Intro to Sound Waves (Brain Dump)

Still needs some work, but this combination is seeming like a good intro to sound waves:

(Note: Students have been learning about springs, oscillations, transverse waves on strings, etc)

Discussion: Is air springy?

Students were asked to come up with a list of ways that we interact with air in our lives or make use of air in technology. Students are then asked to consider the question: “is air springy?” If not, why not? If so, how so? (We’ve talked a lot about things that act springy but aren’t springs). Then, what is it about air that can makes it seem so unspringy? When does the springiness of air become more apparent? What properties of air give it this springy quality?

Pressure Gauge Exploration : Students then explore the springiness of air using a vernier gas pressure gauge and a syringe. Students are asked questions about “Equilibrium” pressure, what happens when you compress air / expand air. Why? How does that amount of pressure change relate to amount you stretch or compress the pocket of air in the syringe? What happens when “let go”? How is a pocket of air similar / different than a spring? Students are then guided to make pressure oscillation graphs by pushing and pulling the syringe in out with high frequency, low frequency, high amplitude and low amplitude.

Visualizing compression waves on speakers, slinkies, and a video.

A few demos are done around visualizing– including watching a speaker move in and out, using longitudes waves on slinkies to help visualize what happens when a compressed pocket is not trapped / confined. Using slinkies to model showing compressions pulses, rarefaction pulses, and then sinusoidal longitudes waves as a model for a speaker going in and out at a certain pace.

Then watch this short video, that gives us another visualization.

Playing with Microphones:

Students are then introduced to vernier microphone as a pressure (force) sensor that has the advantage of being able to collect data very rapidly. Students are asked to explore using their voice to create different sounds and see what types of pressure oscillations take place at the microphone. Students are asked to try to make a sound that makes a oscillation at the microphone that is as sinusoidal as possible and to determine the frequency and period.

Students are introduced to range of human hearing, and we why don’t “hear” the low frequency pressure oscillations in the syringe.

Designing an Experiment to measure speed of sound.

Students are then introduced to following notion:. That Sound can be hard to investigate for two reasons… high frequencies and high wave speed (the video claims > 700 mph). The microphone is a technology that helps us get access what’s happening on a short time scale (to measure those high frequencies), but that it can also be used as a tool for measuring the speed of sound.

Students are introduced to Triggering in Logger Pro, and how to make a good pulse by snapping their fingers. Students are showed how to set up a PVC pipe to record sound pulses and their echoes. Studens are asked to design an experiment to estimate the speed of sound from the echo timing data.

Refining our Technique:

Because students are not careful about identifying pulse timing carefully and only typically use 2 data points, we get decent errors and spread in our data. Also some groups will not use correct distance. This gives us an opportunity to clarify the experimental situation and to refine our data experimental technique.

1. Careful data marking–how to identify a consistent feature of pulse, including how pulse inverts upon reflection.

2. Plotting distance vs time of many echoes to use slope.

Using this technique we get really good data for speed of sound. There then are a bunch of lab questions to press students to make connections, including,

– what would and wouldn’t change with a longer PVC pipe.

– what would and wouldn’t change on a warmer day (from text reading)

– to calculate the wavelength for sounds Ross human hearing range based on their speed is sound.

Blah blah blah…

Brain Dump on Wave Speed on Guitar strings Problem Redesign

For next time around, here’s the problem I’m redesigning for students to work on during no our first day of traveling waves, which comes after various explorations, discussions, exercises, clicker questions. For context, the day only focuses on making different shapes pulses, measuring wave speed on slinkies, and what factors do and don’t effect wave speed. A lot of this will then orient us to following question. How fast do pulses travel along guitar strings when you pluck it?

Each group will be given a loose guitar string sample and the manufacturers recommended tensions. A guitar will be at front of room for examining and measuring, as is a scale and meter sticks.

The subgoal will

Need to be determining the linear mass density of their string, its wavespeed, the echo time on their string of the guitar (pulse to travel down and back once), and finally how many echoes reach the bridge of the guitar in a second (echo frequency).

Students will add their data and calculations to a large table at front whiteboard in the room. They will need to double check at least one other group’s work.

Follow ups:

1. There will be questions to structure our talk about patterns in data. What patterns do you notice? Which strings had fastest moving pulses, why? How does echo time compare across strings? Etc.

2. We will listen to each string using microphone in Logger Pro and compare period for microphone oscillation to predicted echo time- a gentle pluck can be used to keep the higher harmonics subdued. The two should be close to the same –> I can conspire for them to be the same if needed.

3. Follow up questions might ask about how the acts of using tuning peg and fretting change the echo time, but in different ways.

Still needs lots of detailing, but I just needed to get the overall idea down.

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