Line Arrays and Subwoofers

01Point source vs line source

A single speaker box spreads its sound in a sphere. That is why it dies off so fast with distance. A tall row of drivers spreads sound differently, and it holds its level much further. This one idea is the reason line arrays exist.

Why the line source loses less

A point source sends energy out in every direction, so each doubling of distance spreads the same energy over four times the area: −6 dB. A tall line of drivers that work together pushes the sound out sideways and not up and down. The energy spreads over twice the area, not four times: −3 dB.

The catch: the line only behaves like a line in its near field. Past a certain distance it turns back into a point source and loses 6 dB per doubling again. For a small array that distance is not far, so the real gain is a few dB, not a miracle. The gain is real, and it is cheap, because the sound lands on the audience and not on the ceiling or the floor.

What "throw" means

Throw is how far a speaker can send usable sound before it is too quiet or too muddy. A long throw means the back row hears nearly what the front row hears. Line arrays are the long-throw tool.

02What a line array is

A line array is a vertical stack of identical speaker elements, hung or stacked so close together that their sound combines into one tall source. The elements are not spread out. They touch, and that is what lets them couple.

The three things that define it

• Identical elements, tight spacing. Close enough that the sound of neighbouring boxes adds up and does not fight.
• Narrow vertical, wide horizontal. The stack shapes the sound into a wedge: tall and thin when seen from the side, wide when seen from above. It lands on people and misses ceilings and floors.
• Splay angle. The small angle between one element and the next. It is set with pins or a bar on the hardware. More splay widens the vertical coverage and lowers the level. Less splay narrows it and throws further.

Why small arrays suit small rooms

You do not need ten boxes. Two to four small elements already give a tidy wedge for a hall of 100 to 300 people. In a low room the benefit is simple: less sound hits the ceiling, so less echo and less feedback.

03Column arrays

A column array is the small-event version of a line array: a sub on the floor and a thin vertical column of small drivers on top. One person can set it up in minutes. For speeches, corporate functions and a DJ for a modest crowd, it is often the right answer.

Why they are tidy

• Even coverage. People near the column and people at the back hear a similar level.
• Less spill to the walls. The beam is narrow vertically, so less goes up to the ceiling.
• Small footprint. The column is slim and sits on the sub. Nothing blocks the audience's view.

Their limits

Be honest with the client and with yourself. A column array is not very loud. It is not a dance-floor system for 500 people. A short column has a short near field, so it cannot throw far. Sound check shows you the limit: if the front is comfortable and the back is quiet, the room is too big for it.

04True line-array elements

A real line-array element is bigger than a column driver. A typical one is a pair of 8" woofers and a horn in one angled box, built so that it stacks with its neighbours at a set angle. It is much louder than a column and throws much further.

Ground stack or flown

These elements are used two ways. Flown means hung from a truss or a motor. Ground-stacked means built up from the floor, usually two to four elements high on a sub. At a small event the crew ground-stack. Ground stacks are shorter, so they cover a smaller area, but they are safer to build and cheaper to run.

The J curve

The splay between elements is set with pins. Top elements: almost no splay. Lower elements: more. That gives the J shape. The result: every row of seats gets roughly the same level, because each slice of audience has its own part of the array aimed at it.

05Subwoofers

A subwoofer ("sub") handles the lowest part of the sound: kick drum, bass guitar, the thump you feel in the chest. It plays below about 100 Hz. Tops play everything above. The point where one hands over to the other is the crossover point.

Crossover points

Most small PAs cross over between 80 and 120 Hz. Lower crossover: subs do less, tops do more. Higher crossover: subs do more, and they start to be heard as a source in the room. If a powered sub has a switch or a preset, use it. Do not invent your own number.

Why bass goes everywhere

A low note has a long wavelength. A 60 Hz note is about 5.7 m long from peak to peak. A sub box is much smaller than that, so it cannot aim its sound. Bass spreads out in all directions, behind the box too. That is why you can feel bass on stage, in the next room, and in the car park.

Why subs sit on the floor

That free gain is called boundary gain. A sub on the floor gains up to 6 dB. A sub against a wall gains more. A sub in a corner gains the most, but the bass can turn boomy and uneven. A sub on a stand in free air gains nothing.

In the B&W kit: JBL single-8" and double-8" powered subs.

06Two subs: together or apart

This is the lesson that separates crew who know what they are doing from crew who just carry boxes. Where you put two subs changes the sound more than which subs you buy.

Together: coupling

Put two subs side by side, touching. They act like one bigger sub. Their sound waves line up and add: twice the sound pressure, which is +6 dB, for the same amplifier on each box. No cancellation. The pattern is smooth across the room. This is the default for a small PA.

Apart: power alley and comb filtering

Put the same two subs 6 m apart (one each side of the stage). Standing in the exact middle, both are the same distance away, so they add: +6 dB. A "power alley" down the centre. But move sideways and one sub gets closer than the other. At some spots the bass from one arrives half a wavelength late and cancels the other. That is a dead zone. Further out, it adds again. The result is a series of loud and dead strips across the room. This is comb filtering.

In the B&W kit: stack the JBL subs side by side, touching, and treat them as one big sub.

07Cardioid sub setups

Bass goes everywhere, including onto the stage. That causes muddy sound for the band, feedback in vocal mics and noise in the building behind. A cardioid sub setup uses two or more subs and a delay to steer bass forward and cancel it behind. The name comes from the heart-shaped pattern.

Two ways to do it

What you get

• Less bass on stage. Performers hear themselves instead of boom.
• Less feedback into mics. Quiet low end behind the subs means fewer problems with open mics.
• Tighter sound. Less bass bouncing off the back wall and returning.

08Setting up and tuning

A sub that is not lined up with the tops can make the system sound worse than tops alone. Tuning takes ten minutes. Do it every time the subs move.

Aligning subs with tops

1. Pre-set both tops and subs to the manufacturer's recommended crossover. Set subs and tops at a safe starting level.
2. Play music with a steady bass line, or a test tone around the crossover frequency. Stand where the audience will be.
3. Walk slowly toward the stage. Listen in the crossover region, around 80 to 120 Hz, where the subs and tops both play.
4. Flip the polarity switch on the subs. One position is clearly fuller. The other sounds thin, as if a chunk is missing.
5. Leave it on the louder, fuller position. If neither is clear, ask for a technician to set a delay.

Tops and subs: the rules

• High-pass the tops when subs are present. Around 80 to 100 Hz. The tops stop wasting power on bass, and they get louder and cleaner.
• Aux-fed subs get their own mix on an aux send, so you choose what goes to the subs. A sub that gets the vocal mic gets boom. Full-range feeding sends the whole mix to the sub, and the sub's own crossover does the filtering. Fine for a DJ; for live speech use the aux.
• Limiter and DSP presets. Many powered speakers have a built-in limiter and a preset menu. Leave the limiter on. It protects the drivers.

Preset names vary between makers. Read the one-page quick guide for the box in front of you.

The listening routine

1. Pink noise at low level. Walk the room. It should sound the same everywhere. Listen for spots that go thin.
2. A familiar track you know well, with real bass. Does the kick punch and the bass line stay clear?
3. A voice. Speak into the mic from the podium. Words should be clear and the feedback should hold at working level.

09Rigging and safety for ground stacks

Speakers fall on people. A stacked array is heavy and tall. The risk is highest in wind, on soft ground, and in the last ten minutes of a long day. Ground-stacking is crew work. It still has rules.

Ground-stack checklist

1. Is the ground flat and firm?
2. Is the stack within the maker's rated height and weight?
3. Are all pins, plates or bars fitted and locked?
4. Are the straps tight, and tied off to something solid?
5. Is the stack out of the public's reach, with a barrier if needed?
6. Has the wind been checked, and is there a plan if it rises?
7. Are cables relieved, routed and covered?
8. Are two people on hand to lift and lower? Never one.