Information about sound wave and a few concept questions

1. Sound Waves
For Dummies
By Brigette Wee

2. Sound
• As a sound wave passes through a medium, it
creates different regions with varying
pressure:
When medium is compressed, pressure is
higher than ambient pressure
 When medium is stretched (rarefaction),
pressure is lower than ambient pressure

3. Speed of Sound
• Depends on the medium
• Can be expressed as:
 v= (B/ρ)^(1/2)
Where B is called bulk modulus
 B=-V (Δ p/Δ V)

4. Displacement Amplitude
• Sound wave function:

• The relationship between the speed of sound
wave (v), the frequency (f), and the wave
length (λ):
v=fλ=w/k

5. Relationships
• The relationship between displacement,
pressure, and intensity:


6. Intensity
• The energy the that a sound wave delivers:
I=P/A
P is the rate at which the energy is delivered
A is the area the wave is striking on

7. Now onto some concept
questions!!

8. Concept question #1
An airplane 1km above sea water explodes. The
sound wave travels through air then encounters
the sea water. Compare the wavelength of
sound in air and water.
a) λair>λwater
b) λair=λwater
c) λair<λwater

9. Answer to #1
Answer is c)
-The speed of sound in air is 343m/s (a fact at 20o C)
-The speed of sound in water is 1482m/s (a fact at 20o C)
-The time between wave peaks does not change so we know that
frequency for both medium is the same (Note: the frequency does not
change when a sound wave moves from one medium to another)
-We can now use the formula v=fλ
-rearrange it to get λ=v/f
-And plug in the velocity and use a random λ for both cases for
simplicity if not given (but they have to be the same!)
-You’ll see that λwater>λair

10. Concept question #2
• An airplane 1km above sea water explodes.
The sound wave travels through air then
encounters the sea water. If we decrease the
λwater by 10%, compare the speed in air and
water at 20o C in this scenario to the scenario
in question 1.
a) V2>V1
b) V2=V1
c) V2<V1

11. Answer to 2
-Answer is B
- The speed of sound wave does not depend on the
frequency or wavelength of a medium
- Although the speed of a sound wave can be calculated
from the product of the wavelength and the frequency,
altering either λ or f does not change the speed of the
sound in that particular medium.
-The speed of sounds in a medium can only be altered by
the temperature.
-Remember from the previous question that:
-The speed of sound in air is 343m/s (a fact at 20o C)
-The speed of sound in water is 1482m/s (a fact at 20o C)

12. • The end. I hope you learned a
lot!