How It Works - Loudspeakers (Part 7)
Last month, David Mellor explained how loudspeaker drive units work and how manufacturers sometimes try to pull the acoustic wadding over our ears. Part 2 tells the story of how the loudspeaker works as a system, the reason for crossovers, and gives a couple of hints on how to make BIY (bodge-it-yourself) cabinets.
In last month's article, David Mellor explained how loudspeaker drive units work and how manufacturers sometimes try to pull the acoustic wadding over our ears and make us think that their speakers sound more 'impressive' than their rivals. Part 2 tells the story of how the loudspeaker works as a system and gives a couple of hints on how to make BIY (bodge-it-yourself) cabinets.
Like any other sound engineer or musician, I am interested in getting the best possible sound off the record, tape or CD and into my earholes. It's fairly straightforward to buy a decent amplifier - or any other electronic audio component - just by looking for a good paper specification. With speakers it is different. All the paperwork in the world will not tell you what a speaker sounds like, and decisions made in haste in a hi-fi showroom are often much regretted when transferred to the studio or living room.
If you read last month's article, you will know how bass response on some speakers is often artificially 'pumped up' to give a sound which is more initially impressive but later tends to induce symptoms of listener fatigue, which a double dose of Sanatogen can do little to cure. This month, I hope to shed a little more light on loudspeaker anatomy which will illuminate the goings-on inside that elegant, handcrafted, simulated teak box.
Not as risky as getting over the Berlin Wall but a necessary component of a loudspeaker system. (When I talk about a loudspeaker system, I mean the box and all the components inside it, including the drivers.) As we saw in Part 1, one drive unit is not enough for an adequate frequency response. Domestic loudspeakers often have two drivers, one low frequency and one high frequency - otherwise known as woofer and tweeter. Speakers with three or more drivers are not necessarily better, as I shall explain later.
The important thing is to direct appropriate frequencies to the appropriate driver. If high frequencies (HF) go to the bass unit then distortion will result. If low frequencies (LF) go to the tweeter then BANG! A week's beer-money gone up in smoke. Crossovers come in two flavours: passive and active. The passive crossover is the most common because it costs a lot less to produce, but can still provide excellent results. Figure 1 shows the difference.
As you can see, the passive crossover comes between the amplifier and the drivers. You usually do not see it because it is mounted inside the enclosure. It consists of ten or so high wattage resistors, capacitors and inductors which sort out the frequencies, low from high. Unfortunately, it is not possible to choose a frequency (eg. 3,000Hz) and simply say "All lower frequencies go this way, all higher frequencies go that". There is a certain rate of 'roll-off' as shown in Figure 2. For reasons not unconnected with the laws of physics, it is common for crossovers to be specified as either 12dB per octave, 18dB per octave or 24dB per octave. This means, for a 12dB/octave crossover, that if the crossover frequency is 3,000Hz, then at 6,000Hz the output to the woofer will be reduced by 12dB (decibels). At 12,000Hz the output will be 24dB down. Conversely, at 1,500Hz the output to the tweeter will be 12dB down, etc.
So you see, some frequencies do go the wrong way. The steeper the roll-off, the more complex the crossover - and the more difficult it is to build. Usually, an 18dB/octave crossover is adequate but 24dB is sometimes used.
The problem with the passive crossover is the size of the components. If you have a 100 watt speaker, then the crossover has to be capable of handling all this and passing it on to the drive units - without wasting too much power. High wattage components which have sufficient accuracy are expensive - also, high wattage inductors can create distortion. The solution to this, usually employed in big PA systems and studio monitors, is the active crossover.
The difference between active and passive in this case is the positioning of the crossover (see Figure 1). An active crossover is usually a mains-powered device but it doesn't have to be. It could be built up from passive components, but it usually isn't.
The advantage of going 'active' is that the signal before the power amplifiers is only at line level. This means that it is easier to work on. A roll-off of 24dB/octave is also the norm. Another advantage is that the full power of the amplifier is passed on to the speaker. It doesn't have to fight its way through a labyrinth of hostile components to get to its destination. Additionally, having it more closely coupled to the speaker - electrically speaking - gives better performance. (Watch out for 'How It Works - The Power Amplifier' for more on this point.)
Many people feel, almost instinctively, that the active crossover must be better than the passive. This would indeed be the case if the active crossover was matched to the rest of the system. You see, when a responsible manufacturer designs a loudspeaker system, he takes into account all the components of the system - drivers, crossover and enclosure. Many drivers, for instance, are in all respects good - apart from a slightly uneven frequency response. This is no problem because extra components can be added to the crossover to compensate. Active crossovers are normally made by separate manufacturers so they cannot be accurately matched to the system with which they will be used.
There are speakers on the market which accept a line level source from a preamp, and have inside the enclosure an active crossover and a power amp matched to each driver. This is the only proper way of doing things but it remains to be seen whether the idea will catch on more than it has so far.
I hope it is clear that the need for active crossovers in large PA and studio systems is for reasons of power and efficiency. As a practitioner of theatre sound, I ought not to say that ultimate accuracy in sound is not as important as in a domestic or studio setting, but it isn't. Don't tell anyone I said that!
I can't leave the subject of crossovers without a word on how difficult it is to design them - VERY. If you know a bit of electronics and have the right source books, then active crossovers are straightforward enough. Ordinary common-or-garden passive crossovers are different. It is impossible to calculate component values. Well, I exaggerate slightly, it is quite possible to calculate the values from a knowledge of the crossover frequencies required and the driver impedance. The only snag is that the answers will be wrong! Drive units are, electrically, very complicated beasts. Cottage-industry manufacturers have to use manual trial-and-error techniques to get their component values right; the big boys make their mistakes on large number-crunching computers.
Not the Royal Enclosure at Ascot but the thing that keeps a loudspeaker's innards in.
There is more to the enclosure than nailing a few bits of plywood together but this is one area where the home constructor can beat the commercial manufacturer. It is expensive to build a high quality enclosure. It takes time and requires close attention to detail. So how do manufacturers turn out high quality cabinets so cheaply?
The answer is they don't. In years gone by, enclosures were craftsman-built and expensive. Like all other fields, production engineers have been busy finding out ways to do it more and more cheaply [Don't you mean 'cost-effective' - Ed.]. Take, for instance, a common tool used to make a speaker box - the V-groove machine.
It doesn't come as an attachment to a Black & Decker - this is heavy industry stuff. What you do is feed a sheet of wood in at one end. A series of V-shaped grooves, which go nearly all the way through the wood, are cut according to a computerised plan. The wood comes out the other end and it is folded into a box! No bracing or anything fancy like that. A smear of glue if you are lucky. Forgive my cynicism. I agree that it is a nice idea that people who want reasonable quality speakers can have them cheaply. But advertising copywriters do sometimes go slightly over the top when describing budget equipment. There is no magic speaker dust that can be sprinkled to make things okay. If you want good speakers, you are going to have to pay a lot of money for the workmanship necessary.
So what makes a good enclosure? Obviously, the volume of air space has to be right for the particular bass driver and the response curve you are aiming for. (See Part 1 of this article and the references to find out how difficult this is to work out!) Assuming you have this information, whether you worked it out yourself or took it from manufacturers' data, all you have to do is build a box which doesn't leak air or have any undue resonances.
I am not going to tell you how to do it, because that would be an article in itself, but I have listed a couple of addresses at the end where you can get plans and components. It is worth saying what happens if the enclosure is not properly built, however.
It should go without saying that a box whose function in life is to separate the backs of the drivers from the front should be air-tight (unless it is a vented enclosure, in which case the vent should be the only opening). Air-leaks will spoil the careful calculations that have gone into the design, giving unpredictable results.
Panel resonances will always be a problem. This happens when the walls of the enclosure are excited by a particular frequency they happen to like, and it colours the sound. It is not possible to make panels stiff enough for this not to occur, so vibrations must either be damped out or be made to occur at a frequency where they are not troublesome. Damping is commonly achieved with bitumen-impregnated felt pads, which I can tell you are extremely messy things to play with.
The enclosure as a whole is also liable to 'wobble', so internal bracing of some form is essential to prevent this. Some commercial loudspeakers, such as the B&W Matrix series, have extremely elaborate methods of achieving this.
This is where we learn how to blow up speakers. What do you mean it's easy?
It is sometimes all too easy to damage a speaker. However, a little knowledge of the mechanisms involved will help in avoiding too frequent occurrence of that dreaded silence - or that even more dreaded rasping speaker noise.
There are two speaker damage modes. One is over-excursion, the other is thermal failure.
Over-excursion happens suddenly and unpredictably. A sudden transient and the cone moves so far that its suspension is damaged. What happens then is that the speaker still works and makes a noise, but it is now scraping its voice coil against the sides of the magnet gap. This is not pleasant, although I understand a number of Heavy Metal bands incorporate the effect into their stage act!
Thermal failure takes a bit longer to occur. When the speaker is driven for a period of time above its rated power, then the voice coil will get hotter and hotter and eventually melt. Result - no sound at all from the driver. To help prevent this, some drivers use square-section copper wire which withstands heat better. Others use a magnetic fluid - ferrofluid - around the coil to conduct away excess heat.
However you damage your driver, the only cure is a trip to the manufacturer for a re-cone. Either that or a complete driver replacement.
As far as I am concerned, the ideal driver is one with a tough suspension and good thermal capacity which is limited by distortion above its rated wattage. I have used loudspeakers with drivers like this and it is comforting to know that if you get to a point where you are hearing distortion, then you can back off the power, safe in the knowledge that there was always ample 'headroom' before damage could occur from either cause.
There is a third damage mode worth a mention which happens to tweeters. Suppose you have a low power amplifier which you drive hard. Distortion will be produced, which has a lot of high frequency content, and is fed straight to the tweeter. Not surprisingly, the tweeter objects and downs tools. This accounts for the legend that a low power amplifier is more likely to blow your speakers than a high power amplifier. It's true.
Someone made a lot of money out of a song called 'There Are More Questions Than Answers' but in this case Q & A come in a one-to-one ratio. I have to admit that I made up the questions myself but if anyone would care to suggest any more, then I shall try to cover them when I get around to writing 'How It Works - Monitoring and Room Acoustics'.
First question from the back of the class please:
Q Why is there wadding inside my speaker?
A Woolly jumpers are jolly good things for stuffing inside bass drums but don't try it with your speaker. The usual material is called BAF (Bonded Acetate Fibre) wadding and is acoustically transparent. Without going into a lecture on more of those troublesome laws of physics, wadding has the effect of making the speaker cabinet apparently bigger. How much bigger is difficult to calculate without experiment but the right amount of wadding, in general, is a 'good thing'.
Q I like to have my speakers on their sides because they look more professional! Why don't they sound as good?
A It depends whether you are a side-to-sider or an up-and-downer. Most sound engineers have to move from side-to-side to operate different controls on the mixing desk. Some get cramp in their legs and have to stand up occasionally. Either way there are problems.
As I said last month, if two drivers radiate the same sound simultaneously, then at certain angles (which differ according to frequency) there will be interference and cancellation between the two soundfields. In a two-driver speaker system, this will occur precisely at the crossover frequency. The optimum listening position is on a line directed at right angles to the point mid-way between the drivers. With vertically-mounted drivers, cancellations will occur above and below this line but not on either side. With horizontally-mounted drivers, the cancellations occur at the sides but you can bob up and down to your heart's content.
Horizontal mounting will cause trouble with the stereo image because you now have four drivers (or more) in the same plane as the sound image, instead of the optimum two. The moral of the story is mount vertical, then buy a large vice and put your head in it.
Q Are three-driver systems always better than two-driver systems?
A The bigger the better, right? Actually, it depends where you are. If you are at home or in a small studio, then two-way systems would be a more favourable bet.
There is a myth that bigger drivers give lower bass. This is not entirely true. An 8-inch driver can give as low a bass as you are ever likely to need, but I must stress that this depends on its design. A good one can also operate up to 4kHz or more with reasonable sonic quality. Above this frequency a tweeter can take over. 12-inch drivers, on the other hand, are very much more restricted in the upper range of their frequency response, making it essential to use a mid-range unit.
The advantage of having two drivers only is that there is only one crossover point. Nasty things happen in the crossover region and there is no point in compounding the problem by having more. The advantage of having a three-way system with a 12-inch bass driver is that it will go much louder. An 8-incher will not give enough acoustic power to satisfy a pro studio but I find mine quite loud enough for domestic use.
Q Can any decent bass driver be mounted in an appropriate enclosure to give a smooth frequency response?
A This goes back to the pumped-up or 'one note' bass that I was talking about last month. Some drivers are made so that it is impossible to get a smooth bass response out of them. Reputable drive unit manufacturers make and sell them to el cheapo 'boom box' builders. These drivers have not been made in a slipshod way, however, they have been designed to be like that to satisfy certain elements of the market.
Q Do speakers wear out?
A Surprisingly, it is possible for a speaker to wear out. It will not get fainter and fainter but the driver and enclosure parameters may alter with time. Damping materials on the cone and on the inside of the cabinet walls may lose their flexibility and not operate as they should, having a worsening effect on sound quality. This should not occur to any great extent with modern units.
Q What was the deliberate mistake in last month's article?
A Both the references at the foot of the article are to be found in Volume 21 of the JAES, not as stated. My mistake.
I can't finish without describing an experience I had trying out some top-of-the-range speakers.
I had the opportunity of comparing the B&W 801, Quad ESL 63, Proac Studio 3 and Celestion 600 speakers over a period of about a fortnight. The first three, by the way, are getting on for £2,000 a pair. The Celestions are a very tiny £700+ quid's worth.
As they were delivered, one pair at a time, and fired up, there were exclamations of "Ooh" and "Wow" and the like. Then we got around to comparative tests. The ear can play some funny tricks. When you hear such high quality speakers in turn, you realise how different they are. One speaker would point out the faults in another and, eventually, no speaker could be judged to be better than any other. I came away not liking loudspeakers at all.
I can't remember who first said this but it wasn't me: "The best way to buy a speaker is to choose one at random, then learn to live with it." I am not suggesting that this is completely accurate but there is certainly the odd grain of truth in there. I think I'll go back to using headphones.
Two companies, both of which I have used in the past, who can supply plans and parts for DIY speaker builders are:
Wilmslow Audio, (Contact Details).
Falcon Electronics, (Contact Details).
Feature by David Mellor
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