The sound of thunder

Original post by NJA

Hi physics peeps,

I watched Richard Hammond on our weather on tv and the explanation of the sound of thunder was a little incomplete. The lightning cases an explosion that causes a shock wave, but so does an earthquake shaking the ground.

What actually causes the noise?

Rapidly expanding plasma/gas

I don't think the frequency range of earthquakes corresponds to frequencies we perceive as sound - the sound you hear when an earthquake happens is generally the sound of stuff being shaken by the earthquake rattling or breaking.

also I think there's an impedance mismatch - most earthquake energy reflects off the boundary with air

Reply 2

OP

Original post by Joinedup

Rapidly expanding plasma/gas...

thanks but again that doesn't fully explain how the sound arises, what creates the sound wave?

Maybe I need to speak to the chemistry peeps?
Perhaps they can give an equation.

Reply 3

Original post by NJA

thanks but again that doesn't fully explain how the sound arises, what creates the sound wave?

Maybe I need to speak to the chemistry peeps?
Perhaps they can give an equation.

Infinitesimal pressure disturbances

Reply 4

OP

Original post by pleasedtobeatyou

Infinitesimal pressure disturbances

But what actually creates sound?
Friction of air molecules?
Release of electrons?

...?

Reply 5

Original post by NJA

But what actually creates sound?
Friction of air molecules?
Release of electrons?

...?

Sound is essentially created by changes in pressure propagating as a wave. So, each particle vibrating and these pressure disturbances are picked up by your ear which is interpreted as sound.

Reply 6

lerjj

13

Original post by NJA

But what actually creates sound?
Friction of air molecules?
Release of electrons?

...?

Your ear is basically a really sensitive air pressure detector thingy. An initial vibration causes a compressed region of air, which returns to it's equilibrium pressure by expanding outwards, thus compressing the air next to it and so on.

You get a pressure wave that propagates to your ear, your ear is basically a really sensitive air pressure detector thingy and hey presto- you hear a sound!

Reply 7

OP

pleasedtobeatyou and lerjj - thanks.
I guess that's true for all explosions?

One other thing, is this the same as a sonic boom?

Reply 8

Original post by NJA

pleasedtobeatyou and lerjj - thanks.
I guess that's true for all explosions?

One other thing, is this the same as a sonic boom?

A sonic boom is still a sound wave but they only occur when the object emitting the pressure waves is travelling quicker than the local speed of sound. The waves at the front of the moving object compress together.

The sound waves now don't propagate in your typical concentric circular fashion but in a cone shape. When you are outside of this cone, you don't feel the effects of the pressure disturbances i.e. the sound.

Only when the pressure waves from the cone cross you, do you actually hear the sound. The combined strength of each wave leads to a bigger "sound" and hence the boom.

Reply 9

OP

Original post by pleasedtobeatyou

A sonic boom is still a sound wave but they only occur when the object emitting the pressure waves is travelling quicker than the local speed of sound. ...

OK, so, doesn't lightning cause the air to expand faster than the speed of sound?

Reply 10

Original post by NJA

OK, so, doesn't lightning cause the air to expand faster than the speed of sound?

Well, if the conditions were right. Sometimes you hear archetypal crack accompanying thunder but other times you only hear a slight rumble in the background.

Reply 11

OP

Original post by pleasedtobeatyou

Well, if the conditions were right. Sometimes you hear archetypal crack accompanying thunder but other times you only hear a slight rumble in the background.

ah that explains the difference between crack and rumble.
Thanks.

Reply 12

Joinedup

I think the crack/rumble is mainly a distance effect - the sound can only travel at the speed of sound but the lightening emitting the sound has considerable length so it takes different lengths of time for the sound from different distances to reach you.

http://www.weatherdudes.com/facts_display.php?fact_id=46

FWIW mostly I've heard rumbling thunder cos most of the time I'm not near the lightening strike - a few times it's been fairly close and then it's a bang.

Reply 13

OP

Original post by Joinedup

I think the crack/rumble is mainly a distance effect - the sound can only travel at the speed of sound but the lightening emitting the sound has considerable length so it takes different lengths of time for the sound from different distances to reach you.

http://www.weatherdudes.com/facts_display.php?fact_id=46

FWIW mostly I've heard rumbling thunder cos most of the time I'm not near the lightening strike - a few times it's been fairly close and then it's a bang.

Ok, good stuff.

P.S. no shortage of songs about lighning and thunder.

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Reply 14

Joinedup

Media+these+days+_77c8631540372bce01f4014c5d93d40c.gif161.6KB

Original post by NJA

Ok, good stuff.

P.S. no shortage of songs about lighning and thunder.

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this is more my speed tbh...

anyway check this out, you might see when some lightening storms are coming in your direction. http://www.blitzortung.org/Webpages/index.php?lang=en

Reply 15

Phichi

11

Original post by NJA

thanks but again that doesn't fully explain how the sound arises, what creates the sound wave?

Maybe I need to speak to the chemistry peeps?
Perhaps they can give an equation.

Sorry for the off topic comment, but this really crushed my soul.

Reply 16

OP

Original post by Phichi

Sorry for the off topic comment, but this really crushed my soul.

I don't get it.
Do physics & chemistry disciples get along like Daniel & Mark from The Apprentice?

Reply 17

Phichi

11

Original post by NJA

I don't get it.
Do physics & chemistry disciples get along like Daniel & Mark from The Apprentice?

You could say yes, and then again no, haha! But, the question lies more within the physics realm of explanation, so asking Chemistry would be, blasphemy!

Reply 18

OP