Physics 114 – Lecture 35

Physics 114 – Lecture 35 • Chapter 12Sound • Parameters associated with our sense of hearing: note sound → longitudinal waves propagated in a medium • Source, transmission (longitudinal waves), detector • §12.1 Characteristics of Sound • Wave motion • Speed of sound in air at room temperature is roughly, • v ≈ (331 + 0.60 T) m/s • At room temp, T = 200C, v = (331 + 0.60 X 20) m/s = 343 m/s L35-s1,12

Physics 114 – Lecture 35 • Study Example 12.1, distance from a lightning strike • Loudness is related to intensity (energy/unit time/unit area • Pitch – high, like flute or low, like a string bass. The physical quantity specifying this is the frequency, f = v/λ • Audible range roughly 20 Hz to 20,000 Hz – c.f., 1Hz = 1 cycle/s • Sound waves with wavelengths, λ > 20,000 Hz are said to be ultrasonic, c.f., dogs, bats, … can hear such sounds L35-s2,12

Physics 114 – Lecture 35 • Applications of ultrasound – medicine, autofocusing devices, … • Study Example 12.2 • Infrasonic waves – f < 20 Hz, e.g., earthquakes, thunder, volcanoes, heavy equipment, … can cause injuries L35-s3,12

Physics 114 – Lecture 35 • §12.2 Intensity of Sound: Decibels • Human ear can hear sounds with an intensity range from 10-12 W/m2 to about 1 W/m2 • Sound above the latter intensity becomes painful • The human ear does not perceive loudness to be proportional to the intensity, e.g., a sound that is increased in intensity by a factor of 10 is perceived by the ear to be only about twice as loud • It is therefore more realistic to give sound intensity levels on a logarithmic scale L35-s4,12

Physics 114 – Lecture 35 • The resulting unit is known as the bel • The commonly used unit is the decibel (db) which is one tenth of a bel and is defined as follows, • Where I0 = 1.0 X 10-12 W/m2 , the minimum intensity audible to a good ear • Thus for I = 1.0 X 10-10 W/m2 L35-s5,12

Physics 114 – Lecture 35 • Study examples 12.3, 12.4, 12.5 • §12.4 Sources of Sound: Vibrating Strings and Air Columns • String instruments, standing waves L35-s6,12

Physics 114 – Lecture 35 • Open tubes or pipes L35-s7,12

Physics 114 – Lecture 35 • Tubes or pipes closed at one end L35-s8,12

Physics 114 – Lecture 35 • §12.7 Doppler Effect • The pitch or frequency of a sound is affected by motion of the source or/and of the detector relative to the medium in which the sound is transmitted L35-s9,12

Physics 114 – Lecture 35 • Source moving towards observer • T = 1/f = λ/v • λ´ = λ – ds = λ – vsT = λ – vs λ/v • λ´ = λ ( 1 – vs/v) • v/f ´ = (v/f) ( 1 – vs/v) • Note that if vs > 0, f´ > f L35-s10,12

Physics 114 – Lecture 35 • Moving observer, velocity, v, stationary source • f´ = f ( 1 + vo/v ) for motion approaching source • f´ = f ( 1 - vo/v ) for motion receding from source L35-s11,12

Physics 114 – Lecture 35 • In general for motion of both the source, vs, and the observer, vo , • where the upper sign indicates motion of the source approaching the observer or of the observer approaching the source L35-s12,12

Physics 114 – Lecture 35