Sound Wave Patterns
Amanda Stone, Cassie Leone, Mary Flack
Background information Δ Return to Top
Much of today’s technology utilizes sound waves to create antisound in order to fight noise with noise. For example, militaries have used antisound to make the waves cancel each other and make the engines quieter in jets and airplanes. In cars, engineers are utilizing antisound devices to make quieter mufflers.
Sound waves alternate high and low frequencies (Fighting Noise with Noise, 2008). Interference is when two or more waves meet at the same point as the same time. The two different types of interference are constructive and destructive. Antisound occurs when the crest of one wave overlaps a trough of another (Gale Encyclopedia of Science, 2008). This is called destructive interference, and cancels the wave motion and sound heard. Amplified sound occurs when two waves arrive in phase, which is when the crests arrive together. This amplifies the sound, and causes constructive interference.
Antisound devices cannot function in open rooms because the waves bounce off the walls and create interference, making it extremely difficult to cancel (Today‘s Science, 2008).
High pitched sounds are more easily absorbed by sound barriers than low pitched sounds. For example, when you are listening to music, it is much easier to hear the bass of the music than the lyrics or singing through a wall or automobile. Most antisound devices work best with low pitch sounds (Giancoli, 1980). However, when you are in the room, the human ear is more sensitive to high frequencies rather than low ones (A Weighing, 2008).
Many antisound devices use a microphone that takes in the sound being produced, inputs it into a computer, and then the computer calculates the antisound wave and transmits it through the speakers. When steady sound waves are used, the computer doesn’t have to change the antisound wave frequency as much, making it easier to broadcast (Fox, 1992). Our calculations will be measured in decibels, which is the unit used to measure the loudness of sound (Decibel, 2000).
Statement of Problem Δ Return to Top
Discover where sound waves are canceled and amplified to see if we can create antisound.
Statement of Hypothesis Δ Return to Top
We believe that we will find a pattern of constructive and destructive interference that is manifested by change in the decibel meter.
Method Δ Return to Top
In order to discover a pattern of sounds, an experiment that wouldn’t create interference had to be constructed. Since interference is created by sound waves bouncing off objects or walls, the enclosed area would need to absorb the sound. Our solution was to line a cardboard box with foam, which absorbs sound. The box needed to be big enough to hold both speakers, and be large enough to get a clear picture of the sound wave patterns, if any. The dimensions of our box were 8.5 cm by 42 cm around, and 43 cm deep.
If the position of the speakers changed during the experiment, the sound waves wouldn’t be consistent. To prevent this, we used Velcro to keep the speakers stationary. They were placed at equal angles from either side of the box.
Along the left and right side of the box, markings were drawn in two-centimeter increments. Then a string was marked in the same two-centimeter increments for 42 cm. The string and the box markings were used to make a consistent grid to gather clear data.
The speakers were hooked up to a computer and set at a consistent volume. Using the Test Tone Generator program, a 2000 Hz frequency was emitted from both speakers. The string was taped to the top marking on either side of the box, with the highest and lowest markings being 32 and 12 from the bottom of the box, respectively. Using a decibel meter, which was also covered in foam to not interfere with the sound waves, we measured the decibel reading across the box at every marking on the string from left to right. The decibel meter was set for A-Weighting and between 50 and 100 dB. The power was set to DC and the response was set to slow. We recorded the decibel reading for each mark and continued this procedure for each increment down the box.
Calculations Δ Return to Top
λ = V / f v = 343m/s f = 2000Hz λ = .1715
We used the speed of sound for our velocity, and the test tone generator provided a frequency of 2000Hz.
(L1 – L2) / λ = n λ = .1715 n = 0, constructive n = .5, destructive
We found the lowest points on our graph and found their location in the box. We picked a point in the middle of the speaker and measured from the points we found to both speakers. From that, we received two lengths and inserted them into the formula above. We found the second lowest point and the two highest points, and repeated the calculations.
Results Δ Return to Top
To better analyze our data, we made a 3-D graph. The y-axis is the loudness of the sound, or decibel reading, and the x-axis is the position across the box horizontally in centimeters. The depth is represented on the z-axis. The different decibel readings are represented by the range of colors. We determined that the lowest points on the graph are where destructive interference occurred, and the highest points we believed to be constructive interference. We determined where in the box those four points were, and then we measured from a set spot on each speaker to the corresponding high and low points. Using the calculations above, we found the two lowest points to equal 53.6 and 83.4 and the two highest points were 75.2 and 90.9. In order to be a destructive interference, our numbers needed to end in .5 and to be constructive they needed to end in .0. Taking to account error, the two lowest points were closest to 53.5 and 83.5 and the two highest were closest to 75.0 and 91.0.
The uncertainty in our experiment includes the uncontrollable loudness of the room and the decibel reader itself accounted for minimal interference with sound waves. Since the room we were experimenting in was not entirely silent, the talking from people in the room affected the decibel reading. We also did not have the entire decibel meter or our hands foamed, so those surfaces provided places for the sound waves to bounce off. When we measured the distances from the speakers to the low and high points, we used a string and a ruler, which creates unavoidable human error.
Conclusion Δ Return to Top
Our hypothesis was that there would be a pattern of constructive and destructive interference in the foam box. As shown in the graph, our data definitely showed a trend of alternating constructive and destructive interference. Thus our hypothesis was proven to be correct.
Our results might have shown a different pattern if we had made a three dimensional grid. Instead of making a horizontal and vertical plane, in the future we would do across depth and width multiple times. Also, we learned that ear protection is essential.
Bibliography Δ Return to Top
“A-Weighing.” DiracDelta Science and Engineering Encyclopedia Version 2.3. 2008. 18 Nov. 2008. <http://www.diracdelta.co.uk/sciences/source/a/weighing/source.
“Decibel.” The Columbia Encyclopedia. The Columbian University Press, 2000. 10587. General Center Gold. Gale. Tualatin High School. 20 Nov. 2008. <http://find.galegroup.com/ips/start.do?prodId=IPS>.
“Fighting Noise with Noise.” Today’s Science on File. Feb. 1993. Today’s Science. Facts On File News Services. 29 Oct. 2008. <http://www.2facts.com>.
Fox, Barry. “Technology: Antisound makes it all quiet on the Western front.” New Scientist. December 5, 1992 <http://environment.newscientist.com/article/mg13618504.100-technology-antisound-makes-it-all-quiet-on-the-westernfront-.html>.
Giancoli, Douglas C. Physics: Principles with Applications. 5th ed. Pages 333-334, 362-364. Prentice Hall 1980.
“Interference.” The Gale Encyclopedia of Science. Eds. Brenda Lerner and K. Lerner. Vol. 3. 4th ed. Detroit: Thompson Gale, 2008.
"Sound." Encyclopedia. Reproduced in Today's Science. Facts On File News Services. 30 Oct. 2008. <http://www.2facts.com>.
Links Δ Return to Top
http://en.wikipedia.org/wiki/Sound - Wikipedia page about sound and sound waves.
http://www.umanitoba.ca/faculties/arts/linguistics/russell/138/sec4/acoust1.htm - University of Manitoba website on sound waves.
http://hyperphysics.phy-astr.gsu.edu/Hbase/Sound/db.html - Additional information about decibels.
http://www.ndt-ed.org/EducationResources/HighSchool/Sound/interference.htm - NDT Resource Center website about sound wave interference.
http://www.phys.unsw.edu.au/jw/dB.html - Webpage about basics of decibel measurements and a-weighting.
http://electronics.howstuffworks.com/gadgets/audio-music/noise-canceling-headphone3.htm - Article about applying anti-sound in today's technology.