# Sound waves through air column

1. Aug 4, 2012

### tastoon

hello guys, i'm having problems understanding why if we were to have a tuning fork that vibrates at a particular natural frequency, it will resonate at many different lengths of air column (which means different natural frequency of air column). Does this mean that the natural frequency of the tuning fork does not need to be equal to the natural frequency of the air column for resonance to occur?

2. Aug 4, 2012

### Bobbywhy

tastoon, Welcome to Physics Forums!
To search for answers to your question I entered the search terms "tuning fork resonate air column" and got lots of hits. Here are two sites which may help explain the difference between "natural frequency of the tuning fork" and "resonant frequency of the open tube". The key is that odd harmonics resonate also.

"The various possible frequencies at which a tube may resonate are definite and fixed in value. They depend upon the length of the pipe and the velocity of sound in air. In a cylinder of air, the length (L) of the column of molecules vibrating must be some odd number multiple of the wavelength of the tuning fork oscillation:
n l = 4 L
In this equation: n = an odd integer (1, 3, 5, ..); l is the wavelength of the sound wave; and L is the length of the air column."
http://chemmovies.unl.edu/chemistry/dochem/DoChem028.html [Broken]

A simple experiment you can do to search for odd harmonics of tuning fork:
http://www.lhup.edu/~dsimanek/scenario/labman2/soundvel.htm

Cheers,
Bobbywhy

Last edited by a moderator: May 6, 2017
3. Aug 4, 2012

### tastoon

I still don't get it =( . If the frequency of the sound wave entering the tube equals to the reflected sound wave's frequency, won't resonance always occur since driving frequency = reflected frequency?

From http://chemmovies.unl.edu/chemistry/dochem/DoChem028.html [Broken], it states that, "It is observed when the sound waves reflected from the closed end of the tube return to the top of the tube in phase with the new waves being produced by the tuning fork. The direct and reflected waves combine their effects. Very noticeable increases in sound intensity can be heard at certain frequencies in conjunction with certain tube lengths." However, isn't the reflected sound wave produced will always be in phase with the incoming sound wave?

Last edited by a moderator: May 6, 2017