11
$\begingroup$

Rolling thunders always come cracking in with a high frequency, and the sound grows more deep as the rolling advances, ending with the deep sound of a roaring lion. Why? This is not the same question than the one of the extended sound but about the way we hear the rolling thunder from the beginning till the end. The sound starts with a higher frequency than that with which it ends and I was wondering why.

$\endgroup$
8
  • $\begingroup$ Acoustic dispersion $\endgroup$ Jun 3 '16 at 16:36
  • 1
    $\begingroup$ @JohnRennie: Air is not really a dispersive medium $\endgroup$
    – ACuriousMind
    Jun 3 '16 at 16:42
  • 5
    $\begingroup$ Possible duplicate of Extended sound of thunder $\endgroup$ Jun 3 '16 at 16:45
  • 1
    $\begingroup$ This is speculation on my part, but my intuition is that the initial stroke creates a high-pressure plasma in a narrow column of air, which in turn creates a shock wave as it expands out into the normal pressure air. This wave is not high-frequency per se, but rather a uniform contribution from all frequencies, much like a clap. (This is just the Fourier transform of a delta function.) $\endgroup$ Jun 3 '16 at 18:06
  • 1
    $\begingroup$ After the initial stroke, the air in the column has expanded away, leaving a low pressure region in the column. The surrounding air then collapses on the column, producing the low rumble as it alternately expands and contracts in the column until it restores pressure equilibrium with the surrounding air. $\endgroup$ Jun 3 '16 at 18:08
2
$\begingroup$

I am going to steal from an answer to another question.

... a variety of sounds are heard following a lightning strike is not due to dispersion, but rather the multiple branches of the pre-strike, the main strike, and the extended distances covered by the lightning, plus, sometimes, echos. ...

In the quote there is a link to a page where they talk about why thunder sounds the way it does.

$\endgroup$
2
  • $\begingroup$ I still don´t exactly understand why the sound (or maybe it´s just my imagination) goes from high to low. By the way, is it true that a flash goes back and forth, very quickly, but slow enough for the eye to see? Like short stroboscope? $\endgroup$ Jun 4 '16 at 11:04
  • $\begingroup$ After a huge thunderstorm on th vulcanic island Ischia I saw thosands of little electric discharges around the central mountain top (Monte Epomeo). Just a little anecdote. But why....? I still don´t know. $\endgroup$ Jun 4 '16 at 12:11
2
$\begingroup$

Higher frequencies are attenuated (absorbed) more strongly than lower frequencies. Here are a couple of attenuation values from this table of sound-wave attenuation (90% relative humidity):

f (kHz)   a (dB/km)
1         5.3
2         9
4         20
8         63

The discharge itself will generate a wide spectrum of frequencies, from infrasonic all the way to ultrasonic. Your ear is much more sensitive to high frequencies (1 kHz-8 kHz range) than to lower frequencies, which causes you to perceive the sound as "crackling", i.e., dominated by high frequencies. The sound of rolling thunder will be well below 1 kHz.

Different parts of a lightning discharge will generally be at different distances from your ears. Suppose that the nearest part of the strike is at 0.5 km distance and the furthest part is at 3 km distance. The parts of the strike at 3 km distance will have lost most of the high-frequency components, so only the low-frequency part is left. Because it's farther away, it will arrive later.

Update I took a video from youtube,Lightning & Very Loud Thunder, and did some analysis of the sound in Audacity. The numbers on top indicate the time in seconds, counting from the beginning of the video. The lightning strike was at around 41.0 s; the thunder starts at 44.5 s. I have scaled the amplitude to be roughly constant over the duration of the thunder.

thunder, scaled for constant amplitude

Indeed, the spectrum seems to be more or less white with a fairly sharp cutoff frequency, which drops from around 5 kHz at the beginning (3.5 s = 1.0 km distance) to about 1 kHz at at 9 s (3 km). So the transition from crackling to roaring sound is not imagination; it's really there. Moreover, it's also roughly consistent with the attenuation characteristics of air in the table above.

For the sake of completeness, here is the un-normalized signal spectrum.

Spectrogram thunder

$\endgroup$
2
  • $\begingroup$ So the far away sounds are dominated by the low, roaring thunder. Problem solved! $\endgroup$ Jun 5 '16 at 4:53
  • $\begingroup$ @descheleschilder - updated answer (I'm not sure whether you get a notification of the change). $\endgroup$ Jun 5 '16 at 19:56

Not the answer you're looking for? Browse other questions tagged or ask your own question.