BIEB 166 Lecture 14 (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 14 Sound fields - Sound is the propagation of a local perturbation in molecular density - Sound is detected by comparing local pressure exerted by medium molecules relative to that of some ambient reference level (ears and microphones do this) - This spatial distribution of deviations from ambient pressure levels at any given instant of time is called the sound field Pressure wave - Molecules of air moving near the sound source when sound is produced (i.e. hitting a drum) - Short distance away, the air molecule motions are translated to pressure ○ The molecules will no longer travelling any appreciable distance (now vibrating) - Pressure component - Near-field component Ultrasound and Infrasound - Most human can hear 22kHz 1. Ultrasound - Extremely high frequency sounds (>100kHz) - Reflects back very detailed information - Rapidly absorbed by the environment (doesn't transmit very far) - Used by animals such as bats/dolphins ○ Echolocate ○ Image the environment 2. Infrasound - Extremely low frequency sound (below 5Hz) - Carries great distances Elephant - Frequencies are too low for human to detect - They lift up their legs when they generate sound ○ The infrasound may perceive through substrate, and being picked up in part through the legs of the animal 1. Mating of Females - Cheeks are vibrating when the females are ready to mate ○ Sound couldn’t be heard - Elephant female has a very long gestation period ○ Elephants scattered from miles seem to know when they're ready to mate ○ They produced some kind of signals to inform the other males - Play back the low frequencies ○ Male elephants from kilometers away they broken to a run towards the speakers Longitudinal vs. Transverse waves 1. Longitudinal - Air molecules are compressed or not compressed Vocal cord vibrating as human speaks 2. Transverse wave - Vibration transmitted through a substrate - How a surface is vibrating (going up and down) Elephants creating vibrational communication Free Field - Infinite medium in which there is nothing to block or reflect the sound - Free of obstructions - The sound would propagate outward - No echoes - No echoes - Ideal situation of propagating sound Water waves (liquid medium) VS Rayleigh wave (solid medium) Relative amplitude: decibel scale (dB units) - Sound covers a wide range of pressures, and the dB scales is the convenient way to express the range of pressures - Relative amplitude ○ How loud something is relative to a certain reference - dB = 20 log10(P measuredreference ○ The decibel measurement is always relative. Therefore you have to specify a P reference level. - Every 20dB increase in pressure corresponds to a 10 fold increase in pressure 60 dB increase = 1000 times increase in pressure (10 ) - For sounds in air, a common reference is 2 x 10 dynes/cm2. This reference is denoted as dB SPL Rustling leaves = 10 dB SPL Describing simple sound waves Wave form 1. Sine wave - Amplitude: loudness of the sound Time measures 1. Period = T - Duration of 1 cycle - Crest to crests/trough to troughs 2. Phase - Relative shift between peaks - Measured in degrees or radians - Out of phase: 180 degrees phase shift Noise cancelling headphone - Playing sound that is exactly the same but is 180 degrees out of phase 3. Frequency = f - Number of cycles per second - The unit of frequency is called the Hertz, abbreviated Hz (cycles/sec) - f = 1/T Spatial measures 1. Wavelength - How long does it take in the periodic oscillation/disturbance of the air for the wave to go from one p
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