L35-s1,12 Physics 114 – Lecture 35 Chapter 12 Sound 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 ≈ ( T) m/s At room temp, T = 20 0 C, v = ( X 20) m/s = 343 m/s

L35-s2,12 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-s3,12 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-s4,12 Physics 114 – Lecture 35 §12.2 Intensity of Sound: Decibels Human ear can hear sounds with an intensity range from W/m 2 to about 1 W/m 2 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-s5,12 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 I 0 = 1.0 X W/m 2, the minimum intensity audible to a good ear Thus for I = 1.0 X W/m 2

L35-s6,12 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-s7,12 Physics 114 – Lecture 35 Open tubes or pipes

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

L35-s9,12 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-s10,12 Physics 114 – Lecture 35 Source moving towards observer T = 1/f = λ/v λ´ = λ – d s = λ – v s T = λ – v s λ/v λ´ = λ ( 1 – v s /v) v/f ´ = (v/f) ( 1 – v s /v) Note that if v s > 0, f´ > f

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

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