This presentation will describe a method of measuring the speed
of sound. The method involves using tuning forks and listening
for resonance in closed pipes. Click on a topic below to view its contents.
 Purpose
 Materials
 Theoretical Derivation
 Procedure
 Data Collection & Results
 Conclusion
The purpose of this laboratory experiment is to measure
the speed of sound in air.
Sonar, echo location, medical ultrasound all take advantage of
the “Doppler Effect” to produce useful results.
The Doppler effect is the perceived change in frequency of
sound from its source depending on the relative motion
between the source and observer (listener).
The Doppler effect exists because the speed of sound is
finite. Knowing of the speed of sound is critical in
understanding the Doppler effect and applications of sound!
The following materials are needed
to complete this lab:
 2000 ml Graduated cylinder
 Meter stick
 ½ inch diameter PVC pipe
 Tuning Forks –
256Hz, 426Hz, 512Hz, 1024Hz
 Water Supply
 Paper towels
 Paper, pencil, calculator & patience
L
1
4

L
3
4

L
5

L
7

4
4
The pipe above is closed at its bottom end. Resonance occurs when
standing waves form in the pipe. In this case a node is at the
closed end and an anti-node at the open end. The first four
simplest resonant waves are shown. The simplest one is called the
fundamental. The subsequent waves are called harmonics. Notice
the pattern between the resonant wavelengths and the fixed
length of the closed pipe. Only odd harmonics fit.
If we let n = 1, 3, 5, 7, … we can write a general expression for the
resonant, standing, harmonic wavelengths as:
n
L 
4
Algebraically substituting the general expression of the
“resonant wave” into the basic “wave equation” results in:
n
L 
4
Do the algebra !
Substituted into 
 4 L
v  f
 n
v  f
; n = 1, 3, 5, 7, … (odd harmonics)
This reveals an expression for the speed of sound in terms
of the resonant length of a closed pipe, the frequency of
the sound source, and the nth harmonic.
1.
Fill the graduated cylinder 7/8 full of water.
2. Insert the PVC pipe into the water filled
cylinder.
3. Activate the tuning fork and hold it ¼ inch
above the PVC pipe.
4. Slowly lift the PVC pipe & the tuning fork
out of the water filled cylinder.
5. Listen for resonance and stop.
6. Measure the effective pipe length from the
water level to bottom of tuning fork.
7. Record:
a.
Frequency of tuning fork
b. Harmonic number
c.
Effective pipe length
8. Repeat for next harmonic (some frequencies only have
one harmonic because the pipe is too short).
9. Change tuning fork frequency and repeat procedure.
Frequency (Hz)
Water level (m)
Fork level (m)
Effective length (m)
nth harmonic
Speed of sound (m/s)
256
0.10
0.44
0.34
1
348
Make a data table to record your measurements.
Be sure to include proper units of measure and to be
organized.