B&O Tech: How to Make a Loudspeaker Driver (A primer)

#11 in a series of articles about the technology behind Bang & Olufsen loudspeakers

 

I realised this week that I’ve been throwing around words like ” voice coil”, “suspension”, “surround” and “spider” when I talk about loudspeakers, but many people don’t know what these things are – or how a loudspeaker driver works in general… So, this week, I thought I might back up a couple of steps and talk about some basics. There’s nothing here about Bang & Olufsen loudspeakers specifically – it’s just an introduction to how loudspeaker drivers (like woofers, for example) work.

Back in 1820, a Danish guy named Hans Christian Ørsted was in the middle of giving a lecture when he noticed that, when he switched a circuit on and off, a compass sitting nearby on the desk moved a little. Since he had been poking around with experiments in electricity and magnetism for years, it didn’t take him long to put two and two together and come up with the idea that, when you run electrical current through a wire, you get a magnetic field around it. (Interestingly, not only did Ørsted figure this out (unless you believe that Romagnosi did it), but he also wrote papers on aesthetics – and he was the first person to isolate the element aluminium – and he founded Den Polytekniske Læreanstalt which, today we call the Danish Technical University. So, he had a big influence on B&O in many respects.)

Nowadays, we know that, by putting current through a wire, you produce a magnetic field that has magnetic lines of force that encircle the wire. The direction of the lines of force are dependent on the direction of current, and the extent by which the magnetic lines of force extend away from the wire is dependent on the amount of current.

Depending on whether or not you believe Benjamin Franklin you can use your right or left hand to determine the direction of the lines of force. Figure 1, below, shows a right hand (which indicates that we believe Benjamin Franklin and we say that current runs from the positive terminal of a battery to the negative terminal, which is, in fact, incorrect.). The thumb points in the direction of the current and your other fingers wrap around the wire in the same direction as the magnetic lines of force (which go from North to South on a magnet).

 

The right hand rule shows that, when you put current through a wire, you get a magnetic field around it.
Fig 1. The right hand rule shows the direction of the magnetic lines of force around a wire as a result of putting current through it.

 

If you take that same wire, keep running current through it, and coil it up like a spring, you can make a slightly more useful magnet that actually has a North pole and a South pole. Again, you can use your right hand to figure out which end is which – you wrap your fingers around the spring in the same direction of the current going through the wire, and your thumb will be pointing towards the North pole of the magnet, as is shown in Figure 2.

 

If you have a coil of wire and you put current in it, you get a magnetic field. Note that, if you run the current the other way, the magnetic poles will reverse, so North will be on the left of this diagram.
Fig 2. If you have a coil of wire and you put current in it, you get a magnetic field. Note that, if you run the current the other way (by reversing the battery), the magnetic poles will reverse, so North will be on the left of this diagram.

 

Now, if you take two magnets and you put them end-to-end with the North of one facing the North of the other (or South to South), they’ll push each other apart. If you put them North-to-South, they’ll pull each other together.

So, let’s do something weird. We’ll make a coil of wire, and we’ll put it in a strange-looking permanent magnet that is a bit like a horseshoe magnet that has been wrapped around itself to make a circular plug in the middle which is one pole (say, North) and a ring around it which is the other pole (say, South) as is shown in Figure 3.

 

A coil of wire about to be put in the gap of a strange-looking magnet.
Fig 3. A coil of wire about to be put in the gap of a strange-looking permanent magnet.

 

Now, if I put current through the wire, I’ll make a magnetic field around it that will either push against or pull towards the magnetic field of the permanent magnet around it. In other words, if it’s free to move, it will.

Now, since a loudspeaker, generally, is a thing that is used to convert electrical energy into acoustical energy; and, since in order to create acoustical energy (i.e. make noises) we need to move air molecules; we can use this strange device we’ve made in Figure 3 to our advantage. However, let’s be a little more methodical about this…

So, let’s build a dynamic moving coil loudspeaker driver, bit by bit. We’ll start by talking about its name. The “dynamic” part means that the basic principle that does the work is electromagnetism (as opposed to electrostatics or some esoteric methods like using plasma, tesla coils, or cats). The “moving coil” part is because, uh, the part of the device that moves in the magnetic field of the permanent magnet is a coil of wire.

What we want to do is to put the wires of the coil inside a magnetic field that is as strong as we can make it (within reason, of course). The easiest way to do this is to make the “gap” the coil sits in as small as possible (and, of course, to use as strong a magnet as we can fit or lift or afford to buy). So, let’s make a small gap for the coil to sit in.

We start by making a “bottom plate” and connect a “pole piece” – this results in a shape that looks like a disc with a cylinder. It’s made out of soft iron because soft iron is a really good magnetic conductor. (In other words, if you stick a magnet on a piece of soft iron, the soft iron basically becomes an extension of the magnet without losing very much magnetic force.) That bottom plate and pole piece assembly is shown in Figure 4, below. I’ve made it red just to keep things clear later. It’s usually not red in real life.

 

The bottom plate and the pole piece, both typically made of soft iron.
Fig 4. The bottom plate and the pole piece, both typically made of soft iron.

 

As you can see already, the “plug” in the middle of the magnet in Figure 3 is already visible as part of the pole piece in Figure 4. However, in order to make the strength of the magnetic field greater (in other words, in order to concentrate the magnetic lines of force) we want to make the gap (where the coil is going to sit) narrower. This can be done by making the cylinder on the pole piece a little bigger in diameter – but only where the coil of wire will be. That’s done by putting a ring around it, as is shown by the blue part in Figure 5, below.

A ring has been added around the pole piece to reduce the gap width.
Fig 5. A ring has been added around the pole piece to reduce the gap width. (Note that the gap doesn’t exist yet – we’ll need to put in a couple of more pieces first.)

 

Now we add the magnet as you can see in Figure 6.. This looks like a ring that sits on the disc part of the pole piece. The top of the ring is one pole (say, South) and the bottom is the other pole (say, North) of the magnet. However, this means that the North pole of the magnet is extended to the top of the cylinder on the pole piece because (as I said earlier) the soft iron is a good magnetic conductor.

 

The blue ring is the permanent magnet, typically made of ferrite or neodymium.
Fig 6. The blue ring is the permanent magnet, typically made of ferrite or neodymium.

 

You can see in Figure 6 that the gap between the top of the pole piece and the magnet is pretty big, so let’s make it smaller by putting a “top plate” on the top of the magnet. This is another disc of soft iron, where the hole is just a wee bit bigger than the diameter of the ring around the top of the pole piece as shown in Figure 7. This means that the South pole of the magnet is now the inside edge of the hole in that disc, so we’ve made a circular gap (between the top plate and the ring on the pole piece) that is very small, and therefore has a very concentrated magnetic field.

 

The top plate, also made of soft iron.
Fig 7. The top plate, also made of soft iron.

 

Unfortunately, we can’t just make a coil of wire and stick it in the gap and hope that it’s going to behave. Instead, we take a roll of cardboard (or something else) – a bit like the cardboard tube in the middle of a roll of toilet tissue – and wrap the coil of wire on that. That cardboard roll that supports the coil is called the “former” – it’s shown in Figure 8.

 

The light blue tube is the former, around which the voice coil is wound.
Fig 8. The light blue tube is the former, around which the voice coil is wound. You can’t see the voice coil because it’s hidden by the top plate. (Actually, you can’t see it because I didn’t draw it – but if I had, you’d just see some wires sticking out from the gap – depending on the type of coil we had.)

 

One little extra piece of information here. Since the voice coil, sitting in a magnetic field is the system that essentially converts electrical energy into movement, we call it the loudspeaker driver’s “motor”. Of course, it isn’t a motor that causes something to spin – but it does cause something to move.

Great. Now we have the coil of wire (the “voice coil”) wrapped around the former, sitting in the magnetic field. So far so good. Now we can put current through the wire and it will want to move in or out of the magnetic (depending on which direction we’re sending the current in). Now, our first problem is that, even if the voice coil and former moved out and in, there is nothing there to push and pull the air molecules in front of it – so it won’t make a lot of noise. So, let’s start putting up a surface that can move some air. We’ll start by putting on a “dust cap” which seals off the end of the former. This is the bump that you see on the front of a woofer in the middle of the cone – so we’re starting to get out to the visible “pretty face” of the loudspeaker driver. The dust cap is shown in Figure 9. Note that the dust cap is not always the same diameter as the former. Note as well that it is usually, but not always convex. Note as well that some drivers don’t have a discrete dust cap (like the BeoLab 3 woofer, for example).

 

The dust cap has been added to the front of the former.
Fig 9. The dust cap has been added to the front of the former.

 

Now we have a problem. We can put current into the coil and get it to move, but there is nothing there to stabilise it. What we need is something to make sure that it doesn’t fall down when you put the loudspeaker on its edge (as most are…). So, we’ll put in a stabiliser. It has to keep the former centred in the magnetic gap, but it also has to be flexible to allow the former to move in and out of the magnet. This part of the loudspeaker is called the “spider” – it looks like a disc that has wiggles in it that can stretch as the former moves in and out. This spider is shown attached to the former in Figure 10. Note that its outside will attached to something else, later.

 

The spider has been added. It is glued to the former, but is not attached to the coil or the top plate.
Fig 10. The spider has been added. It is glued to the former, but is not attached to the coil or the top plate.

 

Welcome to later. Now we need a frame to attach the outside edge of the spider and some other parts of the loudspeaker to – as well as to allow us to attach the whole loudspeaker to a cabinet. This part is called the “basket” – it doesn’t do much other than act as a structural support for everything – a bit like the steel beams in a building. The basket is shown in Figure 11. It may be interesting to note that the basket for an automotive loudspeaker driver is a little different from one for a home loudspeaker because it has to be able to deal with the possibility of a nasty accident. For example, a friend who knows such things once told me that it’s a bad idea to put a woofer intended for a home loudspeaker in a car door because if you’re ever in a side impact collision, it’s not inconceivable that the magnet will rip away from the basket, shoot across the car and cut your legs off. So now I’ve warned you…

 

The basket is glued and/or riveted to the top plate.
Fig 11. The basket is glued and/or riveted to the top plate. In addition, the outside edge of the spider is glued to the basket.

 

Now we can put the rest of the loudspeaker parts on. We attach a “diaphragm” or “cone” which makes the moving surface bigger. That’s the medium-dark green part in Figure 12. If we left it at that, when we moved the loudspeaker in and out of the magnet, it would sag, because the spider isn’t strong enough to keep the whole thing vertical. So, we add a “surround” which is usually made of foam or rubber (or fabric, in the old days). The surround is a flexible ring that is glued to the basket and the edge of the diaphragm. It’s the lightest green thing in Figure 12.

 

An entire moving coil loudspeaker. The green ring is the surround and the greyish-purple ring inside it is the diaphragm or speaker cone, glued to the top of the former.
Fig 12. An entire moving coil loudspeaker. The light green ring is the surround and the darker green ring inside it is the diaphragm or speaker cone, glued to the top of the former and the dust cap.

 

So, now when you put current through the voice coil, it pushes out of (or pulls into) the magnet and moves the former, dust cap and diaphragm with it. This causes the spider and the surround (usually grouped into what we call the “suspension”) to stretch.

 

A cross section of a (not very) simplified model of a moving coil dynamic loudspeaker driver.
Fig 13. A cross section of a (not very) simplified model of a moving coil dynamic loudspeaker driver.

 

If we take the device in Figure 12 and cut it in half, we get a cross section like the one shown in Figure 13.  And, just to prove that I’m not lying, I cut apart a real woofer  – it’s shown in Figure 14. And then, not satisfied that I had done enough damage, I did it again to a BeoLab 3 woofer – those photos are in Figures 15 to 19. Another good example is this picture.

 

An actual moving coil dynamic loudspeaker, after I was very mean to it.
Fig 14. An actual moving coil dynamic loudspeaker, after I was very mean to it.

 

A BeoLab 3 woofer - after I was finished with it...
Fig 15. A BeoLab 3 woofer – after I was finished with it… You can see here that this particular loudspeaker driver does not have a separate dust cap and diaphragm. Also, you’ll notice that there is a flared cone that is used to connect the former to the outside edge of the diaphragm.

 

A BeoLab 3 woofer, showing some of the components.
Fig 16. A BeoLab 3 woofer, showing some of the components.

 

A BeoLab 3 woofer, showing some more of the components.
Fig 17. A BeoLab 3 woofer, showing some more of the components. The magnet assembly is hidden inside the silver can at the bottom of the photo.

 

A BeoLab 3 woofer, showing some more of the components.
Fig 18. A BeoLab 3 woofer, showing some more of the components.

 

A BeoLab 3 woofer, showing some more of the components.
Fig 19. A BeoLab 3 woofer, showing some one more component.

 

 

That’s about it for this week. If you want to do a little more digging for yourself, you can look into things like the difference between overhung and underhung voice coils, neodymium vs ferrite, or just watch some relaxing, cool, tangentially-related videos like this one or this one or this one or this one. Or maybe just this.

 Addendum

For the purposes of this explanation, I said that the top of the pole piece is the North pole of the permanent magnet, and the top plate’s inner edge is the South pole. However, there is no fixed convention for this. Manufacturers will almost always ensure that, when you put a positive voltage on the positive terminal of the loudspeaker, the diaphragm will move outwards. However, the north/south-ness of the magnet and the direction the voice coil is wound, and which end of the wire goes to which terminal vary not only from manufacturer to manufacturer, but model to model within one manufacturer’s portfolio.

  1. Very interesting write-up. It’s sort of amazing that the mechanical parts live as long as they do, given the “abuse” they’re subjected to.
    I’ve seen issues with “foam rot” in the surround and that obviously affects the sound. However, is there a gradual degradation of the drivers performance as the moving parts are gradually worn out, or does it typically perform very well until it suddenly breaks?
    Put another way, could a keen listener such as you tell how “old” the driver is based on it’s sound?

  2. Hi Mattias,

    There is no question that, as a driver ages, its characteristics change. Foam rot will result not only in a change in the “compliance” (an engineer’s version of “springy-ness”) but also cause air to leak in and out of the cabinet. I’ve even seen a case where a loudspeaker sounded “weird” because we left it stored in the basement for a couple of years. Turned out that it the problem was that the rubber surround on the woofer (it was a 2-way loudspeaker) stiffened because it hadn’t moved for a long time. The solution was to just play pink noise (or music) through them overnight to get the rubber to loosen up a bit. At the next listening session, they sounded fine.

    I should be quite specific here – in both the before- and after- cases of the rubber surround story, the loudspeakers were behind a curtain and the people listening to them didn’t know what they were. In the first case, the speakers were rated quite low (which came as a surprise, since they’re pretty good loudspeakers). In the second case everyone in the room thought they sounded fine. Both listening sessions were blind, and we played exactly the same tracks at exactly the same levels through the same chain of gear – same group of people all sitting in the same seats in the same room. The only thing that changed was how recently the loudspeakers played music before the listening session started.

    On the other hand, I have no doubt that, if you own a pair of loudspeakers and you listen to them every day, they will age, and thus slowly change their response – but you’ll adjust with them. So, your reference will always sound “normal” until something is REALLY wrong…

    Not a very clear answer – but I’m not sure there is one… Sorry.

    Cheers
    -geoff

  3. Geoff, thank you for your contributions to the audio knowledge base. I am very curious as to why a driver like a Scanspeak Illuminator can reproduce so much more detail of the original signal when the fundamental design principles are just like less expensive drivers. I guess my question is how is a driver with superior detail retrieval designed and constructed? Also, what other benefits do you see when working with high end drivers?

  4. Hi,

    I can only answer part of this question – the other part of my answer is “I don’t know”. However, the answer that I do know is rather long – so if you don’t mind waiting a couple of days, I’d like to turn this into my next blog posting.

    Hope that’s okay with you.

    If you’re impatient, you can go searching for information about the intersection of the search terms “BL curve”, “linearity” and “underhung” This is what I’ll discuss in relation to the BL90 midrange drivers.

    Cheers
    -geoff

  5. Greg Quiring says:

    Very good article! What about the part where the wires from the coil get glued to the dust cap and then have braided wires soldered to them and then to the terminals? I’m trying to build one from scratch and make it like a real speaker & not one of those foam plate jobs like the King of Random has on his site. Also, why aren’t neodymium magnets the standard now? Do you have any advice on building a speaker from scratch?

  6. Hi Greg,

    1. Could you be more specific about your lead wire question? Typically, the ends of the voice coil are connected to braided wire and both are stuck to the former (for example, using a dab of glue). It is not unusual for this to be done in the same area where the spider is glued to the former, since there’s glue there anyway… The reason the braided wire is used to run from the coil to the leads is that they’re flexible (and they have to flex when the driver moves in and out) and the reasons you don’t use braided wire for the coil is both due to weight and efficiency (due to packing density). Maybe this answers your question – if not, please give me a shout.

    2. Neodymium can have a higher magnetic strength than ferrite for the same volume of material. So, if you want a smaller and lighter loudspeaker with the same Bl curve, then neodymium is the better choice. It is also shiny – so if your your magnets are visible, then magpies (and perhaps some people) will think they’re prettier. However, neodymium also has disadvantages when compared to ferrite. It’s more expensive and its price is more volatile. (Even when you consider that you need less of it to produce the same magnetic field as ferrite, NdFeB is still more expensive by a factor of 2 or 3.) It’s also less tolerant of high temperatures (meaning that it will lose its magnetic strength if heated up. (If you want the numbers, neodymium starts to lose magnetism at about 80 deg C whereas ferrite can go up to about 250 deg C.) This page has a good summary of the pro’s and con’s of the two materials.

    3. I have only four things to say regarding building a loudspeaker from scratch:

    – Don’t let your ears get seduced by the amount of work that you did on them. You might have worked long and hard, and they sound bad… But don’t let this deter you from trying

    – Use measurements – but don’t trust everything you see. Measurements can be distracting sometimes.

    – Don’t do it because you want to save money. You won’t. Don’t even do it because you think you can make a better loudspeaker than a mass-produced one. Do it because you want to learn how loudspeakers are made… And be prepared to make another, different one afterwards…

    – “In order to make chocolate chip cookies from scratch, first you have to create the universe.” -Carl Sagan

    Hope this helps!

    cheers
    -geoff

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