Sound Outdoors

Outdoors there is almost nothing to send sound back at you. The wave leaves the stacks, spreads, and keeps going until it runs out of energy, and that condition — no reflecting boundaries worth counting — is what acousticians call a free field. It is the simplest acoustic environment there is, and the least forgiving.

Bands treat an outdoor gig as the easy version of a club: no boomy corner, no ringing room, nothing to fight. Half of that is true. The other half is that a room does real work for you, holding the level up at the back, and outside nothing does that work.

Free Field Means Nothing Comes Back

Indoors, what reaches your ears is the direct sound plus hundreds of reflections arriving milliseconds later, and past a few metres those reflections are most of what you hear. Outdoors the direct sound arrives once and there is almost nothing behind it.

Real sites only approximate that. The ground is a reflector, so you almost always get one strong bounce, and grass, dirt and asphalt each absorb it differently. A wall behind the audience, a hillside, the back of the stage — each puts a reflection into a picture the arithmetic below assumes is empty.

Six Decibels Per Doubling, and It Compounds Fast

In a free field the direct sound from a point source drops 6 dB every time you double your distance from it. The cause is geometry: the same energy spreads over a sphere, doubling the radius quadruples the area that energy has to cover, so any fixed patch of it collects a quarter as much power, which is a 6 dB drop. You already know the price of 6 dB — half the sound pressure, four times the amplifier power.

Walk it out. Say the PA measures 105 dB SPL at 5 m, out at the front rail. At 10 m you have 99 dB, at 20 m 93, at 40 m 87, and at 80 m (260 ft) you have 81. Four doublings and 24 dB gone, across a field most bands would call modest. Going by the rule that ten decibels is roughly half the loudness, the back of that field hears under a quarter of what the front row gets.

Recovering 24 dB takes about 250 times the power, which is why outdoor rigs dwarf the club systems that reach the same apparent level. Indoors the reverberant field props the level up past a few metres, so the back of a bar is nowhere near 24 dB down. Outdoors nothing props it up. Your options are more power, delay speakers partway down the field, or boxes that are not point sources — a line array loses closer to 3 dB per doubling through its near field, which is most of why festivals hang them.

The Air Itself Is a Low-Pass Filter

Distance is not the only thing taking level off. Air absorbs sound as it travels, converting it to heat through molecular relaxation in oxygen and nitrogen, and the effect depends heavily on frequency: negligible at the bottom, brutal at the top.

At 20°C and 30% relative humidity, 250Hz loses about 0.14 dB over 100 m, which rounds to nothing. Over that same 100 m, 4kHz loses roughly 5 dB and 8kHz roughly 17. On the field above, the listener at the back is down a further 4 dB at 4kHz and 13 dB at 8kHz, on top of the 24 dB the distance took. A mix that reads right at the desk sounds blanketed at the fence: the top was filtered off the whole way out and the bottom was not.

The humidity part gets stated backwards constantly. The intuition is that muggy air is thick and soaks up the highs; the physics runs the other way, and dry air absorbs high frequencies far more than damp air does. At 20°C, 4kHz loses about 11 dB per 100 m at 10% relative humidity and only about 2 dB at 80%, because water vapour speeds up the relaxation behind the loss and pushes the worst of the absorption above the range you care about. It is not perfectly tidy — at 8kHz absorption peaks near 20% humidity rather than at the driest end — but the rule holds. The drier the day, the duller the far field.

Wind Doesn't Carry Sound. It Bends It.

Wind is the effect people describe worst, because "the wind carried it off" is not what happens. Sound moves at about 343 m/s at 20°C, and a brisk 13.4 m/s wind (30 mph) blowing straight downfield makes that 356 m/s. Under four percent. On its own it would be inaudible.

What matters is that wind speed changes with height. Friction slows the air near the ground, so sound heading downwind travels faster at the top of its wavefront than at the bottom. A wavefront whose top outruns its bottom tips over, and the ray bends toward the slower air — downward, back to the ground. Go upwind and the gradient reverses: the top is held back, the ray bends up, and past a certain distance there is an acoustic shadow. That is why an upwind audience sounds shortchanged while a downwind neighbour two fields away is dialling the noise line.

Temperature bends sound the same way, since sound moves faster through warm air than cold. On a sunny afternoon the ground is hot and the air cools with height, so the upper air is slower, rays bend upward and the far field thins. After sunset the ground sheds its heat and the layer above it can end up warmer than the air at head height — a temperature inversion. The fast air is now on top, rays bend back down, and the show reaches much further than it did in daylight.

Feedback Outdoors Is Pure Geometry

Indoors, feedback is partly the room's decision. The reverberant field puts PA sound into every microphone wherever you place it, and the frequency that rings is usually a peak in the combined response of room and system. Outdoors there is no reverberant field to do that. What arrives at a mic from the PA is the straight-line path plus whatever the ground throws up, and both of those are decided by where you put things — which turns gain before feedback into a question of distance and aiming.

Push a vocal channel up during soundcheck and the wedge eventually starts singing on its own. A microphone cannot tell a singer from a speaker; it adds up whatever pressure reaches the diaphragm. Once the PA's share of that arrives as strongly as the singer's, each trip round the loop is a full-size copy of the last one, and the note builds instead of dying away. Everything that buys you room is a matter of where things stand and where they point:

• Source to mic — halve the distance and you gain 6 dB, which is the whole argument for singers working the mic close.
• Speaker to mic — double the distance and you gain another 6 dB, which is why mains get flown, pushed downstage and angled away from the vocal position.
• Directivity — aim a directional box so the mics fall outside its coverage, and a cardioid mic so the boxes sit behind it. Reckon on about 6 dB in practice: both drift toward omnidirectional as frequency falls, and a 40Hz wave is 8.6 m long with no cabinet anywhere near that size.
• Open mics — every doubling of live channels costs 3 dB, so eight open mics have eaten 9 dB before anyone sings a note.

Do not park the system at that edge. Nothing is howling just underneath it, but the notes that almost survive a lap hang in the air a beat too long, and that is the hollow, tubey quality on a vocal that engineers chase around an EQ all night and never catch. Come down about 6 dB. You get the voice back, and you get somewhere to put the chorus when the band turns up louder than they did in soundcheck.

Working an Outdoor Stage

• Budget the back of the field, not the front — the level at the far edge decides whether the rig is big enough, and it is not the level you hear at the desk.
• Stop trying to EQ the top back at the console — boosting 8kHz makes the front rows harsh long before it brightens the fence. Delay speakers are the fix.
• Check the humidity, not just the rain — a dry, hot day dulls the far field noticeably more than a damp one.
• Find out who is downwind — the complaint call comes from whichever neighbourhood the gradient is aiming at, which may be nowhere near your audience.
• Expect the level to change after dark — an evening inversion pushes the same rig further than it reached in daylight, so re-check the perimeter once the sun is down.
• Move mics and boxes before you reach for a filter — outdoors the feedback path is a straight line, so a metre of repositioning buys more than an EQ will.
• Mute channels you are not using — with no room to blame, open-mic count is one of the few feedback variables entirely in your hands.