Collecting Sound Waves

1
Collecting Sound Waves
Acoustical Analysis of A Crescent Valley Classroom
Chaz Matteson and Andrew Marshall Special
thanks to Miss McKeen and Mr. Sherwin for the use
of their classrooms.
The Project
Acoustics is important in the design of any room
where communication is important, such as a
classroom. Humans can perceive three different
aspects of sound loudness, pitch, and quality.
These perceived qualities correspond with
qualities of the sound waves. Loudness
corresponds with intensity, which is expressed in
decibels. It signifies the amount of sound waves
flowing through a certain area. Pitch
corresponds with the frequency of the wave.
Frequency is expressed in Hertz (Hz). The
frequency of a sound wave is the number of
compressions passing through a given point in one
second. The relative ratios of frequency also
have effects on the quality of sound. A
frequency ratio of 21 represents an octave in
Western music. Sound quality usually
corresponds with sound pressure, but not always.
Sound quality helps humans distinguish the source
of a given sound. Sound pressure is the force
that vibrating molecules of air exert on an
object. It is usually measured in atmospheres,
or Newtons/meter2. Humans have a limited range
of hearing. There is a threshold of hearing,
where humans start perceiving sound, and a
threshold of pain, where sound becomes harmful.
The human threshold of hearing starts at 0
decibels, 20 Hz, or 2 10-10 atmospheres. The
threshold of pain starts at 130 decibels, 20,000
Hz, or 0.0006 atmospheres. This experiment is to
investigate how chairs and tables in a classroom
affect sound pressure. The frequencies that make
up the sound pressure will also be analyzed to
see if there is a general correlation between
expressed frequencies and a rooms contents. It
is expected that the range of the sound pressure
will decrease when there are more obstacles in
the room. The frequencies that are strongest are
expected to change based on the rooms contents.

• Points A-E shown in Figure 1 show where the
  Vernier microphone was held during data
  collection.
• The source of sound for the experiments was a
  concert B flat pitch being played on a Getzen
  Eterna cornet at a consistent volume to the best
  of the musicians ability as show in Picture 1.
• The Vernier microphone was connected to a laptop
  running Logger Pro 3.3.1 and graphing sound
  pressure over time.
• Two measurements were taken at each point for
  each room configuration
• One collected 200 data points in 0.5 seconds
• The other collected 200 pieces of data in 2
  seconds
• The experiment was performed with two changing
  variables
• The presence of chairs in the room.
• The presence of tables in the room.
• The two measurements above were taken at points
  A-E when the room had both chairs and tables as
  shown in Picture 3, only chairs, only tables, and
  without chairs and tables as shown in Picture 2.

Picture 2
Picture 3
Picture 1
Acoustical Comparisons
Connecting the Waves

• Table 1 shows that the emptier the room was, the
  lower the minimum recorded pressure. The empty
  room and the room with just chairs had low
  minimums whereas the full room and the room with
  only tables had high minimum values. Likewise,
  the maximum value was also larger in the emptier
  rooms. The mean was relatively the same
  throughout all the tests.
• The standard deviation for each room got smaller
  the fuller the room was. This shows that there
  was more variation in sound pressure in the
  emptier rooms.
• There does not seem to be a correlation between
  the frequencies recorded and the contents in the
  room. Frequency was affected, however, by how
  direct the sound was. Our recordings picked up
  more frequencies as the sound got more and more
  direct.

The collected data has shown several interesting
correlations. The range of recorded sound
pressure did decrease as the room was filled with
more obstacles. However, the chairs did not
impede the sound pressure nearly as much as the
tables. As shown by the data, the chairs have
roughly the same recorded pressures and standard
deviations as the empty room this data is
recorded in Table 1. Likewise, the tables have
nearly the same results as a full room. Thus,
the chairs have little effect on the diffusion of
sound pressure in a room. The frequencies
recorded yielded unforeseen results. The
presence of chairs or tables in a room did not
seem to have a constant effect on how the
frequencies were expressed. However, the
direction of the sound did have an effect on the
frequencies that were recorded. As shown by our
data, the most direct soundthe sound at point
Cconsistently had a wider range and greater
magnitude of recorded frequencies. This is
probably because the direct sound was recorded
before bouncing off of as many walls, tables, or
chairs. This would give less of a chance for
minor frequencies to drop out of the
spectrum. These results can show how different
classroom layouts can affect different parameters
of sound. A layout minimizing table usage will
have less of an effect on the sound pressure in
the room. Likewise, a layout that allows for
direct reception of sound from a sound source
will ensure that the full ranges of frequencies
are received. A good example of this is shown in
many lecture rooms where chairs have small
writing spaces that are attached to the side of a
chair rather than bulky tables.
Table 1