MadSci Network: Physics

Re: Why do squash balls sometimes make a weird high pitched noise when hit?

Date: Tue Nov 8 14:12:04 2005
Posted By: Kenneth Beck, Senior Research Scientist, Chemistry and Physics of Complex Systems, Pacific Northwest National Laboratory
Area of science: Physics
ID: 1128868583.Ph

Dear No Name Entered,

You probably know squash was invented in England in the 19th Century and 
initially was restricted to school and university play.  I myself learn 
to play at the University of Pennsylvania.   With the instructional 
prompting of my postdoctoral adviser I became an adequate player, as she 
was faculty champion.  And I have also heard the sound you described, 
although I cannot recall the specific conditions that generated it.  So, 
I’m giving you a mission!

Squash balls are made from two pieces of highly durable rubber compound 
glued together. Different type balls are provided for the varying 
conditions and standards of play:  beginning players are able to use 
balls that are bouncier and larger than those used by more experienced 
players. Colored dots on the ball indicate the level of elasticity, 
or “speed”.  A bouncier ball is said to be "fast" whereas a less bouncy 
ball is said to be "slow".

The ball becomes more bouncy as the temperature of the ball increases. 
Pro players hit harder and have longer rallies, and so play with a ball 
at a much hotter temperature than amateurs. The "faster" balls actually 
allow amateurs to play the game with the same amount of ball bounce as 
the pros.  Many club players end up playing dead ball squash because they 
use a ball which isn't suited for their level of play.
The color code for squash balls is:

Double Yellow - Extra Super Slow 
Yellow - Super Slow 
Green or White - Slow 
Red - Medium 
Blue – Fast

The yellow dot you describe is a super slow ball that previous to the 
year 2000 was considered only for championship tourneys.

What does this super slow designation mean thermodynamically?  It means 
that about 75% of the kinetic energy impacted to the ball by your raquet 
on a hit is eliminated and lost through heat and sound dissipation.  
Roughfully, only 25% of the kinetic energy is store as potential energy 
through ball strain for use on the rebound.

As the ball heats up over the course of play, the elasticity of the ball 
increases, allowing more energy to be stored as ball strain.  It begins 
to respond more like a green or red dot ball and rebound further.  An 
interesting study was done comparing a Double Yellow and Yellow dot 
ball.  The URL is here…


According to this investigation, the room temperature (unplayed) balls 
rebound distances off the wall are as expected.  However, the balls hit  
at “playing temperature”  rebounded further…the Double Yellow actually 
rebounding further than the Yellow dot at comparable temperatures…

	          	Playing Temperature     Room Temperature

Double-yellow           664.50 cm               475.25 cm 
Yellow-dot       	624.25 cm               531.25 cm 

So, the yellow dot is a particular slow ball, even when hot. 
Thermodynamically, we could say it is a particularly good dissipation 
sink for kinetic energy, producing alot of heat and alot of sound.

So, ok.  Why does it make the high pitch whine you only hear in sci-fi 
movies? It does this because under some circumstances, heat dissipation 
is not fast enough, or efficient enough, during the volley to remove the 
excess kinetic energy given the squash ball. The only avenue left for the 
little yellow dot guy is to produce high frequency sound. What 
circumstances are those? It may be a couple of things, which you can 
investigate. So, your mission is to answer these questions:

What are the condition of your squash courts?  Are they dry or humid? Are 
the wall and air temperatures generally cool or warm?
For another, what move did you make when the ball “sounds off”?  Did you 
just put some weird spin on the ball, placing it a mode more conducive to 
sound dissipation than heat dissipation?
And after it “sounds off”, is it still as “fast” on the next rebound, or 
has it perceptively become slower with less bounce?  In other words, does 
the heat build up on the yellow dot squash ball to the point it must 
release energy as sound?  Does it then cool down and become slower…less 
bouncy?  Does it feel cooler after emitting sound?

Sorry I have no easy answers for you No Name Entered, but perhaps you can 
find them for us!

Hope this helps in your investigation,

---* Dr. Ken Beck

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