Choosing Speakers To Drive Your Cabs

author: Phil Starr date: 02/19/2010 category: gear maintenance
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Release your inner nerd. Join me in a world of technical specs and numbers. Well that's the health warning over, I want to write about some of the issues you will need to know about if you want to build speakers as good as or better than commercial units. This article is going to be more technical than anything I have ever written for UG so it will be really good to have your thoughts on whether I have gone over the top. How Speakers Work Speakers are driven by a motor system consisting of a coil sitting inside a magnet. When your amp puts electricity through the coil it creates an electromagnetic field which pushes against the speaker's magnetic field and the coil will move. The coil is fixed to the cone which pushes against the air which is made to move in time with the music. The cone is returned to rest by the suspension which acts as a spring. Everything is held in place by the chassis, an open metal frame which surrounds the cone. This is all very simple but there are already loads of variables which will affect how the speaker sounds. Big cones will move more air but be heavier and harder to push. Stiff suspensions will tune the speaker to higher notes and heavy cones to lower ones. Big magnets will give you extra force. Long coils mean more bass but lower efficiency and so on. Celestion, Fane, Eminence and Jenson etc. have made dozens of compromises In designing the speaker so how do you choose which one is the compromise that suits you? The answer lies in the Thiele Small specs that most manufacturers provide. If I do a good job by the end of this article you should be able to read a spec sheet and make a guess as to how a speaker will sound. The Magnet System Most magnets are made of a ceramic material called ferrite containing loads of iron. It is cheap to manufacture and holds its magnetism well, the only disadvantage is that it is heavy. Alternatives are the old fashioned Alnico alloy and the newer and much lighter Neodymium alloy. Alnico is expensive and loses its magnetism but some guitarists think it makes a sweeter sounding speaker. I play bass. I don't believe in the healing power of crystals or the magic power of poor magnets. Neodymium is really promising as magnets of the same power can be much smaller and you can also make super power magnets of a practical size. It costs a fortune though. Whatever magnet you have it will be fixed so that the magnetism is concentrated into a cylindrical gap where the voice coil sits. A big magnet lets you do two things increase the magnetic field or make the gap bigger which in turn can make the speaker louder or can improve the control the speaker has over the air it moves making it sound more accurate and crisper. Eventually though a really huge magnet will get too tight a grip on a speaker's movement and, for reasons buried deep in the maths, will throttle the bass. This is called over damping and I'll deal with damping later. Generally though the magnet is the most expensive part of the speaker so the bigger the magnet the better the speaker. The magnetic gap will have to be designed to match the coil shape. A big coil diameter means more power handling and a better mechanical join to the cone but reduces the magnetic field and efficiency with a particular magnet. A short voice coil means more of it is in the magnetic field and the speaker will be louder a long coil means the speaker can move further before the coil leaves the magnet and so bass will be better but speaker designers can also lengthen the magnetic gap and this will change how long the coil stays in the magnetic field. There is a trade off between bass and efficiency in other words but a bigger magnet can give you both, at a cost. The Cone And Suspension. OK we've done the motor now here's the transmission. The cone will usually be made of paper' though this may be impregnated with Kevlar, carbon fibre or special grades of plant fibres and coated with metal foil or formed wholly in metal. Don't look down on paper though iut is still the best all round material. Big cones shift more air giving better efficiency and better bass. With big cones though the sound takes longer to travel from one part of the cone than another and the sounds will cancel each other out at higher frequencies. In practice even an 8 speaker has cancellation problems from 1000Hz upwards and this is made more acute by the problems of making a big cone move back and forth over 1000 times a second. It is big cones for bass and little cones for top end. There is a solution to this though. As the speaker moves at high frequencies the middle of the cone responds first and the outside of the cone reacts later as the impulse spreads out. The cone bends or flexes. This can be controlled by cone designers so that the high frequencies are only radiated by the centre of the cone and the bass by the whole cone. Get the design right and you get a flat frequency response or a mid boost just where you want it giving a lovely vintage Marshall' sound. Generally curved sided cones show the designer is trying to get an extended frequency out of a speaker. The Chassis OK we've done the motor and the transmission, now let's look at the bodywork. The choice is between pressed steel and cast aluminium or magnesium. Pressed steel is cheap and is certainly strong enough for small speakers though because it is less rigid than cast frames the chassis can be quite big and this affects the sound. Aluminium is my favourite material for chassis, being rigid it can be much thinner and open around the back of the speaker and it maintains its alignment meaning the speaker will last longer; it is also lighter. Speakers get very hot in use which is the main reason they break down. Aluminium is an excellent conductor of heat and will help to cool the magnet and voice coil. Go for aluminium if you can, especially for big speakers (15) and big magnets. Frequency Response Your speakers frequency response is what is going to determine most of the character of your cab and you can't use tone controls or modellers to entirely remove this character or to add anything that isn't there. For guitar speakers you will want a frequency range of 80Hz-6000Hz. Most guitar speakers have a definite hump at about 2-3000Hz which gives the so called British' sound and another hump in the low frequencies will give you a warm' sound. If you heavily use a modeller you might want to go for a flatter more uncoloured sound. Bass speakers should run from 40-5000Hz and most players prefer a flat, uncoloured response. If you have a B string you might want to go down to 33Hz. But.... most bass sound is in the harmonics and there are many successful bass designs that reproduce almost no low bass. Celestion's specialist 10 bass speakers have a cut off point almost an octave above bottom E and sound fine. The bottom end of the frequency response is determined by the resonant frequency fs. The cone mass resonates against the springiness of the suspension. Just like a guitar string a heavy string and a slack suspension means low and slow, tighten things up and use a thinner string or a lighter cone and the resonance rises. At resonance the speaker goes into reverse the coil resonating in the gap generates an electrical current opposite to the amp (back emf) and the impedance of your 8ohm speaker rises to 30 ohms or more. Below resonance the output of your speaker falls quickly so the resonant frequency is the lowest practical limit of your speaker in most designs. So again there is a trade off, you can make the cone heavier to get more bass but the efficiency will suffer. Matching The Cab The main job your cab does apart from being a box to carry your speaker is to stop the air at the back of your speaker from interfering with the sound you want from the front. At the very moment the cone moves forwards to create the first note it reduces the pressure behind and sucks air in. Without a cabinet the air would simply move round from the front of the speaker and very little sound would escape. The cab stops this happening. For Instrument amps you really have a choice of three types of cab, open back, sealed and ported (vented or bass reflex). Subs, horns and exotic hi fi designs are beyond this article. Open back cabs simply make the air behind the speaker go on a journey round the cab delaying it a little. This gives all but the low notes time to escape from the front of the cone. You can just reach E on a guitar with an open cab but it won't do the bottom octave of a 4 string bass. Sealed cabs trap the rear waves inside the box. Ported cabs use a clever trick with resonance to delay the bass notes so that the port radiates the rear sounds from the speaker out of the port in time with the front sounds and doubles the bass. It only does this at the very lowest frequencies below E and so you don't use ports in a guitar speaker except as a gimmick. The cab has two other effects on the speaker which changes the design. The air in the cab acts as a load on the speaker's magnet and also as a spring and damper (shock absorber). The three types of cab each need a different design of speaker to work with. Here is what to look for. Open backed speakers have no spring and little load or damping. Because they can't have deep bass they dump all the problems of trying to reproduce bass and go for volume. Open backs have stiff suspensions short voice coils and light cones. They only need small magnets. Closed cabs are dominated by the spring of the air in the cab and the load and damping of this air. Because the rear bass is absorbed the speaker has to work harder for bass and closed cab speakers have longer voice coils heavier cones (to tune lower) and softer suspensions. They often have bigger magnets. Reflex cabs need speakers with bigger magnets to improve damping but the bass output from the port means that you can get away with a shorter voice coil for the same bass output. The lack of a load on the back of the speaker means it needs to be designed to stop it flapping about too much at very low frequencies so the suspension has to be stiffer and the maximum excursion is limited. Using the wrong speaker in a cab can mean it might get damaged in use and will mean it sounds weaker than it should do. By and large you can trust manufacturers to tell you what cabinet their speakers work best in and you can always email them with enquiries. Looking At The Specs. I've chosen to look at the specs of a couple of speakers from Eminence simply because they are probably the most widely distributed speakers worldwide and because they release great spec sheets. If you want to look then the links are: Texas Heat, Delta 12A. I'll concentrate on the Texas Heat as it is designed for Guitar and I'll use the Delta which is a general purpose/PA design for comparison. Power Rating Eminence use an EIA 426A rating measured in free air. This means that they passed a signal supposed to mimic the content of real music through several samples of this speaker for several hours at full power and they didn't break. This is true RMS power and is really about the speakers ability to resist overheating. The Delta is rated at 400W and is also given a Peak' rating of 800W. Peak ratings have no legal or accepted scientific definition so take it with a pinch of salt but you could reasonably say that the Delta will handle 800W peaks for a fraction of a second without distorting horridly and you could use an 800W PA amp with it so long as you don't run into levels where the amp clips. You wouldn't give this rating to a guitar speaker because distortion is almost a given and driving the Heat with a 300W amp will end in tears. For guitar always at least have at least 100W of speaker handling for every 100W of amplifier power. Resonant Frequency (fs) This will become very important if you design a bass reflex cabinet but for all speakers it tells you the lowest frequencies you can reproduce. Eminence have chosen 79Hz carefully as it is just below bottom E on a guitar so it will reproduce the whole frequency range justifying their claims of a warm tone for this speaker. The delta has an fs of 55Hz which means it will reach down much lower but at the expense of a heavier cone (Mms) of 39g as opposed to 31g for the Heat. Having a high resonance means you can get away with a shorter voice coil and makes the speaker more efficient(louder) Sensitivity This figure tells you how loud the speaker will be at 1m with a 1W signal. This ranges from roughly 92-102dB (dB are decibels) for most live sound speakers with anything above 96 being good. To put this in context an extra 3dB is the same as doubling the power of your amp and the difference between 92 and 102 dB/W is the same as ten times the power. Use this link if you want more on sensitivity. Frequency Range For the Texas Heat 70-5000Hz is quoted but much more usefully they also provide a frequency graph and you can see that these are only the points where the output has fallen 6dB below the average. The bass plots are identical for both speakers this is because of the way the plot was measured which they tell you about on the bottom of the sheet. The Heat shows a large dropout at about 1600Hz and a peak at about 2500Hz which are both pretty much in the upper mids. This is exactly where the cone is starting to flex and if you look at the photo you can see how the designers have controlled this by having the cone centre plain and ribbed the outside. This is what gives the speaker the British flavor that they claim. (I'm pleased to see that Texas Heat is considered British.) The peak hits right on the frequencies where the ear is most sensitive and with 107dB/Watt at this point it is really going to sing out on certain notes. I also like the way the upper frequency plot falls gradually which shows distortion is being controlled. Frequency response curves will tell you a lot about how a speaker will sound but in the end you have to listen as well as there are dozens of variables which affect sound. Eminence are one manufacturer who provide sound clips so you can listen to this. Magnet Weight As a piece of raw data this doesn't tell you much as the way the magnet is used is just as important as its size. Big magnets drive the speaker harder increasing sensitivity and they control the cone better but this is traded against lower frequencies and longer voice coils amongst other things. The Texas Heat has only a moderate magnet for a 12 speaker and the Delta has almost twice the magnet but the sensitivity is almost the same. The extra magnet strength is being used to drive a heavier cone and a longer voice coil whist keeping the sensitivity high amongst other things. Generally speaking though a big magnet increases damping which has a big impact on the sound. Damping (Qts) This is one of the most important features of loudspeaker design and it is a figure you must have to design a successful speaker. Having said that it is really a concept arising out of the way speaker behaviour is modelled mathematically so it is tricky to describe in words. I'll give you my version which I'll try to keep clear. Damping or Q is about how accurately the speaker movement tracks the electrical signal from your amp and sums up all the restoring forces acting on it. If the damping forces are too little then at resonance the speaker cone will move excessively and there will be boomy bass but no deep bass. If you increase the damping then the bass becomes cleaner and more controlled but increase damping too much and the bass is squeezed out. It may go low and clean but at reduced volumes. A well damped speaker will respond to a click by shooting forwards and then coming straight back and stopping. A poorly damped speaker will shoot forward ok but will overshoot the return and go on backwards and forwards a few times before coming to rest. This means your speaker is still ringing as the next note comes along and the sound is muddied. The damping is in three parts, the mechanical Q or Qms, the electromagnetic Q or Qes and the extra damping of the cabinet. Cabinet design is really all about getting just about the right amount of extra damping from the cabinet to get the total Q spot on. The total Q of the whole system is called Qtc with high Q meaning poor damping. Qtc of 0.5 gives the best damping but weak bass but is used in midrange speakers. Qtc=0.7 gives the flattest response and is the usual design for bass and PA and some guitar speakers. Qtc=1.1 gives the maximum power handling and efficiency but at the expense of poorer transients and a bass hump and less deep bass which is sometimes described as boomy or one note' bass. Qtc=1.1 is still acceptable though, and there are many successful designs like this. The figure we are really interested in though is the Qts or total Q of the speaker which is a combination of the mechanical and electrical damping. It is dominated by the action of the speaker magnet so this is the best way of judging just what the magnet is doing and whether it is big enough. If Qts is below 0.4 then you have a big magnet, below 0.3 and it is huge. If Qts is above 0.7 the magnet is small and it is going to be difficult to create a cab with a flat response. If Qts is high you will need a bigger cab to add damping and low Qts is what you need for a compact cab. The Texas Heat has a Qts of0.65 which is fairly high but well within what you would look for as a guitar speaker, and it will give you a nice warm fat tone in a normal cabinet which is exactly what Eminence tell you. The Delta 12A has a total Q of 0.43 so it will give a tighter sound. The other factor is that you really need a Qts below 0.4 to design a good vented cabinet with a flat response. Eminence are recommending only a vented cabinet for this speaker so I would say that the magnet is only just big enough for this speakers intended use despite being nearly twice the size of the Heat's magnet. The reason for the recommendation is that there are lots of other features in speaker design which will make it more suitable for reflex (vented) or sealed enclosures. A year ago Eminence were saying this speaker was suitable for either type of cab so they have either changed their minds or the design. Excursion limit (Xmax) The loudspeaker coil is going to be longer than the magnet gap so that as it moves backwards and forwards the same length of coil stays in the gap maintaining power. Some speakers pack as much of the coil into the gap as possible which makes them more efficient as current is wasted outside the magnetic field. Other speakers have longer voice coils so that they can move further and this gives them better bass. The distance they can move before the coil leaves the magnet is called Xmax. Once the coil starts to leave the magnet there will be less force pushing the speaker and the bass will become compressed. Eventually the speaker will reach the limits of the suspension and will stop moving. This is a very important design feature for reflex speakers which have a tendency to flap around at subsonic frequencies and which would otherwise be damaged. The Texas Heat has an Xmax of 0.8mm meaning it can only travel through 1.6mm without distorting. This is OK though in a guitar speaker as the bass is limited to bottom E and you gain a very loud speaker even though the magnet is quite small. The Delta has an Xmax of 2.4mm which is still quite low, I would look for at least 4mm for a bass or PA speaker. However it is intended for a vented or reflex cab and this will reduce its excursion at the resonant frequency. Eminence helpfully give a Xlim figure of 9.9mm for this speaker telling us where the mechanical limit is. Notice that multiplying the cone area by Xmax gives you Vd (not the disease) which tells you how much air the speaker can shift and it is this that determines the ultimate bass output. Compliance (Cms) When a speaker cone is moved it experiences resistance mainly from the suspension. Some speakers move more easily than others and are said to be compliant. The compliance (Cms) can be measured directly in mm/N, how far it moves under a given force. The Texas heat has a compliance of 0.13mm/N which is quite stiff compared with the Delta at 0.21mm/N. A compliant or soft suspension has a lot of implications for the speaker. It reduces the resonant frequency and increases the efficiency but the lack of a built in spring means the cabinet has to compensate and add spring. A stiff suspension means that the speaker can be used in an open backed cabinet but a really soft suspension means you must have a sealed cabinet. Speakers for vented designs usually fall between these two extremes. The compliance also helps dictate the size of the cabinet and to calculate this you need another figure, Vas the equivalent volume. Equivalent Volume (Vas) Two things affect how easy the speaker is to move, the compliance or softness of the suspension and the surface area of the cone. When you design a cabinet this has to be balanced against the volume of the air in the cab. It is easier to combine these into a single figure; the equivalent volume or Vas. This is the volume of air which is as easy to move as the speaker and it takes into account both the mechanical compliance and the size of the speaker cone. The bigger the Vas the bigger the cabinet you are going to need. If you go back to the damping bit then you can see that the total Q of speaker and cabinet is Qtc and the speaker alone is Qts. For a sealed box the volume of the box is calculated by dividing Vas by alpha and alpha= (Qtc/Qts)2-1. So there you are, by just knowing Qts and Vas you can calculate the size of a sealed cab and tweak its bass response by tweaking Qtc. To give you some idea of how this is all used; I recently helped someone to find a speaker to fit a cab they had built. The speakers with 20oz magnets had high Qts values and needed big cabs but the 38oz magnets gave low Qts and we found several that fitted. I guess for most musicians this is all a bit confusing but don't worry you'll soon absorb the ideas if you start to use them. If not you'll have at least a little insight into what the guys at Marshall, Peavey and so on spend their time worrying about. To sum up, speakers are optimised for different uses and to match particular cabs. The most important thing is their sound and for everything except the bass you need to see a frequency plot. Better still is to listen to them. For the bass you need to know their resonant frequency and their Q. Big magnets give low Q and extended clean and fast bass, small magnets give warm bass but can be woolly and you wouldn't choose them for ultimate bass, great for guitar though. Have a good look at the speakers efficiency, 96 dB/W is good for bass and anything of 99dB/W or over good for guitar. For bass or PA you'll need Xmax to be high (4mm or over) or you will run out of bass at high power levels. Finally to calculate cabinet sizes you'll need Vas, Qts and fs figures (Qes and Xmax will be useful for some design programs). This all ties in with some other articles I've written for UG for which some of the links are: For Choosing cabinets For speaker efficiency For impedance and power handling, doctorproaudio.com That's it then, I hope it was useful and not too baffling. My next article in this series will be about actually designing cabs using all this information. Have fun.
More Phil Starr columns:
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+ A Guide To Live Sound Speakers And Amps Gear Maintenance 02/07/2011
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