How Everything Works
How Everything Works How Everything Works

Question 1068

How does a steam whistle work? — DR
As far as I know, a steam whistle is just a whistle that's blown by steam rather than air. The principle behind a whistle is straightforward: the air inside the whistle is driven into intense vibration by the stream of gas blown across a slot-shaped opening. This stream of gas is directed at the sharp edge on the far side of the opening and might or might not actually enter the whistle. If air happens to be flowing out of the slot-shaped opening as the stream flows across the slot, the outgoing air will deflect the stream outward and that stream won't enter the whistle. But if air happens to be flowing into the slot as the stream crosses the slot, the stream will be deflected into the whistle. This situation leads to an amplifying effect: if any air is flowing into the slot, the whole stream of gas will flow into the slot. If any air is flowing out of the slot, the whole stream of gas will flow out of the slot.

Now air inside the whistle is never perfectly still—it's always sloshing back and forth at least a tiny bit, much like water sloshes in a basin. As a result, there is always a little motion of air in or out of the slot. When the stream of gas begins to blow across the slot, it amplifies any tiny motions of air inside the whistle so that they become more and more vigorous. Soon the air inside the whistle is vibrating intensely and the resulting pressure fluctuations radiate outward from the whistle as sound.

This same principle is active in many other musical devices, including pipe organs and flutes. In a steam whistle, the stream of gas that drives this vibration is steam rather than air. Water is heated in a boiler until it forms moderately high-pressure steam and then the steam is released through a valve to a large whistle, which sounds loudly.


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