MadSci Network: Engineering

Re: How does sound travel?

Date: Tue Apr 15 10:03:36 2003
Posted By: Aurelio Ramos, Grad student, Computer Engineering
Area of science: Engineering
ID: 1049247654.Eg

Sound travels in different ways depending on the medium. The medium is the 
material the sound is using to travel. Air is the most common. When sound 
travels thru the air it normally originates from somewhere in space, like 
a speaker or a person's mouth, and then the waves travel in spherical 
ripples, this is similar to water waves: those travel in circular ripples 
because the surface of water is flat. The ripples of sound waves in air 
are made of air compressed closer together, while the ripples of surface 
waves in water are simply zones where the water surface is elevated. So 
not all ripples are the same.

The speed of sound in air at normal room temperature is about 1000 feet 
per second. So if you stand facing a wall that is about 5 basketball 
fields away from you (that is about 500 feet) and you clap your hands, the 
echo will come back at you after traveling 500 feet twice (one way to the 
wall and one way back to you) and the echo will be about one second late.

Waves can also travel thru solids and liquids. In liquids, water travels 
more or less the same way as it does thru air. The ripples in this case 
are not on the surface of the liquid, but rather in the volume, as 
compression, just like it does in air. Sound is not normally transmitted 
by liquid surface waves. The speed of sound in liquids can sometimes be 
faster or slower depending on the density and compliance (compressibility) 
of the liquid

But now, to the important question you have: Solids, like doors, walls and 
glasses!. Water travels thru solids just like it does in air and liquids, 
but it travels much faster. Although its speed varies from solid to solid. 
In general, sound waves travel thru rigid solids much better than squishy 
solids. Rigid solids are said to have low compliance. Squishy things, like 
rubber have a higher compliance. Compliance is the "softness" 
or "bounciness" of a solid. Also, some solids are squishy in a different 
way, they don't really bounce that well. That property makes such solids 
(like pillows and sand) very bad at transmitting sound, because they 
absorb the energy of waves and turn it into mild heat. If you fill a sand 
bag, and press it against a door, then put your ear, you will not hear as 
much as you do with a glass. You cannot hear that much of anything with a 

So: as far as glasses goes, all glasses are made of a rigid solid: Glass. 
That is a good start, because rigid means they will work well right away 
at transmitting sound. But, keep in mind, for the glass to work it has to 
be pressed against your ear. Then sound reaches you inner ear NOT thru the 
air, but thru you bones!! yes, that's right, the glass trick works because 
the glass is pressed hard against your skull bone, without that pressure 
it wont work. And, come to think about it, if there is not enough 
pressure, the sound must first go from the glass to your outer ear (pretty 
squishy stuff) and then thru your skull. If you don't press hard, the 
sound is disipated by the squishy ear and skin.

So, I believe when you do your experiment, you will find that almost all 
glasses are plenty rigid for transmitting sound, but some glasses have 
their bottoms shaped in a way that makes it easy to press against the 
skull. The winning glass will probably be quite comftable to put a lot of 
pressure against with your ear. That being said, I think you will find 
little variation between glasses. You could also try plastic cups and see 
how they work. They should work but not as good as real glass.

Another thing to try is to put the glass on the door and press with other 
parts of your head, like your forehead. You should be able to hear some 
sound, no matter what part of your head you use. This shows again that the 
sound that comes from a glass travels thru your bones, not air.

Hope this answers your question.

Your mad scientist,

Aurelio Ramos.

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