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The Electrical Part of a Loudspeaker.

The cross section of a Loudspeaker

The cross section of a Loudspeaker

Dearly beloved we are gathered here today for a divorce. Well, sorta. It is not just a simple divorce we are going to keep the parts we want and deal with the other parts at a later date once we want to complicate our life.   Seriously, the best way to understand a system is to break it down into parts.  But, always the big but, we do have to remember the other parts exist. The mechanical and the electrical are coupled… never to be parted in real life.  What affects one affects the other, but it is a whole lot easier to deal with one thing at a time.  What we are going to do is called a mind experiment.  It is a good way to break things into bite sized parts.

To break the system apart, imagine the cone and surround does not exist, but has been replace by 3 wires and weak springs going from the front of the voice coil to the frame.  Why three?  The wires are placed 120 degrees apart and will perfectly balance the coil centered on the inner pole piece.  (Refer to “Three is right for me”– it works in mechanical systems as well as electrical systems.)   What we have done by doing this is remove the “pumping action” of the cone. Because the pumping action is the main reason we want a loudspeaker, it will be back.

Movement caused by Energizing the Voice Coil

Movement caused by Energizing the Voice Coil

Now for the simple and easy part, refer to the second picture. If the coil in energized so the front of the coil is a south pole the voice coil will move backwards.  This is because the north pole of the field magnet repels the north pole of the voice coil and attracts the south pole of the voice coil.  If the polarity of the signal going into the voice coil is reversed, the poles on the voice coil will be reversed and the direction of the movement will be in the opposite direction.  (Even though I divorced the mechanical, it is back.  You can’t live with ’em and you can’t live without ’em.)

How much and how fast will it move depends upon the strength of the field produced by the magnet, the strength of the field produced by the voice coil, and the stiffness of the spider.  If the spider is “stiff” a large force will be required to move the coil.  If we have a large and strong magnet, we will have more force.  Finally, a large current is flowing in the voice coil will produce more force than a small current will.  Also, now you can see why the air gap distance is important.  The smaller the gap the greater the magnetic field produced by the same size magnet.

So far, life is simple; Now it is time to start adding complications and disharmony to our happy couple.  What happens if the coil is just setting there resting but some outside force comes along and makes the coil move.  For example, lets say we simply press on the front of the coil and move it toward the back.  The coil is suspended in a magnetic field and the movement will cause a voltage to be produced in the coil.  This is not really a big deal if we are using a high impedance voltmeter or amplifier input connected to the speaker terminals.  However, if the speaker is connected to a low resistance moving it will not be easy.  The reason for this is the current that attempts to flow due to the movement will create a magnetic field in the voice coil.  If we are pushing to coil toward the rear (left in picture 2) the current will create a magnetic field such that the front of the coil will be a north pole.   This means that if the load impedance is low it will “put the brakes on movement”.   And we thought we had completely divorced the two?

Loudspeaker Circuit

Loudspeaker Circuit

The last picture on this post is a circuit representing a loudspeaker connected to an amplifier. Since the voice coil is a coil, it obviously has some inductance. Because the coil is made of wire, it has some resistance and as I have described above when it moves in the magnetic field it produces a voltage. That is the hardest part to wrap your head around. EVEN WHEN THE COIL IS BEING BOSSED AROUND BY THE SOURCE, IT IS ACTING LIKE A GENERATOR. That is something we will be having to cope with when we deal with all motors. The voltage being generated by the speaker motor will be in the opposite direction of the source voltage because of the movement of the voice coil. The term we will grow to love is C.E.M.F. for Counter ElectroMotive Force. The amplifier like all real devices has a source impedance, which I have drawn as a resistor, but may include some reactive elements as well. (Refer to the previous post “Technical Datasheets and Real Power Sources“.)   Again this is a “mind experiment”.  The resistance and the inductance is part of the coil so it is impossible to split these from the actual coil, but this is very helpful for our understanding.

You probably have noticed I did not put voltage polarity signs on anything except the two voltage sources. I probably should not have even put the signs on those either because we really don’t know those unless we know what is happening in the system at a particular time. The one thing we do know is the voltage at the terminals of the loudspeaker and the amplifier.  If we assume the bottom of the diagram is the reference voltage of 0V.   Then the voltage at the top wire is equal to the voltage source minu the voltage across the source resistance.   Similarly, this voltage is equal to the voltage produced by the coil windings  minus the voltage dropped by the inductance and the coil resistance.

Right now things are probably about as clear as mud, so we will make an example.   For the moment assume the inductance is zero so we can deal with a simple DC problem.  The coil resistance is about 7.5 ohms (A realistic number).   We will apply a battery to the terminals, and will not allow the coil to move.  The batter has a natural voltage of 1.6 V and an internal resistance of 1 Ohm.   The current that will flow will be 1.6V/(1 + 7.5) Ω or 0.188 A.   Now if we release the coil and allow it to move and we say that in a the early moments before it starts fighting the spring tension of the spider it moves fast enough to produce 1,2V.   Now our current is  (1.6-1.2)V / (1+ 7.5)Ω = 0.047 A.   A whole lot less!

The point of all of this is our mechanical load affect the amount of power drawn by the the motor and the amount of power required from the source.   The mechanical system is tightly coupled to the electrical system!

Although we are doing some “hand waving” in describing this system and picking and choosing what to keep and what to throw out, this is a real thing dealt with in speakers.   The impedance of the amplifier is important to dampen a speaker’s movement. and there is an actual calculation using the ratio between to the amplifier impedance and the speaker impedance to determine this value.   For those of you that really want to get into speaker design, I will have some sources in future post once I explain the basic concepts.

By the way. Way back there when I was in third grade, the teacher left the left the room to go to the office. The room P.A. system allowed her to turn the loudspeaker into a microphone and listen to the room while she was gone. We were busted for making noise. More than likely the amplifier used for that purpose had a high input impedance so the speaker moved freely and produced a relatively high voltage for small sounds.

The next loudspeaker post will deal with the speaker mechanics.

Gary
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The Electrical Part of a Loudspeaker.” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

 


 

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