1. Assignment #3
Sound
2/17/11

2. 1. Vocal cords of frog involve two separate membranes

3. 1. How do A and C differ?
High frequency (1-2 kHz) is generated by vocal cords (lighter upstream membranes) which are gated on and off by glottis at Hz - amplitude modulation
Use glottis to amplitude modulate sounds from vocal cords
C) Leave glottis open while frequency modulate vocal cords

4. 2a) Sound losses in water
Water absorption = dB / 100 m
So between 100 and 200 m the sound amplitude decreases by 0.008dB
Relative amplitude = dB = 10 log(I200m/I100m)
= log10(I200m/I100m)
I200m/I100m = = 0.998
So only 0.2% loss due to water absorption

5. Alternative way to solve
Between source and 100m
0.008dB * 100m = 0.008dB = 10 log (I2/I1)
100m
I2/I1 = = = Isource/I100m
Think about what is I1 and I2 in this case.

6. Alternative way to solve
Between source and 100m
0.008dB * 100m = 0.008dB = 10 log (I2/I1)
100m
I2/I1 = = = Isource/I100m
Between source and 200m
0.008dB * 200m = 0.016dB = 10 log (I2/I1)
I2/I1 = = = Isource/I200m

7. Alternative way to solve
Between source and 100m
0.008dB * 100m = 0.008dB = 10 log (I2/I1)
100m
I2/I1 = = = Isource/I100m
Between source and 200m
0.008dB * 200m = 0.016dB = 10 log (I2/I1)
I2/I1 = = = Isource/I200m
So from this we can figure

8. 2b) Sound losses in air Air absorption = -1.2dB / 100 m
So between 100 and 200 m the sound amplitude decreases by 1.2 dB
Relative amplitude = -1.2dB = 10 log(I200m/I100m)
-0.12 = log10(I200m/I100m)
I200m/I100m = = 0.758
So lose 24.1% due to air absorption

9. 2c) Which media transmits better
Water loss is 0.2%
Air loss is 25.4%
So water transmits far better

10. 2d) Spreading loss Since intensity falls off with 1/distance2
In going from 100 to 200 m
So in terms of relative amplitude
So -6dB per 100m

11. 2d) Spreading loss Since intensity falls off with 1/distance2
In going from 200 to 400 m
So in terms of relative amplitude
So -6dB per 200m which is -3dB per 100m

12. Compare losses in air at different distances
So close to sound source, spreading loss is most important
Further away, absorption is biggest loss

13. Sound losses will depend on the sum of spreading loss and absorption
- as well as any other losses
sum

14. 3a. Talking around a campfire
Factors that might impact getting sound from one person to another
Sound absorption
Sound refraction - density gradients of cold air and hot gas of fire will bend sound
Sound scattering
Noise from fire

15. Losses are a function of frequency
Lecture 4 slide 28
Losses are a function of frequency
Higher frequencies have higher losses
Loss at 10 kHz is 10x loss at 1 kHz
Loss
Frequency

16. Scattering summary Lecture 4 slide 40 Fig 2.6
Bigger objects, more scattering. Shorter wavelength, more scattering.
For 1 kHz, wavelength is 33 cm. So for objects < 5 cm, Rayleigh and objects>3.3 m, simple scattering. In between get complex scattering pattern.
Fig 2.6

17. Scattering summary Fig 2.6 Lecture 4 slide 40 Large Wavelength Small
Bigger objects, more scattering. Shorter wavelength, more scattering.
For 1 kHz, wavelength is 33 cm. So for objects < 5 cm, Rayleigh and objects>3.3 m, simple scattering. In between get complex scattering pattern.
Large Wavelength Small
Small Frequency Large

18. 1. Global attenuation All frequencies decrease equally
Lecture 4 slide 22
1. Global attenuation
All frequencies decrease equally
Close to source Further away

19. 3b+c. Frequency effects Absorption is higher for higher frequencies
Scattering is worse for higher frequencies
Mosquito’s sound won’t carry as well as friend
Bear will carry better
Spreading loss is the same for all frequencies

20. 4. Wavelengths vs body sizes
Since we know the sound frequencies, we can convert these to wavelengths using the appropriate sound velocity
c / f
Where I used
c = 344 m/s for air
1513 m/s for water
Guesstimate the body sizes:

21. Animal
Frequency Hz
Wavelength,  (m)
Body length (m)
/body length
Range?
Mosquito
500
0.69
0.002
344
Short
Cicada
2000
0.17
0.02
8.6
Bat
100,000
0.0034
0.1
0.03
Long
Frog
4000
0.086
0.05
1.72
Human
400
0.86
1.5
0.6
Fish
100
15.1
0.3
50.4
Whale 20
0.76
Dolphin
50000 3
0.01

22. 4. Animal sounds
So many animals do use frequencies that they can generate at high intensity for long distance communication
However some do not
Mosquito - have you ever heard a mosquito across a room?
Cicada - I think they overpower us with their numbers since any one of them can’t generate a lot of sound
Fish - generally communicate over short distances

23. 5. Aquatic animals In aquatic habitat, high freq can be better
Avoid notch due to surface reflection
Avoid low frequency noise
Attenuation is worse at high freq but not too bad since low overall attenuation in water
Fish - exception in using low freq
Whales - also use low freq for long range