The EM Poll

P.A. 101

Mar 14, 2008 3:24 PM, By Emile Menasche

           

HOW TO CHOOSE THE RIGHT SOUND SYSTEM FOR YOUR NEEDS WITHOUT TEARS

Divide and Conquer

In many club P.A. systems, each speaker cabinet includes both a low-frequency driver (the woofer) and a mid/high-frequency driver (a horn or tweeter), and you simply connect the output from your power amp to one jack on the speaker cabinet. Inside the speaker cabinet is a circuit called a crossover, which uses filters to divide the signal according to frequency, sending the portion of the signal that is above the split point (called the crossover point) to the high-frequency driver and routing the portion below the crossover point to the low-frequency driver. In most cases, the crossover is a passive circuit, and the crossover point is preset at the factory to match the speakers and the cabinet's acoustics. Such a system is easy to use, but it does not offer much flexibility, and it usually is not intended for use in larger, more sophisticated sound systems.

If you want to be able to adjust the crossover to suit your tastes-tweak the crossover point or slope, for instance-and especially if you want to use separate bass and high-frequency cabinets, you need an external crossover. In many cases, these crossovers are active (powered) units, and their sound quality is generally superior to that of most passive crossovers. If you go this route, you need to separately power the low- and high-frequency drivers, which is called biamping.

With biamping, you use an outboard crossover to split the signal before it reaches the power amp, and you feed the high- and low-frequency information to separate channels or amplifiers. If you are using separate bass and mid/high cabinets, you have to biamp, but biamping also is a way to get more efficiency from two-way speaker cabinets that allow you to power their bass and high-frequency drivers separately, bypassing the internal crossover.

If you want the ultimate in sound-system efficiency and accuracy-and are willing to haul a lot more gear-you can use a triamped system. With triamping, you assemble a three-way system comprising (for each side) a bass bin, midrange cabinet, high-frequency cabinet, and three-way external crossover. The principle is the same as with biamping: each cabinet is driven by a separate power amp channel. There are a number of ways to amplify a triamped system; for instance, you could run three stereo power amps in dual mono-one each for the left/right bass bins, midrange cabinets, and high-frequency cabinets. Alternatively, you could dedicate a power amp to each bass bin, running in bridged mode, which is a more efficient way to power those big cabinets but requires a fourth power amp. You could even dedicate an amp to each of the six cabinets (the left and right lows, mids, and highs) if necessary.

Speak to Me

Speakers deliver the sound to your audience, so they're among the most vital components in your system. In the bad old days, speaker horns were big, and bass cabinets were bulky and weighed a ton. Can anyone say hernia? Fortunately, speaker design has come a long way, and today's cabinets are lighter, easier to position, and have wider frequency responses than ever before.

Typical stage speakers are two-way enclosures, with separate low- and high-frequency drivers (in contrast to the typical guitar speaker, which has a single full-range driver with limited frequency response). A speaker enclosure is like a self-contained universe: the size and type of the drivers influence its sound, but so do environmental factors such as the size and makeup of the cabinet. As noted in the discussion of biamping, you could use separate low- and high-frequency cabinets, but this would mean more hauling and wiring, and it is not as popular an approach for club sound as it once was.

When choosing speakers for your amps, remember to factor in power handling, as discussed earlier. It is also important to make sure the impedance of the speaker matches the output impedance of the amplifier. If you're planning to run more than one speaker on each amplifier channel, make sure the total impedance of all the speakers matches the amp's impedance. Otherwise, you're asking for trouble in the form of distorted sound and/or damage to the amp and speakers.

Thanks to the miracle of modern design, loudspeakers have become much easier to position on stage. You can mount most small to midsize enclosures on a stand, which allows you to raise them above the stage, giving the sound waves a clearer path to the back of the room. Although speaker stands aren't absolutely necessary, they're a great investment. Conventional wisdom says to position the house speakers on either side of the stage, slightly in front of the performers and their microphones. If you position your speaker behind a microphone, you'll be in for a long night of feedback.

With the speakers positioned in front of the musicians, though, how can the members of the band hear themselves and each other? Stage monitors-also known as wedges-are designed to project sound back to the performers without interfering with the mix heard in the house. (Be forewarned that if you turn the monitors up too loud in a small venue, they will interfere with the sound out front.) The typical stage monitor sits on the floor and points up at the performer. Other monitoring options, such as in-ear wireless monitors, are becoming more affordable for working musicians. (For a review of in-ear monitors, see "Stick It in Your Ear" in the April 2000 issue of Onstage.)

In most cases, you'll want to feed the stage monitors a different mix than the one heard in the house. The number of separate monitor mixes you can create depends on two factors: the number of sends in your board (each send can create a separate mix), and the number of available channels of power-you'll need a separate power source to deliver each mix to its respective monitor speakers.

No matter what kind of speakers you use, connect them to your power amps with the best speaker cable you can afford, and never use instrument cables. Good cables make an audible difference in the sound and can also save headaches at the gig; there's nothing more frustrating than being sunk by a bad cable. Cable that's too light a gauge for the task at hand restricts the signal flow, causing significant power loss and potentially damaging your system. The longer the cable run, the more power you lose, so in choosing the right cable, you have to consider the amount of power being handled, the impedance of the load (that is, the speakers), and the length of the cable run.

In general, you should use at least 16 AWG (American wire gauge) speaker cable, but that is pretty light. In most cases, 14 AWG cable is a good choice for carrying moderate power levels over relatively short cable runs (say, 25 feet or so). But you will see a significant performance increase if you go to 12 gauge cable or heavier, especially if you have 100-foot runs or need to push a couple of hundred watts down the line. Large, high-power rigs often use 10 gauge or even heavier cable, and if you want to go that way, fine, but you probably won't need that for the average club P.A. In contrast, instrument cables are usually between 26 and 22 gauge, which is clearly inadequate for handling speaker-level signals. Unlike instrument cables, speaker cables do not need to be shielded.

Powered speakers are another option. Although generally used as monitors, they sometimes-especially in very small systems-can be used as mains.