RESONANCE LAB
Resonance occurs when a vibrating object causes another object to vibrate at its natural frequency. We
often depend on resonance to hear sound waves. Musical instruments use resonance to amplify the intensity of
vibrations, thereby amplifying the volume of sound. Sound waves moving through a pipe cause the pipe to
vibrate at the frequency and that resonance can amplify the sound to a more easily heard volume.
Today we will use a resonance tube to calculate the speed of sound in air. A
resonance tube is a very thin-walled glass tube that can amplify low energy sound
waves. These tubes are very fragile and expensive, so please use care while you
handle them! We will use the tube as a closed-end pipe to determine the wavelength
of the sound waves from a tuning fork. The length of the resonance tube will be adjusted
by submerging it in water, effectively creating a closed-end pipe. Resonance will occur if
a vibrating tuning fork is held above the tube and the length of tube corresponds with the
wavelength of the fork’s frequency. The resonance will occur when the length of the
L
tube is equal to ! !, as shown in the diagram to the right. (In a laboratory situation, a
correction factor must be added to the length of the resonating air column. The correction
factor is .4 times the diameter of the column.)
!/4
Once the wavelength of the frequency is calculated, the speed of sound can be
found using the wave formula:
v= f!
We will find the velocity of sound based on resonance and compare that to the predicted velocity of sound
based on the room temperature.
Before you do the lab, read the procedure thoroughly. Use the attached data table for recording necessary
data. Corrected L is the length of the air column plus the correction factor. Remember to be as accurate as
possible and measure as precisely as the measuring device will allow. Write in ink on a separate sheet of paper
for your lab write-up. Each lab group needs to turn in one lab report.
MATERIALS
Resonance Tube
Large graduated cylinder
Tuning forks – You will need four. Take one at a time and trade with other groups after you are done.
Tuning fork mallet or rubber stopper to activate fork
Water
PROCEDURE
Record the room temperature from the thermometer on the front counter. Obtain lab materials and fill the
graduated cylinder most of the way with water. Measure and record the diameter of the resonance tube. Place
the resonance tube gently in the graduated cylinder. One person should gently strike the tuning fork with a
mallet and hold it approximately a centimeter above the open top of the resonance tube. The same person
should slowly raise both the fork and tube, maintaining a consistent distance between the two. BE CAREFUL
NOT TO ALLOW THE TUBE TO COME IN CONTACT WITH THE VIBRATING TUNING FORK, AS
THIS WILL SHATTER THE TUBE! Observe by listening for when the tube resonates at its maximum volume.
At this height, the person holding the tube should stop and another group member should measure accurately
from the top of the tube to the water level. Once the data is recorded, a different group member should repeat
the procedure. This should be done by all members of the lab group, and the results of each trial should be
recorded. Once all the data is recorded, calculate the average speed of sound using that frequency. The data
should be collected for three tuning forks of different frequencies.
CALCULATIONS - show all work!
1. Show your calculation for the correction factor. (note the tube diameter somewhere)
2. Show one calculation for the speed of sound for each of the tuning forks.
3. Calculate the average speed of sound for each of the frequencies.
4. Using the room temperature, calculate the accepted speed of sound.
5. Calculate % error for each of the average v’s:
accepted-experimental
%error =
!100
accepted
6. Calculate the third harmonic of a 1.00 meter tube when the speed of sound is 366 m/s.
QUESTIONS
1. Give at least four other examples of resonance, i.e. where one object’s vibration starts another object
vibrating.
2. Name two musical instruments which act as closed-end pipes.
3. Are you surprised by the accuracy of your speed of sound calculations? Why or why not?
4. If you calculated a very large difference between your experimental speed of sound and the actual, come
up with a reasonable explanation why and use that explanation to recalculate the speed of sound. (HINT:
This most often happens with the high frequency tuning forks.)
Tube Diameter (m) =
f (Hz)
room temp ( C)=
L (m)
corrected L (m)
! (m)
v (m/s)
average v