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Thread: Wave unit Problem-Need Help

  1. #1 Wave unit Problem-Need Help 
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    Hey Everyone...

    Can anyone please help me and explain me the two problems... (below)
    that would be much appreciated because I have a test soon and
    I am clueless :S Thank you



    1. A 600Hz sound is produced, reflects from a wall that is 800m away, and the echo is heard after 4.92s. Find: the sound period, speed and wavelength and the air temperature.

    2. A loud 600Hz sound is produced in 15 degrees celcius in air. 2.25s later the echo is heard. Find: Speed of sound, period and waelength, distance to the wall the sound was reflected from.


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  3. #2 Re: Wave unit Problem-Need Help 
    . DrRocket's Avatar
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    Quote Originally Posted by Jack280
    Hey Everyone...

    Can anyone please help me and explain me the two problems... (below)
    that would be much appreciated because I have a test soon and
    I am clueless :S Thank you



    1. A 600Hz sound is produced, reflects from a wall that is 800m away, and the echo is heard after 4.92s. Find: the sound period, speed and wavelength and the air temperature.

    2. A loud 600Hz sound is produced in 15 degrees celcius in air. 2.25s later the echo is heard. Find: Speed of sound, period and waelength, distance to the wall the sound was reflected from.
    Why don't you show whatever work you have done to attempt these problems and then we can comment on that work.

    For starts how about figuring out the speed in the first problem, then tell us what you know about the relationship between speed, frequency and wavelength.

    For the second problem, I am curious as to what the significance is to you of the fact that it is a LOUD sound. Is this a high school physics class, a freshman university physics class or an advanced class in the dynamics of compressible fluids? What do you know abou the speed of sound in a gas (relationship with universal gas constant, temperature and ratio of specific heats for instance)?


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  4. #3 My answers... 
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    Sorry about that...
    and this is a high school question...

    1. First of all, find the speed of sound, I used the formula:
    v=331m/s + 0.59 m/s/degree celcius (temp)

    so I plug in the values: v = 331m/s+ 0.59m/s/degree celcius (15 degress celcius)
    = 339.85 m/s in air

    To find the period I use this equation: T = 1 divided by f (frequency)
    = 1 divided by 600Hz
    = 0.0016s

    To find the wavelength I use this equation: Wavelength (lamda) = v divided by f
    = 339.85 divided by 600Hz
    = 0.57m

    2. Given Information: Frequency=600Hz
    distace of wall = 800m
    echo is heard after = 4.92 seconds (s)

    Find: Period, speed, wavelength and the air temperature

    Find Avergae Velocity: V = d over t
    = 800m ivided by 4.92 seconds
    = 162.6 m/s


    ok, The only thing I can for this question is to find the air temperature:

    Temp = v minus 331m/s divided by 0.59
    = (162.6 m/s - 331) divided by 0.59
    = - 285.4 dgrees celcius


    I have no idea how to solve (for the first question) the distance that the sound travels to the wall, and back... Also I am not sure of my answers are correct... please help. And for question number 2, I don't know how to solve for speed, wavelength and period.


    Much Appreciated
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  5. #4 Re: My answers... 
    . DrRocket's Avatar
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    Quote Originally Posted by Jack280
    Sorry about that...
    and this is a high school question...

    1. First of all, find the speed of sound, I used the formula:
    v=331m/s + 0.59 m/s/degree celcius (temp)

    so I plug in the values: v = 331m/s+ 0.59m/s/degree celcius (15 degress celcius)
    = 339.85 m/s in air

    To find the period I use this equation: T = 1 divided by f (frequency)
    = 1 divided by 600Hz
    = 0.0016s

    To find the wavelength I use this equation: Wavelength (lamda) = v divided by f
    = 339.85 divided by 600Hz
    = 0.57m

    2. Given Information: Frequency=600Hz
    distace of wall = 800m
    echo is heard after = 4.92 seconds (s)

    Find: Period, speed, wavelength and the air temperature

    Find Avergae Velocity: V = d over t
    = 800m ivided by 4.92 seconds
    = 162.6 m/s


    ok, The only thing I can for this question is to find the air temperature:

    Temp = v minus 331m/s divided by 0.59
    = (162.6 m/s - 331) divided by 0.59
    = - 285.4 dgrees celcius


    I have no idea how to solve (for the first question) the distance that the sound travels to the wall, and back... Also I am not sure of my answers are correct... please help. And for question number 2, I don't know how to solve for speed, wavelength and period.


    Much Appreciated
    You have some of it. But your speed is wrong, so the things that depend on it are wrong also. If the wall is 800 m away, and if the sound wave travels from the source to the wall, reflects and returns to the source, how far did the sound travel ?

    For the second problem you will apply the same set of equations, but in a different order. Try solving for the various quantities in the order in which they are requested. You have provided all the equations that are needed in your attempt at the first problem. Start with speed, use the relationship that you gave for speed as a function of air temperature.

    And you don't have to worry about the distinction between instantaneous velocity and average velocity. Sound will travel at a constant speed given constant air temperature.
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