Why does it make a "pa" sound?

If it is not the sound made by the people upstairs, it is the sound made by the reinforced concrete of the building itself through the expansion and contraction of the steel bars. This is a normal phenomenon, there is no need to worry. Every building has this phenomenon.

If we are sure that there is no man-made noise, 1 is caused by thermal expansion and contraction, probably when you feel the room is cold at night. At this time, it was quite quiet and loud, some like footsteps, some like the sound of stones hitting the floor, and so on.

Because the floor is a drumhead, it rises when heated during the day, and suddenly collapses and vibrates at night when it is cool enough to make a sound. This situation often occurs in houses in some remote places, which makes people suspicious.

Water hammer action

As we know, as soon as the valve is opened, the water from some faucets will be sprayed out, which shows that the water flow rate is very fast, in other words, the water has sufficient momentum.

Therefore, when the valve closes quickly, the change of momentum will cause the pressure in the water pipe to change sharply, thus causing the pressure to impact the pipe wall.

This impact makes the pipe wall vibrate and make a sound, just like a hammer hitting the pipe wall. This phenomenon is called water hammer phenomenon.

At the same time, water has a certain degree of compressibility, and when the sound passes through the concrete wall, it will sound like a marble falling.

Because, the water will bounce around in the water pipe, and then gradually reduce the impact force, just like the marble bouncing back to its original position, gradually reduce the "da-da" sound.

So, if you have ever heard such a sound, it may be because there is a water pipe on the ceiling of your house or through the wall of your house, and then someone quickly turns off the faucet or flushes the water in the building, which leads to the phenomenon of water hammer.

This phenomenon is enough to explain the sound of "marble falling to the ground" I encountered, because the water pipes in our row houses are more complicated and supply many households at the same time. But many people don't agree with this explanation.

Other situations of "marble falling"

In addition to this water hammer phenomenon in the pipeline, there are actually other phenomena that can also cause sound, such as the bursting of bubbles trapped in water pipes and so on.

Fast-flowing water can always produce some bubbles, which will burst in the water pipe and produce echoes. However, this usually only produces a single sound. It's not the sound of marbles falling.

Pictured: Porous plate building materials

In addition to water pipes, some people think that the ceiling or the floor where you are located is at work.

Because the ceiling used in previous buildings was made of perforated plate (as shown in the above picture), and this perforated plate may crack, which may come from thermal expansion and contraction, or from the instability of its own structure, and then the pebbles inside will bounce in the holes and produce sound.