How It Works - Loudspeakers (Part 6)
Another in our 'How It Works' series. David Mellor kicks off a two-part investigation into the practicalities of bins, horns, tweeters and woofers. Baffled? You won't be once you've read this!
It is often said that a big loudspeaker is better than a small loudspeaker, therefore a long article on the subject should be better than a short one. In the first part of two, David Mellor explains how speakers work and how not to be fooled by hi-fi salesmen.
Legend has it that when Mr Rice and Mr Kellogg invented the loudspeaker as we know it today, they started by rolling up an empty cornflakes packet into a cone. Whether or not this is true (it isn't), the loudspeaker is fundamentally a very simple device. The question is, why have engineers taken so long to get it to work properly? Does it, indeed, reproduce sound in as natural a way as the original instrument? My aim in this article is to explain some of the weapons in the speaker-maker's armoury and some of the very difficult problems he has to face.
As I shall explain later, a loudspeaker has to operate as an entire system. To understand that system we have to see how the various components work. The obvious starting point is the bit that actually makes the noise - the drive unit or, simply, driver.
The most common and most useful type of driver is the moving coil kind. If you read 'How It Works: the Microphone' (SOS July '87), then you will know that the moving coil microphone works in the same way as a dynamo. It should come as no surprise then to learn that the moving coil driver works just like an electric motor. It doesn't go round and round (at least, I have never seen one that does) but the ingredients are the same. Recipe as follows:
Take one magnet and one coil of wire. Pass a current through the coil while it is close to the magnet.
It's no way to get good scrambled eggs but the end product of this cookery class is motion.
In any situation where you have a magnet and a current-carrying coil, the field of the magnet and the field produced by the coil interact to produce a force between them. If you feed the coil with an audio signal then it will move backwards and forwards according to the audio waveform. Figure 1 shows what a real moving coil driver looks like.
Here, the coil (or voice coil as it is sometimes known) is attached to a cardboard or plastic cone, so that when it moves it shifts air backwards and forwards. The cone and the coil have to be flexibly suspended in some way so that they can freely move, while keeping the coil centred between the poles of the magnet. Later on, when I explain how to blow up your speaker, we shall see what happens when the coil is not accurately centred in the gap.
You are probably all familiar enough with loudspeakers to know that Figure 1 is a diagram of a bass, driver. High frequency drivers look a bit different, but the principles are the same. One question which arises in after-gig chat from time to time is, why does the cone have to be a cone? Why won't a Swiss roll do?
The ideal loudspeaker of whatever shape would act as a rigid piston. All parts would move in and out to the same extent rather than flexing and bending. As all the motive force in the moving coil driver is produced at one point, it is essential that the diaphragm is stiff, so that the force is transmitted throughout its length and breadth. It so happens that the cone is a good shape for this - up to a point.
It's now time to examine why drivers are divided into different sorts - bass, midrange and high frequency (or woofers, squawkers and tweeters, if you are into farmyard impressions).
Bass drivers are big. It's not always true to say the bigger the better, but they do have to be a certain size. If you think about low frequency and high frequency sound waves, low frequencies vibrate slowly, high frequencies much faster. Therefore, for a loudspeaker to be able to produce equal energy at both ends of the frequency spectrum it will have to create bigger vibrations at low frequencies, and over a wider area. There also happens to be an imbalance in the natural sounds of life towards the bass end. The bass driver has to be big and be able to travel in and out a long way. (Have a 'good excursion' as it is called.) The question now is, can a large speaker which is good for bass also produce high frequency sound? Yes it can - but not very well. As I said earlier, the speaker cone is rigid but only up to a point. As frequency increases, a situation arises where the cone will no longer act as a rigid piston but will exhibit what is known as 'breakup'.
This does not mean that it is making those nasty rasping noises we all know - they come in Part 2! It means that parts of the cone are moving inwards while the rest moves out, ie. the cone is bending.
Traditional paper cones fall down badly in this respect. A 200mm cone would find its first breakup mode at around 750Hz. Irregularities in frequency response above breakup mean that the driver is not producing an accurate representation of the input signal. Fortunately, newer materials have better performance. A stiffer cone will have a higher breakup frequency. Other materials can damp out this sort of vibration. It is currently possible to produce a 300mm driver with a smooth, controllable response up to 4kHz with these.
Another problem about trying to get decent high frequencies out of a bass driver is the directionality effect. This is dependent purely on the diameter of the driver and there is nothing the loudspeaker designer can do about it. There are two rules: (1) The larger the driver, the more directional it is. (2) The higher the frequency, the greater the directionality. So, if you try to get a 10kHz tone out of an 18-inch driver, chances are that it will be focused into a laser-tight beam which will scythe through your audience at round about kneecap level (slight exaggeration).
High frequency moving coil drivers are more usually dome-shaped than cone-shaped these days, for reasons of breakup control. Obviously, they are not much good at producing low frequencies - their size limits that.
These days 25mm seems to be a popular diameter, though they do vary from less than 20mm to over 35mm. Once again, the larger it is, the more directional. It's a compromise that has to be made between this factor and the capability to produce a lot of sound. One interesting point is the use of more than one high frequency unit in some PA speakers. Good idea or bad? Well, yes, you do get more level - but you also get increased directionality. At certain angles the outputs will cancel each other out completely. This is not a good idea. More on this topic in Part 2 next month.
I can't leave the subject of drivers without a mention of the good old horn. (Bin - as in 'bass bin' - is a commonly used term for a folded-up horn). The moving coil drivers I have been talking about are, strictly speaking, direct radiator moving coil drivers. They fire straight into the atmosphere. By fixing a 'spout' on the front of a moving coil unit, as in Figure 2, it can be made more efficient. Any type of waveform - sound, light or even Hawaiian surf - finds it difficult to pass from one medium to another. You are probably familiar with the coating process that cuts down reflections from a camera lens. It makes it 'easier' for the light to cross the boundary between air and glass. Well, it's the same with the horn. It couples vibrations in the diaphragm to the air in front.
Although a horn unit can produce a lot of sound for very little input (its efficiency is high), it nevertheless has a couple of problems. If it is to be an effective low frequency horn it has got to be big. I mean BIG. For instance, to be useful at 40Hz (the lowest note of a bass guitar) the mouth of the horn will need to be around six square metres and the length will be considerable. Smaller, high frequency (HF) horns are more often used, especially in studio monitors - but it is a fact of life that techniques used to get a good high-end response also increase distortion. Thankfully, the trend is towards dome type HF units.
Although moving coil drive units are by far the most widely used, there are other ways of pumping air. (The electrostatic loudspeaker being well known.) Whatever the method of operation, there is always the problem that high and low frequencies need separate drivers. I shall describe next month how two or more drivers are made to work together.
I am being very careful to distinguish between the drive units and the whole loudspeaker. A loudspeaker is a system which includes drivers, an enclosure of some sort, and other necessary components.
There cannot be any reader of this magazine who has not at some stage at least thought of building a loudspeaker or two. It's easy isn't it? All you need are a few bits of wood and a Black and Decker.
Actually, this is not the case. If you are at all concerned about sound quality, the enclosure has to be as carefully designed as the driver - and competently constructed. First of all, let's consider why there must be a box at all.
Bass, and most midrange, drivers do not operate in one direction only. They blast out just as much sound from the back as they do from the front. Unless steps are taken to prevent it, sound from the back will leak around to the front and cancel it out, causing a severe loss of bass frequencies.
One answer might be to mount the driver on a large flat sheet of wood - called a baffle. (It baffles me where these names come from.) The larger this piece of wood is, the further down the frequency range goes the problem. Unfortunately, the baffle has to be so large as to be completely impractical, so another solution has to be found.
How about folding the baffle around the speaker so that it becomes a closed box? This could be considered to be an 'infinite' baffle. In fact, a speaker in a completely sealed box is often called an infinite baffle speaker. I would say that 'closed box' is a better term. For a closed box to work the box has to be pretty big, because as the speaker cone moves in and out the air inside the box becomes either compressed or rarified, acting against the motion of the cone. I bet you can guess the result - a loss of bass response. I'll tell you now, with loudspeakers you are always in a 'no-win' situation. There is always a three-way trade-off between size, bass response and efficiency. You can make a speaker small with a good bass response but it will need a terrific amount of amplifier watts to get any level out of it. Alternatively, you can have good bass and efficiency but the speaker has to be the size of a house.
Small closed-box speakers, where the 'springiness' of the air inside is taken into account, are known as acoustic suspension types. In effect, the air is used to help the speaker. 'Acoustic suspension' is really just a term for the proper design of small closed-box speakers, so it doesn't get around the 'no-win' situation I have described but proper use of the design rules can result in useful-sounding speakers of liveable-with dimensions.
It's all very interesting so far, but this is where it gets really interesting. Did you know that loudspeaker designers often try to pull the wool (or acoustic wadding) over our eyes? Read on...
A competent designer can select a driver and construct a box to get several different types of frequency response. (It's the all-important bass end I am talking about here.) Suppose you were given the choice of two identically-sized speakers, one of which could respond to a lower frequency than the other, which would you choose? Well, you don't get something for nothing so there must be some sort of problem with the speaker with the apparently better bass. Look at Figure 3, it shows two typical low frequency speaker response curves.
You will notice that the second curve maintains its response to a lower frequency, at the expense of a 'hump' just below 100Hz. Both these responses can be obtained from the same size box, although the driver specifications would be slightly different. It turns out that there are a whole range of low-end responses at the command of the designer. They don't just happen by accident. The benefit of the second speaker is more bass, with the bad effect that it tends to be a 'one-note' sort of bass. Every thud of the bass drum excites the system resonance - just below 100Hz here - and it sounds off at that frequency. Figure 4 shows something else.
These are the same two speakers but this time the response (in the time domain) to an impulsive sound like a drum beat is shown. The first speaker, the one with the smooth bass, responds to the impulse then quickly settles down. The second one responds but the cone oscillates for a period of time after the initial 'thud'. This oscillation occurs at the resonant frequency of the system.
Now go into a hi-fi shop and listen to all the speakers on demonstration. I can tell you that it is hard to find a speaker that doesn't display a peaked-up characteristic to a greater or lesser extent, although it is quite possible to design one. Many people are fooled by the initial impression of a lot of bass. Don't be one of them.
I should mention that very often the designer is able to 'tame' the peak in the frequency response so that it looks flat on paper. The speaker will still, however, act as an energy storage system, and the oscillation will remain.
An alternative to the closed box is the 'vented box' or bass reflex design.
In this case, the air inside the enclosure is connected to the outside world by a cardboard tube. This has the effect of tuning the enclosure to a particular low frequency, which extends the bass response. Once again, it is up to the designer to decide whether to have smooth bass or 'one-note' bass. With the bass reflex system, it is just a little easier to be careless and end up with the latter!
Size for size - and correctly designed - the bass reflex system will have better bass and be more efficient than the closed box, although some manufacturers would claim better sound quality for the closed box type.
I am sure I must have put a lot of people off building their own speakers. It can be done, there are two ways. Trial and measurement - which means you have to get a lot of things wrong before you start getting things right. You also have to have good measuring equipment and techniques. The other way is by calculation. If you really want to build an accurate speaker yourself, this is the way to go. I would suggest that you find a good technical library and look up a couple of papers on the subject which I have listed below. They are tough going, but it is the real stuff.
The above paragraph does not, of course, apply to anyone building a reggae style 'sound system'! Get yourself down to the Notting Hill Carnival later this year if you want to hear some massive sounds!
There is so much more to say on the subject of loudspeakers that I can't fit it all into one episode, so you will have to wait till the next issue. Treats in store include crossover networks, enclosure construction, speaker testing and evaluation, why you shouldn't turn a speaker on its side - and more. Listen out for it.
The two technical papers I mentioned above are both by R.H. Small and are to be found in the Journal of the Audio Engineering Society:
'Closed box loudspeaker systems, part 2, synthesis'- Vol 21 No 1 (1973)
'Vented box loudspeaker systems, part 3, synthesis' -Vol 21 No 7 (1973).
Feature by David Mellor
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