BIEB 166 Lecture 17 (WI13)

4 Pages
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Department
Biol/Ecology, Behavior, & Evol
Course Code
BIEB 166
Professor
James Nieh

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Description
Lecture 17 Frequency domain sources of signal distortion during propagation Global attenuation 1. Spreading losses - When sound is being produced, it is spreading out to the environment - Even there is nothing blocking the sound, the sound will decrease in loudness as it travel further from the sound source - These are the same for all frequencies ○ Each frequency is affected equally - In air, lose 6dB in pressure per doubling of distance Pattern loss 1. Differential medium absorption - Different objects that are absorbing sound in the environment - Usually higher for higher frequencies Lacewing vibrational communication - Sound transmitting through a substrate through vibration - Medium is the plant stem - Plant vibrational attenuation ○ Spectrum ○ Shows three different spectra on top of each other ○ Measuring at different locations from the source ○ Furthering away from the sound source, more attenuation - Sound is being absorbed by the medium - In each of the individual spectra, there is a peak at the same frequency (2kHz) ○ Frequencies that are not 2kHz are being absorbed more than the 2kHz frequency ○ This illustrates a characteristic of resonance (resonance is at around 2kHz) - The High frequencies, they are absorbed more rapidly - The animal would prefer to communicate at 2kHz 2. Scattering loss - The kind of scattering depends upon the relative size of the wavelengths of the propagating sound, and that of scattering objects - Caused by different forces in the environment Wind scatter - There is 2-35dB attenuation per 100m distance due to wind scatter - Depends on how much the wind is blowing 3. Boundary reflections - Controlled by changes in the acoustic impedance at a boundary - The difference in impedance between mediums causes the sound wave to reflect back - Problem that constraints animal communication - Area of low density to area of high density ○ Reflection of sound More in "patterns" + "water" 4. Refraction - Sound bend as it transmitted to the environment - In terrestrial environments, sounds propagate further when the ground is colder than the air ○ At night  Male may be better to sing at night in order to attract female from longer distances ○ When the caller is below the canopy of a higher forest ○ When the caller is upwind of the receiver  In front of the wind, relative to receiver Patterns of sound propagation Patterns of sound propagation Direct waves - Going directly from the signalers to the ear of the receiver Reflected wave - Bounce off wall/ground and travel to the receiver Boundary waves - Only carry low frequencies and are only detectable when sender and receiver are both close to the ground (both at that boundary layer) - Boundary wave is created in part by the interference of the direct wave and the reflected wave (out of phase, cancel each other) - Intermediate frequencies, (e.g. about 500 Hz), are too high to be conducted by boundary waves, and too large in wavelength to be scattered by irregularities in the surface. Bird singing 1. One singing on a rock, the other one listening on a tree - The receiver first hear the sound that propagated by direct wave - Also hearing a reflected wave (reflected by the ground) 2. When the receiver (female) is now sitting on the ground - She would hear the boundary wave, that is a product of sound moving along the boundary between the air and the ground and what is left over when the direct and reflected wave after cancelling each other out Sound propagation in water In deep aquatic environments - Sounds produced near the surface (fish/whale underneath the water but near the surface) propagate poorly because there is always a phase shift (could result in cancellation of sound) when sound travelling in water reflects off of the surface - Instead of the lower frequencies are propagating, the direct and reflected waves cancel them out and only the higher frequencies are left (different than the land boundary wave example) ○ Land boundary example: only low frequencies propagate ○ Water example: only high frequencies propagate In very shallow water - Standing wave reflections between the water’s surface and the bottom result in the cancellation of all frequencies below some critical value ○ The critical value depends on the depth of water ○ The shallower the water and the softer the bottom, the higher this cutoff frequency - Animals in shallow water must use higher frequencies or none at all to avoid the cancellation (phase shift cancellation) Properties of sound 1. Medium - Among different states of matter (solid, liquid or gas), sound speed positively correlated with density of the state of matter - Within a state of matter, the inertial properties of the medium will influence sound speed ○ Inertial = resistance of a material to movement - Within each state of matter, sound speed is negatively correlated with density because less dense materials have less inertial - Thus, in air, as the temperature goes down, so does the speed of sound ○ The density of the medium decreases, the speed of sound should increase 2. Energy - When sound travels faster, in a given medium, it loses energy mor
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