Sound Signal Propagation and Reception

propagation

movement of sound waves throughout a medium

signal degradation

change in an acoustic signal as it moves through its environment

examples of signal degradation

• reduction in signal amplitude

• Filtering out of higher frequencies

• Reverberations or echoes that blur temporal (time) patter of signals

reduces

• on a hot day

• into oncoming wind

• sounds at top or on a hot day go upwards → ________ range at which vocalizations can be heard, or the “active space”

increases

• clear night/dawn

• into the direction the wind is moving

• sounds that are produced below the canopy → _______ an animal's “active space” (“dawn chorus”)

Right under the canopy if it’s a hot day

If you were an animal calling in a rainforest, where would the ideal canopy location be if you wanted your signal to transmit the farthest?

attenuation

reduction in energy→ if you were standing next to someone yelling it would be very loud, but if you are standing across a football field from them it would be quieter

lower frequencies

how animals use attenuation so their calls can travel longer distances

higher frequencies

__________ attenuate faster than lower frequencies

farther

longer wavelengths travel _______ without losing energy (lower frequencies)

scattering

affects propagated signal by filtering out certain frequencies → occurs when objects in environment have different acoustic impedance from rest of the medium

open field

slow decline in amplitude

Deciduous forest with leaves

greatest amount of scattering because it causes high frequencies sounds to not travel far → snow can also cause this (eerily quiet)

reverberation

sounds recorded at distance from source can be distorted by echoes

ambient noise changes by environment

• Different environments dominated by different frequencies of background noise

• Can affect propagation of animal sounds

Traffic noise is low in frequency so it travels farther

How would you quantify the ambient noise in an urban area?

Birds in an urban environment have higher frequency songs because the birds in urban environments are increasing the frequency of their vocalizations to overcome the traffic noise. Calling louder or higher to overcome the traffic noise

How does environment affect song characteristics (great tit)?

mechanoreceptors

All receptors used to detect sound are generalized __________: responds to mechanical pressure or distortion

At close range (near field)

can detect and track direction of passing waves, can do this with a single sensor → crustacean statocyst, fish ear with otolith or insect antennae

At farther range (far field)

can only detect passing sound waves. Relies on more than one sensor to track direction. → insect pressure differential ears or vertebrate pressure detector ears

tonotopy

when different regions respond to different frequencies

inversely

The emitted call frequency of an animal scales _______ with body size (bigger body size; lower the frequency)

higher, lower

Small animals= _______ frequency vocalizations

Larger animals= _______ frequency vocalizations