The Physics
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Opus in profectus

Intensity

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Practice

practice problem 1

Determine the loudest sound possible (the sound with the greatest intensity level) in decibels…
  1. on the surface of the Earth
  2. in the ocean, near the equator, 1000 m below seal level (in the equatorial SOFAR channel)

solution

Sound is a variation in pressure that ranges from…

Pmin = P0 − ∆P  to  Pmax = P0 + ∆P

Where…

P =  instantaneous pressure
P0 =  static pressure
P =  maximum dynamic pressure

No sound wave may vary in pressure more than the static pressure of the medium itself. If it did, the minimum pressure would be less than zero, which is impossible. Sound intensity levels are computed relative to an agreed upon standard reference pressure. This number is different for gases and liquids.

  1. For this part, we'll use the standard value of atmospheric pressure (101,325 Pa) and the reference pressure in air (20 μPa).
     
    LP =  20 log 
    P
    P0
    LP =  20 log 
    101,325 Pa
    20 × 10−6 Pa
    LP =  194 db  
     
     
  2. For this part, we need to first compute the pressure at depth.
       
    P =  P0 + ρgh
    P =  (101,325 Pa) + (1,025 kg/m3)(9.8 m/s2)(1,000 m)
    P =  10,146,325 Pa
       
    Then we need to compare this pressure to the reference pressure in water (1 µPa).
     
    LP =  20 log 
    P
    P0
    LP =  20 log 
    10,146,325 Pa
    1 × 10−6 Pa
    LP =  260 db  
     
     

practice problem 2

Normal conversation has an intensity level of around 60 dB. Determine the…
  1. pressure amplitude
  2. density amplitude
  3. displacement amplitude
  4. velocity amplitude
  5. acceleration amplitude

solution

The solutions.
  1. For pressure amplitude, compare 60 dB to the reference intensity of 20 μPa at 0 dB. Start with the equation for pressure level in decibels.
     
    I =  P2max
    v

    Solve for Pmax. (I'm going to drop the subscript P from the level symbol L as well as the unit reminder [dB].)

    L
    20
     = log  Pmax
    P0
    10L/20
     =  Pmax
    P0
    Pmax  = ∆P010L/20  
     

    Substitute and compute (a.k.a. plug and chug).

    Pmax =  (20 µPa)1060 dB/20
    Pmax =  (20 µPa)103
    Pmax =  20 mPa
     
    Just a tree place decimal shift written as a prefix change from micro to milli.
  2. We have an equation that relates density amplitude to pressure amplitude.
     
    ∆ρ =  P
    v2

    Let's assume the conversation is taking place in air at room temperature (20 ℃) with a speed of sound of 343 m/s. Divide and conquor!

      ∆ρ =  (0.020 Pa)
    (343 m/s)2
      ∆ρ =  2.5 × 10−7 kg/m3
     
     
    I can't relate to numbers this small.
  3. Before we can compute the displacement amplitude, we'll need the intensity. The density of air at room temperature (20 ℃) is 1.207 kg/m3.
       
     
    I =  P2max
    v
      I =  (0.020 Pa)2
    2(343 m/s)(1.207 kg/m3)
      I =  4.8 × 10−7 W/m2
     

    Intensity allows us to find displacement amplitude.

    I =  2ρƒ2vΔx2max  
     
    Δxmax =  I    
    2ρƒ2v    

    Human speech doesn't occur at just one frequency. Early Twentieth Century telephone engineers determined that most of the power in human speech fit between 300 and 3400 Hz. Let's take the geometric mean of this range…

    √(300 Hz × 3400 Hz) ≈ 1000 Hz

    …and continue.

      Δxmax =  4.8 × 10−7 W/m2
      2(1.207 kg/m3)(1000 Hz)2(343 m/s)
      Δxmax =  7.7 × 10−9m = 7.7 nm
     
     
    A nanometer is about ten atoms in a row. This means the air molecules driving your eardrum only wiggle back and forth a distance of about 150 atoms when listening to human speech. (I doubled the displacement amplitude since they are moving back and forth.)
  4. Velocity amplitude is most easily computed from displacement amplitude. 
       
    vmax =  2πƒ ∆xmax
    vmax =  2π(1000 Hz)(7.7 × 10−9m)
    vmax =  4.8 × 10−5 m/s
       
    Another small number that means nothing to me.
  5. For acceleration amplitude.
       
    amax =  2πƒ ∆vmax
    amax =  2π(1000 Hz)(4.8 × 10−5m/s)
    amax =  0.30 m/s2
       
    Not such a small number anymore. Not a lot when compared to gravity, but not microscopic like the other amplitudes.

practice problem 3

Write something different.

solution

Answer it.

practice problem 4

Write something completely different.

solution

Answer it.