MadSci Network: Physics

Re: In which does sound travel faster Wood or Glass, Why?

Date: Fri May 10 13:18:44 2002
Posted By: David Smith, Faculty Geology, Environmental Science
Area of science: Physics
ID: 1018281561.Ph


I found some data-from a page at a site on pianos and tuners (who should 
care about sound and wood)- that indicates that not only is the type of 
wood important, but so is the direction in which the sound is traveling:  Using this site and converting to 
metric units, I found that in many woods, the speed of sound is close to 
4500 meters per second (m/s) along the grain and 1500 m/s across the grain. 
 The Physics book I have on my desk gives the speed of sound in Pyrex glass 
as 5640 m/s (Serway and Jewett, 2002, Principles of Physics, 3rd Edition, 
Volume 1)  Other types of glass would be very similar, but perhaps 
not exactly the same.  Because glass has the same properties in all 
directions (isotropic in physics-speak)the speed of sound in any 
particular piece of glass is the same in all directions.  So, from this 
research, sound would appear to be slightly faster in glass than along the 
grain of wood and significantly faster in glass than across the grain of 

The reason has to do primarily with properties of the material called 
density and elasticity.  Density describes how much mass is in a 
specific-sized block.  Mass is hard to define, but one way to think about 
it is as a measure of resistance to motion.  Since sound waves involve 
moving the atoms of a material back and forth, more dense materials will 
tend to resist this motion.  Therefore, as density goes up, speed of sound 
generally goes down.  Elasticity is the ease with which a material can be 
stretched or will spring back once it is stretched.  It is measured by 
something called the "bulk modulus," which relates the force applied to an 
object to the amount that the size of the object changes.  If a small force 
causes a big change, the bulk modulus is small and the material is easily 
squashed.  If a big force causes a small change, the bulk modulus is large 
and the object is stiff.  Stiffer objects are better transmitters of sound 
waves and so as elastic modulus goes up, the speed of sound goes up, too.  
These relationships are not linear.  Instead, speed is proportional to the 
square root of the bulk modulus divided by the square root of the density.

An excellent reference to all things physics is the HyperPhysics web site:

An interesting discussion of techniques to measure the speed of sound is 
found at along with a number 
of references to other resources.  The vocabulary in this is pretty 
technical, but it's expressing relatively simple ideas.  If you don't let 
the big words scare you off and you get some help to understand them, you 
should be able to get the idea.

Berg, Richard E. and Stork, David G., The physics of sound, Prentice Hall, 
Inc., 1982 is a book written for beginning students and might also be 
useful for you to read.  If your library doesn't have it, maybe they can 
get it for you through a process called inter-library loan.

As for not being good in science:  every human is born good in science - 
babies do science to survive - they experiment with their world and see 
what happens.  Not everyone is good at what school teachers, popular 
culture, and others call science, but many scientists know that what goes 
on in school science classes is often not much like what real scientists 
do.  It's especially unfortunate that girls are often expected to be poor 
in science (and in math, which is a vital tool of science) by both their 
peers and their teachers.  If you like the idea of experimenting, then have 
a good time with it.  Don't let anybody (especially not yourself!) tell you 
that you aren't good at this or that you can't do it.  Some fun sound 
experiments to try can be found at:

David L. Smith
Associate Professor of Geology, Environmental Science, and Physics
La Salle University, Philadelphia, PA

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