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Sound wave

  1. Nov 5, 2011 #1
    I noticed that once I was talking to a friend close to me and then a vehicle passed by with a huge sound and then I merely heard what he said.

    Can someone explain me why ? even though that friend was close to me ?

    Does it have any connection with the Doppler effect ?
  2. jcsd
  3. Nov 5, 2011 #2


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    You probably were surrounded by "white noise" while that speeding car passed by. The noise it generates has a masking effect and drowns out conversations, so it's no surprise you barely heard what your freind said. Although there is a doppler effect when a moving sound source passes you, it does not account for what you have asked about. For a great article on the sound a passing car generates see:


    Be sure to use google and wikipedia to find answers to your questions...they are your friends.
    Last edited by a moderator: Apr 26, 2017
  4. Nov 5, 2011 #3
    He said "merely" not "barely". As it is the question is ambiguous. The way it's phrased now implies that all he heard was his friend speaking and he did not hear the truck at all.
  5. Nov 5, 2011 #4


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    Dr Morbius, of course you are correct. I did assume the OP's meaning. Jadaav needs to rewrite his question in correct and clear English.
  6. Nov 6, 2011 #5
    Right, then I rewrite. Thought it was clear what I first wrote but never mind.

    The other day I was talking to a friend who was near me at about not more than 2m, along the main road. Some time later, a lorry passed by and I couldn't hear what he said during that particular moment. I could only see his lips moving.

    When the lorry went, then I could hear him again. I wanted to know how is it possible that I couldn't hear my friend talking but instead heard the sound of the lorry ? What happened to the sound wave which came from my friend ?

    I hope, I'm clear this time.:smile:
  7. Nov 6, 2011 #6
    Last edited by a moderator: Apr 26, 2017
  8. Nov 6, 2011 #7
    you said: 'a vehicle passed by with a huge sound'

    The 'huge' sound from the vehicle did nothing to your friend's sound. It cannot!
    the amplitude induced at your eardrum by the vehicle's 'huge' sound was high; much more than your friend's sound. So even though both the sounds reached your eardrum, you simply could not differentiate between the two: your eardrums were already too stressed and could not afford to vibrate for your friend's sound simultaneously.

    comments welcome.
  9. Nov 6, 2011 #8


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    You couldn’t hear your friend when the lorry passed because his voice was masked by the loud (white) noise from the passing lorry. Once the lorry passed by you could hear him again. Try this website for a good explanation:


    Here is a sample paragraph from the above site:

    Noise from tire-roadway interface and engine exhaust noise.

    This category includes delivery vans, such as UPS and Federal Express trucks, large sport utility vehicles with knobby tires, large diesel engine trucks, some tow-trucks, city transit and school buses with under vehicle exhaust, moving vans (U-haul-type trucks), small to medium recreational motor homes and other larger trucks with the exhaust located under the vehicle. Typical noise levels for medium trucks are 80 to 82 dBA at 55 mph at 50 feet.
    Last edited by a moderator: Apr 26, 2017
  10. Nov 6, 2011 #9


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    Also, consider what I like to call "contrast". (Or signal to noise ratio)
    Before the vehicle, your friend's voice (the signal) is much higher than the background (the noise). When the truck passed by the background increased so high that the signal to noise ratio was much much lower than it was before. Similar to being able to clearly see something black on a white background compared to barely being able to see the same thing when it is a very very light gray instead.(Contrast!)

    So, assuming the sound wave made it to your ear without being disrupted, and your ear still processed your friends voice, the difference between his voice and the truck was so small that you simply couldn't make it out.
  11. Nov 8, 2011 #10
    Right got it now.

    So when the truck/lorry came near, its amplitude that is, its loudness increased. Thus it overcome the loudness of my friend's voice. My ear did process both sounds but I simply couldn't differentiate between them.

    Is it right ?
  12. Nov 8, 2011 #11
    Jadaav: try to develop confidence; physics is not something to memorize.
  13. Nov 8, 2011 #12


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    Yes, Jadaav, you've got it now!
  14. Nov 9, 2011 #13
    That's the reason I like physics so much:D You just need to understand it:)

    Thanks to all of you for helping me.
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