A case study on how speakers work

[Vaughan Odendaa]

Hi there,

In the past few weeks I've been posing questions that on the surface are pretty easy to understand but to understand the "why" or "how" behind it becomes far more fascinating.

At least I am fascinated by it. But to understand this fully I need to ask more questions.

A week ago I posed a question because I was unsure about digital signals being AC as members at AVSforum seem to think that digital signals are AC with a DC bias offset. But the bottom line is that the signal that runs from the cables to your speaker is AC. It has to be otherwise the cone would not move according to the direction of current, and no sound would result.

Apparently DC can carry no information so it would be fallacious to state that digital signals are DC. At least I would think so. But the DC signal that the amp amplifies is turned into AC by the transistors I would imagine.

That is that.

So now I ask a different question concerning two things so that I understand it fully. One is current flow and the other are two magnetics in the speaker attracting and repelling on another.

Current will reverse polarity depending on frequency. Music is obviously dynamic and consists of thousands of frequencies and so current will reverse polarity thousands of times per second or hundreds of times, depending on the musical signal.

My question is, the current that changes direction is a product of the electromagnet attracting and repelling (because it has a north and a south pole) the positive north pole of the permanent magnet and repels on the negative pole of the electromagnet.

Because I was thinking about this the other day and I was getting slightly confused with two things. On the one hand, we have two magnets, one a temporary electromagnet that can only exist or produce an electromagnetic field if it is interacting with a permanent magnetic field.

So we have repulsion and attraction. But then we also have current flow reversing direction based on polarity of the signal. So we have the positive cycle and the negative cycle of current flow and this will cause the cone to move out and in depending on the cycle.

So in other words, there are two parts to the equation. I just need to understand how they fit in a bit better. Is the cone changing direction because of the current flow and is the current flow direction caused by the two magnets attracting and repelling.

If we had both EM and PM and a DC current then direction of the coil would only move in one direction. Because there can be no attraction and repulsion because current in this case has a non-varying polarity ?

If someone more knowledgable in this area could explain to me how the current flow direction (AC) works in conjunction with the two magnetics, and in what step this occurs, I would really appreciate it.

Thanks for hearing me out !

--Regards,

 

[Vaughan Odendaa]

Anyone ?

--Regards,

 

[Haoleb]

Apparently DC can carry no information so it would be fallacious to state that digital signals are DC. At least I would think so. But the DC signal that the amp amplifies is turned into AC by the transistors I would imagine.

Digital signals do not go into the negative voltage domain, therefore how can they be AC.

Im not sure what you mean by DC signal that the amp amplifies. You cannot feed a digital signal directly into an amplifier without first going through a DAC, which turns the digital signal into an analog AC waveform. basically all the transistors in an amplifier do is take the incoming signal and make it bigger.

 

[Vaughan Odendaa]

But in order for speakers to reproduce those signals it has to be converted to AC otherwise there wouldn't be sound, correct ?

not sure what you mean by DC signal that the amp amplifies. You cannot feed a digital signal directly into an amplifier without first going through a DAC, which turns the digital signal into an analog AC waveform.

Although badly worded, that is what I mean't. It is converted to AC. The question arose because cable vendors have claimed in the past that wire is directional, but how can wire be directional if the signal is AC.

There is one person I spoke to who countered this and said that digital signals are DC and so only flow in one direction. But it's not the same thing obviously, because musical signals are AC by definition.

--Regards,

 

[Haoleb]

Im not even going to comment on directional cables besides the fact that some, are by design- directional. This has to do with the shielding being connected at only one end. Obviously the cable will still work when connected up the other way too, but the drain is meant to be connected at the source end only IIRC.

 

[mtrycrafts]

Anyone ?

--Regards,

One comment on that DC and digital. If that signal pulses with the code, how many zeroes and 1s in a row, that is a signal transmission. like your spark plug firing at different time intervals. Another circuit can time it and translate it.

 

[MDS]

There is one person I spoke to who countered this and said that digital signals are DC and so only flow in one direction. But it's not the same thing obviously, because musical signals are AC by definition.

This is a case of mixing metaphors. There are umpteen different 'modulation' schemes for digital signals. You are dealing with binary numbers - zero and one - and there are myriad ways to define zero and one. The modulation scheme determines what represents a zero and what represents a one.

The simplest example I like to use is PWM (Pulse Width Modulation). The 'width' of the pulse determines whether the value is a zero or a one. In that case the voltage is constant but the digital values are encoded in the width of the pulse. A 20us pulse could be the zero value and a 50 us pulse could be the one value.

This is not related to AC vs DC for a speaker. Digital values mean nothing to an amplifier or a speaker. The zeros and ones are accumulated and then converted to analog by a digital to analog converter. The accumulated values (the 'bit depth') are essentially the amplitude of the signal at points in time. The DAC converts those numbers to an analog waveform in an inverse fashion to an ADC that 'samples' the analog values and assigns them a number to represent their amplitude at that point in time.

The key thing to remember is that digital values are DATA that represent the real world analaog equivalent whereas an analog signal IS the real world signal itself.

 

[Vaughan Odendaa]

Okay cool. That is sorted. Could you please comment on my primary question ?

Thanks.

--Regards,

 

[no. 5]

So in other words, there are two parts to the equation. I just need to understand how they fit in a bit better. Is the cone changing direction because of the current flow and is the current flow direction caused by the two magnets attracting and repelling.

Let's see how good my understanding on speaker function is!

As far as I know, the current flow direction (polarity?) is governed by the amplifier, and the cone changes direction depending on the direction of the current coming from the amplifier.

 

[Vaughan Odendaa]

Yes, current flow direction is governed by amplifier. Correct. Cone changes direction based on the direction of current flow as current flow changes polarity so does the polor orientation of the coil.

But the coil is producing an EM field and it is being used in conjunction with a permanent magnetic field because the EM field by itself is useless. It's an alternating field that can only exist when current is applied.

Both magnets have north and south poles except the voice coils N and S poles can be changed depending on current flow. My question was as follows ; just as current flow changes polarity of VC and of course, the cone, the magnetic forces of attraction and repulsion are also happening because the south end of the VC is attracting the south pole of the PM and the north end of the VC is repelling the magnetic field.

Attraction and repulsion.

On one hand we have current flow and the other is opposing magnetic forces between two magnets. So my primary question is, in what stage does 'what' occur. I just need someone to put this into perspective for me so that I can understand this.

Thanks.

--Regards,

 

[mtrycrafts]

Yes, current flow direction is governed by amplifier. Correct. Cone changes direction based on the direction of current flow as current flow changes polarity so does the polor orientation of the coil.

The the distance the cone moves in a direction is also signal strength dependent. so, if you vary the signal intensity in one direction flow, the cone will travel from the minimum distance with minimum signal strength to max distance with max signal, if it goes down in signal strength but still in the same direction, the cone still has to move accordingly.
If you reverse the signal flow, the cone moves from the neutral cone position to the other direction.

But the coil is producing an EM field and it is being used in conjunction with a permanent magnetic field because the EM field by itself is useless. It's an alternating field that can only exist when current is applied.

You get a magnetic field from a direct current. That is how cars are picked up by that huge magnet

My question was as follows ; just as current flow changes polarity of VC and of course, the cone, the magnetic forces of attraction and repulsion are also happening because the south end of the VC is attracting the south pole of the PM and the north end of the VC is repelling the magnetic field.

Like poles repel each other not attract. So, the current flows in one direction, you have the north pole at one end of the coil. This field interacts with the permanent magnet, pushes away or pulls closer to it. when the field reverse, the opposite happens. And, the amount of current the distance the voice coil travels.

Does this help better explain?

 

[Ampdog]

Vaughan Odendaa,

Perhaps I can offer an alternative explanation. Your problem is perhaps the same as that of many thinking students at first contact with the moving coil loudspeaker. They see a centre pole that is (say) N by virtue of the permanent magnet in the construction, making the ring or outer pole S. If one first take the voice coil away from the gap, a (dc) current flowing in that coil will naturally cause one end to be N and the other to be S. If now that coil is inserted into the gap, the question arises why the S end of the coil (say the top) is not attracted to the N centre pole and the N end to the ring, in a sort of twisting way so to speak. Allowing the coil to move in and out only in a constant magnetic field, seems to bring nothing nearer to or further from the fixed poles.

If that is your problem, you must forget about poles in the coil caused by a current. The thing to remember is that, empirically, if a straight wire is suspended in a magnetic gap of uniform intensity and a current is turned on through it, it will move in one direction perpendicular to the gap or magnetic field (not to one or other pole). If that current is reversed, the movement will be in the opposite direction. This is one of Fleming's basic rules of electricity, current and motion. The same applies if the magnetic field is circular and the wire in the form of a coil. I hope this helps you.

Allow me just generally to warn about urban legends about cables - they abound! Yes, you are correct; there is obviously not such a thing as a directional cable for ac. As Haoleb mentioned some cables are called "directional" with regard to which end the shield is connected. To my mind the term is misleading as you observed, but it is used.

 

[Vaughan Odendaa]

If I understand things, when current flows through the voice coil it's attacted towards the permanent magnet which is then repelled by said magnet and this varies on the polarity of said current.

What say you ?

--Regards,

 

[Vaughan Odendaa]

Ampdog, excellent post btw. Are you an engineer ?

--Regards,

 

[Vaughan Odendaa]

If no magnet existed and there was just a voice coil with current flowing though it, then I would imagine it would move in one direction ? Regardless if the signal is AC, it won't move the other way because there is no permanent magnet to repell the coil's motion.

This is where my confusion was.

--Regards,

 

[mtrycrafts]

If no magnet existed and there was just a voice coil with current flowing though it, then I would imagine it would move in one direction ? Regardless if the signal is AC, it won't move the other way because there is no permanent magnet to repell the coil's motion.

This is where my confusion was.

--Regards,

No permanent magnet, no way for that magnetic field created by the voice coil to push/pull to., no movement. It doesn't really matter how physically the two magnets are built. One filed uses the other to move, if it can. Certainly the fixed magnet cannot do the moving and if would not create sound. So, the lighter mass of the voice coil moves when it creates a field.

 

[Vaughan Odendaa]

Thanks ! Got it.

--Regards,

 

[nick1000000]

If no magnet existed and there was just a voice coil with current flowing though it, then I would imagine it would move in one direction ? Regardless if the signal is AC, it won't move the other way because there is no permanent magnet to repell the coil's motion.

This is where my confusion was.

--Regards,

If you have no magnet then the voice coil would act just like another wire. And it would probably get really hot. But yes there would be no movement from the cone.
What you were talking about earlier with the difference of using DC (direct current) and AC (alternating current) for speakers can be described like this:
DC moves the electrons through a voice coil in one direction which creates a magnetic field in one direction. It just runs current from one end of the voice coil to the other. AC on the other hand is where the electrons move back and forth. The change in direction depends on the Hzs. When you run AC through the voice coil it creates a magnetic field in the magnet that alternates with the current.
Without AC the cone would have no movement like many have said before.