What's happening to a cone driver when over driven with bass?

[Boscoe]

Just wondering I'm reading a book at the moment on loudspeakers which talks about no linear distortions when the cone breaks up when high volumes are being played then you get fluctuations in the cone.

Is this what's happening or something different?

Also is the a website that could describe to me all the distortions that are associated with speakers only and one that can explain what all the parameters mean?

Thanks
Boscoe

[chris661]

The two (main) distortion thingies in loudspeakers seem to be as follows. I think. I'm not an expert on this, it's just what I've either observed or can picture happening in my head. If someone else comes along and tells you otherwise, even I'd believe them first.

Non-linearities in the movement. This can be down to suspension being asked to move further that it's designed for, or the voice coil leaving the magnetic gap. The resulting sound isn't really objectionable until the voice coil smacks the back plate. That said, some drivers suffer more than others. Many note that the Fostex FE*** range tend to pass linear excursion limits with no obvious degrading of the sound.

Next up, cone break-up. This occurs when the cone no longer acts as a piston. The outer rim of the cone may be delayed relative to the centre of the cone. The whole cone would flex, and you'd end up with phase issues (happens with larger drivers to give less high frequency output relative to when the entire cone is radiating at lower frequencies). Another way would be standing waves on the cone. If a frequency is being reproduced at sufficient amplitude, where the wavelength (or half wavelength) co-incides with the distance of the voice coil to the cone edge, the cone will flex in a non-piston-like fashion. This gives strange peaks in the output, too, but only occur when you're playing it loud.

Chris

[Boscoe]

Non-linearities in the movement. This can be down to suspension being asked to move further that it's designed for, or the voice coil leaving the magnetic gap. The resulting sound isn't really objectionable until the voice coil smacks the back plate. That said, some drivers suffer more than others. Many note that the Fostex FE*** range tend to pass linear excursion limits with no obvious degrading of the sound.

^This is when the driver just exceeds its xmax and starts hitting the magnet and can damage the surround, suspension and the coil. I know this one!

You described non-linear distortion in your second paragraphand it sound about right to what I've been reading! So is this what causes that awful crackle at high volume/bass?

[chris661]

There's usually a few mm between non-linear excursion (often leaving the magnetic gap, when BL is down to ~70%) and hitting the back plate.
Xmax is linear excursion, Xmech is when you're hitting the back plate.

The awful crackle could be all sorts. I know with the drivers I've tried (full-rangers), they tend to "scream" on vocals and guitar solos. It sounds terrible. Really peaky response. No crackle though... That sounds like it's more of a mechanical problem. These full-rangers were crossed over to prevent excursion becomming an issue.

[Xoc1]

Hopefully the crackle that you are experieriencing is nothing more serious than occaisional amplifier clipping.
Take it as a reminder to turm it down, or buy a bigger amp, befiore you so some serious damage.

[Boscoe]

Originally Posted by Xoc1

Hopefully the crackle that you are experieriencing is nothing more serious than occaisional amplifier clipping.
Take it as a reminder to turm it down, or buy a bigger amp, befiore you so some serious damage.

Yeh that would be good if they actually went louder! I always assumed it was too much bass!

[Xoc1]

It's difficult to be more specific without knowing some details about your system.
I never manage to clip my amp, but it does deliver 150W per channel!

[Boscoe]

It's difficult to be more specific without knowing some details about your system.
I never manage to clip my amp, but it does deliver 150W per channel!

Well I came to the conclusion that it was the speakers becuase I measure the voltage of 20 volts which meant 45W was going into the driver at 30Hz (this is the point at which it started sounding bad) then this corresponded to about 1mm over the xmax and with the speaker rated at 50W I assumed it was that.

[Soundminded]

Originally Posted by Boscoe

Just wondering I'm reading a book at the moment on loudspeakers which talks about no linear distortions when the cone breaks up when high volumes are being played then you get fluctuations in the cone.

Is this what's happening or something different?

Also is the a website that could describe to me all the distortions that are associated with speakers only and one that can explain what all the parameters mean?

Thanks
Boscoe

As the speaker is driven harder, an imbalance between the forces restoring the cone around the inner and outer circumference of the suspension or between one point in either circumference and another will cause a shearing force on the cone tending to twist or flex it. To reduce this possibility, cones should be made of strong materials that are inherently rigid and do not deform easily. In this regared, aluminum and plastic are better than most paper but they do not damp resonances within the cone nearly as effectively as paper. When the cone flexes, it breaks up into vibrational modes just the way a drum head does. This results in severe harmonic distortion, 30% or more. (Acoustic suspension systems have an inherent advantage in this regard since the restoring force is the force of the trapped air spread uniformly over the cone. The force density is lower the pressure at any moment is the same at all points.

Beyond that are the mechanical limits of the speaker. When the helical voice coil bottoms out, it gets flattened and is no longer helical. Time for a new voice coil. Often a rasping sound will be heard as the coil scrapes its way back and forth. The suspension and the cone can often be torn by forcing it to travel too far. Finally, when enough current passes through the voice coil wire, it will heat up to the point where it will melt. Then there is no sound at all. And they can and do catch fire from the heat sometimes.