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

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

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

Anuran sound production 2 1 Primary membrane (1) 100-200 Hz Second set of membranes (2) which vibrate and are key to frequencies produced Thinner with higher frequency vibration 500-2000 Hz

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 100-200 Hz - amplitude modulation

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

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

Frog calls http://www.youtube.com/watch?v=EJngsbcdIQs http://www.youtube.com/watch?v=rIhx5pIcK0A&feature=related toad http://www.youtube.com/watch?v=LnjJ6mMIYrA chorus frog

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

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

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

Emperor penguin Two frequencies are not harmonically related

Xeno-canto - penguin calls Free online recordings

Song of brown headed cowbird Frequency Pressure L Pressure R

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

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.

Bird songs

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

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

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

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

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

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

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

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!!

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

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

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

2a. Pattern loss by medium absorption At each frequency you can calculate a loss Example: 1 kHz loss in water is 0.008 dB / 100m loss in air is 1.2 dB / 100m Relative amplitude (dB) = 20 log (Pafter /Pbefore) Change frequency spectrum

Losses in water Relative amplitude (dB) = 20 log (Pafter /Pbefore) = -0.008 dB (per 100 m) Pbefore 100 meters Pafter 1 0.999

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 1 0.87

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? 1 0.993

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

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

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.

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

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

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

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

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

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

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

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

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.

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.

For lower frequencies, get a notch in signal amplitude where destructive interference fd=c/(2D) Typically occurs at 300-800 Hz so ground animals call at higher freqs to avoid this

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

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

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.

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

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.

SOFAR (SOund Fixing And Ranging) channel Sounds of certain wavelength fit best and propagate best in this channel fmin=1.8 x 105 / d1.5 d=100 m fmin= 180 Hz

Noise - where does it come from?

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

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.

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

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

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

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.