Intensity of Sound_Worksheet

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					                                               Intensity of Sound

Sound Levels:

Commonly measured in Decibels (dB), the relative intensity of sound is based on a logarithmic scale, much
like the Richter scale for measuring earthquakes and the pH scale for measuring acidity.

The equation for measuring the relative intensity of sound is:

                                                dB = 10 Log
              –12      2
where I0 = 10 W/cm which is known as the threshold of hearing, or the lowest level of sound the
“average” person can hear.


   1. What is the relative intensity of a sound which has an intensity of 10-9 W/cm2?
   2. What is the intensity of a sound which has a relative intensity 110 dB?
   3. A pneumatic drill has the same intensity at 2 feet away as a jet plane’s engine has a 1000 ft away, 200
      dB. What is the intensity of this sound in W/cm2?
                                                 Doppler Effect

                                    Moving Source, Stationary Observer:

Source approaching the listener;                     Source moving away from the listener;
[aka listener in front (Lf)]                         [aka listener behind (Lb)]

                                                                  
fLf = fs  V                                        fLb = fs  V 
         V V                                               V V 
             s                                                  s 

                                    Stationary Source, Moving Observer:

Listener approaching the source;                     Listener moving away from the source;
[aka listener closing (Lc)]                          [aka listener opening (Lo)]

           V  Vs                                              V  Vs 
fLc = fs 
                                                   fLo = fs 
                                                                      
          V                                                  V 


    1. While you are sitting at a railroad crossing waiting for an approaching train, the engineer sounds the
       whistle (f = 500 Hz).
          a. If the train is approaching at 20 m/s and the air temperature is 20 C, what frequency will you
          b. After the train passes you the whistle sounds again, now what frequency will you hear?
    2. You are driving through the neighborhood and pass a car whose alarm is going off (f = 700 Hz).
          a. If you are obeying the speed limit of 25 mph (11.3 m/s), what frequency will you hear as you
               approach the car?
          b. What frequency will you hear after you pass the car?
    3. On the freeway, you are driving at a law abiding 60 mph (100 km/h) when an obnoxious driver passes
       you at 80 mph (133 km/h) blowing his horn (f = 100 Hz), what frequency will you hear as he passes
       you? (hint: with what relative velocity does he pass you, naturally if he is obnoxious the driver is male)

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