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Basic physics question

  1. Jan 20, 2012 #1

    I am a primary school teacher and have just been observed by my headteacher and a "senior consultant" in a lesson on sound.

    I asked the children to find the best insulator of an alarm clock from a range of materials wrapped around it.

    The material that worked best was the newspaper. It was the thickest material as there were many pages.

    The kids said that it was the best insulator as it was the thickest.
    I agreed with them.

    I was later told that I mislead the children (ten year olds) and it has nothing to do with the thickness of the material to muffle a sound.

    Are they right? If there is any evidence that backs my theory that the thicker the material, the better it insulates, please point me in the right direction to show them evidence.

  2. jcsd
  3. Jan 20, 2012 #2
    You are definitely correct; but it should also be noted that the particular material properties play a large roll as-well.

    Their perspective doesn't make any sense. Compare a single sheet of paper covering the alarm clock, to a stack of papers (e.g. a newspaper)... done.

    Obviously the thicker the walls in a building, the less you hear your neighbors, right?
    The further you are underwater, the harder it is to hear something from the surface?

    The reasoning:
    Sound waves carry energy. How loud they are is related to how energetic the waves are (louder = more energy). The more material between a detector (e.g. an ear) and the source, the more energy dissipation occurs, and the lesser energy is transmitted.
  4. Jan 20, 2012 #3


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    It wasn't the best insulator because it was the thickest.

    A 1mm "thick" vacuum would probably be a much better sound insulator than 10mm of newspaper. But I guess most primary schools don't have access to a bell jar and a vacuum pump.

    Of course it is true (most of the time!) that for one particular material, a thicker layer will absorb more sound than a thin layer.

    They way to prove or disprove the theory is to use different thicknesses of different materials, so you can compare both different thicknesses of the same material, and the same thickness of different materials.

    That would be a nice example of how science works: you do an experiment, propose an explanation (hypothesis) for the reslts, and then devise some more experiments that might show the explanation was wrong.
  5. Jan 20, 2012 #4
    Hello, Jusdat.

    First let me correct an oversight and welcome you to Physics Forums.

    Then I would like to express sympathy with your predicament.
    The transmission/blocking of sound is very complicated and you are talking about primary school level.
    I can see the explanation with the newspaper as an attempt to talk about the quantity of material between the sound source and the listener.
    This is correct and good.
    However formal concepts about 'quantity of material (matter) are not introduced at primary level.
    I clearly remember a BBC (primary) schools science programme about the bouyancy where the narrator explained that
    " the reason objects float is that they weigh less than water"
    After the programme I was told off for saying that the reason is that the objects were less dense than water.

    Not politically correct to go against the syllabus. Density is not included in the primary syllabus.

    Here we have a similar dilemma. One factor in sound reduction is density. In general you require a much greater thickness of cardboard than of lead to achieve the same reduction. However if you weighed a barrier of lead and another of cardboard showing a similar sound reduction you would find that they have very similar weights. Lead is more dense than cardboard.

    It should also be pointed out that there are other factors than density at work, which is why pillows and bags of sand also work well.

    But at primary level?

    Out of interest what was the 'Official Explanation'
  6. Jan 21, 2012 #5
    Thanks for your responses.

    There are many factors at work and I understand that density, air between materials and the type of mateiral all play there part.

    I was observed for half an hour, they wanted to see 'significant progress' which is almost impossible given their age and the nature of the topic.

    I've had no official explanation as of yet.

    It's common practice to find fault in lessons, they either say its 'too easy' or 'too hard.' Even though there are guidelines to what makes a good lesson, it ends up being their subjective opinion.
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