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 performing DJ. You also need to understand a bit about what happens after the signal leaves your controller (or the club's mixer). In any venue with more than a couple of speakers, 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 system.
This stuff is important because while sometimes clubs 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 even 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?
Loudspeaker management systems (LMSs) serve to manage multi-speaker systems so you can get better performance out of your speaker layout. When you DJ, you typically want the loudest, cleanest sound you can get, in the widest area you can manage out on the dancefloor. 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
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).
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.
This is the operation that decides how much of each frequency gets sent to the speakers. It's the same thing as your DJ 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 the DJ 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.
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
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).
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?
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 knob. 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.
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 performing DJ". 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 you sound great when you DJ in any half-decent venue... and also the importance of (back to the beginning again) always DJing out of the red!