Sound Ch 14

Producing a Sound Wave Sound waves Caused by vibrations Require a medium in order to propagate Are longitudinal waves https://www.youtube.com/watch?v=Si-OYX20FRs

Characteristics of Sound Waves Diffraction: All waves bend when they pass around the edges of an obstacle

Characteristics of Sound Waves Diffraction

Characteristics of Sound Waves Diffraction

Characteristics of Sound Waves Infrasonic Not audible, below 20 Hz volcanoes, avalanches, earthquakes and meteorites. The eruption of the Fuego volcano in Guatemala produced infrasonic sound in excess of 120 decibels in the range below 10Hz. vocalizations of elephants were measured to have frequencies as low as 14 Hz which were detectable at a range of 10 km. Observations of elephant behavior suggests that they responded to the waves through the ground before they heard them in the air

Characteristics of Sound Waves Ultrasonic above the frequencies of audible sound, over 20,000 Hz communication and navigation by bats, dolphins, etc… Ultrasound imaging or sonography is often used in medicine, testing of products and structures, cleaning, mixing, and to accelerate chemical processes. Animals (e.g. bats and porpoises) use ultrasound for locating prey and obstacles.

Characteristics of Sound Waves

Characteristics of Sound Waves Sound level is measured in decibels (dB)

Characteristics of Sound Waves https://www.youtube.com/watch?v=PAf3p9otexI http://www.telegraph.co.uk/news/science/science-video/11498768/Watch-fire-put-out-using-sound-waves.html https://www.youtube.com/watch?v=1P3FsLMKwJE

Characteristics of Sound Waves https://www.youtube.com/watch?v=6hE6KjLUkiw https://www.youtube.com/watch?v=qNf9nzvnd1k

Characteristics of Sound Waves Piezoelectric effect Conversion of mechanical energy to electrical energy A reversible reaction

Characteristics of Sound Waves X rays vs ultrasound as a diagnostic tool

Characteristics of Sound Waves X rays vs ultrasound as a diagnostic tool

The Speed of Sound The speed of a sound wave in a fluid depends upon the fluid’s compressibility and inertia The speed of sound also depends upon the temperature of the medium In air Vs = speed of sound in air at zero Celsius

Energy and intensity of sound waves Intensity: the rate at which energy flows through the surface E = energy A = area P = power Units: W/m2

Energy and intensity of sound waves Intensity level (decibel level) Relative intensity of sound, compared to the threshold of hearing 1.0 x 10-12 W/m2

Energy and intensity of sound waves https://www.youtube.com/watch?v=9VK4IAAFrPo 3 Waves\Doppler Effect.ppt

Earthquakes

pH

Intensity in Decibels Relative intensity of a sound is intensity level or decibel level (β) Units decibels (dB) The intensity of a given sound is compared to the faintest sound detectable (I0) (1.0 x 10-12 W/m2) Notice that this a logarithmic scale, multiplied by ten

Intensity in Decibels Notice that this a logarithmic scale, multiplied by ten

Interference of Sound Waves What is wave interference?

Wave anatomy Nodes and Antinodes Node: no displacement from neutral position Antinode: maximum displacement from neutral position (crests, troughs) Nodes: destructive interference Antinodes: constructive interference

Interference of Sound Waves

Interference of Sound Waves If two speakers are hooked up to the same sound source producing a monotone sound, then a sound interference pattern can be observed within the room.

Interference of Sound Waves

Interference of Sound Waves Path Difference (PD) = r1 – r2 For antinodes, constructive interference PD = nλ n = 0, 1, 2, 3… (n) nodal line number

Interference of Sound Waves Path Difference (PD) = r1 – r2 For nodes, destructive interference PD = (n + ½) λ n = 0, 1, 2, 3…

Interference of Sound Waves

Interference of Sound Waves Two point sources, 3.0 cm apart, are generating periodic waves in phase. A point on the third antinodal line of the wave pattern is 10.0 cm from one source and 8.0 cm from the other source. What is the wavelength? PD = 2.0 cm Are the conditions constructive or destructive? Therefore PD = and λ = λ = 2.0 cm / 3 = .67 cm

Interference of Sound Waves http://www.acoustics.salford.ac.uk/feschools/waves/super3.php

Standing Waves String All points on the string vibrate with the same frequency, but with different amplitudes The ends of the string must have nodes, but the total number of nodes and antinodes can vary

Standing Waves String All points on the string vibrate with the same frequency, but with different amplitudes The ends of the string must have nodes, but the total number of nodes and antinodes can vary

Standing Waves String The lowest frequency of vibration is the fundamental frequency or the first harmonic.

Standing Waves String The next, higher frequency is the second harmonic

Standing Waves String The next, higher frequency is the third harmonic

Standing Waves String What is the pattern of frequency and harmonics?

Standing Waves String What is the pattern of frequency and harmonics?

Standing Waves String Wavelength of a standing wave The frequency of a standing wave can also be calculated

Standing Waves String The wave equation can be helpful when working on these types of problems

Standing Waves String The wave equation can be helpful when working on these types of problems https://www.youtube.com/watch?v=-gr7KmTOrx0 https://www.youtube.com/watch?v=BbPgy4sHYTw

Forced vibrations and resonance Objects have natural frequencies that they vibrate at Pendulums Wine glasses Bridges Buildings…

Forced vibrations and resonance A vibrating object can stimulate vibration in another object The initial vibration must be at the second object’s natural frequency https://www.youtube.com/watch?v=vNuDxc9tZMk https://www.youtube.com/watch?v=BE827gwnnk4

Standing waves in air columns Describe a standing wave on a string 1st Harmonic 2nd Harmonic 3rd Harmonic

Standing waves in air columns https://www.youtube.com/watch?v=s2ctpx0zYgk

Standing waves in air columns An air column (or pipe) can be open at both ends or closed at one end

Standing waves in air columns Open vs closed end piped instruments https://www.youtube.com/watch?v=N5Ch2NThFvY#t=12

Standing waves in air columns