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Discussion in 'Amps and Cabs [BG]' started by semborg, Apr 19, 2005.
What is it technically exacly happening when a cab "Farts"?
how to prevent it?
When a power amp runs out of power it clips the signal (aka turned up too loud). The speaker reacts to the clipped signal as if it were DC current and locks into position during the duration of the clip. That creates the farting sound you hear.
Clipping isn't the only effect....
If the cabinet does not control speaker movement well, the speaker may run into movement limits at low frequencies, which is a form of "mechanical clipping" that adds lower order harmonics.
There may be added "breakup modes" if the cone starts to physically bend out of shape due to the opposing forces of voice coil and suspension.
A speaker driver below its system limits may start to "double", producing second harmonic distortion.
BTW, the speaker doesn't "lock into position" with a clipped input. It simply follows what is now a harmonic-rich square wave input to it.
Some amplifiers don't clip smoothly, and may have some form of DC as a side effect of clipping. But the speaker does not see DC just because of clipping.
The farting that most people describe is typically some product of the speakers reaching their excursion limits, sometimes accompanied by amp clipping, sometimes not...
Some power amps don't sound right unless they're clipping a bit..... Old QSC 1500s on Martin bass bins, for instance. The bit of low frequency distortion gave a lovely thwack to a kick drum.....
As for clipping locking the driver in place, I think I understand where that misconception comes from. People see graphical representations of perfect square waves and logically surmise that the drivers follow that trajectory. Of course, real signals are way more complex than that and don't really ever get close to a pure square wave. Cone inertia would prevent the speaker from stopping dead even under pure square waves.
It expells excessive air and a noxious sound out of it's "port". Just feed it the right mix of power and it will stop, and your friends and neighbors will thank you.
Think so? Try this experiment. Hook a 9v battery to a speaker...now try and move the cone. Let me know what you discover.
Of course DC for several seconds sustained will allow the cone to stop.
"DC" for a fraction of a second shortly followed by "DC" of the opposite phase will not allow the cone to stop accelerating.
Please note I say "acceleration" and not "motion", since the cone has to stop "moving" at each peak of any wave in order to change direction. The cone would theoretically not be accelerating at the zero-point of the wave, where its velocity would be a maximum. This is more readily comprehended for a standard sine-wave, although it holds true at this basic level for any non-complex wave.
At any rate, for such a signal as a clipped musical signal, the driver does not see a constant DC voltage for long enough to become "stuck". As Mark rightly said, the speaker's inertia doesn't allow for that during such a short period of time. This holds true not just for clipped signals either.
You will discover that the cone "appears" to be held firmly. That is an illusion. it is held "sorta" firmly, between the action of the current in the coil, and the restraining force of the suspension.
For the "real story", short the speaker terminals, and try to move the cone. You will find there is some resistance, but you can move it.
In reality, very little movement is needed to create a sound that can be "heard". And there will be SOME "overswing" of the cone, no matter how "tight" is seems.
The magnetic coupling of coil and magnet cannot stop the cone "dead" any more than you can swing a 2 kilo hammer and stop it instantly without ANY overswing (unless you hit something).
Math/physics-wise, there cannot be any restraining force by the magnetic field unless there is some cone movement to induce a current in the voice coil.
You can make that as small as you please with bigger magnets, but there must be "some". And that "some" is the "overswing" that very likely is hearable with a practical coil and magnet system.
This has been more than adequately dealt with by Jerrold and BassIan but I have a few comments.
Of course a connecting a 9V will make the cone jump out to a static position and stay there, DC creates a static field.
But if you got some sensitive measurement equipment and measure the position of the cone at steady state and then measure what happens in the few milliseconds after you connect the battery you will see that the cone overshoots its steady state position and then oscillates about the steady state position a few times before settling down. It actually takes a significant amount of time to settle down, depending on the mass of the cone, etc. (tens of ms for big cones). Things with mass cannot stop instantaneously.
It's not exactly difficult to push it back either, the fields aren't that strong.
Even under sever clipping, speakers don't stop moving for any significant amount of time for the above reason and because at any time there are many frequencies driving the speaker together, so even if it could stop at the crest of a clipped low frequency wave, there would be a lot of other information going to it that would keep it moving, regardless.
Regarding amplifier clipping as being a pure square wave at one frequency or basing AC speaker behavior on DC behaviour is far too simplistic to be of any use.
Okay, I give up, but one thing is clear...during clipped conditions whether the duration is 1 millisecond or 100, the amp has lost control of the cone and the sound that comes out is not musical, but total audio nonsense which sounds like a fart.
The amp has not lost control of anything. Its output section simply cannot produce a greater voltage swing. When driven to this condition, any output device generates strange harmonic distortion, which in the case of many (read most) solid-state devices are not coherant/pleasant to the ear. The cone of the speaker is simply following the signal, be it a distorted or clean one, it is just moving mechanically to reproduce whatever voltage function it's being sent. The accuracy of this mechanical reproduction, as was covered above, is limited by the capabilities of the driver.
Now, if the speaker driver goes into a non-linear segment of its travel, you will start to hear driver distortion. This is where the driver's cone has gone outside of its linear zone either because the voice coil is out of the gap, or the suspension has begun to reach its limits. At the extreme case, the voice coil may bottom out on the magnet structure, or the suspension may reach its mechanical limits. This can also cause "farting".