02/05/2017
A Pro Audio Guide To Loudspeaker Management
Systems
The loudspeaker management system is what
slots between audio gears and loudspeakers to
ensure the sound system plays as sweetly as
possible. Nearly 99.9% of live sound audio
practitioners in the Nigeria audio industry
employs their usage.
While knowing to keep your levels out of the red
is great, it's not enough if you want to be fully
competent as a live sound audio Engineer. You
also need to understand a bit about what
happens after the signal leaves your mixing
console. In any venue with more than a couple of
loudspeakers, that is likely to involve a
"loudspeaker management system" (LMS). In
today's article, I'll give you all you need to know
to understand the LMS element of a PA/Sound
Reinforcement system.
This stuff is important because while sometimes
the client give you an audio engineer to take care
of it all for you, often they don't, and the
management may expect you to be able to do
some setting up (and fix any problems) with the
sound "post mixer". At the very least, it's worth
understanding what the LMS is doing, so you can
appreciate its importance, and evn decide if you
want to add one to your own gear as and when
it's time for you to buy a PA of your own.
What are loudspeaker management systems?
Loundspeaker management systems (LMS) serve
to manage multi-speaker systems so you can get
better performance out of your speaker layout.
When you as the mix engineer, you typically want
the loudest, cleanest sound you can get, in the
widest area you can manage out on the event.
Well-tuned multi-speaker systems sound better
because they're tasking specific speakers to do
specific jobs.
Essentially, LMSs are a collection of (formerly sold
separately) speaker management components.
The most basic system would include two
loudspeakers and a subwoofer using crossover,
limiter and EQ (I'll get to these individual things
later in the article). When you use more than two
loudspeakers (typically surrounding the dance
area, and essentially pointed at each other), you
may want to use a delay as well.
An LMS is likely to be a 19" rack-mounted
component in the same cabinet that houses the
amplifiers for the club's speaker system, and
takes the signal from the mixer before it goes to
the amplifiers and then speakers. Let's take a look
at its functions:
Main loundspeaker management system
functions
Crossover
This is the operation that directs bands of
frequencies to specific speakers. They can be two,
three or even four-way, indicating the number of
"splits". With a two-way crossover, you can direct
only the low-end of your music to the sub-woofer
(s), and everything above that to your main
loudspeakers; in the case of three and four-way
crossovers, your lows go to your sub, your mids
(low and high-mids in the case of four-way) to
your loudspeakers, and your highs only to your
tweeters (in or out of the loudspeaker cabinets, as
long as you have a separate input for each
speaker available).
When you do this, you'll direct the full power of
your amplifier to specific speakers thereby giving
you better-performing speakers, cleaner sound,
and more loudness (when done right, with all the
proper gear).
Limiter/compressor
This is the operation that regulates how much
power you can push to your speakers. This is used
if the amplifiers are capable of pushing the
speakers past the "clip" point (be driven to the
point of distortion). This is also important if you
have an upper-end volume (dB) limit for your
space, due to environmental regulations, for
instance.
Limiters / compressors are the reason why it's
just pointless to push your controller / mixer into
the red, because doing so would more than likely
engage the limiter / compressor in the LMS, so the
music doesn't get any louder, just less clear as the
limiter / compressor increasingly works to stop
you overloading the speakers.
Equalisation
This is the operation that decides how much of
each frequency gets sent to the loudspeakers. It's
the same thing as your EQs (low, mid, high) only
with a lot more control over the range. This is also
very important for balancing the "weight" of each
range of sounds. Remember that when you set
the EQs on your LMS, it's the maximum your
system will output, not an average.
Getting the EQ right is something the PA company
that sets up an installation rig will ideally spend
some time doing, and done properly, it should
mean you can play with his or her EQ "flat", not
having to compensate (for too much or too little
bass, for example) by always having an EQ in the
mixer or controller set somewhere away from
central.
Delay
This is the operation that times multi-speaker
systems to get the sound to the same location at
the same time. When you point two (or more)
speakers at each other, you're almost guaranteed
to get "phase cancellation", and that actually
makes your music quieter. When you properly set
a delay on the sound for each speaker, you'll get
convergence, which results in a wider coverage
area than your two-loudspeaker system, and sets
your sub-woofer to be in time with the rest of
your music.
Optional loudspeaker management system
functions
Auto-feedback suppression
This is the operation that actively cancels the
"squeal" you get when your microphones are
resonated by your speakers (bass and/or
loudspeaker). When this is turned on, your LMS
runs an active EQ process and quickly cancels
anything that spikes on the EQ curve. You'd want
to have this on whenever you have people
recklessly operating a microphone anywhere near
a loudspeaker, but it's typically not desired when
you don't.
Temperature and humidity compensation
This is an operation that extends low-end
frequencies. Humidity does introduce some
changes to the sound (because it's changing the
air density), and this is designed to correct that.
However, when your room temperature /
humidity goes up, that's typically a result of more
dancing bodies, which requires its own tuning
(one of the jobs of the audio engineer on the
night).
Subharmonic synthesis
This is another operation that extends low-end
frequencies, this time really deep ones. By
extending the low-end in this way, it's possible to
produce true tummy-churning sub-bass, but this
comes at the cost of power for the rest of the low-
end. You'll want to make sure your subwoofer can
produce these sounds in the first place if you're
going to engage this, because it won't do you any
good to extend the bass if the cabinets cannot
handle it.
What's NOT (normally) in the box?
Amplifiers
These are the physical boxes in your rack (or in
your powered speakers) that sends amplified
(increased wattage) sound to your speakers. They
take signal and power in, and the amplifier
increases the signal by the amount you've turned
the k**b. Without a limiter in-line, you're very
likely to do damage to your speakers if the
amplifier can output more than the speakers can
handle (refer to your spec. sheets for these
numbers).
In-speaker crossovers / limiters
This is a hardware feature in some speakers
which either balances the frequency range
automatically or limits how much output the
speaker will emit before self-limiting. Internally
crossovers and limiters are normally crude safety
features, but they're better than nothing. If you
use an LMS, it's best you disable these features
because they will complicate the tuning process.
Setting up an LMS
Usually this is done in clubs by whoever set up the
PA system, but if you want to assemble your own
PA system and add an LMS (and I thoroughly
recommend you do so), you're going to want to
know a bit about how to do it.
While it's too long a subject to cover completely in
this article, and your instructions with your LMS
will get you started, one thing to be aware of is
the extra gear you need for tuning the delay
properly. In order to use digital delay in your LMS,
you need an "RTA mic" (it's a specially designed
microphone for this purpose), a mic stand, and
enough XLR cable to reach the the microphone in
its testing location.
Another thing to bear in mind is that most LMSs
have profiles you can save and restore at another
time, as well as auto-restore when the power is
turned off and restored, so make sure you save
your settings when you've got them right, or you'll
lose all your hard work! Finally, do make sure to
get an LMS that suits your needs, especially when
it comes to number of speakers. For instance, if
you have four loudspeakers and a sub-woofer,
make sure you buy a unit that has one XLR output
per speaker so you can include all your gear
under the LMS's control.
FINALLY...
I started this article by saying that "while knowing
to keep your levels out of the red is great, it's not
enough if you want to be fully competent as a live
sound audio engineer. The reason I said that is
that really, if you're serious, you do need to at
least understand what's going on at all stages of
the process with your sound, even if you don't
have the experience to make adjustments in
things like the LMS.
Hopefully now you do at least have some
appreciation of the work that goes into making
your sound great when you're engineering for live
sound in any half-decent venue... and also the
importance of (back to the beginning again)
always engineering out of the red!
Has this article helped you? Do you have anything
to add to the information here? Please feel free
to let us know in the comments.
Written by Engr Stephen Aluko
