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The question: "Why is a kettle loud?" has been asked and answered many times, including on here:

Why is boiling water loud, then quiet?
What is the reason for this pattern of noise in electric kettles?
Thunderf00t: Why is a kettle louder BEFORE it boils?

The answer given is that vapour bubbles are being created at the bottom, then collapsing as they rise into the cooler water. This seems plausible.

Question:

However, why is it that my induction hob can boil a cup of water faster than my kettle, and do it almost silently?

I used a spectrum analyser app to measure the sound volume in both cases. As you can see, the kettle is considerably louder. In actual fact, most of the noise of the induction hob is actually electrical noise (presumably to do with the alternating magnetic field turning the pan into a speaker).

The pan had a non-stick coating.

Red lines are the peak during the experiment. Green lines are current sound level when screen shot was taken.

Sound of an induction hob boiling water. Sound of a kettle boiling water. Sounds of kettle and induction hob boilind water.

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  • $\begingroup$ Great charts. +1 $\endgroup$
    – Time4Tea
    Sep 1, 2019 at 16:48
  • $\begingroup$ There is no (or little) vertical temperature gradient with induction. $\endgroup$
    – safesphere
    Sep 1, 2019 at 17:25
  • $\begingroup$ @safesphere - Why do you say that? As far as I can see, both cases are identical. The water is sitting on a metal surface which is being heated. Water at the bottom, touching the metal will be heated, while water at the top surface will be cooler. The vertical temperature gradient is in the water, not in the metal. $\endgroup$
    – Rocketmagnet
    Sep 1, 2019 at 18:20
  • $\begingroup$ I am not sure of your setup. You haven't provided the details. Perhaps your pot side walls are heated by the induction and this effectively removes the temperature gradient? $\endgroup$
    – safesphere
    Sep 1, 2019 at 18:23
  • $\begingroup$ Electric kettles (usually?) have tubular heating elements immersed in the water, which have a smaller surface area than your pan, so they run hotter if they are delivering the same power. Is yours like that? $\endgroup$
    – Keith McClary
    Sep 3, 2019 at 2:31

1 Answer 1

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The roar of the kettle which is on the verge of switching into "full rolling boil" mode comes from rapid expansion of vapor explosions at the hot surface of the kettle which briefly protrude into slightly cooler water outside the boundary layer, which quenches the vapor and causes the bubbles to very rapidly collapse.

Each of those individual vapor explosions is nucleated at a pit or scratch in the kettle surface. The capacity of the kettle to make a roar depends on the pit density (pits per unit area) and on their size, where larger pits begin nucleating sooner (i.e., at lower surface temperatures) than smaller pits.

If you deactivate the pits by deaerating them with detergent in the water, or by otherwise removing the stuck air in them which causes them to act as nucleation sites, then you can suppress the roar phase.

A microscopic examination of the surface of the induction hob will probably reveal fewer pits in its surface. This explanation is testable by observing the surface of the immersed hob soon after turning on the electricity to it, because the nucleation pits will first act as exsolvation sites for dissolved air in the water before they act as nucleation sites for the vaporization process. Their presence is hence revealed by the appearance of air bubbles at specific sites on the hot surface.

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  • $\begingroup$ Thanks for the answer. Just to be pedantic, the induction hob is the stove top, while the water is in a pan on top of the hob. $\endgroup$
    – Rocketmagnet
    Sep 1, 2019 at 18:18
  • $\begingroup$ I guess could test this by trying to find a scratced up pan to see if that's louder. $\endgroup$
    – Rocketmagnet
    Sep 1, 2019 at 18:19

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