Grow Room Ducting and Noise: How to Get Full Airflow and a Quiet Tent

Two complaints come up more than any others once a tent is running: it is louder than expected, and it does not shift as much air as the box promised. They are the same problem wearing two hats, and the fan is rarely at fault. The number printed on an extractor is measured with nothing attached to it, and almost everything you hear is made by air moving fast through an awkward path rather than by the motor. This guide covers what your duct run actually costs you in airflow, which ducting to use where, how to fit it so it does not leak or rattle, and the order to work through when you want the room quiet.

Why your fan never delivers its rated airflow

Extractor fans are advertised with a single headline figure — 200 m³/h, 400 m³/h, 800 m³/h. That number is a free-air rating: the fan spinning with nothing attached to either end, pushing against no resistance at all. It is a fair way to compare two fans against each other. It is not a prediction of what will come out of your tent.

The moment you bolt a carbon filter to the inlet and a length of duct to the outlet, the fan has to work against static pressure — the resistance of everything the air has to squeeze through. Every fan has a curve describing this: as resistance rises, delivered airflow falls, and past a certain point it falls away sharply. Your filter, your duct, your bends and the tent itself all sit somewhere on that curve, and where you land is decided by how you built the run rather than by which fan you bought.

This is why two people with identical fans get completely different results. One has a filter, a taut two-metre duct and a single gentle bend. The other has the same fan, the same filter, six metres of slack duct looped round a shelf, two sharp right angles and a step down to a smaller diameter to fit through a window board. The second one is not broken. It is doing exactly what the physics says it will.

What each part of the run costs you

You cannot calculate your exact system resistance without manufacturer pressure-drop data and a fan curve, and for a single tent that is overkill. What you can do is know the running order — which parts of the run are expensive and which are cheap — so that when you are short of airflow you go after the big items first.

The practical conclusion is the same one our fan and filter sizing guide reaches from the other direction: buy a fan with headroom and run it slowly. A larger fan at 60% is quieter, cooler, longer-lived and still ahead of a smaller fan screaming at 100% against the same filter.

Which ducting to use

All flexible ducting carries air. What separates the types is how much noise passes through the wall, how well it holds its shape, and how much abuse it survives. Airflow-wise there is far less between them than the price difference suggests — what you are mostly buying, as you go up the range, is quiet.

Bare aluminium

The cheapest and lightest option, and perfectly good air-wise if you keep it taut. Two caveats: the thin wall does almost nothing to stop noise passing through it, so a long run of aluminium duct across a loft becomes a long thin loudspeaker; and it crushes and tears easily, so it does not enjoy being handled. Ideal for short outlet stubs and for runs somewhere nobody listens.

Combi ducting

Aluminium bonded to a tougher outer layer. It holds its shape better, survives being moved, and the black version is light-proof — which matters when a duct passes through a lit room and into a tent that needs a genuine dark period. A sensible default for most tents.

Acoustic ducting

Two skins with an insulating layer between them. It cuts the noise radiating out through the duct wall noticeably, and the insulation reduces heat transfer as a side benefit — useful if the duct runs through a room you are trying to keep cool. It is bulkier and stiffer, so it needs a more generous bend radius than the bare stuff.

Heavy acoustic ducting

The top of the range, using a much heavier multi-layer construction. This is what you fit when the run passes a bedroom wall and everything else has already been done. It is expensive and stiff, and it is worth it in exactly one situation: when noise through the duct wall is the remaining problem.

Fitting it so it does not leak, sag or rattle

A duct run assembled carelessly loses air at every joint, and a leak on the outlet side of a carbon filter puts unfiltered air straight back into the room. None of this is difficult; it just has to actually be done.

1. Pull every length taut before you cut it. Flexible duct is delivered compressed and it is tempting to leave it that way. Slack, concertinaed duct is the single most common airflow fault in a home tent, and stretching it costs nothing.
2. Clamp every joint. A jubilee clip round the duct where it meets a spigot is the standard fixing and holds indefinitely. For joints you open regularly — filter to fan, fan to silencer — a padded fast clamp is quicker and seals better on flanged fittings.
3. Then tape the joint. Use foil or metal-weave tape, not cloth "duck" tape. Cloth tape dries out in a warm room, the adhesive lets go, and it usually does so in the loft in the middle of a cycle.
4. Support long horizontal runs. An unsupported duct sags into a U, and a U is a bend you did not plan plus a place for condensate to collect. A loop of wire or a strap every metre or so is enough.
5. Use a wall plate where the duct passes through a wall or window board. It gives the duct a rigid spigot to clamp to, stops the edge of the hole chafing through the duct, and means the run is not hanging off the aperture.
6. Never hang the fan on the ducting. The duct is not a structural member. The fan hangs from the frame or the ceiling on its own straps, and the duct simply connects to it.

Browse fans, filters & environment

Where the noise actually comes from

"The fan is loud" is a diagnosis that hides three completely separate problems with three completely separate fixes. Working out which one you have takes about a minute and saves you buying the wrong thing.

Aerodynamic noise — the hiss

This is air being dragged through the impeller and shoved down a duct: broadband rushing, hissing and whistling. It is the noise most people are actually complaining about, and the important property is that it rises very steeply with air speed — far more steeply than the airflow itself does. That single fact is why slowing a fan down is such a good deal, and it is covered in its own section below.

Mechanical noise — the hum

Bearing whine, motor hum and blade tone: narrower, more tonal, and it does not change much when you dim the fan. If your fan is quiet in the hand but the room is loud, this is not your problem. If it is unpleasant even at low speed, it is the fan itself, and the answer is either to enclose it or replace it.

Structure-borne vibration — the one you feel

The fan is a spinning mass. Bolt it to a tent frame, sit it on a shelf, or stand it on a floor, and that vibration is conducted into the building and re-radiated somewhere else entirely — often a room away, often at night, and often by a neighbour. Two things are worth knowing about it: a duct silencer does nothing for it, because the noise is not travelling in the air; and it is usually the cheapest of the three to fix, because breaking the contact costs nothing.

The quieting ladder, cheapest first

Work down this list in order. People routinely buy a silencer first and are disappointed, because the problem was a fan resting on a chipboard shelf, and no silencer ever built will help with that.

1. Hang the fan so it touches nothing rigid. Bungees, rubber straps or rope from the tent's top bars, with the fan floating in mid-air and the ducting flexible enough not to transmit vibration into whatever it connects to. Free, takes five minutes, and is frequently the single biggest improvement available.
2. Pull the duct taut, shorten it, and lose a bend. Also free. Attacks the airflow problem and the aerodynamic noise problem simultaneously.
3. Slow the fan down. Cheap, immediate, and the subject of the next section.
4. Fit a duct silencer. A lined cylinder that goes inline — most usefully on the fan's outlet, and on the inlet as well if the tent-side noise is the issue. It absorbs airborne noise travelling down the duct and it costs you a little airflow in exchange, which is a trade almost everyone is happy to make.
5. Replace the noisy section with acoustic ducting. Only the section that passes somewhere it matters.
6. Change the fan. A silenced EC fan or a fan already enclosed in an acoustic box starts from a much lower baseline than a bare inline fan, and an EC motor is quieter at part load than an AC motor throttled to the same airflow. This is the expensive option, so it belongs at the bottom of the list — but if you are buying the fan anyway, buy the quiet one.

Slowing the fan down is the cheapest real fix

Because aerodynamic noise climbs so much faster than airflow does, backing a fan off is an unusually good bargain: you give up a modest slice of extraction and get a disproportionate amount of quiet back. It also cuts the running cost and takes load off the motor. If you sized the fan with headroom in the first place — as you should have — you will not miss the difference.

AC fans

An AC extractor needs a purpose-built fan speed controller. Do not use a household lamp dimmer; they are not designed for inductive motor loads and the result ranges from a horrible growl to a cooked controller. Proper units are voltage or triac controllers built for motors, and the good ones have a minimum speed adjustment — set it just above the point where the motor starts to hum, and you avoid the low-speed stall growl entirely.

EC fans

EC motors are speed-controlled by a low-voltage 0–10V signal rather than by choking the supply. They stay smooth and efficient right down the range, draw considerably less power at part load, and are markedly quieter at reduced speed than an AC motor throttled to the same airflow. If you are buying a fan and quiet is a priority, this is the single most effective thing you can specify.

Let a controller do it for you

Better still, hand the decision to a temperature or humidity controller. Rather than running flat out around the clock, the fan ramps only as far as it needs to in order to hold your target — which means it spends most of the day well below maximum, and most of the night nearly silent. Our humidity guide covers the targets to set, and the summer heat guide covers what to do when the fan alone cannot cope.

Intake, negative pressure and moving the air inside

An extractor cannot pull out air that cannot get in. This is the half of the system almost nobody plans, and it quietly caps what the rest of it can achieve.

If your tent runs on passive intakes — open vents rather than a second fan — those openings need to be substantially larger in total area than the extract duct. A common working rule of thumb is around twice the area of the outlet, and more is better. Undersize them and the fan spends its energy trying to pull a vacuum instead of moving air: the walls suck visibly inward, the fan gets loud, and the temperature stubbornly refuses to come down no matter what speed you set.

One more distinction worth being clear about: extraction moves air out of the tent; it does not move air around inside it. Those are different jobs. Air exchange controls temperature and humidity; internal circulation stops still pockets forming around the leaves, evens out the temperature between canopy and floor, and keeps the plants moving. A diffuser fitted to a duct outlet inside the tent spreads incoming air across the space instead of firing a jet at whatever is unlucky enough to be in front of it.

Browse the full ventilation range

Six mistakes that cost airflow, quiet, or both

1. Taking the rated m³/h at face value. It is a free-air figure. Assume you will get meaningfully less once a filter and a duct run are attached, and size accordingly. This is the single most common reason a correctly-chosen fan feels underpowered.
2. Leaving the duct slack and coiled. Delivered compressed, installed compressed, and then blamed on the fan. Pulling it taut and losing the spare loop is free and often the biggest airflow gain available.
3. Sealing joints with cloth duct tape. It looks the part on day one. Six weeks in a warm loft later the adhesive has given up and the joint is open — usually the joint on the outlet side of the filter, which is the one that matters.
4. Reducing the duct diameter to suit an existing hole. Resistance rises roughly with the square of air velocity, so a step down costs far more than it appears to. Make the hole bigger, or move the outlet.
5. Standing the fan on something solid. A shelf, a floor, a joist or a tent pole turns the whole structure into a sounding board. Hang it on straps and the problem frequently disappears without spending anything.
6. Silencing the outlet and forgetting the intake. You can make the exhaust side perfectly quiet and still be kept awake by air whistling in through an undersized passive vent. Noise, like airflow, is a property of the whole loop.

Quick answers

Why is my extraction weaker than the fan's rating?

Because the rating is a free-air figure, measured with nothing attached. Once a carbon filter, a length of ducting and a couple of bends are in the way, the fan is working against static pressure and delivers less — how much less depends entirely on your run. The biggest single contributor is usually the carbon filter, followed by slack or over-long ducting and sharp 90-degree bends. Pull the duct taut, shorten it, remove a bend if you can, and clean the filter's pre-filter sleeve before you conclude the fan is faulty.

Does acoustic ducting really make a difference?

For noise radiating out through the duct wall, yes, and it is very noticeable on a long run passing somewhere people sleep. What it does not do is silence the fan itself or stop vibration travelling through the building, so if the fan is sitting on a shelf you will not hear much improvement. Ducting is sold by the metre, so the efficient approach is cheap duct for the sections inside the tent and acoustic duct only for the stretch that crosses somewhere quiet.

Should the silencer go before or after the fan?

The outlet side is the usual choice, because that is where the noise is being pushed out toward the rest of the building. If the noise inside the grow room is what bothers you, a second silencer on the inlet side helps there too. Either way a silencer adds a little resistance and costs a small amount of airflow, which is a trade nearly everyone accepts. It is worth being clear that a silencer only treats airborne noise travelling down the duct — it does nothing at all for vibration passing into the structure.

How much quieter is an EC fan than an AC fan?

The useful comparison is at part load rather than flat out. EC motors are speed-controlled with a 0–10V signal and run smoothly and efficiently right down the range, whereas an AC motor throttled to the same airflow tends to be noisier and can hum or growl if driven too slowly. In practice that means an EC fan sized with headroom and run at half speed is considerably quieter than an equivalent AC setup, as well as using less electricity. If you are choosing a fan and noise matters, EC is the specification to look for.

Can I use a lamp dimmer to slow my fan down?

No. Household lamp dimmers are designed for resistive lighting loads, not for inductive motors, and the results range from a loud growl to overheating the dimmer. Use a purpose-built fan speed controller for an AC fan — ideally one with an adjustable minimum speed, so you can set the floor just above the point where the motor starts to hum — or a 0–10V controller for an EC fan. Check the controller's current rating covers the fan or fans you are connecting.

How big do my passive intake vents need to be?

Larger than most people expect. A common rule of thumb is a total intake area of around twice the cross-section of your extract duct, and erring generously does no harm. If the tent walls are sucked hard inward and creased, the fan is loud, and the temperature will not drop however fast you run it, the intake is the bottleneck rather than the fan. Opening a second passive vent is usually a five-minute job and it can transform how the whole system behaves.

My neighbours can hear the fan but I can barely hear it. Why?

That pattern is almost always structure-borne vibration rather than airborne noise. The fan is in contact with something rigid — a shelf, a floor, a joist, a tent pole — and the vibration is being conducted through the building and re-radiated as sound somewhere else. Airborne noise fades quickly with distance and through walls; vibration does not. The fix is to break every rigid contact: hang the fan on bungees or rubber straps so it floats, keep the ducting flexible either side of it, and make sure nothing rests against a wall. It costs nothing and it is usually the whole answer.