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Our First Motor – A loudspeaker.

The dross section of a Loudspeaker

The dross section of a Loudspeaker

It is time to get back into some theory. Almost any project will end up producing some kind of mechanical motion so the understanding of electrical motors and magnetic devices is very important. My way of learning and teaching is to learn one thing very well and then as I branch out I remember how the different device is similar…but different.  I am not saying my teaching is exceptional, but that I teach by exception.  (Yeah, I know, my puns are definitely NOT exceptional.)
The first motor we will consider is a loudspeaker.  I will use the words speaker and loudspeaker interchangeably throughout this post.  (Yes, I am a typical lazy American, butchering the language in the name of convenience.  🙂 ).   One way of thinking of a loudspeaker is a combination motor and air pump.  Tonight’s post is about the components of a loudspeaker and especially the concept of a magnetic circuit.  In future posts, we will talk some about the mechanical half of it and then a very little about the design of speaker enclosures.  Speaker enclosures is a very very big topic and although I have built one or two I am no expert but I will try to point you toward books and sites that claim to be.  (I have a somewhat jaundiced view toward much of what is sold as audiophile.  I will also discuss some of that in the future also.)
Before talking about the loudspeaker, we probably need to discuss what sound is and what I ,mean by “pumping air”.  Sound is a series of compressed air and decompressed air approaching the sound detecting device, our ear or a microphone.  The decompressed air is portion is called a rarefaction. This series of compressions and rarefactions happens between 20 Hz to 20,000 Hz in sound heard by humans.
The first diagram is a cut-away cross sectional view of a typical loudspeaker.  It may be necessary to expand the picture to see all the components clearly.  To do that simply click on the picture and it will expand to full size, then use your browser back arrow to come back to this page.  Assume the speaker is circular when looking at it from the front.  Most speakers are circular, but some oval shaped speakers do exist and are used primarily in automobiles.
The base component is a metal frame used to hold everything together and used to mount the speaker in an enclosure.  The metal frame is stiff enough to withstand the vibration but it has some very large holes throughout it so the back of the speaker cone can move freely without trapping air.  The cone is the part that moves and “pumps” the air, creating the compressions and rarefactions of the air for the sound wave.  The cone is made of a stiff but light material so it does not deform while moving the air.  Usually is material is a fairly thick paper, but I have seen plastic cones in some speakers, especially ones in automobiles that may get wet and are exposed to sunlight, heat and cold.  Because the cone is designed to move as one piece, specifications for speakers often refer to this as a piston and the diameter of the cone is called piston diameter.

Around the front edge of the cone is a flexible ring of material called the surround.  This serves several functions.  First, it fastens the cone to the frame and keeps the cone aligned so no part of the cone rubs the non-moving parts. Just as important it is very flexible and allows the cone to move freely.  In my experience, this is also the part that most often fails in a loudspeaker.   At the back of the speaker there is a coil wound on a core and attached to the cone.  This coil called the voice coil is the part of a loudspeaker I am calling a motor.  A circular spring is also attached to the winding core and the metal frame.  This basically looks like a crinkly disk with a hole in it and serves several functions.  First, it keeps the back side of the cone and the voice coil aligned to prevent rubbing of the voice coil on the rest of the speaker.   The second function is that it is a spring causing the speaker to maintain a rest position.  This rest position is ideally with the voice coil centred in the area between the pole pieces.

The final part of the speaker assembly is the magnet structure.  This and the voice coil together form the complete motor for the loudspeaker and that is the main purpose of this blog post.   The magnet in the picture is again a disk with a hole in it made of magnetized material.  One side of the disk will be the north pole and the other side will be the south pole.  We will say the side toward the front of the speaker is the north pole, but I have no way of knowing for sure that is correct without tearing into a speaker and then it may not be true for all speakers.

A close up of the magnetic structure showing the air gap.

A close up of the magnetic structure showing the air gap.

The main concept I want to get understood tonight is the concept of a magnetic circuit. Way back there, in a post called DC electricity and Ohms law I talked about an electrical circuit and how the amount of current through a circuit depended upon two things, the amount of voltage produced by a source and the amount of resistance to the flow of current in the circuit. This became Ohms Law and expressed by the equation I = V / R, where I is the current in Amps, V is the electromotive force expressed in Volts, and R is the resistance expressed in Ohms.  The field strength in a magnetic circuit follows a similar form.  The problem with a magnetic circuit is the values are not easily measure like those in electricity with a voltmeter.  However, the relationship is exactly the same.   The similar equation is Φ = F / Rm where Φ is the magnetic field strength, F is the magnetomotive force (magnet strength), and Rm is the reluctance of the magnetic path.
Iron is a good conductor of magnetic fields (low reluctance) and is a way of directing the magnetic field to the place where it will be useful.  The problem is it is impossible to have a part move through a solid piece of iron, so it is necessary to have an air gap between the pole pieces.  This air gap reduces the magnetic field because it increases the reluctance.  We will be living with this air gap problem throughout our discussion of motors.

We are only part way through the discussion of the electrical and magnetic part of a loudspeaker, but this post is becoming very long so I will continue with it in a upcoming post. For now I will leave you with a site with a better picture of a loudspeaker.  Note they refer to the pole pieces as motor front plate and motor back plate,   I chose my name of pole pieces because that will be used throughout our discussion of motors. Please note I am creating a new category called “Electromechanical” for these posts
Finally, in a little bit of my off-the-wall random wanderings I am embedding a rock video.  At approximately 3:35 in the video is a good picture of some speakers “pumping air”.


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Our First Motor – A loudspeaker.” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

The embedded video is not my creation and is owned by WMG and licensed by the standard you tube license, but allowed to be embedded by the owner.

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