Sound Holt 2006 - Chapter 12

Sound is …. An oscillation Longitudinal wave Cannot travel through a vacuum Can be reflected, diffracted and can interfere

Types of Waves Mechanical – a wave that propagates through a deformable elastic medium (needs a medium to travel) 2. Electromagnetic – does not need a medium to travel

Mechanical Waves Longitudinal waves - Waves move parallel to the wave direction EX: Sound Wave

Longitudinal Waves Anatomy

Longitudinal Waves Compression- region of high density and pressure (become crests in electric waves) Rarefaction- region of low density and pressure ( become troughs in electric waves)

Pitch Pitch - a measure of how high or low a sound is perceived to be, depending on the frequency of the sound wave Pitch is equivalent to frequency

Detection of Sound

Ear A sound detector converts the energy of a sound wave into different forms of energy 3 parts of the ear outer ear middle ear inner ear

Outer ear Pinna – ( auricle)- catches sound waves Ear canal- channels sound waves ( the cilia and wax protect) Ear drum- thin membrane that vibrates

Middle Ear 3 Bones malleus ( hammer) incus (anvil) stapes (stirrup) Eustacian tube – equalizes pressure in the middle ear from the throat

Inner Ear Oval Window- thin membrane that vibrates 3 semicircular canals- contains fluid that vibrates Cochlea- snail like project has hair like cells tha when bent produce electrical impulse sent to the auditory nerve Auditory nerve- sends electrical impulse to the brain

Human Hearing 20 vps – 20,000 vps Causes of Deafness Brain damage Extended periods of loud noises Calcification of bones Damage to parts of the ear explosions

Ultrasonic- above 20,000 Hz bats, dolphins, ultrasound medical devices,… Infrasonic- below 20 Hz earthquakes, thunderstorms, wind and waves, explosions, compressors, turbines, low speed fans

Doppler Shift Change in the wavelength of sound emitted by a moving source

Speed of Sound Depends on the medium See page 410 table 1 What two statements can be made about the listings in Table 1?

Ultrasound See story on page 410 Ultrasonic waves can be used to produce images of objects inside the body Sound waves are partially reflected when they reach a boundary between two materials of different densities

Sound Waves travel best in… Materials having molecules close together In solids the worst in gases Warm heavier air – molecules are closer Speed of sound in air at room temperature is 343 m/s

Lightning and Thunder Light waves travel nearly 1 million times faster than sound waves in air Explain the difference in timing of the lightning and thunder.

Speed of Sound Equation v = f l v = velocity in m/s f = frequency Hz ( 1/sec) l = wavelength ( m)

Sources of Sound Loudness indicates the amplitude of the wave Intensity is measured in decibels Sound waves are produced by vibrating objects Echo is a reflected wave

Musical Instruments Sound of a wind instrument is the result of a vibrating column of air Sound of a Stringed instrument is the result of the vibration of a sounding board

Strings on Instruments Higher pitch- tighten string, shorten string, thin strings Lower pitch – loosen string, lengthen string, thick string

Resonance Is a large motion of a system due to excitation

Resonance in pipes Closed pipe resonates when its length is l/4 3l /4 5 l /4 Open pipe resonates when its length is l/2 2l /2 3 l /2

Open Tube Harmonics or resonance lengths

Closed Tube Harmonics or resonance lengths

Resonance - closed A Student is finding the resonance of a closed tube and finds the resonance is spaced by 34.7 cm. The air temp is 23.5 o . What is the speed of sound? What is the frequency of the tuning fork? 0o =331 m/s goes up by 0.6 for every degree 0.6 (23.5) + 331 = 345 m/s Closed: L = l / 4 so l = 4L or v = f (4L) = 4 ( 0.347m) = 1.388 m V = f l f = v / l f = 345 m/s / 1.388m = 248.55 Hz = 249 Hz

Diameter of Resonance Tube Changes the resonance length Correction factor: L = l + 0.4 d Example: 5 cm diameter tube with a resonate length of 46.3cm L = 46.3 cm + 0.4 (5cm) = 48.3 cm = 0.483m

Harmonics Example- Open What are the first three harmonics in a 2.45 m pipe, that is open at both ends? Assume that the speed of sound in air is 345.0 m/s. fn = n v / 2L where n = 1,2,3,…. F1 = 1 ( 345m/s) / 2(2.45) = 70.4 Hz F2 = 2 ( 345m/s) / 2(2.45) = 141 Hz F3 = 3 ( 345m/s) / 2(2.45) = 211Hz

Resonance Example - closed What are the first three harmonics in a 2.45m pipe, when one end is closed? Assume that the speed of sound in air is 345.0 m/s. fn = n v / 4L where n = 1,3,5,…. F1 = 1 ( 345m/s) / 4(2.45) = 35.2 Hz F2 = 3 ( 345m/s) / 4(2.45) = 106 Hz F3 = 5 ( 345m/s) / 4(2.45) = 176 Hz

OR f1 = 1 ( 345m/s) / 4(2.45) = 35.2 Hz f2 = 3 ( 345m/s) / 4(2.45) = 106 Hz or f2 = n f1 = 3 ( 35.2) = 106 Hz f3 = 5 ( 345m/s) / 4(2.45) = 176 Hz or f2 = n f1 = 5 ( 35.2) = 176 Hz

Resonance ( at least one end is open in these types)