is the equivalent to a free air speakers cancellations but that is another issue not spoken of here or normally mentioned..
ps. Ed, even though you can watch a 1 hz wavelength move a cone at anything higher than a limited amount our eyes will not be capable to see the bending modes, they are not something we can see, that is the purpose of the laser systems, to show through differentiation what is really happening right before our eyes. The eyes lie!
ps. Ed, even though you can watch a 1 hz wavelength move a cone at anything higher than a limited amount our eyes will not be capable to see the bending modes, they are not something we can see, that is the purpose of the laser systems, to show through differentiation what is really happening right before our eyes. The eyes lie!
Attachments
Last edited:
No, the bending behaviour is unchanged, just that at 1Hz the period is >> propagation time for bending waves within the structure, so the structure behaves as a single body rather than as a wave system. As driven frequency increases, and period becomes significant in terms of propagation velocity for the structure, so wave behaviour begins. At some point forces within the structure might exceed limits for normal elastic behaviour and so breakup might occur, and perhaps resonant frequencies play a part in this. But the fundamental bending behaviour of the cone remains the same until or unless a limit is reached. The cone is always flexing, and mostly exhibiting wave propagation behaviour to an extent depending on driven frequency and the specific structure.
In my terms, forces within the structure exceed some threshold for linear predictable bending behaviour.Simon7000 said:
Kindy,
That flexing although very small in well made cones actually makes for a more pistonic behavior than a cone!
I was at a tour of the Bose plant when they were about to give me a tour of where they did laser holography to measure cone distortion. I mentioned I had worked with similar stuff and was anxious to see the exact setup. They then declined to give me that part of the tour as they had paid a bit to an expert to get it working and considered it secret sauce. As I might actually understand what they were doing...
When at JBL I was in what they call the "arena" when a test engineer was setting up an interferometer system to measure enclosure vibrations. He set things up but didn't actually run anything until my host (Bill G.) showed back up and told him it was okay to run the test with me present. They at least would show me some secret sauce.
Now W Klippel and I both work on an AES standard so he says hello and shows me his toys just about every time we meet. unfortunately I really have no use for one of his gizmos. I do have the acoustic measurement stuff, but the only mechanical gizmo we use is a box breaker.
It is a large frame with a hydraulic pull cylinder at the top attached to a load cell. You put the loudspeaker with the hanging hardware in the frame and pull until it breaks. (Useful to note how much force it took!)
Really got one good use out of that so far. We were using eight 35 pound 12" woofer boxes in a line array system. A "consultant" wrote a report claiming the mounting hardware was inadequate. (My structural P.E. of course had already reviewed everything and actually had to make a site visit to confirm things after that.
)Now the attachment was four .125" thick by .75" wide straps (four per box) were bolted by 1/4" bolts to each other to form four chains holding up the array. Four anchors for the top box and four from the bottom box as the array curved enough.
Breaking one of the chains required a bit under 2100 pounds of pull. So as expected the "consultant" was a bit off, as the 280 pound load with eight 2100 pound attachment points for some strange reason is still in the air!
One issue to note is that when loudspeaker arrays are flown with chain motors the maximum stress occurs when the lifting stops. Actually as we use chain hoists the up direction has little excess force as it is hard to push a chain. Dropping down when you stop is the maximum impulse.
ES
snup,
I have not studied the new magnetic shocks but believe that there is still a spring setting ride height? The magnetic fluid is changing viscosity due to a magnetic field, the fluid is still passing through an orifice just as a normal shock I think and the change in viscosity changes how fast the fluid can pass through the orifice. I don't know if the orifice is also changing dynamically, they could modulate the size of the orifice with the viscosity and have a very fast acting change of both to modulate the damping, that is still the function of the shock, it is not the load carrying member that is still a spring as far as I understand.
I have not studied the new magnetic shocks but believe that there is still a spring setting ride height? The magnetic fluid is changing viscosity due to a magnetic field, the fluid is still passing through an orifice just as a normal shock I think and the change in viscosity changes how fast the fluid can pass through the orifice. I don't know if the orifice is also changing dynamically, they could modulate the size of the orifice with the viscosity and have a very fast acting change of both to modulate the damping, that is still the function of the shock, it is not the load carrying member that is still a spring as far as I understand.
Appols, jan.didden just trying to guess what on earth you might mean and where the philosophical problem might lie, because I don't see one when definitions are correctly applied.
We ain't arguing that there is bending behavior. It just normally isn't an issue when used in the proper frequency range for the driver. The normal failure mode is actually buckling. JBL used to make a 15" driver for horn loaded applications. In failure the paper like cones actually cracked and pieces dropped out! (Think 30,000 ips velocity on a 6" length!)
The Queen patent requires bending behavior and is tapering the cone thickness to control it.
The older Bose drivers used to have a glue bead on the back of the cone. That was based on the observed behavior.
Now consider the case of a soft dome tweeter and how it actually behaves. There is a good reason it doesn't become directional. Also a good reason why some folks find their sound to be annoying. (And also how to design one to have a built in 20,000 hertz or so low pass filter.)
Last edited:
There probably isn't any cone bending at 1Hz, because the rim suspension is free to move. The cone moves as a single body for reasons set out above in my reply to Simon7000, and the rim suspension accommodates motion. If the rim suspension were fixed, the cone would flex at 1Hz and we would see it. Dynamics become very different as f increases and wave behaviour becomes dominant.
- Status
- Not open for further replies.