Science Buddies: "Ask an Expert"

Hi, I am doing an experiment where I go to different locations and from a half mile distance record the speed of sound. I did this by using walkie-talkies and blowing an air horn on cue, having the other person record how long it took for them to hear the noise using a stopwatch. I still need a few of my questions answered for an “interview” of someone in my projects field of science (physics). The locations I conducted this at were a control (flat empty field), a forest, a hilly area, and a coast, as well as all of these in the rain. I conducted the test 3 times per location and found the average. The results are listed below. I would greatly appreciate a reply to my questions.

Control (flat field): 2.51 seconds
Control/Rain: 2.28 seconds
Forest: 2.72 seconds
Forest/Rain: not completed
Hills: 2.58 seconds
Hills/Rain: 2.41 seconds
Coast: 0.72 seconds
Coast/Rain: not heard at all

1.Are things such as trees and leaves normally known to reduce the sound?

2.When it rains, the sound’s laminar will go under major stress, but sound travels faster in water, do you think the acceleration of the water will outweigh the stress of the laminar?

3.The first time I did the experiment in the forest, I didn’t hear the sound at all, is it possible that all the sound bounced back or got absorbed by the dense population of trees and leaves? (I later went to a slightly less dense forest, and it worked great)

4.Will sound become slower or faster after going through a solid, since sound travels faster in solid yet can also be absorbed into the solid or even be slowed down when some of it bounces back?

5.What do you think of the results for my experiment?

6.Seeing how quickly that sound travels in humid temperatures, such as the coast, how long (distance wise) do you think the sound would travel?

BDLsciencefair12,

Welcome to science buddies and thanks for your questions. You have a very interesting project. Good use of a control experiment (i.e. flat field). One issue I see with your data aquisition is some of the other effects the environment can have on a wave, in your case a sound wave. Things like scattering and absorption. It looks like you have a good set of data, just be sure to discuss the control variables (i.e. wind, humidity, etc...)

To try and answer your questions:

BDLsciencefair12 wrote:1.Are things such as trees and leaves normally known to reduce the sound?

As your sound wave travels away from you, it does so in all directions at a uniform velocity. The energy (power) of the wave reduces as an inverse square of the distance it has propagated (P = pi/d^2). What this means is your signal gets really week really fast as it moves away from you. When it comes in contact with another medium, like a tree, some of the wave will be absorbed, some will be reflected, and some will may pass through. Hard to say what effect you are measuring here, especially given that a forest isn't a single medium.

BDLsciencefair12 wrote:2.When it rains, the sound’s laminar will go under major stress, but sound travels faster in water, do you think the acceleration of the water will outweigh the stress of the laminar?

Your data actually seems to support this hypothisis, with faster times recorded for rainy conditions in locations over the dry conditions in those same locations. However, it seems likely that the energy of the wave would be disipated faster in a higher density atmosphere and therefor not heard as loud or as far, as evidenced by not hearing the air horn signal when raining in certain locations. I think this is also supported by your data in that the time recorded under dry conditions near the ocean (higher humidity) was much faster than dry conditions in an open field.

BDLsciencefair12 wrote:3.The first time I did the experiment in the forest, I didn’t hear the sound at all, is it possible that all the sound bounced back or got absorbed by the dense population of trees and leaves? (I later went to a slightly less dense forest, and it worked great)

This is likely the case. As mentioned earlier, there will be some absorbtion and reflection of the sound wave anytime it comes in contact with a change in medium. However, it is difficult to pinpoint the exact reason due to the shear number of variables involved. For example, how dense is dense? How were the trees in each forest location? Were they big, little, same or different species, young or old growth? How about the size shape of the leaves? How was the gound, was it covered in foliage, how about the dirt, was it soft or hard, wet or dry, etc...?

BDLsciencefair12 wrote:4.Will sound become slower or faster after going through a solid, since sound travels faster in solid yet can also be absorbed into the solid or even be slowed down when some of it bounces back?

The effect the solid has on your sound wave is going to be dependent on the solid. Each material is going to have it's own characteristics of how it allows the wave to propagate. Namely, the mode of propagation; either longitudinal waves, shear waves, surface waves, or plate waves (if along a thin material). How effieciently the wave passes through the material depends on the how well the atomic particles of the solid material allow and hold the formation of these waves. Longitudinal (or compression) waves are going to retain the most energy and propagate most efficiently.

BDLsciencefair12 wrote:5.What do you think of the results for my experiment?

Your results seem to be in line with what would be expected. Since there wasn't much detail, I am unsure about your method of data acquisition. I can see that there may be a fairly large error bias by trying to measure large velocities with a stopwatch. Because the velocity of sound is quite fast (only a few tenths of a second difference between test cases), even small delays in starting/stoping the stopwatch could skew results. For example, your partner needed to receive a cue to start the stopwatch and then upon hearing the sound, stop it. In both cases, they had to perceive and process these cues and then the brain had to send a signal to the hand to physically perform the action of pushing the right button. These actions take up time that may or may not be in the error threashold of the measurements. Additionally, there are holes in your data where some of your conditions were not tested or data not collected. You need to discuss these in your report and how this missing data may or may not skew the results. Also, there needs to be a significant discussion of control variables, like wind conditions, humidity levels, distracting noises in vicinity of testers, etc... These are all things that you'd ideally like to control, but likely couldn't/can't due to resources available.

BDLsciencefair12 wrote:6.Seeing how quickly that sound travels in humid temperatures, such as the coast, how long (distance wise) do you think the sound would travel?

This depends on the strenght of the initial signal. How big was your air horn and what was it's initial energy level. The intensity of your sound wave will follow something called the "Inverse Distance Law: P ~ 1/r", which states that the Sound Pressure (Intensity) of your sound wave is inversely proportional to the distance from the source. Meaning that the sound wave pressure drops off by 50% as it's distance (r) is doubled. The intensity (or energy) of the wave however follows the "Inverse Square Law: I=1/r^2", which says that the Intensity of the wave will be 1/4th as strong at twice the distance from the source and will continue to decrease exponentially in this manner until it is indistinguishable from background noise. In order to predict this, we must know the initial conditions.

Dear "theborg",
Thank you so much for your response, the info you gave me is very helpful. I will take into account the advice you gave me.

Sincerely,

BDLsciencefair12