It is not uncommon that during the night or early in the morning, when the house is absolutely silent, strange noises are heard on the upper floor that simulate that of a handful of marbles when they fall and bounce on the ground for a few seconds, their intensity becoming less and less.
This phenomenon, which has no relation to acoustic parapsychology, occurs in both new and old or refurbished flats and its physical explanation is known as Zhukowski’s water hammer or pulse.
What is the Zhukowski pulse?
Nikolai And Zhukowski (1847-1921) was a Russian engineer, nicknamed by Lenin as the father of Soviet aviation, who is responsible for the construction of the first wind tunnel and the foundation of the first aerodynamic research institute in Europe.
The pulse that bears his name is related to the elastic behavior of fluids. And it is that when a tap or a valve is suddenly closed, the water particles that were close to the closing point stop abruptly, but those that were further back continue to move until they collide with those that precede them.
In the end, it ends up producing a rebound effect and the water tends to occupy the vacuum that had been generated behind the particles, with the consequent increase in pressure and the reverberation of the pipes.
It is precisely this effect, discovered by the Russian scientist more than a century ago, that causes the characteristic rattling.
Much more than a domestic problem
The water hammer, therefore, is nothing more than the overload of pressure that a pipe suffers inside when a column of liquid – which moved with a certain inertia – suddenly stops. The greater the length of the conduit and the higher the velocity of the liquid, the greater the pressure overload suffered by the conduit.
Little by little, the energy of the water is depleted until it finally manages to be distributed evenly, so the duration of the sound will depend on the distance that the water has to travel and the existence or not of a leak point. For its part, the “number of marbles” that generate the sound will depend on the speed of propagation of the wave, which, in turn, is related to the thickness of the material.
Far from being just a domestic problem, the Zhukowski pulse is a challenge for engineers, as it could have serious consequences in installations with long and large diameter pipes.
In this case, it is necessary to install systems that introduce or extract air to avoid the generation of bubbles, which is why they are used piezometric towers, balance chimneys and spring valves capable of absorbing excess energy.
The role of acoustic memory
The Zhukowski pulse is not only produced when a tap is closed, it can also be generated, for example, when a dishwasher or a washing machine stops taking in water, since a valve closes.
To close the circle, to the physical explanation we would have to add the psychological one, our acoustic memory. And it is that, in some way, the brain associates reality –sound- with our knowledge and experiences, trying to give a coherent meaning that fits one of its cognitive categories.