Disclosure statement

Rachael Helene Nolan receives resources from the NSW Bushfire Risk administration Research Hub, funded with the room of Planning, Industry and Environment (formerly OEH) and also has previously received funding from the Australian Government's creation Connections programme in conjunction through GreenCollar.

You are watching: What sound does a fire make

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Why does timber crackle in a fire? – Rocco, period 6 (nearly 7!)

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Hi Rocco, that’s a great question. Ns love sit in front of a fire, listening come it crackle and also pop.

These noises are caused by pockets of trapped vapor suddenly escaping, make a mini explosion!


To understand why this happens, we require to understand what happens when you ar a wooden log top top a fire. First, the hardwood starts getting hotter. Inside the wood are pockets of trapped water and also tree sap, i beg your pardon is the difficult stuff you periodically see on trees.

In the same means water in a kettle heats up and turns into steam, for this reason does the water trapped within the log. So together the fire it s okay hotter, the water and sap inside start to boil and also turn into gas. Together the fire gets also hotter, this gases start to take it up more space and expand (get bigger).

How carry out the gases to explode out?

While the water and sap turn right into steam, something likewise happens to the wood. Wood includes something called cellulose, i beg your pardon is the stuff that plants are largely made the end of.

When cellulose is heated, that starts to rest down, or “decompose”. If you’ve ever before forgotten an apple in your lunchbox over the weekend, and it turns brown and yucky, that method it has decomposed. When something in nature (like a piece of fruit) decomposes, that changes.

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When lumber in a fire gets warm enough, the cellulose inside starts to turn into gas. This is as soon as we view smoke coming out of the wood, occasionally even before that item of wood has actually burst right into flames.

The flames happen when the gas escaping native the wood starts to mix with the oxygen in the air. Oxygen is prefer food for fires – it makes them burn really bright.

As wood burns, the mix of widening gases and cellulose breaking down provides the pockets of trapped heavy steam burst open from the wood, one by one. This is why girlfriend hear the crackling and also popping noises.

So the an ext water and sap there is inside the wood, the noisier the fire will certainly be. If you’ve ever put damp wood on a fire, friend may have noticed it renders a lot more noise than really dry wood.

How does the wood gain water within it?

But just how does water and sap gain inside timber in the an initial place?

Well, lumber isn’t rather as solid together it looks. It has numerous tiny holes, too tiny for our eye to see, and these holes have actually water and sap within them.

We know wood comes from trees. And also when trees space alive, lock stay healthy by transferring water up their trunk v these tiny holes, which are referred to as xylem vessels. As soon as the tree is chopped under to make firewood, over there is tho water trapped within these xylem vessels.

There are other methods water can obtain inside wood. If firewood is left out in the rain, it have the right to soak increase water that way. Or sometimes insects make small holes in the wood, which permit water in.

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Sitting in front of a fire city hall the flames and listening to the hardwood crackle and also pop deserve to be fun. Many of the time the mini explosions the the steam escaping room small.

But periodically they deserve to be big, and also might even cause small chunks that burning timber to fly out of the fire! This is why it’s important always to store a safe distance from a fire, or to use a fireguard.

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The writer thanks her nephews Aldous Nolan (6) and Fergus Nolan (5) because that helping to enhance this answer.

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