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Pro Audio Systems
Gerry Hayden -UK Corespondent


Pro Audio Systems

Introduction

Back in the good old days all our intrepid DJ Dave needed was his double decks complete with built in amplifier and free pair of foul, sorry, FAL speakers. The resultant projected sound quality was often the sonic equivalent to a slap in the face with a wet kipper. Today things are much better, although to listen to some discos you wouldn't know it. In the same way that home Hi-Fi systems have evolved so the modern Pro Audio (PA) system has developed to produce high quality sound output which today’s audiences demand. Let's face it, your sound system on your disco is probably the most important part of the jigsaw. Get it right and you can have a good sound equivalent to or better than most home Hi-Fi systems, not to mention other rival DJ's who've just thrown their sound system together without a thought for the result. Screw it up, however, and you can sound like you're strangling a cat. Your audience will notice, the occasional compliment on how clear the sound is, plus they can actually understand what you're saying over the microphone, can do no end of good for securing future bookings at a gig. I'm a bit of an audiophile myself, having a nice Hi-Fi at home I wanted to get a similar sound quality for my disco. In the summer of 1996 I started looking into upgrading my disco PA system having seen what was possible when I had a look and listen to a fellow mobile DJ's sound system. Home Hi-Fi is one thing, PA systems in large rooms full of people is a different kettle of fish. Bedroom DJ's take note when you try moving into the mobile scene, your 60 Watt amp and Thunder Bass Speakers won't hack it, they aren't designed for your village hall. There's a knack to getting quality sound at high volume with no distortion while preventing your speakers from melting. With the abundance of products we can buy and, more worrying, a minefield of conflicting advice out there from varying retailers it can be daunting finding the best solution, there's more than one. So why should I know any better?, you ask. Well... The proof of the pudding is in the testing, or is it tasting, anyway during the past two years I have run a project upgrading my PA system in stages from a single amp twin speaker 500 Watt system to a 3000 Watt bi-amplified 4 speaker rig that has a very nice sound. Bully for me, I hear you say, but during this time I have learnt a lot, to start with my amp was under powering my speakers. The next few articles in this series will cover PA systems based on my experience as an electronics engineer and a mobile DJ and includes coherent technical advice and information that I have managed to gather together whilst researching this subject, all opinions expressed are in no way linked to the editor of Mobile DJ magazine. If you are thinking of upgrading your sound system now or in the future, read on... if nothing else at least you can avoid some of the pit falls I nearly fell into.

Getting Started, considerations, what to choose and where to buy.

From the start you need a good idea of what you want to achieve, you can then decide how best you can achieve your goals. You can apply this broad reaching statement to anything but we'll stick to PA systems. It's a bit more complicated than popping to your nearest Dixons to buy an off the shelf Hi-Fi. You need an idea of the type and size of the functions are you are going to perform at, the size of the room and on average how many people are going to attend. For example the sound system at a rave with 1000 people will need to be more powerful than at a wedding with only 80 people. Will your system need to be flexible enough to cope with both or are you going to specialise in just one area? You need to consider all the practicalities involved; dimensions, compatibility, weight, portability, reliability, cost and, of course, how it sounds. You may want a 20,000 Watt concert system that has the greatest sound ever, that's great as long as you realise you will also need a three phase power supply, an arctic lorry, five roadies to carry all your gear up the two flights of stairs to get into a venue and a health insurance policy for the double hernia operation you’ll need later on in life.

Cost will be one critical factor, you will no doubt have a budget but you obviously want the best set up at the cheapest price. Are you going to buy an entirely new set up all at once or are you going to expand and upgrade in stages over a period of time? The old saying; “you get what you pay for” rings true here to a point, the improvements you get as you invest more money in to your set up are exponential. Initially you can get noticeably large improvements in quality and performance but as you continue to invest and upgrade the level of improvement gets smaller to the point where you may only notice a slight difference. For example there will be a more noticeable difference between a five hundred Watt sound system and a two thousand Watt sound system than between a five thousand Watt sound system and a ten thousand Watt sound system, (I’ll explain the reason for this in the next article). An audience that will of course notice if you sound crap are less likely to notice the more subtle difference between what you consider to be good and excellent. You need to consider if what you want is a necessary improvement or if you’re just upgrading for the sake of it.

The best solution is to do some research before you step out of your house to go to the local disco shop armed with your credit card. Have a look in the music press especially magazines that advertise equipment for sale and have articles that review the type of equipment you are looking for. You get a good idea about what is available, how good it is and what to look for yourself when you go out to make a purchase. Compile a list of retailers who sell the equipment and what prices they charge, take it from me, it's worth shopping around. You may even consider buying second hand equipment if you feel you can get what you require this way at what will be a much cheaper price. Answers to any questions you have or advice you need can be found by phoning different retailers or even manufacturers directly, many have technical advice services which can give you accurate answers to any technical queries you have.

Hopefully having done your research you’ll have a good idea of what you want to buy and the next step is to go out to audition equipment and do some listening tests. The only snag here is, unless you live in a big city, you're lucky if there's a retailer within a 30 mile radius of where you live. Also some retailers have a limited amount of choice opting to stock only a couple of makes of equipment, so you have to be prepared to travel. To give you an example I currently use 3 retailers to purchase my equipment, my local is based on a farm just outside Salisbury (yes you read that right, a farm). The other two aren’t your run of the mill disco shops but are Pro Audio companies, one based in Reading the other in Manchester, who both specialise in the supply of PA equipment. My personal experience has shown these companies are more knowledgeable and can often sell the equipment at a cheaper price. It stands to reason really, these people supply a range of PA equipment from portable systems for bands and musicians who are out on the road all the time to massive concert rigs for large indoor and outdoor events etc. they are also happy to deal with mobile DJs no matter how small the set up. A good retailer will be happy to let you audition a range of equipment and even let you bring your own sound system in to try it with new equipment and then go home without making a purchase while you make your mind up. Better still some retailers may let you take a piece of equipment out with you when you do a gig to give it a road test, they may charge a small fee which might be refunded if you subsequently make a purchase. At the end of the day you want to be entirely happy with what you buy, the only way to achieve this is to use the equipment and listen to how it sounds.

The mechanics of PA Speakers.

This is the point where we reach what is considered by some to be the most critical components in a PA system, the loudspeakers. Their job is to convert the amplified audio signal you feed them into audible sound waves that can be heard. Entire books have been written just on the subject of loudspeakers, I’ll do my best to explain the most important points for us DJ’s in this and the next couple of articles in this series. To keep it simple speakers work by moving in and out, a bit like a piston, there by pushing air out from the speaker to create sound waves. This is controlled by the audio signal that is fed into the speakers. The measurement of how fast the speakers move in and out is called the frequency and is measured in Hz (pronounced Hertz, as in the car rental company or what love does) so a loudspeaker that is fed with a signal of 100 Hz is moving in and out 100 times a second. How much the speakers move in and out is known as the amplitude, the larger the amplitude the louder the volume. So far so good, now it starts getting complicated... there’s a huge choice of loudspeakers to choose from, for our purposes there are two main categories of interest to us DJ’s one is the full range PA loudspeaker the other is the bass bin.

The full range loudspeaker covers the full audible frequency range, from the low bass frequencies through the mid range frequencies up to the high frequencies, typically 40 Hz to somewhere between 16 KHz (1000 Hz pronounced Kilo Hz) and 20 KHz. These can be two, three or even four way loudspeakers, this describes the number of drivers or transducers contained in a speaker cabinet that produce the sound output. To give an example a two way speaker will contain a large 10 inch, 12 inch or 15 inch driver to produce the bass to low mid range frequencies. The remaining mid range and higher frequencies are dealt with by what’s known as a tweeter or alternatively a compression driver loaded onto a horn. These drivers are quite different because they have to do different jobs, the bass driver only has to be capable of moving in and out at low frequencies but must move large volumes of air to produce a descent volume level so bass drivers need to be quite large or you need lots of them (preferably both). A tweeter or compression driver is the exact opposite, it has to be capable of moving in and out very fast but does not have to move the same volume of air as a bass driver, this will be a lot smaller in size. To give an example of a 3 way speaker; as before a large driver will handle the low bass frequencies, a smaller 6 inch or 8 inch driver will handle the mid range frequencies and a tweeter will deal with the high frequencies. The advantage here is that by splitting the full range of frequencies between more drivers, each designed for a specific frequency range, you get better overall coverage. Bass bins deal exclusively with the low bass frequencies about 20 Hz to somewhere between 150 Hz and 250 Hz. By the nature of the job they do they are large and quite heavy and will generally use one or two 15 or 18 inch drivers.

[ Fig. 1]Now I’m going to make the assumption that to do a disco you need at least a pair of full range loudspeakers, bass bins are optional and I’ll ignore them for now. (You could try doing a disco with just a pair of bass bins but it would sound a bit odd). Seriously, many discos only need to use one pair of full range speakers, see fig 1 and fig 2, and if this is your choice whether you choose 2 or 3 way speakers I recommend that you use speakers which utilise at least a single 15 inch driver for the bass frequencies so you get a descent bass response. Good examples to name a few include Peavey Hi-Sys 2’s and Hi-Sys 5’s, JBL TR125 and EON 15, RCF ART500 and Event 3000’s and Celestion’s Road R1520. Now I’ll emphasise this is my opinion, based on my experience and that of a few other DJ’s I know, and I’m sure there will be those of you out there saying what about this and what about that. And yes I haven’t forgotten the infamous Bose 802’s, although to sound good you need 4 of them and they need the Bose system controller that boosts up the bass frequencies, try running them without it and you’ll see (or should that be hear). I spent a good few years running a pair of Peavey Hi-Sys 1’s, (12 inch driver + horn) and although they sounded good they were always lacking on the bass, a pair of Hi-Sys 115 bass bins sorted the problem out but this meant lugging four speakers to every gig to get what I thought was a descent sound. A fellow DJ purchased a second hand pair of JBL SR4722A’s, (again a 12 inch + horn) and he also found the bass response a bit disappointing. Reading the manufacturer’s specification on these speakers they recommended them for vocal PA use rather than full range PA use. [ Fig. 2]

Of course if you want full range speakers with loads of bass then you can go for speakers with twin 12 or 15 inch bass drivers, see fig. 2, such as the Peavey Hi-Sys 3’s or Hi-Sys 4’s, JBL TR225, SR4731A’s or SR4733A’s and Celestion’s Road R1522. One disadvantage with these speakers is their weight, unless you’re built like Arnold Shwartnegger, Arnold Shwaritsname, Sylvester Stallone they’re quite hard to transport. Having mentioned the bass response a full range loudspeaker must also have a good ability to project the mid range and higher frequencies in large rooms, a good compression driver mounted on a horn will do a good job. This will be critical if you intend to upgrade later on by adding bass bins to your PA system.

How to get the best from your PA speakers - it’s all about position, so they say.

As with everything there are good and bad ways to do things and speakers are particularly awkward sods in this area. For optimum performance loudspeakers need to be placed across the width of the room at one end facing down the length of the room as far apart as possible to give good stereo imaging but at an optimum distance off the ground and from the walls standing on as firm a base as possible to give the best possible solid reference point for sound quality and good bass response...

Meanwhile back on planet earth our DJ has turned up at the wedding which is running late so with 10 minutes to set up he gets stuffed in the corner of the room with the comment “Thought that’ll do ya mate” from some complete audio philistine. Reality hits, but while you’re throwing your kit around like a mad man possessed so you can set up in time you can also get your speakers well positioned without expending too much extra time and effort. To start with try to get a descent distance between them so you get good stereo effect, this is all dependant on how much space you have to work with and the size of the room etc. at least aim to have the speakers positioned either side of your show. To improve bass response the speakers need to be sat on as firm a base as possible, this means not on tables which vibrate or chairs with cushions etc. in direct contact with the floor is a better solution. However, sitting the speakers directly on the floor can lead to a new problem. Remember I said that speakers work by pushing air out to create sound waves, putting people and objects in the way interrupts this air movement. This has a dramatic effect on mid and high frequencies but not so much on bass frequencies which are more penetrating. You see, just to really piss us off the way high frequencies behave is completely different to bass frequencies. Your PA system with the speakers sat on the floor will sound great as long as the area in front of your set up is empty, but, we’re all brilliant DJ’s who have packed dance floors full of people all evening. Not a problem if you’re set up on a high stage, but how often do we get that luxury. Ever noticed when some drunken deaf bloke walks in front of your speakers that the higher frequencies get bounced back in your direction if you’re stood behind your disco? The effect for the audience towards the back of the room is a loss of clarity to your sound, like stuffing a sock in your speakers, so the sound becomes dull and muddy. Worse still with people dancing in and out from in front of your speakers the tone will continually change. As the dance floor fills up so the problem gets worse. You could try compensating by increasing the volume unfortunately this increases the risk of clipping the amplifier so introducing distortion, you also deafen the people directly in front of you. The best solution is to get your speakers up, no not back on the table, but on descent speaker stands. I can’t emphasise this enough, get ‘em up on stands, if you have really nice expensive full range speakers you’ll get more benefit out of them by going to the additional small expense of buying speaker stands. (Alternatively just play crap music that no one will dance to so you’ll have a clear dance floor all evening). The optimum height for your speakers is to have the tweeters or high frequency horn just above head height so the sound will travel well into the room instead of bouncing off the first person in front of your disco. Speaker stands also ensure the speakers remain sat in contact with a good firm base for a descent low frequency response. The benefit of this is that you don’t have to push the volume up so high to keep a descent sound level when the dance floor fills up. In fact speakers positioned this way can sound just as good as a system twice as powerful whose speakers are left lower down on the floor. The only trouble now is we’re back to the weight problem because you now have to lift the speakers up on to the stands. This will not be possible with the larger full range speakers like the ones shown in fig. 2 which often aren’t designed to be pole mounted, and certainly aren’t a normal one man lift as mentioned earlier.

[ Fig. 3]

So, just to summarise this sorry tale we’re looking for a PA speaker system that has a good frequency response for the full audio spectrum. Can produce copious amounts of bass and at the same time have good mid range and high frequency projection over peoples heads, whilst not being physically too heavy. One solution to get around all this is shown in fig. 3, adding bass bins to your PA system. With this configuration the bass bins are sat directly on the floor giving a good bass response and the top speakers are high up where they should be. This represents a good way to upgrade from the PA system shown in fig. 1 and adds extra flexibility to the size of functions you can perform at. However, I have deliberately avoided discussing bass bins fully in this article as I intend to cover this subject in a later part of this series where I will also cover the related topics of using passive and active crossovers, bi-amplification and balanced wiring.

Finally, “don’t take my word for it”, use your ears ! so you can decide for yourself what sounds good and what doesn’t. This goes for when you go out to buy loudspeakers and when setting them up to do a gig, always do a sound check. If you get an opportunity to turn up early for a booking so you can mess about with your sound system at a venue before your audience arrives it can pay dividends. Try your speakers in different positions, next to and away from the walls etc. to see what makes them sound the best that they possibly can. You only need to do this the once because from then onwards you’ll know how to get the best set up for your PA system.

And all I’ve done so far is discussed how best to position your speakers, we’ve still got to connect the buggers to an amplifier - the saga continues in the next part of this series. Until then it’s back to the funny farm for me and strong drinks all round for the rest of you.

Top Tip

Most faults on a mobile disco set up can be attributed to faulty wiring. So check your cables regularly and always carry a few spares

Copyright © 2001 Gerry Hayden.
Gerry's Website

 

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