Title:
Woofers and Tweeters
Version:
July 25, 2007
Authors:
Colleen McLinn, Jack Bradbury, Monica Plisch, Walt Peck,
and Jim Overhiser
Appropriate Level:
Regents Physics or AP Physics
Abstract:
This kit compares animal and speaker sound production to
demonstrate principles about waves and sound. It engages the
students with the question of why big animals generally make
low-pitched sounds. Students further explore the body sizefrequency relationship for 29 common bird species, using
graphing skills and the wavelength-frequency relationship. A
second lesson focuses on the simpler example of sound
production in crickets to explain the problem of destructive
interference for low-frequency sounds produced by a vibrating
acoustic dipole. A slide show presents analogies between how
crickets use a leaf baffle to avoid destructive interference, and
how speakers are housed in a box. The students elaborate on
this idea by investigating the sounds produced by piezoelectric
buzzers of different sizes and without housings. While
reporting back about what they observed, students evaluate
their thoughts on universal challenges to sound production and
strategies for producing more audible sounds.
Time Required:
Two 40-minute class periods
AP Physics
Learning Objective:
Standard 6—Interconnectedness: Common Themes
3.1 Describe the effects of changes in scale on the functioning
of physical, biological, or designed systems.
5.2 Search for multiple trends when analyzing data for patterns,
and identify data that do not fit the trends.
Standard 4—Science: Physical Setting
4.3 Explain variations in wavelength and frequency in terms of
the source of the vibrations that produce them, e.g., molecules,
electrons, and nuclear particles.
ii. draw wave forms with various characteristics
iv. differentiate between transverse and longitudinal waves
vi. predict the superposition of two waves interfering
constructively and destructively (indicating nodes, antinodes,
and standing waves)
NY Standards Met:
Center for Nanoscale Systems Institute for Physics Teachers
632 Clark Hall, Cornell University, Ithaca, NY 14853
www.cns.cornell.edu/cipt/
cns_contact@cornell.edu
7/07
Objectives:
• Students will apply prior knowledge of how sound travels as a longitudinal wave,
involving compressions and rarefactions of air molecules.
• Students will be able to describe how frequency is related to body size, and gain
experience using the wavelength-frequency relationship.
• Students will compare similarities and differences between animal sound production
and speaker sound production, focusing on the concept of an acoustic dipole.
• Students will investigate the effect of destructive interference in a real-world
situation, and be able to describe strategies for minimizing it.
Class Time Required:
These two labs on sound production can be done in sequence, or separately. Part I
focuses on what the general relationship is between body size and sound frequency in
animals, and part II introduces one reason for why.
• Part I: Introduction to body size and sound frequency; bird graphing exercise, one 40minute period
• Part II: Introduction to cricket sound production and destructive interference; speaker
investigation, second 40-minute period
Teacher Preparation Time Required:
30 minutes. Study how the sample spectrograms (teacher background section or
slideshow) represent frequency, amplitude, and time. If you are not already familiar with
how a speaker works, see teacher background section or the websites in the references.
See the teacher background section for information on animal sound production and
acoustic dipoles.
Materials Needed:
The following are available in the kit “Woofers and Tweeters”, available from the CIPT
lending library at: http://www.cns.cornell.edu/cipt
• Slideshow of animal sounds and images
• Small computer speaker to play animal sounds for class
• Slideshow on spectrogram representation of sounds
• Slideshow on cricket sound production and destructive interference
• Box of combs to demonstrate cricket sound production
• Piezo buzzers with stereo phone plugs (boxed and unboxed, more than one size)
Provided by teacher:
• Internet access
• Microsoft Excel or similar program to tabulate data and create graphs
• Clear ruler to place on screen when estimating minimum bird sound frequency
• Computers with freeware program such as NCH Tone Generator, or frequency
generators
• Paper, tape, and scissors
Optional Extension to Part II:
• Sound pressure meter or microphone and sound analysis software
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Teacher Section - Woofers and Tweeters
Lesson Plan:
Engage
• Show clips of big and small animals: introduce question of why small animals make
high-pitched sounds and big animals make low-pitched sounds
• Propose rule that sound frequency is related to body size
Explore
• Do bird graphing exercise to explore this principle
• Gain experience calculating wavelength from frequency
Explain
• Introduce how crickets produce sound and problem of destructive interference
• Brainstorm ways around this problem, and use slideshow to illustrate how cricket
adaptations mimic those of speaker design
Elaborate
• Speaker investigation—students use different types of speakers (boxed or unboxed,
with and without a baffle, large or small) and compare the frequency response of
different speaker designs
Evaluate
• Students report back about their results and answer thought questions about animals
and speakers
Background Information Students Need:
Students should understand general wave properties, including wavelength, frequency,
and amplitude prior to the start of this lesson. If the students are not yet familiar with the
wavelength-frequency relationship, allow extra time to address it before graphing the bird
data. Part II could be used as a review of how sound travels as a longitudinal wave, or
used to introduce this concept by allotting extra time with the animations listed in the
references. Part II is a good application of the principles of destructive interference and
the inverse-square law in action.
Background Information for the Teacher:
Instead of starting with physics and then giving biological examples, this unit is designed
to engage students in wondering why different-sized animals make different sounds, and
then moving into the way in which physics limits and shapes animal sound production
and speaker design. Generally speaking, animal communication includes generation of
vibrations, modification and filtering, coupling to the medium, propagation, and
reception. This lesson focuses on generation of vibrations and propagation (destructive
interference and the inverse square law). It does not explicitly address resonance,
filtering, and amplification of animal sounds, which although those topics would be an
excellent next lesson.
A spectrogram is a graphical display of a recorded sound with time on the
horizontal axis, frequency on the vertical axis, and amplitude of a particular frequency at
a particular time shown by the brightness of the coloring. This allows one to see how
different frequency components appear and disappear during a bird’s song. (Ref. 1)
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Teacher Section - Woofers and Tweeters
From Raven Lite 1.0 Users Manual (Cornell Lab of Ornithology, 2006)
Dipoles
Most animals make sounds by moving something back and forth along a single
axis. A cricket rubs a sharp edge on one wing over a comb-like series of teeth on the
other wing to make a chirping noise. Mammals pop their vocal cords open and closed,
while birds have masses of tissue at the junction of their bronchi and trachea that flutter
into and out of the airstream passing from the lungs to the outside. A mechanism that
moves back and forth to make sounds is called a dipole because it oscillates between two
positions A and B.
How a speaker works
The speaker in our home or car sound system is also a dipole. It has a cone of soft
material that is moved back and forth along one axis by electrical and magnetic forces to
play our music (Ref. 2). When the cone moves one way, it compresses air molecules on
that side, but leaves a rarefaction of molecules on the other side. The compressed
molecules bounce into the next layer of air molecules which bounce into the next layer,
and the disturbance propagates away from the speaker. On the other side of the speaker,
the layer of molecules just outside the rarefaction zone rush in to fill the space leaving a
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Teacher Section - Woofers and Tweeters
rarefaction behind them. The next layer moves in to fill that space, and the rarefaction
disturbance then propagates away from that side of the speaker. This creates longitudinal
waves of sound.
Destructive interference
Dipoles produce a dumbbell-shaped sound field. Due to their back-and-forth
motion, they produce two wavefronts 180 degrees out of phase with each other (Ref. 3).
Destructive interference tends to occur along the edges of the sound radiating device, and
worsens at lower frequencies (longer wavelengths).
For example, if a speaker plays a low frequency sound, then it moves back and
forth slowly. If it plays a high frequency sound, then it moves back and forth rapidly.
Depending on the size of the speaker (e.g., its cross-sectional diameter), a compression
on one side of the speaker might spread around the speaker and “short circuit” the sound
and be eliminated when it mixes with the rarefaction on the other side. This destructive
interference is only likely to happen if the wavelength of the sound being produced is so
big that the beginning part of the compression has time to move around the speaker to the
opposite side when there is still a rarefaction there. Short-circuiting is a problem for
frequencies with wavelengths about the same size as the speaker or larger. (Ref. 4)
Solutions to this problem, and baffles in particular
There are several possible strategies for dealing with this problem of efficient
sound production. 1. Theoretically, animals could only produce high frequencies. But,
these are not very effective for long-distance communication. (Real-world example: you
can hear car bass from a long way away, but not the treble). Short wavelengths tend to be
more susceptible to heat and scattering losses, whereas long wavelengths can bend
around objects better. 2. Use a baffle: physically separate the wavefronts on each side-increasing the pathlength that compressions and rarefactions have to travel in order to
cancel each other out. Several strategies for this have been adopted in different cricket
species, including singing from a hole in a leaf and singing in a tube-like chamber. There
are strong analogies to speaker design, which can be shown through the slideshow (Ref.
5).
The inverse-square law
As the fixed amount of energy imparted to the molecules around the speaker is
passed on to successive molecular layers, it is spread over a greater and greater area.
This means that the amount of sound energy at any point gets smaller and smaller the
further you are from the speaker. Everyone knows from personal experience that sounds
get fainter with distance from the source.
This also explains why it is difficult for a compression from a previous cycle to
get to a later rarefaction and short-circuit it. If it took this compression more than one
cycle to get to the other side of the speaker, it must have traveled a long way. The further
it has to travel, the weaker it will be due to the inverse square law. So, even if it does
arrive just when the rarefaction is most vulnerable, it will be too weak to cause much
interference.
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Teacher Section - Woofers and Tweeters
Tips for Teachers:
Part I is intended to be conducted in a computer lab with internet access, and no
more than two students to a computer. Although not required, it is a good idea to install
Real Player on the computers where students will be researching bird body size and
frequency on About About Birds (Ref. 6). This will allow students to hear the songs as
well as see their graphical representation. The data are for 29 common species of birds,
but for convenience, the Excel spreadsheet templates divide the number of species in
half, so that the class will generate two similar, but not identical graphs.
You may do the graphing investigation with Excel, Logger Pro, or by hand,
whichever method of graphing is most comfortable for your students. You may wish to
go over the first wavelength-frequency conversion as a class, before showing the students
how to enter an equation in Excel to automate the conversion. We suggest printing the
graphs and asking students to draw lines for wavelength = 1x body size and = 2x body
size by hand, but this could also be done on the computer.
Part II involves a hands-on investigation of the frequency range of various
speaker designs. For convenience, we suggest doing this activity in a computer lab with
NCH Tone Generator installed (Ref. 7), and the speakers plugged into the computer’s
headphone jack. It could also be done in a laboratory with a classroom set of function
generators. Either way, be sure to give clear directions about how high to set the gain for
the experiment. From experience, the volume on a computer needs to be turned up at
least ¾ of the way to be able to hear the smaller piezos.
In the hands-on investigation in part II, students make open-ended observations
about destructive interference. We suggest keeping track of the lowest audible frequency
output by four different speaker designs. Since the measuring device used is their ears,
this once again underlines the relationship between the physics of sound transmission and
the biology of auditory communication. We suggest gathering the class data for the four
speaker designs on the board, and then averaging and/or graphing the results as a
mathematics extension.
However, a more quantitative test could be conducted using a sound pressure
level meter or computer microphones and sound analysis software (such as Raven Lite)
to measure and plot the amplitude of sounds for each frequency tested (Ref. 8).
Theoretically, the students would be able to measure the diameter of the speaker and
estimate its wavelength, then look for a drop in amplitude at longer wavelengths than
body size. Realistically, most speakers are designed to avoid destructive interference,
and can output frequencies far below the minimum predicted. Hence, we have designed
this lab to develop a general understanding of the qualitative relationship, without getting
stalled on the specific frequency response curve of the speakers. We selected piezos as a
simplest possible design, but Daniel Russell of Kettering University has developed
several more advanced demos and investigations using baffled and unbaffled cone
speakers for an undergraduate acoustics course (Ref. 9).
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Answers to Student Questions:
Part I:
1. Record your guesses about what species produced each sound.
Answers will vary.
2. What general pattern do you notice?
Larger animals tend to produce lower frequencies.
3. For the next section, you will need to know how to read a spectrogram. Diagram or
describe what a spectrogram represents. How can you tell which sounds are loud?
Frequency is on the y-axis, time on the x-axis, and amplitude (loudness) is shown by how
bold or brightly colored the lines are.
4. Where do most of the actual points fall?
Under the line of wavelength equal to body size.
5. What fraction of species fall on or below the line predicting wavelengths equal to body
size?
All species: 21/29
Group A: 12/15
Group B: 9/14
6. What fraction of species fall above the line predicting wavelengths equal to twice body
size?
All species: 0/29
Group A: 0/15
Group B: 0/14
7. What are some reasons body size might be related to the wavelength of sound an
animal can easily produce?
Anything reasonable—they may say something about resonance, which is okay.
Resonance plays an important role in the ultimate frequencies that are amplified or
filtered out, but is more important for response-driven systems like a ringing guitar
string. Part II focuses on crickets and speakers, which are source-driven systems, and
more affected by the problems of destructive interference along the mid-line of their
dipole sound fields. Here, we’re just trying to get them thinking and identify prior
conceptions.
Part II:
1. What are some sounds you can hear over long distances? Do they have high or low
frequencies?
Car bass, thunder, elephant rumbles. Generally low frequencies
2. Why do sounds get fainter with distance?
A major reason is the inverse-square law—sound energy is spread over a greater area.
3. Why do crickets produce sound? (Think about biology…)
To attract females and compete with other males/establish territories.
4. How do crickets produce sound?
By stridulation—we discuss snowy tree crickets which run a sharp pick on one wing up
and down a comb-like file on the other wing. Other species stridulate between a leg and
the body, one wing and the body, and other similar mechanisms.
5. A cricket’s sound producing mechanism is called a dipole sound source. Draw or
describe what the sound field of a dipole looks like.
It is a dumbbell shape. Any reasonable drawing.
6. Can you think of any dipole sound producers other than speakers and crickets?
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Any reasonable answer—most mammals do. Bird and mammal sound producing
mechanisms move back and forth along one axis too, although their sound apparatus is
in a tube. Fish often use monopoles—expanding and contracting a swim bladder. A
tuning fork is actually a quadrupole.
7. What are some ways that you could get around the problem of destructive interference
occurring at wavelengths that are large relative to body size?
Only use high frequencies, grow bigger (not realistic!), block the two wavefronts from
interacting somehow.
8. Theoretically, how does a leaf baffle help crickets to produce long wavelengths?
It increases the pathlength before compressions from one side encounter rarefactions
from the other side.
9. Why would distance influence your results? (Hint: see number 2.)
The inverse square law—if you are at different distances during the test, it’ll affect what
you can hear or not hear.
10. Before you get started, which speakers do you predict will have the worst problems
with destructive interference? Make some predictions about what minimum frequency
you think the speakers will be able to produce.
Any reasonable answer—hopefully they can see that an unboxed speaker will be the
worst.
11-12. Data Table and Graphs.
See template in student section. Answers will vary.
13. Describe the general patterns shown in your graph. How do unboxed speakers
compare to boxed speakers? How do large speakers compare to small speakers? How do
unboxed speakers perform with and without a baffle?
Depends on the results, theoretically large boxed speakers should be able to put out the
lowest frequencies, and unboxed speakers at the opposite end.
14. How did your results compare to your predictions?
Answers will vary.
15. Does being larger solve the problem of destructive interference in dipole sound
production?
No, destructive interference is a problem of certain wavelengths relative to the size of the
sound producing mechanisms. So, yes, larger animals can produce lower frequencies,
but they will still have some frequencies relative to body size it is hard for them to
produce without destructive interference, within a reasonable scale of measurement. The
dipole applet can show this in more detail.
16. Do you think any properties of the room could affect your results (e.g., amount of
open space, distance to walls, type of walls)? How?
Yes, you can get reflection off of walls. Any reasonable answer, but students should at
least understand why the speaker was suspended in air away from a table.
17. What could you do to make this investigation more quantitative? What data would
you collect, and what graphs would you create?
Use some kind of sound pressure meter to measure amplitude at each frequency for each
speaker type, and graph it. Ideally, this would be the best way to run the experiment, but
preliminary investigations haven’t shown clear patterns.
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References:
1. Information on Spectrograms:
http://www.birds.cornell.edu/brp/the-science-of-sound-1/what-is-a-spectrogram/
2. Video describing how speakers work:
http://electronics.howstuffworks.com/speaker.htm
3. Dipole sound production:
http://www.kettering.edu/~drussell/Demos/rad2/mdq.html
Excellent animations of sound and wave phenomena, including sound production by
monopoles and dipoles under “Sound fields radiated by simple sources.” A dipole
illustrates a cricket or an unboxed speaker, and a monopole illustrates a boxed
loudspeaker. Animations produced by Dr. Dan Russell of Kettering University, and
includes information on fair usage.
4. An interactive dipole applet:
http://www.falstad.com/wavebox/
A great resource for a guided inquiry of destructive interference in dipoles is this applet
produced by Paul Falstad. This link should open the 3D Waves Applet. Once open,
choose “dipole source” under Setup at the top. You can move the slider to simulate
changes in sound frequency and see how lower frequencies increase the size of the blackcolored areas (where destructive interference occurs). Changing source separation
simulates a change in size of sound producing mechanism.
5. Paper describing cricket baffles as analogies to speaker design:
http://facstaff.unca.edu/tforrest/manuscripts/baffling.pdf
Useful images of crickets and speakers, along with descriptive text, from researcher T.G.
Forrest of UNC Asheville. Originally published in Florida Entomologist in 1982.
6. All About Birds:
http://birds.cornell.edu/AllAboutBirds/BirdGuide/
This website is an excellent source of information about North American bird species.
Includes body size information, and also song clips and associated spectrograms that can
be used to estimate the minimum frequency produced by each species.
7. NCH Tone Generator:
http://www.nch.com.au/tonegen
A program for generating sine waves and frequency sweeps for Mac or PC. A
demonstration version can be downloaded for free.
8. Raven Lite:
http://RavenSoundSoftware.com
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Teacher Section - Woofers and Tweeters
A free program for the visualization of sound frequency and amplitude, developed by the
Cornell Lab of Ornithology. May be used instead of your ears or a sound pressure meter
to record amplitude during the speaker investigation.
9. More advanced acoustics demonstrations:
http://www.kettering.edu/~drussell/anvlabs1.html
http://scitation.aip.org/journals/doc/AJPIAS-ft/vol_67/iss_8/660_1.html
Two more detailed demonstrations about monopoles, dipoles, and quadrupoles designed
for undergraduate acoustics students. From Dan Russell of Kettering University. The
latter requires a library subscription to the American Journal of Physics online.
Other useful references:
10. Online archive of animal sounds and videos:
http://www.animalbehaviorarchive.org/
The Macaulay Library at the Cornell Lab of Ornithology has an online archive where
students can search for more animal sounds and view real-time, scrolling spectrograms
displaying frequency and amplitude (after downloading a free Quick-Time component).
Students can search for sounds and videos of birds, insects, frogs, mammals and fish. As
an extension, Find striped and french grunts sounds: these fish make sounds using a
monopole swim bladder, and experience fewer problems with short-circuiting.
11. Animation of Bird vs. Human Sound Production:
http://birds.cornell.edu/MacaulayLibrary/About/bartelsTheater.html
You can view a streaming 30-s animation of sound production in the human larynx
versus avian syrinx by choosing the Language of Birds clip from the list of Bartels
Theater productions by the Cornell Lab of Ornithology’s Macaulay Library.
12. Animations of Cardinal and Cowbird Sound Production, including 3D:
http://www.indiana.edu/~songbird/multi/media_index.html
Dr. Rod Suthers’ Lab at the University of Indiana studies sound production in songbirds.
They have animations of the production of different Cardinal and Cowbird notes, as well
as a very interesting film showing X-rays and 3D modeling of a singing Cardinal. This
film shows how a cardinal changes the size and shape of its upper vocal tract to remove
harmonics and amplify fundamental frequencies.
14. More Sound Wave Animations:
http://www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/StartCD.htm
More excellent animations of waves, including dipoles, from the Institute of Sound and
Vibration Research at the University of Southhampton.
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Teacher Section - Woofers and Tweeters
Woofers and Tweeters
Part I
Your teacher will show you videos of different animals producing sound.
1. Record your guesses about what species produced each sound.
2. What general pattern do you notice?
3. For the next section, you will need to know how to read a spectrogram. Diagram or
describe what a spectrogram represents. How can you tell which sounds are loud?
To further explore the relationship between body size and sound frequency, we will
gather data on 29 species of local birds.
• Go to the Cornell Lab of Ornithology website “All About Birds”
(http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/).
• The class as a whole will collect data the following 29 species.
Tennessee Warbler, American Goldfinch, Black-and-white Warbler, Black-capped
Chickadee, House Finch, White-breasted Nuthatch, Golden-winged Warbler,
Carolina Wren, Veery, Rose-breasted Grosbeak, Northern Cardinal, Red-bellied
Woodpecker, American Robin, Blue Jay, Belted Kingfisher, Greater Yellowlegs,
Horned Grebe, Short-eared Owl, Cooper’s Hawk, Pileated Woodpecker, American
Crow, Great Horned Owl, Mallard Duck, Herring Gull, Golden Eagle, Common
Loon, Canada Goose, Great Blue Heron, Wild Turkey.
• Your teacher may provide a data table template. If not, create a table with three
columns, labeled “Species”, “Body Size”, and “Minimum Frequency”.
• See your teacher for details on which of the species below your pair will research. If
your teacher provided data table templates, record if you are group A or group
B:___________
• Find the relevant page for each species on All About Birds. (Hint: use the
alphabetical drop-down menu to select a species). Find the body size, and record this
value in centimeters in column 2 of your table. Where a range is given, estimate the
mid-point value.
• Using the “Play sound from this species” link, open a window that shows a
spectrogram of that species’ call or song. The spectrogram shows frequency (or
pitch) on the y-axis, time on the x-axis, and indicates the amplitude (or loudness) of
the sound by the brightness of coloration. Your task is to identify the lowest
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Student Section - Woofers and Tweeters
•
•
•
frequency in the spectrogram that is reasonably loud (i.e., a bold yellow color) and
record this number in column 3 of your table, translating from kHz to Hz if necessary.
Create a fourth column in your table and label it “Actual Wavelength.” The
wavelength of a sound can be computed by dividing the speed of sound by the
observed frequency (λ=v/f). The speed of sound in air is 343 m/s. Convert this to
cm/s, and then calculate the values you want for each cell of column 4 by dividing the
speed of sound in air in cm by each species’ column 3 value (the minimum frequency
in Hz). Your teacher may show you how to automate this calculation in Excel after
you do the first one as a class.
Produce a graph plotting column 4 versus column 2. Label your x-axis “Body
Length (cm)” and your y-axis “Maximum Wavelength (cm)”.
Either print your graph and use a ruler and pencil to draw two lines on the graph, or if
no printer is available, do this using Microsoft Word’s drawing tools.** Draw one
line using observed body length as the x value, and the same number as the y value
(for example, a 10 cm bird produces a 10 cm wavelength along this line). This is the
predicted maximum wavelength of sound that a bird could produce if it can only
produce sounds with wavelengths equal to its body size. Draw a second line in which
the y value is twice the x value (10 cm birds produce 20 cm wavelengths). This is the
line on which a bird’s calls would fall if it could produce sounds twice its body size.
∗
4. Where do most of the actual points fall?
5. What fraction of species fall on or below the line predicting wavelengths equal to
body size?
6. What fraction of species fall above the line predicting wavelengths equal to twice
body size?
7. What are some reasons body size might be related to the wavelength of sound an
animal can easily produce?
To do this in Microsoft Excel, move your cursor to the “B” at the top of column 2 and
double-click to highlight the entire column. Hold down “ctrl” and double-click on the
“D” at the top of column 4 to highlight that column as well. Go to “Insert” and “Chart”,
then choose “XY (Scatter)” and the first chart subtype. Hit “next”, then name your chart
“Maximum Wavelength vs. Body Length.”
**
Microsoft Excel’s drawing tools can be found under “View”, “Toolbars”, “Drawing”.
Choose the straight line icon.
∗
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Student Section - Woofers and Tweeters
Part II
Communicating over long distances
1. What are some sounds you can hear over long distances? Do they have high or low
frequencies?
2. Why do sounds get fainter with distance?
Cricket sound production
3. Why do crickets produce sound? (Think about biology…)
4. How do crickets produce sound?
5. A cricket’s sound producing mechanism is called a dipole sound source. Draw or
describe what the sound field of a dipole looks like.
6. Can you think of any dipole sound producers other than speakers and crickets?
Since dipoles sound by moving back and forth along an axis, they produce wavefronts in
two directions 180 degrees out of phase. Generally speaking, crickets, speakers and other
objects that have dipole sound producers have a problem of destructive interference at the
mid-line of their sound fields. This is particularly bad at wavelengths longer than body
size.
You can see this for yourself at: http://www.falstad.com/wavebox/
Choose “dipole source” under set up and investigate different frequencies and source
separations (body sizes).
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7. What are some ways that you could get around the problem of destructive interference
occurring at wavelengths that are large relative to body size?
Despite the challenge of producing long wavelengths, they are often good for longdistance communication. Your teacher will show you some examples of cricket
adaptations for producing sound and how they are similar to speaker design.
8. Theoretically, how does a leaf baffle help crickets to produce long wavelengths?
Speaker sound production
We will demonstrate why a leaf baffle helps a cricket produce longer wavelengths
using small speakers called piezoelectric buzzers. The goal is to play a range of
frequencies within human hearing range (from approximately 20,000 Hz at the upper end
to 20 Hz at the lower end), and compare the sounds produced for four different speaker
designs:
• Small boxed speaker
• Large boxed speaker
• Unboxed speaker
• Unboxed speaker with a paper baffle
Use paper, tape and scissors to create your baffles. Plug the speakers into the headphone
jack of your computer. Be sure to suspend the speakers in air away from the tables and
objects, and keep a constant distance between the speaker and the sound detector (your
ear!) for all test frequencies.
9. Why would distance influence your results? (Hint: see number 2.)
10. Before you get started, which speakers do you predict will have the worst problems
with destructive interference? Make some predictions about what minimum frequency
you think the speakers will be able to produce.
Directions for NCH Tone Generator: Under “Control Panel”, adjust the computer’s
sound to about three-quarters of full volume. Open the Tone Generator program, and
choose “Tone” then “Constant (Continuous).” Sine wave should be the default, but if it
isn’t, select this at the bottom of the “Tone” menu. Highlight “Sine 1 Frequency” and
increase the frequency to 20,000 Hz. Hit “Play.” As you do the test, decrease in small
steps using the decrease button (single down arrow). Use your ear to detect where your
speaker stops producing audible sounds.
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11. Data Table
Minimum
Frequency
Your Group
Small Boxed
Speaker
Large Boxed
Speaker
Unboxed
Unboxed with
Baffle
Class Average*
12. Draw a bar graph on the computer or by hand comparing the average minimum
frequency of different speakers. Use speaker category for the x-axis and minimum
frequency for the y-axis.
Example (Your data may look different!)
Minimum Frequency
(Hz)
Minimum Frequency for Four Types of
Speakers
60
50
40
30
20
10
0
Small
Boxed
Large
Boxed
Unboxed
Unboxed
with Baffle
Speaker Type
*
To do this in Microsoft Excel: Highlight the values to average, then select “Insert”,
“Function” and “AVERAGE.” To calculate this by hand, use (x1 + x2 + x3 + x4)/N
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Student Section - Woofers and Tweeters
13. Describe the general patterns shown in your graph. How do unboxed speakers
compare to boxed speakers? How do large speakers compare to small speakers? How do
unboxed speakers perform with and without a baffle?
14. How did your results compare to your predictions?
15. Does being larger solve the problem of destructive interference in dipole sound
production?
16. Do you think any properties of the room could affect your results (e.g., amount of
open space, distance to walls, type of walls)? How?
17. What could you do to make this investigation more quantitative? What data would
you collect, and what graphs would you create?
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Student Section - Woofers and Tweeters