Physics
PhET Virtual Lab Sound Waves
Spring 2012
Name: ________________________, ____________________________
Pre-Lab reading: Chapter 12: Sound
Procedures: Launch the virtual sound applet
Questions and observations:
Listen to a Single Source tab:
1. Observe the sound waves coming from the speaker (headphones).
a) What do the dark and light bands represent? (Remember, sound waves are longitudinal
waves.)
The light and dark bands represent the compression and rarefaction (high pressure and
low pressure) regions.
The length between each consecutive dark spot is the wave length
b) Why do the waves get lighter with distance from the speaker?
As the waves get further from the source, their intensity drops by one over r^2. They
represent this with color intensity.
c) How does changing the frequency and amplitude affect the depiction of the sound
waves in the simulation? Hold frequency constant and change amplitude. Hold
amplitude constant and change frequency.
Frequency constant and change amplitude, the sound gets louder and the dark waves get
darker.
Hold the amplitude constant and vary frequency, the separation between new waves (dark
spots) decreases as frequency goes up and increases as frequency goes down.
d) How do you think changing the frequency and amplitude affect the sound “heard” by
the listener (you)?
You hear a series of different of pitches and loudness.
e) Which variable created the sensation of high and low pitch? frequency
Which variable created the sensation of loudness? Amplitude.
Select the Measure tab:
2. Press “start” and move the ruler to the center of the speaker.
a) Look at the stopwatch. What do you notice that is strange about it? Why is it
programmed this way?
It isn’t counting seconds like a normal stop watch. It is adjusted to match the scale on
the rule and the actual speed of sound.
b) Describe how you would find the frequency of a wave if the frequency slider
did not have a number display. Test your idea with at least 5 trials (record
them in a data table). Calculate the %error with respect to the frequency
display.
You can do this one of two ways.
1) Synchronize the stop watch with the start of a new wave. Stop the timer after
exactly one wavelength has be emitted. This time is the period and frequency is
one over the period.
2) Use the ruler and measure the wavelength (dark to dark). Then use v=λf. (vsound
= 343m/s)
T1
T2
T3
T4
T5
Tavg
fdisplayed
FCalculated
% error
c) Hit stop and reset, and measure the distance a wave travels in a certain amount of time.
Make a data table and do at least 3 trials. Find the speed of sound using v = d/t.
d) Use the ruler to measure the wavelength of this sound wave. Check the speed
calculated above using v = fλ. Measure λ and use the display on the frequency slider.
e) Using v = fλ as the reference, calculate % error of the calculations from parts d and e
from above:
Select Two-Source Interference tab:
3. Observe the interference pattern made by the sound waves coming from two speakers.
a) Sketch the pattern using shades of gray.
b) Describe what is happening with the waves where you see white spots and dark spots.
They are going through constructive and destructive interferences.
c) Read below
 Observe the two-point source interference pattern shown below. Several points
are marked and labeled with a letter.

An antinodal line extends outward from the sources in the exact center of the
pattern. This antinodal line is referred to as the central antinodal line.

More antinodal lines are present to the left and to the right of the central antinodal
line. These are referred to as the first antinodal line, the second antinodal line,
the third antinodal line (if present), etc.
Draw the other antinodal lines and label them. For additional help: Go to
http://www.physicsclassroom.com/class/light/u12l1b.cfm
D) Read Below

Each antinodal line is separated by a nodal line.

The nodal lines are also named; the first nodal line to the left or to the right of the
central antinodal line is referred to as the first nodal line. The second nodal line
and the third nodal line are found as one moves further to the left and to the right
of the center of the pattern.

Each line in the pattern is assigned a number, known as the order number and
represented by the letter m.

The numbering system associated with this pattern is just as creative as the
naming system. The central antinodal line is assigned an order number of 0.

The first antinodal line is assigned an order number of 1; the second antinodal line
is assigned an order number of 2; the third antinodal line is assigned an order
number of 3; etc. Nodal lines are assigned half-numbers.

The first nodal line is assigned the order number of 0.5. The second nodal line is
assigned the order number of 1.5. Finally, the third nodal line is assigned the
order number of 2.5.
Draw the nodal lines and label them
For additional help: Go to http://www.physicsclassroom.com/class/light/u12l1b.cfm