CONCEPT: Touching a solid ruler to a clock makes the sound louder because:

This may sound strange but: In this case, the ruler is acting like a sound tube: The air around the ruler stops sound from leaking out

Pressing our ear to a solid, for example, a door, allows us to directly hear the pressure waves travelling through the solid. Reference: Primary Connections

Does Sound Travel Better Through Solids?

Many elementary textbooks say that sound travels better through solids and liquids than through air, but they are incorrect: air, solids, and liquids are nearly transparent to sound waves.

Some authors use an experiment to convince us differently - for example:

“Place a solid ruler so it touches both a ticking watch and your ear, and the sound becomes louder.”

Does this prove that wood is better than air at conducting sound?

Not really, because sound has an interesting property not usually mentioned in the books: waves of sound travelling inside a solid will bounce off the air OUTSIDE the solid.

The experiment with the ruler merely proves that a wooden rod can act as a sort of “tube,” and it will guide sounds to your head which would otherwise spread in all directions in the air. A hollow pipe can also be used to guide the ticking sounds to your head, thus illustrating that air is a good conductor after all.

Sound in a solid has difficulty getting past a crack in the solid, just as sound in the air has difficulty getting past a wall. The effect is exploited to test aircraft and bridges to detect otherwise invisible cracks resulting from stress.

Solids, liquids, and air are nearly equal as sound conductors.

It's true that the speed of sound differs in each material, but this does not affect how well they conduct. “Faster” doesn't mean “better.”

It is true that their transparency is not exactly the same, but this only is important when sound travels a relatively great distance through each material.

It's also true that complex combinations of materials conduct sound differently and may act as sound absorbers (examples: water with clouds of bubbles, mixtures of various solids, air filled with rain or snow.)

And lastly: when you strike one object with another, the sound created inside the solid object is louder than the sound created in the surrounding air. So, before we try to prove that solids are better conductors, we had better make sure that we aren't accidentally putting louder sound into the solids in the first place. http://www.eskimo.com/~billb/miscon/miscon4.html#sound

Also see: http://amasci.com/miscon/miscon4.html#sound

This means, using the formulas above (effect of impedance), that when sound reflects off water, only 4/3500 of the intensity makes it through. That is a reduction of almost 30 dB in the intensity of the sound. Note that sound, moving through the water, reflects just as efficiently to stay in the water and let only a tiny amount into the air.

http://www.physics.mcgill.ca/~guymoore/ph224/notes/lecture18.pdf

Total Internal Reflection Total internal reflection can only occur for light passing from a more optically dense medium to a less optically dense one. Typical critical angles include 49o for water, 42o for crown glass and 24o for diamond.

Total internal reflection can occur for sound also.

One application of total internal reflection is found in fibre optics. Good quality glass of high refractive index is coated with a thin layer of glass of lower refractive index. Light is passed into the end of the thin fibre. Any ray of light striking the boundary between the two glass media at an angle greater than the critical angle, will be totally internally reflected along the whole length of the fibre. Light can therefore travel from one end of the fibre to the other without loss. Reference: http://webs.mn.catholic.edu.au/physics/emery/prelim_communication.htm

Perhaps you're thinking of being underwater and knocking two stones together? That's very loud because the sound inside the rock can easily escape out into the water. But lift those two stones up into air, and when you whack them together it's not nearly as loud. The sound doesn't escape; it bounces around inside the rock. Only a tiny bit will get out into air.

Here are the rules underlying it:

Sound in air will bounce off of rock, and remain in the air Sound inside rock will bounce off the air, and remain in the rock. See, the sound has trouble moving from air to solid, but it has just as much trouble moving from solid to air. (And that's why guitars need a sounding board. The sound trapped inside the solid strings can't easily get out into the air.) But water is very dense; almost as dense as solid. Sound can easily jump from a solid and travel out into a liquid. Or it can easily travel from a liquid into a solid.

http://www.reddit.com/r/explainlikeimfive/comments/k2104/eli5_why_does_sound_travel_better_through_liquid/

This means, using the formulas above, that when sound reflects off water, only 4/3500 of the intensity makes it through. That is a reduction of almost 30 dB in the intensity of the sound. Note that sound, moving through the water, reflects just as efficiently to stay in the water and let only a tiny amount into the air.

Source: http://www.physics.mcgill.ca/~guymoore/ph224/notes/lecture18.pdf

CONCEPT: Does sound travel faster/slower than light

Light travels faster than sound and so we see things before we hear them.

See this simple interactive to find out how sound travels: http://www.knowitall.org/nasa/flash/sound/how_sound_travels.swf

CONCEPT: Determining the Distance to a Lightning Bolt:

The speed of sound in air is approximately 330 m/s

Sound waves take approximately three seconds to travel one kilometer.

Using this information, it is possible to measure one's distance from a lightning bolt.

1. Begin counting immediately after you see the flash.
2. Every 3 seconds counted is roughly equivalent to 1 kilometer of distance.