1. Finish Ch 4 Sound production Ch 5 : Sound propagation 2/8/11
Lecture #4
Finish Ch 4 Sound production
Ch 5 : Sound propagation
2/8/11

2. Next few classes Today and Thursday Next week
Finish discussion of sound and hearing
Next week
Start light and vision
Tuesday I am away - Adam Smith will give intro light lecture
Problem set will not be due till Thursday 2/17

3. D. Vertebrate sound production
Membrane in air flow which vibrates
Mammals
Amphibians
Birds

4. Anuran sound production2 1
Primary membrane (1) Hz
Second set of membranes (2) which vibrate and are key to frequencies produced
Thinner with higher frequency vibration Hz

5. Anuran pressure wave and frequency spectrum
Glottis
opens and closes
Vocal cords
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

6. Air is retained and recycled
Air is not expelled
Collected in throat sac
Recycled back to lungs
Filled sac is a resonant coupler, increasing sound transfer by 2-5 fold
If puncture this sac, the sound measured is 2-5 lower

7. Diversity of frog calls
For Freq Modulation of carrier frequency, need longer call so that have time to change frequency
Independently modulate glottis and vocal cords
AM and FM modulation

8. Frog calls http://www.youtube.com/watch?v=EJngsbcdIQs
toad
chorus frog

9. Avian sound production
Birds at rest have lungs partially full
Lots of air sacs which fill with air on inhalation
Can expel air from lungs using muscles

10. Birds have modified junction of bronchi and trachea
A) Chicken B) songbirds
parrot
Bronchi are cartilaginous rings joined by connective tissue. Some of rings are missing and this is where membrane occurs
Membrane forced in by pressure from interclavicular air sac Labium can be rotated in to change diameter of tube
B&C - two sides can be independent
A - only 2-3% of energy as sound; B) 10-15% of energy as sound
C) nocturnal, penguins
Labium

11. Two sides can be operated independently
Can show act independently
Different usage ratios in different birds
Block one of airways, cut nerves controlling muscles
Use tiny sensors to measure airflow on each side

12. Emperor penguin
Two frequencies are not harmonically related

13. Xeno-canto - penguin calls
Free online recordings

14. Song of brown headed cowbird
Frequency
Pressure L
Pressure R

15. Normal song and after nerve cut to right side
Chaffinch
Normal song and after nerve cut to right side
Most song remains
as left lateralization
Spectrogram of brown thrasher
Use two sides
equally
Chaffinch has strong left lateralization so when cut nerve to right side, most of song remains
Brown thrasher uses two sides almost equally
Breath in

16. Brown thrasher
Two notes are independent as they actually beat against each other
Beat frequency is proportional to difference in frequency between two notes
At left dashed line, two notes are further apart in frequency and so show more beats. As two notes get closer together, the number of beats decreases.

17. Bird songs

18. Ch. 5 Sound propagation Distortions altering frequency domain
Distortions altering time domain
What’s a signaller to do?

19. Sound distortion Sounds are detected at a distance
Distorted and degraded on the way
Major constraint on the evolution of animal signals
Also major constraint in collecting data on animal sounds
Degradation btn animal and microphone

20. Distortions modifying frequency spectrum
Global attenuation
Pattern loss
Differential medium absorption
Scattering
Boundary reflections
Refraction
Addition of noise

21. 1. Global attenuation = spreading loss
Sound spreads out equally in all directions
Assumes far from source = far field
Intensity falls of with distance2
If outer sphere is twice the distance from source as inner sphere, sound intensity will drop by 1/4

22. 1. Global attenuation All frequencies decrease equally
Close to source Further away

23. Spreading losses
Intensity drops with distance2 but pressure drops linearly with distance
If distance doubles,
intensity drops by 4
pressure drops by 2

24. Spreading losses
Intensity drops with distance2 but pressure drops linearly with distance
If distance doubles,
intensity drops by 4
pressure drops by 2
Relative amplitude (dB)
= 20 log (Pafter /Pbefore)
= 20 log (0.5) = -6 dB
= 10 log (Iafter / Ibefore) = 10 log (0.25)=-6 dB

25. Let’s measure
In front of class, Bingjie measured 68 dB about 1 m from computer
In back of class, Haena measured 52 dB which was about 8 m from computer
So we measured -16dB change in sound intensity
Since intensity falls as 1/distance2 we predict Iback/Ifront=(1/8)2 = 1/64
Rel amplitude(dB) = 10 log (Iback/Ifront)
=10log(1/64) = -18dB
This is close to the -16dB change that we measured!!

26. Pattern loss By passing through medium sound will change Intensity
Frequency spectrum
Direction

27. 2a. Pattern loss by medium absorption
In absorption, some frequencies decrease more than others
Change frequency spectrum
Where does sound go?
Becomes heat. Some frequencies excite the surrounding molecules and hence are absorbed better than others

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

29. 2a. Pattern loss by medium absorption
At each frequency you can calculate a loss
Example: 1 kHz
loss in water is dB / 100m
loss in air is 1.2 dB / 100m
Relative
amplitude (dB) = 20 log (Pafter /Pbefore)
Change frequency spectrum

30. Losses in water Relative amplitude (dB)
= 20 log (Pafter /Pbefore) = dB (per 100 m)
Pbefore
100 meters
Pafter

31. Losses in air Relative amplitude (dB)
= 20 log (Pafter /Pbefore) = -1.2 dB (per 100 m)
Pbefore
100 meters
Pafter
Very counterintuitive - much more loss in air than in water
Losses are higher as temperature increases.
Losses are lower as humidity increases

32. Losses in air across this room
Losses are -1.2 dB per 100 m
Pbefore
5 meters
Pafter
Could we measure this change across our room?

33. Losses in other media
Losses in salt water are 1050x those in freshwater (8 dB / 100m)
Losses in the ground are 6dB / cm
So this is 10,000 worse than salt water

34. 1+2. Spreading loss + medium absorption
Amplitude of sound

35. 2b. Pattern loss from scattering
Objects in medium will cause sound to scatter
Angular patterns depend on object size relative to sound wavelength
>object
object
object
Here I’ve drawn the wavelength of sound and an object which has a diameter typical of a tree.

36. Types of scattering : Rayleigh scattering
Object< 
Scatters in all directions

37. Types of scattering : Rayleigh scattering
Object< 
Scatters in all directions
Scattering decreases with and increases with object size
Scattering
Scattering
Wavelength
Object

38. Types of scattering : Mie scattering
Object ≈ 
Strong angular dependence due to interference of scattered and diffracted wave
Scattering
Object / Wavelength

39. Types of scattering : Simple scattering
Object > 
Object makes a shadow
Scattering
Object / Wavelength

40. Scattering summary 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

41. What kinds of things scatter sound?
Objects in the environment: trees, fish
Density gradients from wind or temperature variation

42. Objects which scatter Objects in the environment: trees, fish
Density gradients from wind or temperature variation

43. 2c. Pattern loss from boundary reflection
Reflection from objects >>
Air: ground, temperature inversion layers
Water: surface, bottom
Can alter frequency distribution

44. Boundary reflections Three kinds of waves
Direct wave Straight line from sender
Reflected wave Bounces off surface
Boundary wave Travels along surface
If R2 large, most of sound reflected; if it is smaller, some will be in boundary wave
Boundary wave can be sound absorbed by ground, travels along and then reradiates
Can also be surface wave just on surface of ground
All sounds combine additively at receiver. Can get constructive and destructive interference.

45. Boundary reflections Sound at air / ground interface shifts phase
As a result, two waves may destructively interfere if
D = 1/2  but since  = c/f this occurs for
fd=c/(2D)
Boundary wave can be sound absorbed by ground, travels along and then reradiates
Can also be surface wave just on surface of ground
All sounds combine additively at receiver. Can get constructive and destructive interference.

46. For lower frequencies, get a notch in signal amplitude where destructive interference
fd=c/(2D)
Typically occurs at Hz so ground animals call at higher freqs to avoid this

47. 2d. Pattern loss from refraction
When sound crosses boundary, it will refract (change direction)
Low Z
High Z
High Z
Low Z
Bends away from surface
Bends towards surface

48. Pattern loss from refraction
Temperature gradients cause air density gradients - Sound will bend
Day
On left, the ground is warmer than the air and sound will bend upward, creating a shadow where sound does not go
This is also the case if the wind is blowing. In wind, the sound velocity is faster near the ground where wind is slower. So Impedance is higher near ground. As sound moves up, wind speed gets higher and sound velocity gets lower. So sound will bend up.
Vel

49. Pattern loss from refraction
Temperature gradients cause air density gradients - Sound will bend
Day Night
On right, at night, ground temp cools quickly leaving warmer layer of air up above. Then sound velocity and Z is low close to ground and sound will cross into higher Z as it goes up. As a result, sound will be bent back towards the ground. This creates a sound channel close to the ground so sound does not penetrate the upper air layers.
Also occurs in closed forest where canopy warmed by sun creates a warm layer.

50. Refraction in water Similar thing happens in water but inverted
Surface is warmer so sound is faster
Sound bends from high Z (surface) to low
(depth)
In winter, surface is colder. This creates a sound
channel

51. Refraction in water
In deep water, the surface water is warmer and low pressure so high Z. At depths, the high pressure makes Z large. In between, Z is lower so create a sound channel called the SOFAR channel (sound fixing and ranging) This occurs at about 1200 m. Sound can travel for 100s-1000s km. Whale may use this for long distance communication.

52. SOFAR (SOund Fixing And Ranging) channel
Sounds of certain wavelength fit best and propagate best in this channel
fmin=1.8 x 105 / d d=100 m fmin= 180 Hz

53. Noise - where does it come from?

54. 3. Noise Adds new frequencies to the sound Terrestrial sources:
Wind over vegetation, head of receiver
Less in forest than over grasslands
Less in morning, more midday
Insects
Aquatic sources:
Surface, wind, waves
Because of high transmission, noise travels far in water

55. 3. Noise spectra Forest Grasslands Deep ocean Shallow ocean
Verts can try to make signals at frequencies in the low notches of the noise, or increase total output above noise levels.
Fish may not be able to make higher frequencies because limitations of swim bladders so may just have to live with high noise levels below 1 kHz.

56. B. Distortions modifying time domain
Global attenuation
Same as for frequency - decreases with distance
Pattern loss
Addition of noise
Alters temporal pattern of sound

57. B. Distortions modifying time domain
Global attenuation
Same as for frequency - decreases with distance
Pattern loss
Addition of noise
Alters temporal pattern of sound

58. Pattern loss from reverberations
Multiple paths between sender and receiver
Echoes from boundaries and scattering
Especially a problem in forests
Low freq (<1 kHz) reflect off canopy, ground
High freq (>3 kHz) reflect off foliage
So what is best freq for communication?
Best is freq btn 1 and 3 kHz - so that is often range used by birds

59. Reverberations Echoes arrive later and with lower amplitude
Even worse in water than air because sound travels farther
Sender’s signal Received signal
In air, reverberation is 1/2 signal after 100 ms. So don’t want to modulate faster than 10 Hz as need time for reverb to decrease.