|
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]](graphics/fig31.gif) 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]](graphics/fig32.gif)
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]](graphics/fig33.gif)
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 |
