Acoustic Sound Reinforcement Solution
~ Minimal but high quality ~
Copyright ©1999 Howard Coleman
I am writing this to help anyone wanting a faithful amplified sound without being sold loads of unnecessary equipment. The last thing I want is for this page to say - "Hey, look at my cool PA rig!" While in some areas I recommend you spend more (eg microphones) in other areas I recommend you spend less (eg speaker cables and gadgets). I must stress that what I have written applies to live acoustic vocal and instrumental music. It does not necessarily apply to recorded music or rock music.
The Problem
In putting together my sound reinforcement system I was not
prepared to compromise on the naturalness of the sound. I wanted
it to be unobtrusive - completly transparent if possible. I
realised I had to use good quality equipment throughout since any
weak link in the chain would be noticeable. It sounded at first
like it was going to be expensive but I was able to make a number
of savings by not buying anything that wasn't vital. So no
anti-feedback device, no compressor, no monitors, no pick-ups, no
DI boxes and no crossovers.
Use
The main use for my PA at the moment is for
"Live Acoustic Specials" in the large function room of
a pub in Loughborough. This gig involves setting up for about
eight different acts from solo guitarists to a jazz quintet.
Unlike most pub gigs the audience goes to listen. The quality of
the PA is therefore not wasted.
Microphones
I use two Crown
CM-700 microphones. They are superb on paper and in action. Their
frequency response is particularly good without too much
"presence boost". The bottom end has three positions
for bass roll off. What I think is very important for close
miking guitars is that their frequency response stays flat off
axis. This means that sound invariably coming in from the sides
is not coloured as it is with other microphones. No amount of
equalisation can cure a mic that sounds nasal or hollow off axis.
Another important point for live work is that the polar response
pinches in at the sides more than it does with a number of other
cardioid microphones making this mic more directional and
therefore less susceptible to feedback. This means that I can
move the microphones further forward into the direct sound field
(so that the musicians can hear themselves) without feedback
occurring which, in turn, saves money as monitors are not needed.
It took a lot of reading, listening and agonising to realise that
the safe bet of a Shure SM58 would not be good enough for vocals.
I feel that this and similar microphones are essentially rock
mics. They are great for getting the vocalist heard over a band.
The way this is achieved is with a presence peak that, to me,
sounds unnatural. (Listen to the "S"s.) For the
acoustic gig I described above, the vocals are no less important
than the instruments so I jusitified buying a second Crown
CM-700.
I also use a Shure Prologue
16L which I used to rave about before I bought the Crowns. It's
great value for money but is outclassed in all respects. The EQ
on this microphone's channel needs a lot of adjustment as the
microphone's frequency response slopes steadily upwards from bass
to treble.
I use foam pop filters on all the microphones to prevent nasty
collisions with guitars.
Powered Mixer
I use a Peavey
XR 560 which gives uncoloured, clear monophonic sound. The
amplifier power (150W) is plenty for the use I put it to. A
particularly useful feature is the Feedback Locating System which
I use, in conjunction with the 7-band graphic EQ, to reduce the
gain on the frequency bands that might otherwise cause feedback.
Those accustomed to using more bands with finer bandwidth
(frequency range of each band) might be surprised to see that
this is adequate. In fact I would go further and say that its
simplicity is a great advantage as I am more likely to alter the
settings than someone with a more sophisticated setup. I prefer
to adjust the 7-band EQ when setting the equipment up. I do this
simply by turning up the gain until feedback starts, then cutting
the gain of the offending band. I then repeat the process to find
and cut the second offending band. Repeating again usually brings
me back to the first band again. Typically, I end up with two
bands (500 and 2000kHz) cut by only 3dB. Being a bit of a purist
with sound reinforcement I only do what I feel is absolutely
necessary to adjust the EQ as it detracts from the naturalness of
the sound. I regard this as equalising the frequency bands that
are being over-emphasised by the room. I could split hairs and
say that I don't have sufficient control over the EQ to assure
that, but to me the proof of the pudding is in the eating. It
passes the listening test and there have been no complaints so
far. In theory, this tailoring of the frequency response for the
room will need changing with additional people filling the room,
with the temperature changing and with movement of the
microphones. However, in practice I have found these not to be a
concern - possibly because of the broadness of the bands of the
7-band EQ. (Another advantage of simplicity!)
Speakers 
(Click to enlarge.)
I made the loudspeakers myself. They are based on
the WEM Songbird and use single 10" WEM Impact drivers and
Motorola piezo tweeters. No crossovers are needed with these
tweeters which is a good thing as they are often a souce of sound
problems. I got advice and followed plans (1 x 10 + horn
reproducer) from PA guru Charlie Watkins of Watkins Electric Music,
London (Tel/fax: 0181 679 5575) and bought acoustic lenses from
him to make decent copies of his Songbird and to deflect the high
frequencies (tweets?) downwards away from the ceiling. This
allows the walls (which are usually further away) to supply the
earliest reflections, which increases the spaciousness of the
sound (apparent spaciousness of the room). Any reflections from
the ceiling would arrive at both ears of a listener at the same
time. If they arrive from the walls they are more likely to
arrive at the ears at different times as the walls are probably
different distances from the listener. This adds incoherence
which the brain also interprets as spaciousness.
I used MDF instead of chipboard as I could not find chipboard of
sufficiently good quality. The sound from these speakers is
unmatched by anything else I have heard (modest!) and there is
plenty of power handling capacity to spare. One thing Charlie
recommends is to make speakers, or reproducers as he calls them,
look professional for use on the stage. I did this by covering
them with leathercloth, adding plastic corner pieces, extrusions
to the edges and by putting my "C" logo on the front,
cut from brushed aluminium. I mount the speakers on stands which
I aquired second-hand.
Cables and connectors
Again I use partly expensive stuff and partly some cost-effective
alternatives. I use professional quality microphone cables and
connectors - no room for economising there as I want the best in
rejection of spurious noise (mains hum, taxis etc) and I want
reliability. The mic cables are mostly Europa oxygen-free copper
balanced leads and the connectors are mostly Neutrik, some
Switchcraft. I used heat-shrink tubing when fitting the
connectors to give some strain relief at these points.
I could have spent a lot on speaker cables but knew from
experience of a friend's PA system that I didn't need anything
more glorious than 'figure 8' shape hi-fi speaker cable with 2x79
strands of 0.2mm copper (overall size 6x3mm). The expense of
oxygen-free or silver plated copper would not have been justified
for my purpose. I have tested the system by playing familar
recorded music from a CD through it at high volume as well as low
and can honestly say that there is nothing lacking at all. I use
Neutrik jack plugs at the amp end and Neutrik Speakon
connectors at the speaker end. The Speakons are good as the
sockets are air tight in the speaker cabinets. I think XLR
connectors make equally good contact but are not air tight.
Pick-ups
Ha! I don't use any! I am an acoustic purist. For
me a "good sound" is not good enough. I want a faithful
sound. I want to hear the instrument as a whole interacting with
the air - not a simulation of it even if they are fairly similar.
Recommended reading
I found virtually nothing on making PA speakers
on the internet. Most of the advice given about amplifying the
acoustic guitar is biased towards built-in pick-ups and achieving
a "good" or "useful" sound (inkeeping with
the current uplugged trend) rather than a faithful sound. I am
therefore recommending books on the subject as the more
background knowledge you have the more able you will be to judge
which way to go.
Last revised: May 22, 2003.