A Guide To Live Sound Speakers And Amps

author: Phil Starr date: 02/07/2011 category: gear maintenance
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Hey, you've made it. You're a real band doing real gigs in front of real audiences. Now you need a proper PA. This aims to be the first part of a guide which will help you set up and run a full blown PA. I've already written a guide to a simpler system and how to run it the links are at the bottom. You'll need speakers, amps, a mixer, mic's and all the leads and a guide to setting it all up and getting the best out of it. I'm going to start with the speakers because that will determine what amp you use and will in turn affect the rest of your choices. It's a great time to buy speakers as design has improved to the point where one engineering solution has evolved which works for 90% of our needs. Nearly all bands end up with a pair of smallish boxes on stands, each with a single speaker and a horn for the top notes. The speakers come in a range of sizes for different bands and if you need to be really loud you add a bass bin or two. All you've got to do is choose the speaker size you need and how you want to run your amplification. Choosing the Speakers There isn't a single PA system that solves every problem and you need to give a little thought to what you want your PA to do. What sort of music do you play? Are you a metal/rock band or a folk duo? Will your PA just carry vocals or the whole band including the kick drum and bass? Where will you perform and what volume levels will you need? You are going to need different stuff to do different jobs. There are also the little practical problems like can you lift it and will it fit into your vehicle. There is no point dragging round a much bigger system than you really need. Pretty much the whole range comes with a 8,10,12 or 15 speaker and a horn to cope with the higher frequencies. The smaller speakers will generally give you more clarity in the midrange and a better vocal and acoustic guitar sound but they won't go as loud as a big speaker and the bass will be limited. Move up to the 15 units and you have a speaker that may be able to cope with the volume of a full rock band, but unless you spend a lot of money you will lose the clarity of a smaller speaker, because neither a horn nor a 15 cover the middle very well. Even if you opt for a 15 unit it may still struggle with bass and kick drum if you are really loud or play in bigger venues. So, if you want to run everything through the PA then at some stage you should add a bass bin (or sub) to free up the main speaker to cope with everything else. 8 or 10 + horn Ideal for vocals in an acoustic band, or for voice and guitar. These speakers usually give good vocal clarity and are easy to transport. Limited sound output except for high end designs. A really high end 10 unit will match a mid-price 12 speaker but at a price, worth looking at if weight and portability are an issue and you can afford it. 12+horn Good all round solution for many bands especially if limited to vocals and guitars. These speakers can achieve high sound levels and can be used successfully for the whole band if you add subs. Vocals are usually fairly clean and you can often lift them unaided onto a stand. They'll also pack into the boot of a family car. Our band opted for this solution (Yamaha S112V's +S118 subs for bigger venues) 15+horn High sound output levels and will cope with bass and drums at medium levels. Any decent design will give enough output to fill a 200 seat venue for most bands though subs might be needed if you are very loud. You will need to be built like an ox to lift these babies onto the stands. 2x15+ horn designs are also available. These are big heavy speakers designed for disco's or for full on rock bands where their bass heavy nature is an asset. A single pair of these will cover most of your gigs on their own but you aren't going to fit them in many family cars. Another solution around is to add a mid range speaker (usually a 5 or 6 unit) to a 15+horn, like the Mackie 1530 which does give lovely sweet vocals. Adding a mid-range speaker is such a great idea for cleaning up vocals that it probably should be a lot more common. Active or Passive? An active speaker is simply one with an amp built in. Most speakers are passive and you need to buy an amp to drive them, but an increasing number have an amplifier built in. You can also buy active mixing desks where the amp is built into the mixer. So, your choice is active speakers with a passive desk, passive speakers with an active desk or passive speakers, separate PA amp and a passive desk. I'll concentrate on the arguments for and against active speakers for now but similar arguments apply to active mixers. Active speakers: Having the amp built in means that the amp exactly matches the speaker meaning one less decision for you and less chance of a blown speaker. Protection circuits are usually built in too and sometimes even digital processing to exactly match amp and speaker. One less thing to carry and forget No speaker leads needed, though you will need a mic lead to run to the speaker. The speakers need to be plugged into the mains. Usually have an active crossover so potential for a better and very slightly louder sound. If anything goes wrong you've lost speaker and amp. Passive speakers: You need to match amplifier and speaker. More flexible, you can change amp or speaker and upgrade in stages. Only one lead running to the speaker. Less impact if anything goes wrong. Lighter, but one more box to carry. To me it is fairly simple, if you are the sort of musician who just wants to get on and play and who wants to run a PA with minimum involvement with technology then get an active system. It's the plug and play solution with a perfect matching of amp and speaker. Some even have computers built in (DSP) which will sort out your tone settings and even some feedback problems without you having to do anything. For acoustic acts there is usually the chance to mix one mic and one instrument so you may only need the active speaker! If you are a technophile who is always looking to upgrade then go for separates; , as you grow in expertise and/or funds you can change components and upgrade at less cost and there are lots of add ins you can use. It really is more flexible. How much Power Do I Need? You are really asking two questions here, how loud do I need to be and how should I match an amp to my speakers? Loudness is measured in decibels or dB, not in watts and speakers vary greatly in their ability to turn watts into sound. Some speakers are just louder than others so you need less wattage. Using the most efficient speakers could easily be the same as using 4x the power in watts. This is why I suggest you choose the speakers first. If you are not used to thinking in terms of decibels have a look at my article Making it Loud' http://www.ultimate-guitar.com/columns/gear_maintenance/making_it_loud.html To give you an idea of what you are looking for I'd say 115-120 dB is enough for an essentially acoustic set, 120dB+ is enough for vocals with a band and 125dB will carry a fully mixed band so long as you don't push too much bass through. There is a word of caution here though. The manufacturers don't all rate their speakers in the same way, so you need to check before comparing figures or at least be a little sceptical about some of the claims. The most common trick is to rate the output with the peak power the speaker can take. Most speakers can handle four times their rated power for a second or so, especially if there is no deep bass and so the manufacturers give a figure their speaker will only give at four times the rated power. You won't achieve this figure without using, say, a 1000W amp with a 250W speaker. One example of this is JBL 305 Eon which claims a max. output of 128dB, enough for most bands needs if you can achieve it. In reality the Eon is a 250W speaker and it has good sensitivity. At its rated 250W though it will only give 122dB which isn't quite enough to cope if you have bass and kick going through the PA. To be fair most manufacturers such as Wharfedale and Electrovoice do exactly the same trick, but Mackie give a rating at the RMS (EIA) power so their 123dB speaker is actually a little louder than the 128dB JBL. Remember that you only get the manufacturers rated output if you use the same amp wattage and you may need to knock 6dB off the maximum claimed in the adverts. Some manufacturers give a one watt sensitivity figure. If they do you can work out the max output from this by adding 20db for 100W and 3dB for each doubling after that so a 200W speaker with a 98dB/W sensitivity will give a 121dB max. Again this is explained in more detail. I'll run through some of the products available in a minute to give you an idea of the choices you will face but first a little reality check. You still need to be sceptical of those ads! A good big speaker will go louder than a good little one because ultimately it shifts more air. You can make a little speaker move more air though, by giving it a long voice coil so it moves backwards and forwards more but this means lower sound levels as only a bit of the voice coil is working at any one time. You can also make a speaker louder with a stronger magnet but this comes with a fancy price tag. If you want deep bass you need more air movement than anything else. What this means is that you can have cheap speakers, loud speakers or deep speakers but not all at the same time. Small speakers will have to compromise over bass or efficiency. So will cheap speakers. I'll try to illustrate all this with some real examples of the choices you'll have to make. I'll look at the Wharfedale range of speakers. (I'm not recommending them particularly, other manufacturers like Peavey, JBL and Mackie all do similar ranges but Wharfedale cover the whole range and I have all the specs to hand.) You can check them out here. I'll start by looking where you probably should, at the 12 speakers. There are 4 speakers in the range. The budget SVP's (179 for a pair at present), the mid-range EVP's (399) and Titans (279) with a moulded plastic cab, and the more expensive DLX (499). The ratings are:
SVP12    150W   50-20,000Hz   124.5dB
Titan    300W   50-20,000Hz   128dB
EVP      300W   50-20,000Hz   128dB
DLX      400W   60-20,000Hz   129dB
All these ratings are at the max peak power so if you use them at their continuous ratings you will have to knock 6dB off. This makes the cheap SVP's 118.5dB/W enough for an acoustic act or as floor monitors but they will struggle with vocals with a rock band. You could use them with a 500W amp if you make sure it never clips but I would be slightly concerned about long term reliability. The Titans and EVP's would handle all your vocals and some extra instruments on top but I wouldn't put bass and kick through these without some subs. The DLX would give you a little extra volume. But, remember there are differences between speakers other than just how loud they are. The cheaper models usually trade efficiency for bass response and they usually have shorter voice coils so they run out of puff in the bass before they reach their rated output. The quality of the crossovers and horns will differ too. The SVP12's have a 1 horn driver, the EVP's a 1.4 unit and the DLX's a 2 unit giving a whole extra octave in the critical midrange compared with the SVP's. Clearly the vocals will be much sweeter and the bass a lot fuller with the better speakers, you get what you pay for. Moving up to the 15's The SVP's are 200W and 126dB max, the EVP's are 250W and 128dB and the DLX's 500W and 131dB max. By moving to 15's you've gained volume so none of these should struggle with vocals and bits of other instruments. You might even be able to put a touch of bass through the EVP's so long as it isn't too strong. The DLX's should deal with almost everything. You'll have to look elsewhere for 10 speakers as Wharfedale don't make them in these ranges, the Titan 8's which come in light weight plastic cabs and are rated 121dB for the active speaker and 124 dB for the passive (remember to knock 6dB off the passives) 70-20,000Hz. An output of 115-120dB is however good enough for an acoustic act and these are much smaller and lighter, so if it is just you and a guitar these would be quite adequate and at a pinch you could use them as vocal monitors for a band. Buying an Amplifier. PA power amps have settled down to a pretty standard form and the cost per watt is incredible. Even cheap stuff is pretty reliable, my Behringer has never let me down, and the better brands like Crown and QSC are fantastic. You get a stereo amp in a 19 rack mount case with two volume controls on the front and a load of indicator lights. Inside the amps are class H or digital and power may come from a huge and heavy toroidal transformer or a lightweight switched mode power supply. Most PA amps have the facility to bridge' combining the two amps into a single mono amp with up to 4x the power (in practice usually twice the power into double the impedance). Some amps will even have a crossover built in which will divide the signal electronically between your main speakers and the subs. This could save you 100 on a separate crossover. Here is a typical example of a good quality typical PA amp The amp has to match your speaker in two ways. It has to match the impedance (the ohms) of the speaker and the power (the watts) has to be right. Matching the ohms should be easy, the speaker will be either 4 or 8ohms, and a PA amp will almost always be solid state which will be happy with either impedance. The power considerations are a little more complex. Speakers are always happy with less power than they can handle so if you are running 100W into a 400W speaker and it is loud enough, no problem. Unlike guitar amps however it is perfectly normal to run PA speakers with much more power than their RMS rating. The reasoning is this; speakers are rated after testing with a continuous signal but music by its very nature varies in volume so speakers can easily handle short term peaks without overheating. Since the whole point of a PA is to give an undistorted sound and amp distortion sounds horrible it is better to have plenty of power in reserve. It is ok to have headroom of 6dB which is 4x the power so you can have a 1200W amp running a 300W speaker so long as you don't go into clipping. This is why almost all PA amps have clipping indicators, if you are running an over-powered amp into a speaker you really must not let the clip light come on. Most of the big hire companies compromise with 2X the rated power. The reality is that with, say 500W per side, the average band playing to a couple of hundred people don't need any extra power. As a rule of thumb, if the PA is as powerful as all the instrument amps put together (count the drums as 100W) then it should be loud enough. The other thing to beware of is to check the power rating of the amp into the speakers you are using. First you need to check the RMS rating (which is usually in the manual if you can't find it anywhere else) and secondly check the rating into the speaker you are going to actually use. For example my amp is advertised as being 2400W. The manual explains that is actually 500W per channel into my 8ohm speakers and to get 2400W I need 2ohm speakers which no-one actually makes. Any rating that says peak' is advertisers deception. To summarise you can comfortably run PA speakers with twice the rated power so long as you avoid any distortion but much less than this is acceptable if you can be loud enough with the speakers you are using. If all this choice is too complicated then go for active speakers where the manufacturer has done the matching for you. I hope you've got a pretty good idea of the things to think about when narrowing down your choice of main PA speakers. You'll have to think about how to match your speaker to the job it is going to do, whether to go for a built in amp and, if not, what amp to match it with to get the best out of it. You should stand a chance of seeing through some of the advertising hype and you'll know what to expect if you can afford to pay a little more. I've tried to keep it simple though and there is loads of stuff I've left out, if you have any questions then ask away and I'll do my best. Good luck with your choices.
More Phil Starr columns:
+ Start a Band/Join a Band. Part 1 The Guide To 02/07/2014
+ The History of Power Amps The History Of 04/26/2013
+ A Guide To Amplifier Classes The Guide To 02/29/2012
+ What The xxx Is The Sound Engineer Up To Gear Maintenance 12/20/2011
+ Sound Good In The Rehearsal Room Junkyard 08/24/2010
+ Choosing Speakers To Drive Your Cabs Gear Maintenance 02/19/2010
+ view all
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