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Planning Thoughts for physics 2 (sound waves)

September 15, 2017

In 2nd semester physics, we are moving on to sound waves and with that energy, power, and intensity. 

My goals is to juxtapose two things we know… one from everyday life and another from our last unit on oscillations. 

— what can damage our hearing at a loud concert (volume, proximity, pitch, duration)

– what makes oscillations more energetic (amplitude and frequency) 

We will look at oscillations at different proximitues to graph amplitude vs distance from source.
We will also look at different frequency graphs of same amplitude and of same duration. 

This will raise two questions for us: Why does amplitude go down with distance? How do higher frequencies of similar amplitude transmit more total energy in same duration? 

–> I’m going to lean on some invention tasks and direct instruction to get us learning about and distinguishing energy, power, and intensity. 

We will return to the questions above once we have the intensity tool. 

I want /need to navigate this toward problem solving of the type where a speaker has a certain power output, how much energy is being delivered to ear each second depending on how far they are from the source. 

5 Comments leave one →
  1. September 15, 2017 6:21 pm

    I’m curious how you will justify this question: “How do higher frequencies of similar amplitude transmit more total energy in same duration?” Perhaps you are using a different definition of amplitude than I do?

    • September 15, 2017 7:36 pm

      I’m open to being wrong here. But yes, I do think it depends critically on what definition of amplitude you are using. Sort of like this — https://physics.info/intensity/ . We are talking about physical displacement, and so I think it does have a frequency dependance. we made sense of in class by boot strapping from what we know about oscillators E = 1/2kA^2… any give oscillation depends only on the amplitude, but higher frequencies mean that energy cycle is passing through more times per second. Maybe this is a bad way of talking about it.

  2. September 16, 2017 4:18 am

    Sound is usually measured as a fluctuation in pressure, not as a displacement. The displacement equation in the physics.info article involves frequency, but the pressure equation does not.

    • September 16, 2017 1:01 pm

      Yes, I agree. That’s why I linked to that. We explicitly talk about the displacement, because we are trying to be more explicitly drawing connections to mechanical oscillators, whose energy depends on both amplitude and frequency. It could be a good idea to bring up that pressure amplitudes are not frequency dependent and why.

    • September 27, 2017 10:45 am

      We decided as a class that higher frequencies produce higher pressure for same displacement. This makes sense because the displacements are occurring in less space, so the displacements allow particles to bunch up more. So higher frequencies of same pressure are same energy. That’s how we are making sense of it for now.

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